22810820 Data Communication and Networking

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DATA COMMUNICATION AND NETWORKINGChapter – 1

Fundamental of Data Communications1. Introduction

Data Communication is a system consisting of carries and related devices used to transport data from one point to another. Communication means to convey a message, an idea, a picture or speech that is received and understood clearly and correctly by the person for whom it is conveyed.Ancient Methods of Communication and Their Demerits – Message were sent in olden times either through horse riders or by using pigeons. There was no surety that the messenger will be able to convey the message exactly in the same form as told to him verbally.Electronic Methods of Communication – With the invention of telephone instrument and the communication satellites, the means of electronic communication has become very popular in India even though the cost of installation and maintenance of telephones is still very high and beyond the means of a common man.Limitations of Telephonic Communication:

(a) Both the sender and the receiver of the message should be available at the same time and should speak the same language to understand.

(b) Telephone communication is not a secured means of communication because anyone can overhear the message.

(c) It is not suitable to send picture or any other type of message except a spoken message.(d) It is affected by the electrical interference or by the people digging roads etc.(e) It is still quite costly to make a telephone call outside the city or the country.

Computerized Communication – Since the time computers have started playing an important role in the field of communications. The main reason for this is that computers can send data extremely fast. They can even transmit pictures and sound in a much secured manner. Further, PCs can send information on the existing telephone line.Advantage of Computerized Communication –

(a) Telephonic calls, using Internet, can be made to any part of the world with the same expenses as a local telephone call made within the city.

(b) Pictures, sound and written matter can be sent within minutes and a confirmation about it reaching at the destination can be obtained immediately.

(c) Message can be sent in coded form so that they are not understood by anybody else except the person who is sending and the person who is receiving them.

(d) Message can be sent in any language from any place of world to any place.(e) Users need not take highly specialized training for sending or receiving message.

2. Communication Systems A Communication system is the combination of hardware, software and data transfer links that

make up a communication facility for transferring data in a cost effective and efficient manner. A communication system itself can be either analog or digital. The technique by which a digital signal is converted to its analog form is known as Modulation. The reverse process i.e. conversion of analog signal to digital signal is known as Demodulation. These processes of conversions carried out by a special device called Modem. Advantage of Digital Transmission over Analog Transmission:–

(a) The voice data, music and images can be interspersed to make more efficient use of the same circuits and equipment.

(b) Much higher transmission rates are possible using telephone lines.(c) Digital transmission is much cheaper than analog transmission.(d) Maintenance of a digital system is easier than maintenance of analog system.(e) A digital signal can pass through an arbitrary number of regenerators in with no loss in signal

and thus travel long distances with no information loss. In contrast, analog signal always suffer some information loss when amplified, and this loss is cumulative.

3. Signal And Data Data in a communication system can be either digital data or analog data.

Digital Data – On the hockey playground, the referee blows a whistle and all the players in the field understand the message instantaneously. The whistle blown in short bursts of high pitched sound like PEE, PE or it may have a long burst PEEEEEEE. The first one is indication to the players to start the game and the second long whistle is to stop the match immediately. The message conveyed by the sound energy in short pulses is very clear to all the players. This is an example of Digital Data Transmission.Analog Data – When we sit in a concert hall where many musical instrument being played by different players. For example say one player to playing sitar and other is playing tabla. This is an analog data communication. Both sitar and tabla are sending sound waves in the same sequence and there is a rhythm and harmony between the two. Any music system conveys the songs in the analog form.Different Characteristics of Analog and Digital Data Communication: -

NOTES BY-BALJEET SINGH SINWAR

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DATA COMMUNICATION AND NETWORKINGItem Analog Transmission Digital TransmissionForm It is in the form of continuous

variable of physical quantitiesIt is in the form of discrete quantities and has binary digits

Cost of transmission Low HighEfficiency Low highMaintenance cost of equipment

High Low

Effect of noise High LowAttenuation High LowExample TV transmission from DoorDarshan Data transmission from hard

disk to memory4. Channel Characteristics

A communication channel provides the medium to move electromagnetic energy from a source to one or more destination points. It is a pathway over which data are transferred between remote devices.Characteristics: -

(a) It should be able to deliver maximum amount of electromagnetic energy from the transmitter to the receiver with minimum cost.

(b) It should not add much noise on the way so that the receiver is able to understand the message correctly.

(c) There should not be any restriction on the distances between the transmitter where the sender is located and the receiver where the signal is received.

Types of Communication Channels: -There are two types of communication channel used in data communication. These are:

(a) A public telephone system(b) A commercial radio station

Both these channels are used for transfer of voice in analog form. The other type of channel is used for the transmission of the data between a PC and a printer. This carries digital data and transmits square waves. The digital signal between a PC and printer also gets attenuated if the distance of the printer is long.Digital Channel Capacity: -

The capacity of a digital channel is the number of data bits a channel conveys in one second. The measurement is in bits per second (bps). It is also known as bit rate of channel. The bit rate of networking ranges from kilobits per second or Kbps to millions of bits per second. The duration of a binary digit determines the bit rate. The shorter duration of bit is the cause of the greater the bps rating of the signal.

Relationship between bit time and bit rate per second: -Bit time (milli

sec)Bit rate per second(bps)

Bit time (milli sec)

Bit rate per second(bps)

3.3 300 .833 1200.416 2400 .104 9600.052 19200Baud and Bit Rate: - Baud is a measure of the digital signaling rate in a channel. Bit rate is a measure of the digital bit values the channel conveys with each baud.

The only way to increase the digital bit rate is to decrease the bit time of the signal. But electrical characteristics of the material used for conveying the bits limit the reduction in the size of the bit time and thus fixing the maximum bit rate per second.Maximum Data Rate of a Channel: - The maximum data rate of a noisy channel whose bandwidth is in Hertz (Hz), and whose signal-to-noise ratio, shown as S/N in decibels, is given by:Maximum number of bits/sec = H1092 (1 + S/N)5. Transmission Modes

There are three modes of data transmission. These are:(a) Simplex – Simplex communication imply a simple method of communication. In simplex

communications mode, there is a one way communication transmission. Television transmission is a very good example of this type of communication.

(b) Half-duplex - In half-duplex mode, both units communicate over the same medium, but only one unit can send at a time. While one is in send mode, the other unit is in receiving mode. It is like two polite people talking to each other—one talks, the other listens, but neither one talks at the same time.

(c) Full-duplex - In a half-duplex system, the line must be "turned around" each time the direction is reversed. This involves a special switching circuit and requires a small amount of time (approximately 150 milliseconds). With high speed capabilities of the computer, this turn-around


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DATA COMMUNICATION AND NETWORKINGtime is unacceptable in many instances. Also, some applications require simultaneous transmission in both directions. In such cases, a full-duplex system is used that allows information to flow simultaneously in both directions on the transmission path. Use of a full-duplex line improves efficiency as the line turn-around time required in a half-duplex arrangement is eliminated. It requires four wires.

6. Asynchronous and Synchronous Transmission Asynchronous Mode: - Asynchronous mode refers to a series of events that take place which are not synchronized one after the other. Asynchronous Transmission: - Asynchronous transmission is often referred to as start-stop transmission because of its nature, that is the sender can send a character at any time convenient and the receiver will accept it. Asynchronous communication lines remain in an idle state until the hardware on the line is ready to transmit. Since the line is idle, a series of bits have to be sent to the receiving node to notify it that there is more data coming. When data is finished, the node has to be notified that the transmission is complete and to go back to an idle state, hence the STOP bits are to be sent. This pattern continues for the duration of the time the link is operative. This is the characteristic of many terminals when on a terminal, the time spent between successive keystrokes would vary. Thus, in asynchronous transmission, data is transmitted character by character at irregular intervals.Synchronous Transmission: - Synchronous devices need not use Start and Stop bits; so coordination between the two nodes, i.e. the sender and the receiver, is handled differently. In synchronous communications, there are two "channels" - one for data and another for link Synchronization. The channel for synchronization uses the integral clock in the hardware for link synchronization between the two nodes when one of the nodes is ready to transmit data, a unique combination of bits called a Sync Character is sent to the receiver. Since the first character will probably get trashed, a second one usually follows to ensure that synchronization is complete.Comparison between Asynchronous and Synchronous Transmission: - Synchronous communications tend to be more expensive than asynchronous ones as the

hardware involved is more costly due to integral clocking mechanism that have to be used as well as more sophisticated engineering efforts.

Synchronous transmission is well suited to remote communication between a computer and such devices as buffered card readers and printers. It is also used for computer to computer communications.

The primary advantage of synchronous transmission is its efficiency. Not only does it eliminate the need for individual start-stop bits on each character, but much higher data rates can be used than with asynchronous transmission.

Asynchronous transmission is well suited to many keyboard type terminals. The advantage of this method is that it does not require any local storage at the terminal or the computer as transmission takes place character by character. Hence it is cheaper to implement.

Efficiency of Data Transmission in Synchronous and Asynchronous Modes: - Asynchronous data incorporates the use of extra framing bits to establish the start and ending (stop) of a data character word. A receiver responds to the data stream when it detects a start bit. A data character is decoded and defined after the stop bit is received and confirmed. Asynchronous data are easier to detect and synchronize, but the efficiency of data transmission is reduced by the addition of framing bits as overhead (no message data) bits.

A comparison of a single character using the two data types is as follows. For this purpose, the ASCII code of the letter E (1000101) is used. The order of transmission is to send the Least Significant Bit (LSB) first. The number of framing bits used for asynchronous data varies depending on the stations in the communication link. For example, suppose we use 1 start and 2 stop bits. This adds 3 more bits to the character 'word. Hence total 10 bits are required to send the letter E using asynchronous data. However, in' the case of synchronous transmission, only 7 bits are required for transmission of the character E.The efficiency of transmission is defined as the ratio of the number of message bits to the total number of transmitted bits:

or % efficiency = to ta lb itsd a ta b its x 100

Cyclic Redundancy Check (CRC): - In synchronous communications, CRC is used to verify the integrity of the entire packet or block of data. Integrity of the packet means whether the complete packet of data is received in its correct form as it was sent at the sending end. In synchronous communications, parity checking is sufficient to ensure data integrity. In high-speed asynchronous communications, single bit corrections are not enough. As each packet is created, a CRC check is placed somewhere in the packet and is verified at the receiving end.

CRC is a calculation method used to check the accuracy of a digital transmission over a communication link. The sending computer uses one of several formulas to calculate a value from the


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DATA COMMUNICATION AND NETWORKINGinformation contained in the data, and this value is appended to the message block before it is sent. The receiving computer performs the same calculation on the same data and should derive the same number. If the two CRCs do not match, indicating that a transmission error has occurred, the receiving computer asks the sending computer to retransmit the data.

This procedure is known as a redundancy check because each transmission includes extra or redundant error-checking values as well as the data itself. A CRC is generated by dividing the total number of bits in the block of data being sent by a predetermined binary number. The remainder is then added to the packet and the packet is transmitted. On the receiving end, the reverse mathematical operation is performed to verify the packet contents. If the computation is successful, the packet is passed to the next step. If it fails, the issuing node is notified and the entire packet is retransmitted.

Common CRC patterns are 12-bit (CRC 12), 16-bit (CRC-16 and CRC-CCITT), and 32-bit (CRC-32)Chapter – 2

Transmission MediaIntroduction – Transmission media is the general term used to describe the data path that forms the physical channel between sender and the receiver1. Guided Media – The Guided Media refer to the media in which the signals are guided through a

solid medium, such as copper wire, optical fiber etc. Examples of guided media are the following:(a) Twisted-pair Wire – A twisted-pair cable consists of two insulated copper wires, typically about 1

mm thick, here the wire are twisted together in a helical manner.Characteristics:

Twisted pair consists of two insulated copper wires. The thickness of coils is about 1 mm. The wires are twisted together. Twisted pair is commonly used in local telephone communication. For digital transmission over short distances up to 1 km.

Advantage: Trained men power is available to repair and service the media. In a telephone system signals can travel several kilometers without amplification, when

twisted pair wires are used. It is used for both i.e. analog transmission as well as digital transmission. It is least expensive. If a portion of a twisted pair cable is damaged the network is not effected very badly.

Disadvantage: This cable has poor protection layer that’s why easily pick up noise. It is likely to break easily.

(b) Co-axial Cable – A Co-axial cable consists of a stiff copper wire as the core, surrounded by an insulating material. Characteristics:

Co-axial cable consists of a copper wire as the core surrounded by an insulating material. It is available in two forms i.e. 50-ohm and 75-ohm. 50-ohm cable is used for digital transmission. 75-ohm cable is used for analog transmission.

Advantage: Co-axial cable is used to span the network to long

distance at higher data rate (bit/s). It is used for both digital and analog transmission. It has higher Band-width. It is in-expensive as compare to fiber optics.

Disadvantage: In respect to twisted pair, it is expensive. Failure of portion, may affect the whole network. Network cannot be extended above 1 km. Bandwidth is not constant, varies according to

length (Distance covers).(c) Fiber Optics – Fiber optics is the newest form of guided

media. This media is superior in data handling and security characteristics. The Fiber Optic transmits light signals rather than electrical signal. Each Fiber has an inner core of glass or plastic and additional resources


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DATA COMMUNICATION AND NETWORKINGrequired at both side i.e. source and destination to convert electrical signal into light signal and vice versa.

In Optical fiber the outer jacket is made up of either polyvinyl chloride (PVC) or teblon. Inside the jacket, Kevlar which is a strong material used in the fabrication of bullet proof vests. Below the Kevlar another plastic coating to protect the fiber is at the centre of the cable and it consists of cladding and core.Characteristics:

Cost: - Fiber Optic cable is more expensive than any other cables. Installation: - Installation of this cable is difficult than any other cables. Bandwidth: - Through fiber optics light signal is passed that’s why chances of attenuation

will be lesser. Fiber Optics cable provides higher Bandwidth than any other cables. Maximum connecting points/node capacity: - If fiber optic is used as communication

channel with Ethernet then up to 75 nodes can be easily connected/installed. Mode of Transmission: - Fiber Optic supports half duplex mode of transmission. In half

duplex mode of transmission, transmission is possible in both direction, but only one direction at a time.

Advantage: This media is superior than any other media use to connect network resources physically. The Bandwidth of this media is higher than any other media. This cable cannot be easily getting noisy. This media is lighter than any other media. Signal cannot be leakage. Greater immunity to tapping.

Disadvantage: Fiber Optics required high skilled people to use. It doesn’t support two ways communication at a time. Cost of this cable is much higher than any other cable. Unidirectional light propagation. Installation and maintenance is typical.

2. Unguided Media – The media in which signal are not guided through a solid medium is called Unguided Media. For example Air is the media through which electromagnetic energy ca flow easily.

There are several methods which are used to send electromagnetic energy through air:(a) Radio Waves – Electromagnetic waves ranging in frequency between 3 KHz and 1 GHz are called

radio waves.Radio waves are omni-directional. When an antenna transmits radio waves, hey are

propagated in all directional. This means that the sending and receiving antenna do not have to be aligned. Omni-directional properly has a disadvantage that radio waves transmitted by one antenna are susceptible interference by another antenna that may send signal using the same frequency or band.

Radio waves are those wave that propagate in the sky mode can travel long distance broadcasting.Characteristics:

Have frequency between 10 KHz to 1 GHz. Radio waves are easy to generate. Radio waves are omni directional. They can travel long distances. They can penetrate buildings easily.

Advantage: Due to low and medium frequency it can penetrates walls, means AM radio can receive a

signals inside a building.Disadvantage:

Due to low & medium frequency these can’t isolate a communication to just inside or outside a building.

Application of radio waves:

Due to omni-directional characteristics of radio wave, it is use in AM and FM radio, TV, Maritime radio, Codeless phone and paging.

(b) Microwaves – Electromagnetic waves having frequency between 1 GHz to 3 GHz are called microwaves.


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DATA COMMUNICATION AND NETWORKINGMicrowaves are unidirectional when an antenna transmits microwaves, they can be

narrowly focused. This means that the sending and receiving antenna need to be aligned. Microwave propagation is line of sight propagation.

There are two types of antenna used for microwave communication:(i) Parabolic dish antenna – It is based on the geometry of parabolic in which every line is parallel

to the line of symmetry or line of sight and reflects off the curve at angles such that on the lines intersect at a common point called the focus.

The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver.

(ii) Horn antenna – It looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmission is collected by the scooped shape of the horn and is deflected down into the same.Characteristics:

Frequencies above 100 MHz.

Microwaves travel in straight line. Microwaves are in expansive as compare to fiber optics system. Microwaves communication is widely used for telephones, television redistribution etc. Microwave system permit data transmission rate above about 16 GHz /sec. Repeaters are used to extend the coverage area.

Advantage: A pair of antennas can be aligned without interfering with another pair of aligned

antennas.Disadvantage:

Very high frequency microwaves can’t penetrate walls, if receivers are inside the building.

Uses of certain portions of band in microwaves require permission from authorities.Application:

Microwaves are used for uni-cast communication such as cellular telephones satellite networks and wireless LANs.

Types of Microwave communication system:There are two types of microwave communication system:

1. Terrestrial : -

Such system used directional parabolic antennas to send and receives signals.

The signals are highly focused and the physical part must be line to sight.

Relay towards are used to extend the signals. Frequency range between 21 to 23 GHz and 4 to 6 GHz. Cost: - Short distance system can be inexpensive but long distance systems can be

expensive. Installation: - In Terrestrial microwave system line of sight maintain line of sight

requirement can make installation difficult. Because antennas must be carefully aligned. Bandwidth capacity: - Data rates are from 1 to 10 m bit/sec.

2. Satellite : -

Satellite microwave system transmits signals between directional parabolic antennas.

Such as also maintain line of sight. One antenna is on a satellite in geo-synchronous orbit (The orbit where the speed of the

satellite matches the earth’s rotation speed), about 36000 kms above the equator. This allows a ground station to aim its antenna at a fixed point in the sky. In satellite communication microwave communication at 6 GHz are transmitted from a

transmitter on earth to a satellite position in space. The signal reaches the satellite and it become weak due to the distance of 36,000 km

traveled. The transponder in a satellite amplifies the weak signal and sends them back to the earth at a frequency of 4 GHz. These signals are received at a receiving station on the earth.

Characteristics: Frequency – Range: 4 GHz – 6 GHz and 11 GHz – 14 GHz Cost – Building and launching such system is extremely expensive. Installation – Extremely difficult and technical. Bandwidth – High and it also depends on the frequency use.


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DATA COMMUNICATION AND NETWORKINGAdvantage:

Satellite communication is a single broadcast or relay station visible from any point of a very large area on the earth.

Satellites used for national transmission are visible from all ports of the country. Transmission and receiving costs are independent from the distance between these

stations. It makes high quality communication. Maintenance cost is less.

Disadvantage: Initial setup cost is very high.

Kepler’s Law: - It defines the period as a function of the distance of the satellite from the centre of the earth i.e.

According to Kepler’s Law,Period = C(distance)3/2

Where ‘C’ is a constant approximately equal to 1/100.‘Period’ has a unit second.

(c) Infrared and Millimetre Wave – In electromagnetic waves ranging in frequency between 3 GHz to 400 THz are called infrared.

The infrared data association (IRDA) an association of sponsoring the use of infrared waves has established standards for communication between devices such as keyboards, mousse, PCs and printers. The recent standard defines a data rate of 4 MB/sec.

Infrared waves are those waves that propagate in the line of sight mode.Characteristics:

Used for short range communication. The remote controls used on television and VCRS, DVDS use infrared communication. They are relatively directional, cheap, and easy to build but do not pass through solid

object. No government license is needed to operate on infrared system like radio system. Infrared communication is used fro indoor wireless LAN. Portable computers with infrared capability can be on the local LAN without having too

physically connected to it.Advantage:

Due to high frequency and short range communication, it prevents interference between one system and another.

Disadvantage: High frequency infrared can’t penetrate walls, if receivers are inside the building. It can’t be used outside the building because the sun’s ray contains infrared waves that

can be interfering with communication.Application:

It can be used for short range communication in a closed area using line of sight propagation.

(d) Light Wave : - A modern application is to connect the LANs in two buildings via lasers mounted on their roof tops. Coherent optical signaling using lasers is inherently unidirectional; so each building needs its own laser and its own photo detector.Advantage:

The bandwidth is very high at very low cost. It is relatively easy to install. It does not require any license.

Disadvantage: Laser beams cannot penetrate rain or thick fog, but they normally work well on sunny

days. Heat from the sun during the daytime causes convection currents to rise up from the roof

of the building.Fiber Optics Communication: - The huge capacity and digital efficiency of optical fibers have made them most appropriate for computer communication. Optical fibers are used to connect work-stations with central processor in a LAN. Main purposes of using optical fibers are to provide safe mechanism with high data rate. Components: - A fiber optics system has three components: -

Light source Transmission medium (channel) Detector


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DATA COMMUNICATION AND NETWORKING Light source – The two commonly use optical sources are Light Emitting Diodes (LEDs) and

Injection Laser Diodes. The LEDs emit a lower level of light (-15 dbm power level) but concentrate it into a tighter cone pattern. The laser diodes emit light at -6 dbm. The pattern of light is shown below:

Transmission medium (channel) – The transmission medium is an ultra-thin fiber of glass. Detector – The detector generates an electrical pulse when light falls on it. By attaching a light

source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it as light pulses, and then reconverts the output to electrical signals art the receiving end.

Categorization into bands of electromagnetic spectrum of Radio wave & Microwave: -(i) Very Low Frequency (VLF) – VLF have range between 3 to 30 Hz and we ground propagation

method. Application of VLF is called long range radio navigation.(ii) Low Frequency (LF) – LF has range between 30 Hz to 300 KHz and use ground propagation

method. LF is used in radio beacons and navigational locators.(iii) Middle Frequency (MF) – MF has range between 300 KHz to 3 MHz and use sky

propagation method. MF is used in AM (Amplitude Modulator).(iv)High Frequency (HF) – HF has range between 3 MHz to 30 MHz and use ground sky propagation

method. HF is used in citizens band (CB), ship/aircraft communication.(v) Very High Frequency (VHF) – It has range between 30 MHz to 300 MHz and used both sky and

line of sight propagation. VHF is used in VHF TV and FM radio.(vi)Ultra High Frequency (UHF) – UHF has range between 300 MHz to 3 GHz and use line of sight

propagation method. UHF is used in UHF TV, cellular phones, paging, satellite.(vii) Super High Frequency (SHF) – SHF has range between 3 GHz to 30 GHz and use line of

sight propagation method. SHF is used in satellite communication.(viii) Extremely High Frequency (EHF) – EHF has range between 30 GHz to 300 GHz and use

line of sight propagation method. It is used in satellite and radar.Chapter – 3

DATA MODEMSModulation: - Modification of one or more characteristic of a carrier wave by an information bearing signal is called ‘Modulation’.

Categorization of Modulation: -1. Amplitude Modulation – In Amplitude Modulation transmission, the carrier signal is modulated so

that its amplitude varies with the changing amplitude of modulating signal. The frequency and phase of the carrier remains the same. Only the amplitude changes to follow the variation in information.

Modulation creates a band width i.e. the twice the bandwidth of modulating signal and covers a range centered in a carrier frequency. However the signal components above and below the carrier frequency carry the exactly the same information. For this reason some implementation discarded one half of the signal and cut the bandwidth in half.

The total bandwidth required for amplitude modulation can be determined from the bandwidth of audio signal.

BAM = 2BWhere B – Bandwidth of Audio

signalBAM – Bandwidth of AM

The federal communication commission (FCC) allows 10 KHz for each amplitude modulation station (AM). AM stations allowed carrier frequency between 530 KHz to 1700 KHz.Advantage:

Easy to implement Used for both digital and analog In the case of digital signal, two different voltage levels are

used i.e. 0 or 1.Disadvantage:

It is affected by the noise signal. That may add up with the information signal. As the strength of the signal decreases in a channel with distance traveled it reaches a

minimum level. Before signal strength goes down, it must be amplified.


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DATA COMMUNICATION AND NETWORKING2. Frequency Modulation – In Frequency Modulation transmission, the frequency of carrier signal is

modulated to follow the changing voltage level of modulating signal. The peak amplitude and phase of carrier signal remains constant but as the amplitude of information system changes, the frequency of carrier changes corresponding.

The actual bandwidth is difficult to determine exactly but it can be solved shown that it is several lines that of analog signal.

BFM = 2(1 + β)BHere β is a factor depends

upon modulation technique.The bandwidth of an audio signal

broadcast in stereo is almost 15 KHz. The Fcc

allows 200 KHz for each station. FM stations are allowed carrier frequency anywhere between 88 MHz to 108 MHz. Advantage:

Frequency modulated is least affected by the noise.Disadvantage:

Needs much higher bandwidth than amplitude modulation.

Use: FM technique is used to convert digital

signal into FM signals.3. Phase Modulation – In phase modulation

transmission the phase of carrier signal is modulated to follow the changing voltage level of modulating signal. The peak of amplitude and frequency of carrier signal remains constant but the amplitude of information signal changes the phase of carrier signal. In frequency modulation the instantaneous change in carrier frequency is proportional to the amplitude of modulating signal while in phase modulation; the instantaneous change in carrier frequency is proportional to the derivative of amplitude of modulating signal.

The actual bandwidth is difficult to determine exactly but it can be shown empirically that it is several times that of analog or modulating signal. Although the formula of same bandwidth for frequency modulation and phase modulation. The value of ‘β’ is lower in the case of phase modulation around 1 narrow band and 3 for wide band.Advantage:

It provides the signal modulation that allows computers to communicate at higher data rates through telephone system.

Disadvantage:

Phase modulation requires two signals with a phase difference between them.Use:

This technique is used to convert colour information in colour television broadcast. This technique is used to convert digital signals into phase modulated (PM) signal.

Analog to Digital Conversion:1. Pulse Code Modulation (PCM): - The common technique in which analog signal changes to digital data (Digitization) is called PCM. A PCM encoder has three procedures:

(a) Analog signal is sampled(b) Sampled signal is quantized(c) Quantized values are encountered as streams of bits.(a) Sampling – The analog signal sampled every Ts second

where Ts second is the sample interval or period. The inverse of the sampling interval is called the sampling rate or sampling frequency and denoted by fs, where fs = 1/Ts.

According to the Nyquist


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DATA COMMUNICATION AND NETWORKINGtheorem, the sampling rate must be at least 2 times the higher frequency contained in the signal.

For elaborating Nyquist theorem we remember some points: Sample a signal only & signal is band-limited. Sampling rate must be at least 2 times the highest frequency, not the bandwidth.

If the analog signal is low-pass, the bandwidth & the highest frequency are the same value.

If the analog signal is band pass, the bandwidth value is lower than the value of the maximum frequency.

(b) Quantization - The result of sampling is a series of pulse with amplitude values between the maximum to minimum amplitudes of the signal. The set of amplitude can be infinite with non-integral values b/w the two limits. These values can’t be used in the encoding process.Steps in Quantization: -

We assume that the original analog signal has instantaneous amplitude between Vmin

& Vmax.

We divide the range into 1 zones, each of height ∆(delta)

i.e. ∆ = 2

Vmin -Vmax

We assign quantized values of 0 to 1-10 to the mid-point of each zone.

We approximate the value of the sample amplitude to the quantized values.

For quantization & encoding, we take a sampled signal and the sample amplitude are between -20 v and + 20 v.

We decided to have eight levels. This means that ∆ = SV

(c) Encoding – After each sample is quantized & no. of bits per sample is decided, each sample can be changed to an n-bit codeword i.e. no. of bits nn = log2L, where L = Quantization level & Bit rate = Sampling rate x no. of bits per sample = fs x nn

In PCM decoder, we first use circuitry to convert the code words into a pulse that holds the amplitude. After the staircase signal is completed, it passes through a low-pass filter to smooth the staircase signal into an analog signal. The filter has the same cut off frequency as the original signal at the sender. If the signal has been sampled at or greater than the Nyquist sampling rate & if there are enough quantization levels, the original signal will be recreated. The maximum and minimum values of the original signal can be achieved by using amplification.


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DATA COMMUNICATION AND NETWORKING3. Delta Modulation (DM) – DM finds the change from previous sample. There are no code words;

bits are sent one after another.Modulator – The Modulator is used at the sender site to create a stream of bits from an analog signal. The process records the small positive or negative changes, called Delta. If the delta is positive, the process records a 1. if it is negative the process records a 0. However the process needs a base against which the analog signal is compared. The modulator builds a second signal that resembles a staircase. Finding the change I then reduced to comparing the input signal with the gradually made staircase signal.Demodulator – The Demodulator takes the digital data & using the staircase maker & the delay unit, creates the analog signal. The created analog signal needs to pass through a low-pass filter for smoothing.

Digital to analog Conversion: - Digital to analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. This is also called Shift Keying.Relationship between Data rate & signal rate – Data or bit rate is the no. of bits per second. Signal or baud rate is the no. of signal elements per second.

In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.S = N x 1/r baud, where S = signal rate, N = data rate, r = no. of data elements carried in one

signal = log2l where l is the type of signal element.Shift Keying: -

Amplitude Shift Keying (ASK) – In amplitude shift keying the amplitude of the carrier signal is varied to create signal elements. Both frequency & phase remain constant while the amplitude changes.Levels of ASK: -

(d) Level o – This is also called Binary ASK or on-off keying & peak amplitude of one signal level is 0 & the other is the same as the amplitude of the carrier frequency.Bandwidth for ASK, B = (1 + d) X S

Where d = value between 0 & 1 S = Signal rate

The formula shows that the required bandwidth has a minimum value of s & a maximum value of 25.

(e) Multilevel ASK (MASK) – Multilevel ASK is that in which more than two levels. We can use 4, 8, 16 or more different amplitude for the signal & modulate the data using 2, 3, 4 or more bits at a time.

Frequency Shift Keying (FSK) – In FSK, the frequency of the carrier signal is varied to represent data. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. Both peak element if the data element changes. Both peak amplitude & phase remain constant for all signal elements.(a) Binary FSK (BFSK) – Binary FSK is considering two frequencies f1 & f2. We use the first

carrier if the data element is 0; we use the second if the data element is 1.

The middle of one bandwidth is f1 & the other is f2. Both f1 & f2 are ∆f apart from the midpoint between the two bands, so the difference between the two frequencies is 2 ∆f.

Bandwidth for FSK, B = (1 + d) X 3 + 2 ∆f(b) Multilevel FSK (MFSK) – Multilevel FSK is that in which more than two frequencies used. To

send 2 bits at a time, we can use four frequencies and so on. Bandwidth for FSK for multilevel,


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DATA COMMUNICATION AND NETWORKINGB = (1 + d) x S + (L – 1) 2 ∆f

Phase Shift Keying (PSK) – In PSK, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude & frequency remain constant as the phase changes. Today, PSK is more common than ASK & FSK.(a) Binary PSK (BPSK) – The simplest PSK is binary PSK,

in which we have only two signal elements, one with a phase of 00 and the other with a phase of 1800.

Bandwidth of PSK, B = (l+ d) S(b) Quadrature PSK (QPSK) – The simplicity of BPSK enticed designers to use 2 bits at a time in

each signal element, thereby decreasing the baud rate & eventually the required bandwidth. The scheme is called quadrature PSK because it uses two separate means out of phase. The incoming bits are first passed through a serial to parallel conversion that sends bit to one modulator and the next bit to the other modulator. If the duration of each bit in the incoming signal is T, the duration of each bit sent to the corresponding BPSK signal is 2T. This means that the bit to each BPSK signal has one half the frequency of the signal.

DPSK – Encoding Technique and CODEC: - MODEMS: - It is a device converts digital signal generated by computer into an analog signal to be carried by public access telephone line. It is also the device converts the analog signals received over a telephone line into digital signal usable by the computer. A modem derives its meaning from modulation and demodulation i.e. a signal modulator and signal demodulator. A modulator converts digital signals into analog and demodulator converts analog signal into digital signal. Modems are classified into many categories. Modem speed range from 300bps to 56kbps. The tasks which modem can perform are:

Automatically dial another modem using touch tone or pulse dialing. Auto answer. Disconnect a telephone connection when data transfer has completed. Automatically speed negotiation between two modems. Converts bits into the form suitable for the LAN. Transfer data reliable. Converts received signal back into bits.

Modem Commands – When a computer wants to make a connection using telephone no. as parameter using tone dialing. The modem then replies with an ok response i.e. it tries to make connection with remote modem if it is not able to make connection it sends message in the form of code.

3 – for no carrier7 – for busy6 - for no dial tone, etc.If it gets connected then it returns a connect code as it sends +++ and then wait for a

command from host computer. In this case command is hang up the connection (ATH). The modem will then return an OK message when it has successfully created a connection.Classification of Modems – Modem can be of the following types:1. Landline Modems – Landline modems are those modems which connect to the public switched

telephone network (PSTN). To connect to PSTN, this modem has a jack known as RJ-11 jack or regular phone jack.

Landline modem can be of following types:(a) Internal Modems – Internal modems are installed within the computer as interface cards.(b) External Modems – External Modems are installed as a separate hardware device, outside the

computer. They are more expensive than the internal modems.(c) PCMCIA Modems – PCMCIA Modems are credit-card sized modems used in laptop computers.

PCMCIA stands for Personal Computer Memory Card International Association.(d) Voice/data/fax Modems – Voice/data/fax Modems which are use for transferring files, sending

and receiving faxes and voice mail using associated software.2. Wireless Modems – Wireless modem are based on webs. Using wireless modem, one can connect to

a network while being mobile. Like Landline Modems wireless modems do not plug into jack. There are very few manufacturers of wireless modem.

3. LAN Modems – LAN Modems allows share remote access to LAN resources. LAN modems are of various types. Depending upon the number of parts, network architecture, memory requirements, security, etc.

Modem Standard – There are two modem standards:1. Bell Modems – The first commercially available modems were developed by Bell Telephone

Company. In the early 70’s they defined the development of the technology and provided the


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DATA COMMUNICATION AND NETWORKINGstandards. Some major Bell modems include the 103/113 series, 202 series, 212 series, 201 series, 208 series and 209 series.

2. ITU-T Modems – Many of today’s modems are based on the standard published by ITU-T. V.21, V.22, V.23, V.22bis, V.32, V.32bis, V.33 and V.34 modems are ITU-T modems.

Modem Protocols – 1. X.25 Protocol – X.25 is an end to end protocol. It acts as an interface between data terminal

equipment (DTE) and data circuit-terminating equipment (DCE).X.25 is a packet-switching protocol that defines the interface between a synchronous packet-

switching host computer and analog dedicated circuits or dial-up switched virtual circuits in the voice-grade public data network.

X.25 allows a variety of devices that are designated as data terminal equipment (DTE) to talk to the public data network (PDN).

2. Triple-X Protocol – X.3, X.28 and X.29 protocols are collectively known as Triple-X protocols. Triple-X protocols are used to connect a dumb terminal to an X.25 network. A dumb terminal is any terminal that does not understand X.25 protocol. X.3 defines a packet assembler/disassembler (PAD). X.28 defines the rules for communication between a dumb terminal and a PAD. X.29 defines relationship between a PAD and a remote terminal.

Protocol Used by Modem for Transferring Files – Some of the protocols used by modem for transferring files are described in the following sub-sections: XMODEM – XMODEM is a file transfer protocol used in telephone-line communication between PCs.

XMODEM protocol requires that one terminal or computer be set up as the sender and other be set up as the receiver. A block of data sent under XMODEM protocol will have the following format:

Start of Header Block Number 1’s Complement of Block Number

128 Data Characters

Checksum bits

Characteristics:• It is easy to implement with a small computer.• It requires manual setup for each file to be transferred.• The error detection technique is unsophisticated and unable to detect reliably the most common

type of transmission error, which is noise burst that can last of the order of 10 milliseconds.• It is a half-duplex protocol.

YMODEM – It is similar to XMODEM, but with some differences. These differences are the following:• A data unit is of 1024 bytes.• Two CANs are sent to abort a transmission.• ITU-T CRC-16 is used for error checking.• Multiple files can be sent simultaneously.

ZMODEM – It combines the features of both XMODEM and YMODEM.

Kermit – It is an also a file transfer protocol like XMODEM. It allows the transmission of control characters as text.

Establishing a Connection – Connection can be of the following three types: Direct Connection between PCs – One PC can call another PC. Modems at both ends of the

connection talk to one another. Connection to a Mainframe Computer – A PC connects a mainframe computer. Connection to an on-line service – On-line service consist of one or more central computers

linked by telephone lines to other small computers spread across the country or world.Chapter – 4

Multichannel Data communicationMultiplexing: - Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.Format of multiplexed system:- Multiplexer is that which combines transmission stream into a single stream i.e. many to one method.

Demultiplexer is that which separates the stream back into its component transmission i.e. one-to-many method & directs them to their corresponding lines. Link refers to the physical path channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many channels.


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DATA COMMUNICATION AND NETWORKINGCategories / techniques of multiplexing: -(a) Frequency-division multiplexing : -

FDM is an analog technique that can be applied when the band-width of a link is greater than the combined band-widths of the signals to be transmitted. In FDM, signals generated by each sending device modulate. Different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel, channels can be separated by strips of unused bandwidth, guard bands to prevent signals from overlapping.

Application of FDM:- FDM is used in AM & FM radio broadcasting with band from 530 to 1700 KHz & 88 to 108

MHz respectively. FDM is used in television broadcasting FDM is used in first generation cellular telephones.

(b) Wavelength division multiplexing : - WDM is an analog technique that is designed to use the high data-rate capability of fiber-optic cable. The optical-fiber data rate is higher than the data rate of metallic transmission cable. Using a fiver-optic cable fox one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one. In WDM, the multiplexing & demultiplexing involve optical signals transmitted through fiber-optic channels. In WDM technology, we want to combine multiple sources light into one single light at the multiplexer & do the reverse at the demultiplexer. The combining & splitting of light source are easily handled by a prism. A prism bends a beam of light based on the angle of incidence & the frequency. Using this technique, a multiplexer can be made to combine several input beams of light, each containing a narrow band of frequencies into one output beam of a wider band of frequencies. A demultiplexer can also be made to reverse the process. Dense WDM can multiplex a very large number of channels by spacing channels very close to one another. It achieves even greater efficiency.Application of WDM: -

WDM is used in SONET network in which multiple optical fiber lines are multiplexed & demultiplexed.

(c) Time–division multiplexing : - TDM is a digital multiplexing technique for combining several low rate channels into one high-rate one.Two schemes for TDM: -1. Synchronous TDM: - In synchronous TDM, the

data flow of each input connection is divided into units, where each input occupies one input time slot. A unit can be 1 bit, 1charecter of 1 block of data. Each input unit becomes one output unit & occupies one output time slot. However the duration of an output time slot is n times shorter. Than the duration of an input time slot. If an input time slot is Ts, the output time slot is T/n & where n is the no. of connections. In other wards, a unit in the output connection has a shorter duration; it travels faster.


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DATA COMMUNICATION AND NETWORKINGIn synchronous TDM, the data rate of the link is n times slots are grouped into frames. A

frame consists of one complete cycle of time slots, with one slot dedicated to each sending device. In a system with n input lines each flame has n slot, with each slot allocated to carrying data from a specific input line.

2. Statistical TDM: - In statistical TDM, slot are dynamically allocated to improve bandwidth efficiency. Only when a input line has a slot’s worth of data to send is it given a slot in the output frame. In statistical TDM, the no. of slots in each frame is less than the no of input lines. The multiplexer checks each input line in round robin fashion: it allocates a slot for an input line if the line has data to send otherwise it skips the line & checks the next line. In statistical TDM, no solt is left empty as long as there are data to be sent by any input line.

Difference between synchronous &statistical TDM: - An output slot in synchronous TDM is totally occupied by data where as in statistical TDM, a slot

need to carry data as well as address of the destination. In statistical TDM, a block of data is usually many bytes while the address is just a few bytes

where as in synchronous TDM, there is no such situation. The frame in statistical TDM need not be synchronized means there is no need of

synchronization bits where as in synchronous TDM must be synchronized. In statistical TDM, the capacity of the link is

normally less than the sum of the capacity of cache channel where as in synchronous TDM, the capacity of the link is equal to the sum of the capacity of channel.

Access technique: - Access technique is basically divided into three groups.(a) Random Access: - In random access of

contention methods, no station is superior to another station & name is assigned the control over the another. No station permits of does not permit, another station to send.Method of random access technique: -

ALHO CSMA (carrier sense multiple access) CSMA/CD (Carrier sense multiple Access with collision detection) CSMA/CA ( Carrier sense multiple access with collision Avoidance )

(b) Controlled Access: - In controlled access, the stations consult one another to find which station has the right to send. A station can’t send unless it has been authorized by other stations.Method of controlled access technique: -

reservation polling token passing

(c) Channelization: - Channelization is a multiple access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations.Method of channelization: -(a) Frequency Division Multiple Access (FDMA): - In FDMA, the available bandwidth is divided

into frequency bands. Each station is allocated a band to send its data in other wards, each band is reserved for a specific station, & it belongs to the station all the time. Each station also uses a band pass filter to confine the transmitter frequencies to prevent station interferences, the allocated bands are separated from one another by small guare bands.

FDMA specifies a predetermined frequency band for the enter period of communication. The means that stream data can easily be used with FDMA.

FDMA is an access method in the data link layer. In each station tells its physical layer to make a band pall signal from the data passed to it. The signals must be created in the allocated band there is no physical multiplexer at the physical layer. The signals created at each station are automatically band pass filtered. They are mixed when they are sent to the common channel.

(b) Time Division Multiple Access (TDMA): - In TDMA, the station share the bandwidth of the channel in time. Each station is allocated to a time slot during which it can send data. Each station transmits its data, in is assigned time slot.


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DATA COMMUNICATION AND NETWORKINGTDMA lies in achieving synchronization between the different stations. Cache station

needs to know the beginning of its slot & the location of its slot this may be difficult because of propagation delays introduced in the system if the station are spread over a large area. To compensate for the delays, inserting guard times synchronization is accomplished by having some synchronization bits at the beginning of each slot.

TDMA is an access method in the data link. The data link layer in each station tells its physical layer to user the allocated time slot there is no physical multiplexer at the physical layer.

(c) Code Division Multiple Access (COMA): - In CDMA, one channel carries all transmission simultaneously but having no timesharing.

CDMA simply means communication with different codes. If a station needs to send a 0 bit, it encodes it as -1 ; if it needs to send a 1 bit, it encodes it as +1 & when a station is idles, it senda no signal which is interpreted as a ‘0’.Properties of orthogonal sequence: -

Each sequence is made of N elements, where N is the no. of stations. If we multiply a sequence by a no; every element in the sequence is multiplied by

that element. This is called multiplication of a sequence by a scalar.i.e. - [ +1+1-1-1 ] = [ +2+2-2-2 ]

If we multiply two equal sequence element by element add the results, we get N, where N is the no. of elements in the each sequence. This is called the inner product of two equal sequences. i.e. - [+1+1-1-1] . [+1+1-1-1] = 1+1+1+1 = 4.

If we multiply two different sequence, element by element & add the results. We get 0. This is called inner product of two different sequences. i.e.: - [+1+1-1-1] . [+1+1+1+1] = 1+1-1-1 = 0

Adding two sequences means adding the corresponding elements. The result is another sequence. i.e. - [+1+1-1-1] + [+1+1+1+1] = [+2+200].

Spread Spectrum: - Spread spectrum combines signals from different sources to bit into a latger bandwidth. It is designed to be used in wireless communication (LANs & WANs).

To achieve bandwidth efficiency, spread spectrum techniques add redundancy; they spread the original spectrum needed for each station. If the required bandwidth for each station is B, spread spectrum expands it to Bss, such that Bss>>B. The expanded bandwidth allows the source to corp. its message, in a protective envelope for a mare secure transmission.Two principles for achieving goals of spread spectrum: -

The bandwidth allocated to each station needs to be. By for larger than what is needed. This allows redundancy.

The expanding of the original bandwidth B to the bandwidth Bss must be done by a process that is independent of the original signal. In other worked. The spreading process occurs after the signal is created by the source.

Two techniques to spread the bandwidth: -(a) Frequency Hopping Spread Spectrum (FHSS): - The FHSS technique uses M different carrier

frequencies that are modulated by the source signal. At one moment, the signal modulates one carrier, frequency, at the next moment, the signal modulates another carrier frequency, although the modulation is done using one carrier frequency at a time, M frequencies are used in the long sun. The bandwidth occupied by a source after spreading is BFHSS >> B.Layout for FHSS: -

Pseudorandom code generator, called Pseudo random noise (PN), creates a K-bit pattern for every happing period Th. He frequency table uses the pattern to find the frequency to be used for this hopping period & passes it to the frequency synthesizes. The frequency synthesizer creates a carrier signal of that frequency & the source signal modulates the carrier signal.


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DATA COMMUNICATION AND NETWORKING(b) Direct Sequence Spread Spectrum (DSSS): - The DSSS technique expands the bandwidth of

the original signal in DSSS. We replace each data. Bit with n bits is assigned a code of n bits, called chips, where the chip sale is n times that of the data bit.Layout of DSSS: -

Digital Hierarchy: - Telephone companies implement TDM through a hierarchy of digital signal, called digital signal service of Digital Hierarchy.

United States, ANSI standard & Europe IIU-I standard is called the synchronous optical network (SONET) & Synchronous. Digital Hierarchy (SDH) respectively. SONET/ SDH are a synchronous n/w using synchronous TDM multiplexing. All clocks in the system are locked to a master clock.

Architecture of SONET/STD: -(a) Signals : - SONET defines a hierarchy of electrical signaling levels called synchronous transport

signals (STSD) each STS levels (STS-1 to STS-192) supports a certain data rate, specified in megabits per sec. the corresponding optical signals are called optical carriers (OCS). SOH Specifies a similar system called a synchronous transport module (STM). STM is intended to be compatible with existing European hierarchies such as E-lines & with STS levels.SONET/SDH rates: -

STS OC Rate (Mbps) STMSTS – 1 OC – 1 S1 – 840STS – 3 OC – 3 155.520 STM – 1STS – 9 OC – 9 466.560 STM – 3

STS – 12 OC – 12 622.080 STM – 4STS – 18 OC – 18 933.120 STM – 6STS – 24 OC – 24 1244.160 STM – 8STS – 36 OC – 36 1866.230 STM – 12STS – 48 OC – 48 2488.320 STM – 16STS – 96 OC – 96 4976.640 STM – 32STS - 192 OC - 192 9983.280 STM - 64

(b) Devices : - Basic devices for SONET transmission:(i) STS Multiplexer/Demultiplexer : - They mark the beginning points & end points of a SONET link.

They provide the interface between an electrical tributary network & the optical network. An STS Multiplexer multiplexes signals from multiple electrical sources & creates the corresponding OC signal. An STS Demultiplexer demultiplexes an optical signal into corresponding electrical signals.

(ii) Regenerator : - It extends the length of the links. A regenerator is a repeater that takes a received optical signal, demodulates it into the corresponding electrical signal, regenerates the electric signal & finally modulates the electric signal into its correspondent optical signal. A SONET regenerator replaces some of the existing information with new information.

(iii) Add/Drop Multiplexer : - It allows insertion and extraction of signal.(iv)Terminals : - It is a device that uses the services of a SONET network.

(c) Connection s: - The devices are connected using section lines & paths.(i) Sections : - A section is the optical link connecting two neighbour devices like Multiplexer to

Multiplexer, multiplexer to regenerator or regenerator to regenerator.(ii) Lines : - A line is the portion of the network between two Multiplexer like STS Multiplexer to

Add/Drop Multiplexer, two add/drop Multiplexer, or two STS Multiplexer.(iii) Path : - A path is the end to end portion of the network between STS Multiplexers.

Network using SONET equipment: -


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DATA COMMUNICATION AND NETWORKING

Chapter – 5Network fundamentals

AN OVERVIEW OF NETWORKINGA network is a group of computers connected in some fashion in order to share resources. A

group of computers in a network provide greater storage capacity and processing power than that by standalone independent machines. In addition to computers, a network also consists of peripheral devices with carriers and data communication devices used for the purpose of exchanging data and information.

By using computer networks, the cost of data transfer can be made cheaper than other conven-tional means like telegrams etc. as computers can send data at a very fast speed. Thus, computers enable us to reduce both cost and time in transferring data. In a network, computers of different make can be connected together and users can work together in a group. Software packages have been developed for group working in Data Base Management (DBMS) and graphical artworks. Also, data from different departments located at distant places can be transferred to and stored on a central computer. This data can then be accessed by the computers located in different departments. The data at the central computer is updated and accessed by all users. This prevents any bottlenecks in the smooth functioning of the organization because all the users will get the latest information (for example, inventory) stored in the central computer.Communication Switching Techniques: - In a WAN two devices are not connected directly but a network of switching nodes provides a transfer path between the two devices. The process of transferring data block from one node to another is called data switching. There are following types of switching techniques: - Circuit Switching – In Circuit Switching, there is a dedicated communication path between the

sending and receiving devices.

A circuit switched network is made of a set of switches connected by physical links in which each link is divided into n-channels.

Circuit switching takes place at the physical layer. In this, data are continuous flow sent by the source station & received by the destination station, although there may be periods of silence. There is no addressing involved during data transfer.

Switching at the physical layer in the traditional telephone network uses the circuit switching approach.Workstation – It is a basically a PC or printer or other sharable resources. Workstation is also called a terminal or data access point of a network.

The dedicated path is a sequence of links between switching nodes.Circuit Switching involves three steps:

(a) Circuit Establishment(b) Signal Transfer(c) Circuit Transfer

Circuit Switching is mainly used for voice based network. It is not effective for data communication.


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DATA COMMUNICATION AND NETWORKING Message Switching – In message switching, it is not necessary to establish a dedicated path

between a sending and receiving devices. In message switching, the sending device spends destination address to the message and passes to the network. The message is then passes through the network from one node to another until reaches to its destination. Each switching nodes receives the message stored it and then transmit it to the next node. Examples of message are email etc.

Packet Switching – Packet Switching combines the advantage of message and circuit switching but it is functionally similar to message switching. There are two approaches to packet switching.

In data communication, the message is going to pass through a packet switched network & it needs to be divided into packets of fixed or variable size.

In packet switching, there is no resource allocation for a packet. This means that there is no reserved bandwidth on the links & there is no schedule processing time for each packet. Resources are allocated on demand. The allocation is done one a first come, first served basis.

In packet switched network, each packet is treated independently of all others. Even if a packet is part of multi-packet transmission, the network treats it as though it existed alone. Packet switching is normally done at the network layer. [Node – Node is a service provides of a network to particular region. The workstations are connected with the node.]

DATAGRAMA datagram is a packet that is sent over a network using a connectionless service, i.e. a network

where the delivery of data does not depend on the maintenance of connections between the communicating computers. In the next Chapter, you will learn that a protocol called User Datagram Protocol (UDP) handles such connectionless services. These services do not guarantee that the datagrams will be delivered without error, without duplication or loss and in the same serial order in which they were sent. They only guarantee a "best effort" delivery of datagram.VIRTUAL CIRCUIT

In a circuit-switching network, making a connection actually means a physical path is established from the source to the destination through the network. In a virtual circuit network, when a circuit is established, what really happens is that the route is chosen from source to destination, and all the switches (that is routers) along the way make table they can route any packets on that virtual circuit. They also have the opportunity to reserve resource for the new circuit.

When a packet comes along, the switch inspects the packet's header to find out which virtual circuit it belongs to. Then it looks up that virtual circuit in its table to determine which communication line to send. They are also known as switched virtual circuit.CONNECTIONLESS AND CONNEC TION ORIENTED COMMUNICATION

In a connection oriented service, a logical connection is established between the two communicating computers. The TCP protocol (discussed in a later Chapter) is used in such a communication. It guarantees error free delivery of messages without loss or duplication. The packets are received in the same serial order in which they were sent. The connection is established by a three-way handshake: In this method, before the sending device can send data to the receiving device, the former must determine the availability of the latter and a network pathway must be discovered on which the data can be sent. This is known as connection establishment. It normally involves the following steps:

The sender sends a connection request packet to the receiver. The receiver, if available, returns a confirmation packet (acknowledgement) to the sender. The sender then returns an acknowledgement of this confirmation packet.

Once the connection is established, the packets are sent in order, and their acknowledgements from the receiving device are also received in that same order. After the communicating computers finish off with the sending and receiving, the connection is terminated. Connection termination also involves full confirmation between both the communicating devices, as in connection establishment.

In a connectionless communication, there is no maintenance of connection between the two devices. Each data packet (preferably called as 'datagram) takes its own path and reaches the destination. There is also no guarantee that datagrams will be received error-free and in the same order in which they are being sent. Also no connection establishment and termination are required.NOTES BY-BALJEET SINGH SINWAR

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DATA COMMUNICATION AND NETWORKINGCELL SWITCHING (ATM)

Many of the problems associated with packet switching are solved by adopting a concept called Cell Switching. A cell is a small data unit of fixed size unlike packets which have variable sizes.1. Cell Network - A network that uses a cell

as the basic unit of data exchange is called a cell network. When packets of different sizes and formats reach a cell network, they are split into small data units of equal length and loaded into cells. The cells are then multiplexed (given in Figure) with other cells and routed through the cell network.

2. Cell Switching - ATM uses switches to route cells from source to destination. It normally uses the following types of switches: VP switch - A virtual path (VP) switch routes the cell using only the Virtual Path Identifier (VPI)

for identification. VPC switch - A VPC switch

routes the cell using both VPI and VCI.

3. Segmentation and Reassembly of Cells - In order to address the issue of segmentation and reassembly, a protocol layer was added in between the ATM and packet protocols such as IP (Internet Protocol). This layer is known as ATM Adaptation Layer (AAL). The AAL header contains the necessary information to reassemble the cells into the original packet.

The packet that passes down to the AAL is encapsulated by adding a header and a trailer. The resulting encapsulated packet is then fragmented into cells.

Packet header consists of the following fields: 8 bit Common Part Indicator (CPI) - It determines which version of the packet format is being

used. Currently only value 0 is defined. 8 bit Beginning tag (Btag) - The Beginning tag must match with the End tag (Etag) of the trailer

for a given packet. This helps in the reassembly of the cells into packet. It prevents the situation in which the loss of the last cell of one packet and the first cell of another packet causes the two packets to be joined into a single packet.

16 bit Buffer Allocation Size (BASize) - It represents as to how much space to allocate for the reassembly.

Packet trailer consists of the following fields: Pad field - Pad field is used to pad the user data to a multiple of 3 bytes. This ensures that the

trailer, along with the 8 bit 0-filled field, is aligned on a 32-bit boundary (4 byte), making it of constant size for efficient processing.

8 bit End tag (Etag) - It contains a value equal to the value in the Btag field. 16 bit Len – It contains the length of the packet.

NETWORK TOPOLOGIESTopology is the method in which networks are physically connected together. Topology

determines the complexity of connecting computer, the strategy for physically expanding the network, in future. There are three type of topology is used: -(i) Bus Topology : - Bus topology is a network geometric

arrangement in which a single connecting line is shared by a number of nodes. In linear bus topology, all computers are connected by a single length of cable with a terminator at each end. Each node is connected to two others except the machines at either end of the cable, which are connected only to one other node. Examples of Bus Topology are Ethernet, Local Talk etc.

Advantage of Bus Topology:


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DATA COMMUNICATION AND NETWORKING(a) This topology is simple, reliable, and easy to use and understand in small sized LANs.(b) This topology requires least amount of cable to connect the computers together.(c) This type of topology is easy to extend.(d) This topology is less expensive than other cabling arrangements.Disadvantage of Bus Topology:(a) There is possibility of collision, thus in this case data packets may be lost.(b) Heavy network traffic can slow down a bus considerably.(c) It is difficult to troubleshoot a bus.

Examples of Bus Topology: -

Ethernet - Ethernet, invented in 1973 by Bob Metcalfe (who later formed a new company called 3 Com, one of the most successful networking companies), was a way to circumvent the limitations of earlier networks. It was based on IEEE (Institute of Electronic and Electric Engineers) standard called 802.3 CSMA/CD, and it provided for ways to manage the crazy situation that occurred when many computers tried to transmit on one wire simultaneously.

LocalTalk - LocalTalk is a data link protocol built into the Macintosh RS-449/RS-422 serial interface. It forms a part of the AppleTalk protocol suite. Appletalk is a suite of networking protocols that work together to provide file and print sharing services to Macintosh networks. AppleTalk enables users to share folders and printers for access by other network users.

(ii) Ring Topology : - Physical layout of Ring Topology based LAN is circular. Means that each workstation is connected with it neighbors. Transmission can be done in only one direction, either clockwise or anticlockwise. It is decided at the time of network design.Token Passing: - It is a mechanism i.e. used with Ring Topology based LAN to make transmission possible between two or more work stations.

Token is nothing but a small program. When network is on, this program starts moving from one node to another in specific direction.

If a work station has to access the service of network, then it has to wait for token. Once token becomes available to the node, it grabs the token make its transmission /work, after completion of transmission, it has to release the token.

Advantage of Ring Topology:(a) Installation cost of Ring Topology may cheaper.(b) No one computer can monopolize the network.(c) The fair sharing of the network allows the network to degrade

gracefully as more users can added.Disadvantage of Ring Topology:(a) Failure of a node may interrupt the system.(b) It is difficult to troubleshoot a ring network.(c) Adding or removing computers disrupts the network.

Example of Ring Topology: -

IBM Token Ring - Ethernet CSMA/CD networks provide a relatively simple way of passing data. However, CSMA/CD breaks down under the pressure exerted by many computers on a network segment. In order to overcome this problem, IBM and the IEEE created another networking standard called IEEE 802.5. 802.5 is more commonly known as Token Ring network topology.

FDDI (Fiber Distributed Data Interface) - FDDI (Fiber Distributed Data Interface) is another ring-based network. FDDI networks run on optical fiber cables instead of copper cabling. It uses fiber-optic cables to implement very fast; reliable networks. FDDI also- uses the 802.5 method of operation.

FDDI is a high performance fiber optic token ring LAN running at 100 Mbps over distances up to 200 km with up to 1000 stations connected.

It can be used in the same way as any of the other LANs, but with its high bandwidth, another common use is as a backbone to connect copper LANs.How FDDI Works? FDDI (like Token Ring) uses token-passing schemes to control network access. But, unlike Token Ring, several FDDI devices can transmit data simultaneously. Like Token Ring, a token is passed around the ring, and the possessor of the token is allowed to transmit FDDI frames. Unlike Token Ring, a FDDI network may have several frames


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DATA COMMUNICATION AND NETWORKINGsimultaneously circulating on the network. This is possible because the possessor of the token may send multiple frames, without waiting for the first frame to circulate all the way around the ring before sending the next frame.

(iii) Star Topology: - Star Topology based LAN based on an electronic device namely Hub. All terminals including server are connected with central Hub. A Hub receives the signal from source and sends it to the destination. Example of Star Network is ATM (Asynchronous transmission Mode). There are two types of Hub: -

(a) Active Hub: - The active hub regenerates the electrical signal and sends it to all connected computers.

(b) Passive Hub: - Passive hub doesn’t generate electrical signal but acts as a connecting point.Advantage of Star Topology:(a) It is easier to modify i.e. easy to add new terminal and easy to remove a particular terminal

from a network. It doesn’t disturb/affect the network.(b) It is easy to troubleshoot.(c) Failure of a single terminal doesn’t interrupt the whole network.(d) To install this network several types of cables can be used.Disadvantage of Star Topology:(a) Failure of central hub interrupts the whole network.(b) It is considered as expensive

network.(c) Additional resources are required

in case of broadcast implementation.

Example of Star Network – ATM (Asynchronous Transmission Mode) - ATM networking is the newest topology available at this time. Unlike others, it can carry both voice and data over network wire or fiber. ATM transmits all packets as 53-byte cells, that have a variety of identifiers on them to determine such things as Quality of Service.

ATM is capable of extremely high-speed routing. At the lowest, it runs at 25 megabits per seconds. At the fastest, it can run up to 622 megabits per second. In addition to its speed, ATM is more complex than either Ethernet or Token Ring. Presently, Fore Systems and IBM have both invested very heavy amount in ATM-to-the-desktop technology. That means, they use ATM to link servers and workstations and are banking on the need for multimedia networks over the next several years.How ATM Works? ATM communicates with cells rather than transmitting frames. Instead of specifying the source and destination addresses of the stations communicating, an ATM cell indicates the path the data will flow through. Small cells all of the same size is used to make it easy for devices to process a cell, so intermediate devices (called switches) can maintain a very high data rate.

On an ATM network, every station is always transmitting. However, most of the cells transmitted are empty cells that can be discarded at the switch. When a cell that is not empty enters the switch, the addresses are read to determine where the cell will go next. The cell is then sent out in the next available slot, according to the type of cell it is.Role of ATM in Internetworks ATM has emerged as one of the technologies for integrating LANs and WANs. ATM can support any traffic type in separate or mixed streams, delay-sensitive traffic, and non-delay-sensitive traffics as shown in Figure. ATM can also scale from low to high speeds. It has been adopted by all the industry's equipment vendors, from LAN to private branch exchange (PBX). With ATM, network designers can integrate LANs and WANs to support emerging applications with economy in the enterprise.

Network designers are deploying ATM technology to migrate from Time Division Multiplexing 1TDM) networks for the following reasons:

To increase WAN bandwidth To improve performance To reduce down time

Chapter – 6Osi model and tcp/ip suite


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Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

OSI STACK

DATA COMMUNICATION AND NETWORKINGNetwork Architecture: - Network Architecture deals with the physical connection, i.e. topologies, access methods and connection protocols. Some examples of network architectures are:

Ethernet Token ring AppleTalk ARCNET ATM etc.

Need for Layered Solutions: - Layered approach provides the following advantages: Each layer needs to know and worry only about the functions in its domain. Functioning of other

layers is hidden from it. Each layer performs a function independent of the other layers. This enables software developer

to develop a software component for a particular layer. A layer can be modified, if needed, without affecting other

layers. Software packages confirming to the standards of a particular

layer are able to use the softwares at other layer to communicate with each other.

Open System Interconnection (OSI) Model: - Main objectives of OSI Models are: -

Allow to interconnected two systems through standard interface.

Each layer performs a well-defined function. The function of each layer should be chosen according to

international standard protocols. Each lower level protocol provides its services to higher-level

protocol. The seven layers of OSI reference Model are: -

1. Physical Layer: - All electrical and mechanical devices, these are used to connect two terminals, two nodes, medium etc. considered as the devices of physical layer.

In other words mostly hardware is categorized into physical layer. These are involved in interconnecting two points of a network as well as carry bit stream.

2. Data Link Layer: - The Data Link Layer is the second layer of OSI Model. The Data Link Layer together with physical layer and the interconnectivity medium provide a data link connection between source station and destination station for reliable transfer of database.

Services provide by Data Link Layer: - Flow Control – Flow Control deals with how to keep the fast sender from over flowing a slow

receiver by buffer at the receiver sides and acknowledgement.Retransmission Strategies

(ix)Stop and Wait – the sender allows one message to be transmitted checked for errors and an appropriate (ack-positive or Nak – negative acknowledgement) returned to the sending station. No other message can be transmitted by the sender until receiving station sends back a reply.

(ii) Sliding Window – The sender (sending station) maintains a sending window that maintains number of frames (packages), it is permitted to send to the receiving station and the receiving station (Receiver) maintains a receiving window that performs some necessary check up. There are two sliding window techniques:(a) Go Back N – This is a sliding window technique. It allows data and control message to be

transmitted continuously without for its acknowledgement from the receiver. In the case of error detection at the receiving slide, the message with error retransmitted, as well as all other frames were transmitted after the erroneous message.

(b) Selective Repeat – this approach is considered as refined approach in contrast to the Go back N. the only message retransmitted is those for which NAK is received. Selective Repeat Mechanism produces greater through put than the Go Back N.

Framing – Some control bits are added to the data packets these are received from network layer. The bits are associated with start and end frames.

Error Detection and Correction Course – Various methods are use for such purpose. Some popular methods are parity bit, checksum etc.

3. Network Layer: - The Network Layer provides services to the transport layer. It can be based on ether virtual circuits or data grams. In both cases its main job is routing packets from source to destination. In case of virtual circuit subnet, a routing decision is made when virtual circuit is setup.

In case of datagram subnet, routing decision is made on every incoming package.


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DATA COMMUNICATION AND NETWORKINGMany Routing algorithms are used in computer Network. All algorithms are categorized into

two categories:(i) Static Routing Algorithm – Static Routing algorithm dies not decide there routing decision on

measurement of current traffic and topology. Where as dynamic routing algorithm changes their routing decisions on current traffic and topology.

The function of Network Layer is routing packets from source machine to destination machine. In most subnets packets will require multiple hops to reach the destination.

Network Layer also manages congestion. To control congestion several algorithms are provided.

When too-many packets are present in the subnet the performance degrades. This situation is called Congestion.

Subnet can be become congested increasing the delay and lowering the through put for packets. Network designers attempt to avoid congestion by proper design. If congestion occurs it must be dealt.

Congestion can be bought about by several factors: - If all of a sudden, streams of packets begin arriving from three for four lines and all need

the same output line, queue is build up. If there is insufficient memory to hold of them, packets will be lost.

Slow processor can also caused congestion. If the routers are slow at performing routing decision.

Low bandwidth lines can also cause congestion.Comparison of virtual circuit and Datagram Subnet: -

Virtual circuits allow packets to contents circuit numbers only, where as datagram allow packets to content full destination address.

Using virtual circuits requires a setup phase, which takes time, where as with datagram such process is not required.

Virtual circuits have some advantages in avoiding congestion with the subnet because resources can be reserved in advance at the time of connection establishment, where as congestion is a potential problem with datagrams.

Virtual circuits have a problem, if a router crashes and loses its memory, all the virtual circuits passing through it will have to aborted, where as such type of problem will not arise with datagrams.

Virtual circuits and datagrams both allow the router to balance the traffic.4. Transport Layer: - This Layer is responsible provide reliable cost effective data transport from

source machine to destination machine. Transport Layer also provides some additional services associated with data transportation. Two protocols are used with transport layer to provide such services. These are: -

TCP (Transmission Control Protocol) – This protocol provides a highly reliable connection oriented end to end transport service associated with the layer no four of OSI-Model. During transportation TCP adds some additional information to the data packet i.e. associated with transportation service, called TCP header.

Source Port no. Destination Port no.Sequence No.

Acknowledgement No.TCP

HeaderLength

Free

Space

URG

ACK

PSH

RST

EYN

FIN Window Size

Checksum Urgent pointerData

Fig: - TCP SegmentSource and Destination Port – Values of this field identifies the local port no.Sequence no. – Sequence no. identifies the current sequence no. of data segment.Data Offset – Values of this field identifies the start of data.Flags – Shows individual status:

(a) URG (Urgent Flag) – Value of this field identifies the urgent pointer. It is considered as Urgent Flag.

(b) ACK (Acknowledgement Flag) – Value of this field associated with acknowledgement of the packet.

(c) PSH (Push Flag) – Push Flag performs push function.(d) RST (Reset Flag) –


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DATA COMMUNICATION AND NETWORKING(e) SYN (Sequence Synchronization Flag) – The value of this field associated with sequence

synchronization.(f) FIN (Final Flag) – End of Transmission Flag.

Window Size – This field contains the values associated with strength of destination station, How many byte (data block) the receiving host can accept at a time.Checksum – Checksum for data end header.Urgent Pointer – The value of this field identifies specific type of data area.

UDP (User Data Protocol) – The protocol supports connectionless transportation with internet protocol. The Protocol encapsulates all the necessary information with data packet to transport them from source to destination, without having to establish a connection.

UDP Segment (the packet i.e. segmented by UDP) consist of header followed by the data in UDP header source code no. and destination port no. are also available like TCP header, which contains address of source machine and address of destination machine. UDP length field contains the value which includes header length + data length. The UDP Checksum field contains the value i.e. helpful in checking the errors at destination station. It functions like checksum of TCP header. Some options fields are also available with UDP header.

5. Session Layer: - Main functionality of this layer is to establish and release the session. The session layer allows users on different machines to establish session between them. Session can allow going both directions. One of the services of the session layer is to manage dialog control.

6. Presentation Layer: - Presentation Layer mainly manages the formats of information.7. Application Layer: - Several protocols are available under this layer. Large number of terminals are

connected with the system and requires verify of services.(i) DNS – Domain Name System is a mechanism which high level name and converts it into

machine understandable form. Domain Name System is helpful in assigning the high level name for several machines. Domain Name System maintains the domains in hierarchical order. This mechanism is implemented with TCP/IP internets. Domain Name categorized into mainly two categorized:(a) Geographic – Example of this is .in, .ch, .jp, .us(b) Non-Geographic – Example of this is .com, .org, .edu, .net, etc.

(ii) Email – Email is a popular service provided by the largest network internet. It is basically a program which enables us to send and receive message to and from world wide.

Email is popular because it provides fast, convenient method of transferring information. The major characteristics of Email are:

Store and forward Delivery time ranging from few sec. to hours Supports multimedia service

Secure mailing Auto processing

(iii) FTP (File Transfer Protocol) – This protocol provides method of transferring files over internet. FTP transfer files to and from a remote network sites. FTP provides several commands to transfer differents formats of files from one host to another. It also supports transferring of compressed files.

(iv)World Wide Web (WWW) – Basically WWW is a system for linking hypertext document. Each document is a web page written in Hypertext Markup language and link between documents is called hyperlink. A browser can display the page by establish a TCP connection to its server. Browser asking for document to the server and closing the connection. In easily days gopher is used in place of WWW.Gopher provides all the implementation in the form of menus. So user has to access the service using such menu items.Characteristics of WWW: -

Fastest growing discovery and retrieval system. Considered as navigational system based on hyperlink. State-less interaction between client and server confirming to http (Hyper Text Transfer

Protocol).(v) Telnet – Telnet is a program that allows us to establish a virtual terminal connection between

two machines using TCP/IP. For this we must have its internet address or host name of computer.

TCP/IP Suite (TCP/IP Protocol): - TCP reference model is implemented with or suitable form those computer network architecture that allow communication across multiple divers network. TCP/IP


http://WWW.Gopher/

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DATA COMMUNICATION AND NETWORKINGnetwork architecture consists of four layers:-

Application Layer: - The Application Layer provides services that can be used by other applications for example protocols have been developed for remote login, for e-mail, for file transfer etc. The application layer program run directly over the transport layer.Transport Layer: - Two basic services are provided by this layer: -

The first service consists of reliable connection oriented transfer of byte stream, which is provided by TCP. The second service consists of connectionless transfer of individual package i.e. provided by UDP. UDP is used for applications that require quick but reliable delivers.

The TCP/IP Model doesn’t require strict layering. In other words the application layer has the option of by passing intermediate layer.Internet Layer: - The Internet layer handles the transfer of information access multiple networks through the use of gateways or routers. It has similar function as network layer of OSI Model. A key aspect of the Internet layer is the definition of global unique address for the machines that are attached with the Internet. The Internet layer provides a single service namely best effort connectionless packet transfer. For this packets are also called data grams. The connectionless approach makes the system robust i.e. if failures occur in the network the packets are routed around the point of failure. There is no need to setup the connection.Network Interface Layer: - Network Interface Layer is concern with the network specific aspects of the transfer of packets. It must deal with port of network layer and data link layer. The network interface layer is particularly concern with the protocols that access the intermediate networks.TCP/IP Services and Application Protocols: - Client Server Model – In this model, transaction responsibilities are divided into two parts: client and

server. Clients rely on servers for services such as file storage, printing, and processing power. Client is a PC running front-end software that knows how to communicate with the server. This model improves performances.

Telnet – Previously Described. File Transfer Protocol (FTP) – Previously described. Trivial File Transfer Protocol (TFTP) – This protocol is nofrills, unauthenticated protocol used to

transfer files. TFTP depends on UDP and often is used to boot diskless workstations. Simple Mail Transfer Protocol (SMTP) – This protocol is a TCP/IP protocol that specifies how

computers exchange electronic mail. It works post office protocol, and is one of the reasons that Internet E-mail functions so well.

Network File System (NFS) – This is a distributed file system protocol suite developed by Sun Microsystems that allow to remote file access across a network. It allows all network users to access shared files stored on computers of different types.

Simple Network Management Protocol (SNMP) – This is a network management protocol for TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security. It specifies how nodes are managed on a network, using agents to monitor network traffic and maintain a management information base.

Domain Name System (DNS) - Previously described. Internet Control Message Protocol (ICMP) – This is an integral part of the IP that handles error and

control message. Specially routers and hosts use ICMP to send reports of problem about datagram, back to the original source that sends to the datagram.

Internet Group Management Protocol (IGMP) – This protocol is mainly used with multicast network to group the members, used by host.

Address Resolution Protocol (ARP) – This protocol is used as a device driver which accepts packets from internet layer and converts the packet into Ethernet format.


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DATA COMMUNICATION AND NETWORKING Reverse Address Resolution Protocol (RARP) – This protocol performs the same function as ARP does

but in reverse, that is given an IP address, it determines the corresponding physical address.Data Transmission By TCP and Ethernet: - Ethernet and TCP/IP works together well. Ethernet provides the physical cabling (layers 1 and 2 of OSI model) and TCP/IP the communications protocol (layers 3 and 4 of OSI model).

TCP/IP uses 32-bit IP addresses to identify a node and the network to which it is attached to. IP addresses are unique, 4 byte addresses that must be assigned to every addressable device or node on the internetwork.

TCP receives the stream of bytes from the upper -Application layer and assembles them into TCP segments, or packets. In the process of assembling, header information is attached at the front of data.

Header information includes a checksum as well as a packet sequence number (if there is more than one segment in the entire message).

A connection (Virtual circuit) is established between the sending and receiving machines. The sending TCP software issues a request for a TCP connection with the receiving machine. The receiving TCP software adds its own unique socket number to the request message and sends it back to the original machine. A connection is thus established between the two machines.

If the message is more- than one TCP segment long, the receiving TCP software reassembles the message using the sequence numbers contained in each segment's TCP header. If a segment is missing or corrupt (which can be determined from the checksum), TCP returns a message with the faulty sequence number in the body. The originating TCP software can then resend the bad segment.

The address resolution protocol (ARP) associates an IP address with the physical address. On a typical physical network, such, as a LAN, each device on a link is identified by a physical or station address usually imprinted on the network interface card (NIC). ARP maintains tables of name-to-address mappings.

RARP (Reverse Address Resolution Protocol) performs the same function as ARP does but in reverse, that is given an IP address, it determines the corresponding physical address.Data Encapsulation: - The data from the application program is encapsulated into the TCP segment. This means that the data in a TCP segment is the data from the application program. The IP datagram encapsulates this TCP segment. Thus, means the TCP segment forms the data part of the IP datagram. The IP datagram is sent along the physical wire, encapsulated in the Ethernet frame (or any other LAN frame). That is, IP datagram is transported as the data of the frame. This process is known as data encapsulation. The same process gets reversed at the receiver side. This is known as data decapsulation. Data Routing: - The IP protocol in the TCP/IP suite is responsible for routing the data packets to its destination. Routing means finding the route (next hop) for a data-gram. If the destination node is on the same network as the source node, the delivery is direct. In direct delivery, the sender can compare the destination address with the addresses of the computers to which it is connected to. If a match is found, the packet is delivered to the same.

If the destination host is not in the same network as the sender, the delivery is indirect. In the indirect delivery, the packet goes from router to router until it reaches the network of the destination host. In indirect delivery, the sender uses the IP address of the destination computer and a routing


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DATA COMMUNICATION AND NETWORKINGtable. The routing table is used to find the IP address of the router to which the packet should be sent to.Error Reporting Mechanism – Internet Control Message Protocol (ICMP): - This is an integral part of the IP that handles error and control message. Specially routers and hosts use ICMP to send reports of problem about datagram, back to the original source that sends to the datagram.

ICMP detects error conditions such as internet work congestion and downed links and notifies IP and upper-layer protocols so packets can be routed avoiding problem areas.

ICMP does not correct errors but simply reports them. ICMP are encapsulated inside IP datagram before they are sent over the network. Five types of errors are reported by ICMP. These are: -

(a) Destination Unreachable(b) Source Quench (Destination overwhelmed with datagrams)(c) Timeout exceeded(d) Parameter problems(e) Redirection

Internet Architecture: - Internet is an interconnection of multiple networks. The word internet (lowercase i) is different from Internet (uppercase I). While internet means network of networks, Internet is the term used to refer to a specific worldwide network, WWW (World Wide Web).

Internetwork (internet) consists of multiple networks in which LANs are attached to other LANs, other communications networks, remote sites, individual stations— and -Wide Area Networks (WANs). It permits data to move freely 'among large numbers of networks and populations.

All internetworks, including the Internet, have a layered architecture. Internet has four layers. These are as follows:(a) Subnetwork Layer - All the machines connected together in a local area network (LAN) reside in this layer.(b) Internetwork Layer - This layer provides the functionality for communi-cations between networks through gateways. Each sub network uses gateways to connect to the other sub networks in the internetwork. The internetwork layer is where data gets transferred from gateway to gateway until it reaches its destination and then passes into the sub network layer. The internetwork layer runs the Internet Protocol (IP).(c) Service Provider Protocol Layer - This layer is responsible for the overall end-to-end communications of the network. This is the layer that runs the Transmission Control Protocol (TCP) and other protocols. It handles the data traffic flow itself and ensures reliability for the message transfer.(d) Application Services Layer - This layer supports the interfaces to the user applications. This layer interfaces to electronic mail, remote file transfers, and remote access.

Chapter – 7Data link protocol

PROTOCOLIn sending data from one place to other place, communication requires at least two devices

working together, one to send and one to receive. Such a basic arrangement requires coordination for an intelligible exchange to occur. For example, in half-duplex transmission, it is essential that only one device transmits at a time. If both devices at the two ends of the link put signals on the line simulta-neously, they collide. The coordination of half-duplex transmission is part of a procedure called line discipline. This line discipline is one of the functions included in the data link layer of OSI model.

Besides line discipline, the other important functions in the data link layer are flow control and error control. These three functions together are known as data link control.TRANSMISSION CONTROL PROCEDURE

Transmission control is a set of procedures used to control transmission and reception of data between two communicating devices. This can be divided into two subgroups, namely:(i) Synchronous Protocols - Synchronous protocols take the whole bit stream and chop it into characters of equal size. Protocols governing synchronous transmission can be divided into two classes. Character-oriented protocols - Character-oriented protocols, also called byte-oriented protocols,

interpret a transmission frame or packet as a succession of characters, each composed of one byte (eight bits). All control information is in the form of an existing character encoding system such as ASCII coding method.

Character-oriented protocols are not as efficient as bit-oriented protocols and therefore are now seldom used.

Bit-oriented protocols - Bit-oriented protocols interpret a transmission frame or packet as a succession of individual bits, made meaningful by their placement in the frame and by their neighboring bits. Control information is in the form of one or more bits.


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DATA COMMUNICATION AND NETWORKING(ii) Asynchronous Protocol - Asynchronous protocol treats each character in a bit stream independently.. Today, these protocols are employed mainly in modems. A variety of asynchronous protocols developed are:

XMODEM - XMODEM protocol has an error checking technique that can be used between microcomputers. It requires that one terminal or computer be set up as the sender and other be set up as the receiver. After the protocol is started, the transmitter waits for the receiver to send a Negative Acknowledge (NAK) character. The receiver meanwhile is set to send NAKs every 10 seconds. When the transmitter detects the first NAK, it begins sending messages as blocks of 128 data characters, surrounded by some protocol control characters. The beginning of each block is signaled by a Start Of Header (SOH) character. This is followed by a block number character in ASCII, followed by the same block number with each bit inverted. The bit inversion, known as the 1's complement, results in the block number being followed by the same block number with each bit inverted. A 128-character piece of the file is sent, followed by a checksum that is the remainder of the sum of all the 128 bytes in the message divided by 255. Mathematically, the XMODEM checksum can be represented as:

CHECKSUM = R[255

128

1∑ rofCharacteASCIIValue ]

in which R is the remainder of the division process. The receiver checks each part of the received block to confirm the following:

Was first character a Start Of Header (SOH)? Was the block number exactly one more than the previous block received? Were exactly 128 characters of data received? Was the locally computed checksum identical to the last character received in the block?If the receiver is satisfied, it sends an Acknowledge (ACK) back to the transmitter, and the

transmitter sends the next block. If not, an NAK is sent, and the transmitter resents the block found in error. This process is continued, block by block, until the entire file is sent and verified. At the end of the data, the transmitter sends an End Of Text character. The receiver replies with an ACK, and the session is terminated.Limitations of XMODEM Protocol - There are several points to consider about the XMODEM

protocol. It is easy to implement with a small computer, but it does require a computer at each end. It requires manual setup for each file to be transferred. The error detection technique (ordinary sum of the data characters) is unsophisticated and

unable to detect reliably the most common type of transmission error, which is a noise burst that can last of the order of 10 milliseconds (the duration of about 12 bits at 1200 bps).

It is a half-duplex protocol; that is, information is sent, and then the sender waits for a reply before sending the next message.

Because operation of the XMODEM protocol generally assumes a full duplex line, it is inefficient in use of the transmission facility.

Support for XMODEM Protocol - In spite of the previously mentioned limitations, the XMODEM protocol and several derivatives are supported by most asynchronous communication pro-;-rams designed for operation on PCs. The rationale for the widespread support of these protocols is dated to the initial placement of the XMODEM protocol into the public domain.

Most of the asynchronous communications programs developed during the early 1980s eventually included XMODEM support. In the late 1980s, several derivatives of the XMODEM protocol gained acceptance due to the increased level of functionality they provided. Some new versions of the XMODEM protocol added CRC error checking. Other versions provided a full-duplex transmission capability with CRC error detection, thus increasing the efficiency of the protocol. Today, almost all communication programs designed for use on PCs support XMODEM and several of its derivatives.

YMODEM - YMODEM Protocol is similar to XMODEM, with the following differences:• The data unit is 1024 bytes.• Multiple files can be sent simultaneously.• ITU-T, CRC-16 is used for error checking.

ZMODEM - ZMODEM is a newer protocol combining features of both XMODEM and YMODEM.

BLAST (Blocked asynchronous transmission) - BLAST (Blocked asynchronous transmission) is more powerful than XMODEM. It is a full-duplex operation with sliding window flow control. It allows the transfer of data and binary files.

CHARACTER ORIENTED PROTOCOLS (COP)There is one type of COP called Binary Synchronous Protocol (Bisync or BSC). BSC stands for

Binary Synchronous Communication protocol. It was developed by IBM in the year 1964. It supports


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DATA COMMUNICATION AND NETWORKINGhalf-duplex transmission, using stop and wait/ARQ (Acknowledgement Request) flow control and error detection. It does not support full-duplex or sliding window protocol transmission.BIT ORIENTED PROTOCOLS (BOP)

SDLC (Synchronous Data Link Control Protocol) and HDLG (High Level Data Link Control Protocol) is the two bit-oriented protocols. SDLC was developed by IBM. HDLC is one of the ISO designed protocols and has become the basis for all bit-oriented protocols in use.Synchronous Data Link Control Protocol (SDLC)

SDLC is a data-link layer protocol developed in the 1970s by IBM for its Systems Network Architecture (SNA) networking environment. It is primarily used in wide area networks (WANs) that use leased lines to connect Mainframe SNA hosts and remote terminals.

In a serial SDLC link, data is sent as a synchronous bit stream divided into frames that contain addressing and control information in addition to the payload of data.

SDLC uses a master/slave architecture in which one station is designated as primary (master) and the remaining stations are secondary (slaves). The primary station establishes and tears down SDLC connections, manages these connections, and polls each secondary station in a specific order to determine whether any secondary station wants to transmit data.

SDLC can be used in a variety of connection topologies such as the following: Direct point-to-point connections between a primary and a secondary station Multipoint connections between a primary and a group of secondary stations.

High Level Data Link Control Protocol (HDLC)This is a protocol to prevent aliasing error. It determines where a true message block begins and

ends and what part of the message is to be included in the CRC (Cyclic Redundancy Check). It uses bit-stuffing for data transparency.

In HDLC, all information is carried by frames that can be of the following types:

Information Frames (1-frames)

Supervisory control sequences (S-frames), or unnumbered command/ responses (U-frames).

Figure shows one information frame as a angular block divided into six fields. These fields are:

A beginning Flag (F1) field.

An address (A) field. It is used to identify ME the terminals. It is of 8 bits.

A control (C) field. It is used for sequence numbers and acknowledgements. It is of 8 bits. An information field (I) or data field contain information. A frame check sequence (FCS) field. It is situ to CRC. A final flag (F2) field.

There are three kinds of controls. Control field Information (I-Frame), Control field for Supervisory (S-Frame) and control field for Unnumbered frame). The contents are shown in Figure. S-frames and U-frames have the same fields except that the I field is left out.

The P/F bits stand for Poll/Final. It is used when a computer is polling a group of terminals. When used as P, the computer is inviting the term send data. All the frames sent by the terminal, except the final one, have the P/F bit set to P. The final one is set to F.Transmission Control Procedure Types: - The following are the different transmission control procedure types:

Non-procedure – Non-procedure protocols are those which do not have any laid down procedure for connecting two PCs together. Examples include Ethernet, Token ring, Token bus, FDDI etc.

Basic procedure - Basic Control Procedure protocols include LAPB (Link Access Procedure, Balanced), LAPD (Link Access Procedure for D channel) and LAPM (Link Access Procedure for Modems).


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DATA COMMUNICATION AND NETWORKINGLAPB is used in balanced configuration of two devices, where both devices are of combined

type. Communication in LAPB is always in asynchronous balanced mode. LAPB is used today in ISDN on B channels.

LAPD is used for control signaling and it also uses asynchronous balanced mode. It is used in ISDN for D channels.

LAPM is designed to do both synchronous and asynchronous conversion, error-detection and retransmission. It was designed to apply HDLC features to modems.

HDLC Procedure - HDLC supports both half duplex and full duplex communication systems. Systems that use HDLC can be characterized by their station types, their configuration and by their response modes.

Stations in HDLC are of the following three types:

Primary Station - A primary station sends commands.

Secondary Station - A secondary station sends responses.

Combined Station - A combined station sends both commands and responses.HDLC supports the following three modes of communication:

Normal Response Mode (NRM), where secondary station needs permission to transmit.

Asynchronous Response Mode (ARM), where secondary station does not need permission to transmit.

Asynchronous Balanced Mode (ABM), where all stations are equal (i.e. all are combined stations) and any station can initiate transmission.

Chapter – 8Local area network (lan)

Introduction: - Local area network (LAN) is a group of computers located in the same room, on the same floor, or in the same building that are connected to form a single network. Local area networks (LANs) allow users to share storage devices, printers, applications, data, and other network resources. They are limited to a specific geographical area, usually less than 2 kilometres in diameter.Advantage of LAN –

Local area networks allow sharing of expensive resources such as laser printers and high-capacity, high-speed mass storage devices among a number of users.

Local area networks allow for high-speed exchange of essential information between key people in an organization.

LANs provide the catalyst to increase the range of potential applications for the IBM PCs. LANs contribute to increased productivity. A LAN installation should be studied closely in the

context of its proposed contribution to the long-range interests of the organization. Disadvantage of LAN –

The financial cost of local area networking is still high in comparison with many other alternatives.

Local area networking software requires memory space in each of the computers used on the network.

Users may have difficulty in learning the network commands. The installation and management of a LAN requires far more technical and administrative skills than installing and managing sev-eral computers that are not networked.

Some control on the part of the user is lost. You may have to share a printer with other users. You may face a situation like.

Some type of security system must be implemented if it is important to protect confidential data.

Many current application programs will not run in a network environment. The program may require too much memory or have other technical constraints.

Characteristics of LANs – LANs work in a restricted geographical area. LANs operate at relatively high speed when compared to the typical wide area networks

currently in use. LAN data transfer speeds may be as high as 80 million bits per second (80 Mbps), or slightly less than 10 million characters per second (10 Mcps). Compare this to the maxi-mum data transfer speed of


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DATA COMMUNICATION AND NETWORKING56 Kbps (7000 Cps) for high-grade telephone company digital trunk lines, or the 1200/2500 bps (120/250 Cps) transmission speed used by most personal computer communications systems.

LANs are private networks, not subject to tariffs or other regulatory controls.BASEBAND Vs BROADBAND

Bandwidth use refers to the ways of allocating the capacity of transmission media. The total media capacity or bandwidth can be divided into channels. A channel is simply a portion of the bandwidth that can be used for transmitting data. The two ways of allocating the capacity of bounded transmission media are the following:1. Baseband - These transmissions use the entire media bandwidth for a single channel. Baseband is

commonly used for digital signaling, although it can also be used for analog signals. Most Local Area Networks use base band signaling.

2. Broadband - These transmissions provide the ability to divide the entire media bandwidth into multiple channels. Since each channel can carry a different analog signal or digital signal, broadband networks support multiple simultaneous conversations over a single transmission medium.

MEDIA ACCESS CONTROL: -The term Media refers to the cabling used for transmitting data from one node to another.

Examples of media in local area networks include Co-axial cabling, Twisted-pair Cabling and Fiber-optic cabling.

MAC (Media Access Control) layer is one of the two sub-layers of data link layer of the OSI model. The other sub-layer is the Logical Link Control (LLC) layer. The MAC layer interfaces with the physical layer below it and provides access to the network interface card (NIC).

The main function of MAC layer is to determine which computer on the network is allowed to use the media (transmission medium) at any given moment.

Media Access Control methods are ways to allow computers to transmit signals over network cabling, while ensuring that only one computer transmits at a time. If two computers simultaneously place signals on the wire, a collision can occur and data might be corrupted unless a method is used to resolve the collision gracefully. Some main media access control methods used in networking are:

Carrier Sense Multiple Access with Collision Detection (CSMA/CD): used in Ethernet networking. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA): used in AppleTalk net-

working. Token passing: used in Token Ring and Fiber Distributed Data Interface (FDDI) networking.

LAN HARDWARE: -Hardware required to implement a LAN, includes he following:

One or more powerful, preferably multiprocessor LAN servers and dumb and/or intelligent terminals.

A transmission medium such as coaxial cabling, twisted-pair cabling, fiber-optic cabling, etc. and their associated equipment such as connectors, patch panels, wall plates and splitters. These days unguided transmission media technologies such as infrared communication, wireless cellular networking, satellite networking etc. are very much in use. In such cases, their associated hardware is also required for establishing the LAN.

The physical connection of a computer to the LAN is made through a Network Interface Card (NIC). This is sometimes also known as Media Access Control (MAC) card. The MAC card is installed in the computer just like a Video Graphics Adapter (VGA) card or a CD-ROM controller card is installed. A laptop computer can access a LAN with a PCMCIA LAN card.

LAN devices, such as repeaters, concentrators, bridges, hubs, switches and multistation access units (MAUs) are also used. Hubs, switches, bridges and repeaters are explained in a later Section.

Equipment for organizing, protecting, and troubleshooting LAN hardware such as racks, cabinets, surge protectors, line conditioners, uninterruptible power supplies (UPSs), KVM switches and cable testers, are needed.

Data storage technologies such as RAID, network-attached storage (NAS), and storage area networks (SANS), and the technologies used to connect them, such as Small Computer System Interface (SCSI) and Fibre Channel. RAID is a technology used to implement fault tolerant storage systems by using data redundancy.

Technologies for securely interfacing private corporate networks with unsecured public ones, such as firewalls, proxy servers, and packet-filtering routers.

LAN OPERATING SYSTEMS: -LAN operating systems are network aware operating systems that support networking. Such an

operating system provides support for multi-user operations as well as administrative, security and network management functions.Jobs done by LAN Operating System -


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DATA COMMUNICATION AND NETWORKING A Network Operating System has to acknowledge and respond to requests from many work-

stations. It has to manage network access, resource allocation and sharing, data protection as well as

error control. It provides for printer, file-system, database and application sharing.

NetWare - NetWare is a LAN operating system designed by Novell Inc. It can be used to connect different offices across a building or across a country.One major advantage of Novell NetWare is the capability to support many users and services. It can support many different types of operating systems such as DOS/Windows, Macintosh, UNIX etc. It is ideal for both medium and large sized LANs.

The disadvantage of NetWare is that it is slightly difficult for both the LAN administrators and users to become proficient in using all its functionality. NetWare can be run over Token Ring, Ethernet, ARCnet and FDDI LANs.Windows NT - The most popular LAN operating system to appear on the market is the Microsoft's Windows NT. It provides connectivity like NetWare.

Advantages of Windows NT are the the following: It is very easy to learn. It is less expensive.

IMPLEMENTING LAN: -Computers in a LAN can be connected in a number of ways. They can be connected via cables,

phone lines or wireless media. Different types of connections are suitable for different types of businesses and budgets. Whatever connection you choose to establish, a Network Interface Card (NIC) must be installed in each computer in the network.1. Implementation of LAN using Coaxial Cables - Coaxial cable is the most common network cable. A

local area network whose nodes reside within a reasonable distance of each other that is on the same floor of a building or in adjacent floors may be implemented using coaxial cables or twisted pair wires.

A special connector called T-connector is used to join work stations in a ring topology. The T-connector is attached to the NIC of each workstation. Cables are attached to the open ends of the T-connectors, to form a chain. The workstations at each end of the chain will have only one cable attached. The open end of the T-connector, in these two workstations, must be capped using a resistor plug. The resistor plug absorbs the signal and prevents distortion.

In bus topology networks, a cable is connected to each NIC and extended to the shared cable. Figure 8.7 illustrates the relationship of the Ethernet hardware components required to connect a workstation to a bus-based coaxial cable.

2. Implementation of LAN using Twisted Pair - Twisted-pair cable is less expensive than coaxial cable, but less durable. However, it is more reliable in the sense that if a portion of a twisted-pair cable is damaged, the entire network is not shut down, as may be the case with coaxial cable.

Twisted-pair wires use connectors called RJ-11 connectors. The RJ connector is inserted into a socket on the NIC.

3. Implementation of LAN using Fiber Optic Cables - Fiber optic cables are becoming popular as a net-working cable because of its high transmission speed. Also, since light signal is not subject to electrical disturbances, it is free from transmission errors. It can transmit reliable signals as far as 10 km. However, it is more expensive to buy, install and maintain fiber optic cables. It also requires special equipments, called fiber line drivers,, to convert electrical signal into light signals.

Fiber optic cable is tailor-made and comes equipped with its own connectors. If you wish to connect a workstation with equipment that is not compatible with fiber optic cable, attach the work-station to fiber-line driver.

4. Implementation of LAN using Wireless Technology - Another alternative for LAN implementation is through wireless technology. The technology consists of hardware that manages the connection using radio or infrared signaling devices. This eliminates the need for cables all together. A wireless network can make use of existing cellular telephony, satellite communication system and paging systems. It may be necessary to implement the LAN using wireless technology in areas where cabling is extremely difficult or impossible. For example, when offices are located across the river or hill side.

FAST LANs: - Fast LANs are LANs that are capable of carrying voice, data and video at 100 Mbps, which is 10

times faster than traditional LANs. They can be implemented using IEEE 100BaseFX or 100BaseT specifications. These standards can be implemented using either copper twisted-pair wires or fiber optics.

100BaseFX is often used for wiring campus backbones. 100BaseFX networks are wired together in a star topology using fiber-optic cabling and fiber-optic hubs or Ethernet switches. Fiber-optic cabling


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DATA COMMUNICATION AND NETWORKINGuses either two-strand multimode or three-strand single-mode optical fiber, with each strand carrying data in opposite directions. Repeaters can be used to extend the length of cabling. With multi-mode fiber-optic , cabling, the maximum allowable distance (using repeaters) is 2 kilometres and with single-mode fiber-optic cabling, maximum distance is 10 kilometres.

The other IEEE standard, 100BaseT, provides data transmission speeds of 100 Mbps. Commonly known as Fast Ethernet, it is used as departmental backbones and in establishing connections to high-speed servers and workstations running bandwidth-intensive applications such as CAD or multimedia programs.

Fast Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) mechanism like other Ethernet technologies. It is generally wired in a star topology using two pairs of wires in category 5 cabling or four pairs of wires in category 3 cabling or a duplex multimode fiber-optic cable.NONSTANDARD LANS: -

Nonstandard LANs are LANs implemented without following any LAN standards. One such implementation is the direct connection of two computers using a serial cable known, as null modem cable. Computers, so connected, can trasfer files by using some supporting software such as Direct Cable Connection accessory of Microsoft Windows 95 and 98, or Laplink software for Microsoft.

Generally null modem cable is based on the RS232 serial transmission interface specifications.EXTENDING LAN: -

Using some specialized devices one can increase the operating range of a LAN. Signals get attenuated with distance. This can be overcome by arranging for a repeater in the path. Repeaters are explained in Section 8.10.2. Similarly, one can extend the LAN to several buildings in a campus by using devices such as bridges and switches.1. Fiber Optic Extension - Repeaters can be used in extending fiber-optic cables in a LAN or

Metropolitan Area Network (MAN). A repeater is a networking component that extends a network by boosting the signal so that it can travel farther along the cabling. Repeaters are used in fiber-optic networks to amplify and regenerate light signals for long-distance cable runs.

2. Repeaters - All transmission media attenuate (weaken) the electromagnetic waves that travel through the media. Attenuation therefore limits the distance any medium can carry data. Adding a device that amplifies the signal can allow it to travel farther, increasing the size of the network. For example, if one is connecting computers that are more than 100 metres apart using Ethernet cable, one will need a device that amplifies signals to ensure data transmission. Devices that amplify signals in this way are called repeaters.

Repeaters fall into the following two categories: Amplifiers - Amplifiers simply amplify the entire incoming signal, i.e both signal and the noise.

These are able to improve upon the analog type of signal only.

Signal-regenerating devices (Repeaters) - Signal regenerating repeaters create an exact dupli-cate of the incoming digital data by identifying it amidst the noise, reconstructing it and passing only the desired information. In this manner, the original signal is duplicated, boosted to its original strength and then sent.

Why Repeaters?Repeaters extend the distance of a single network. So if you are using an Ethernet LAN but need

to go farther than you are usually able to, you can then install a repeater to achieve the added distance. When a repeater is installed, it creates a physical break in the cable. The signal is received on one side of the repeater, regenerated and passed on to the next section of cable.3. Bridges : - Bridges are mainly used to connect two similar local area network. It is also considered

are an device which connect two similar segments in a network.Purpose of bridges:-

Bridges work under data link layer of OSI model. Works on MAC address. [ c medium access control (MAC)] Manages network traffic by filtering packets. Translator from one protocol to another. Protocol.

Type of bridges:-1. Local bridges: - Local bridges are used where the network is being locally segmented means

that the two networks of segments physically close together.2. Remote bridges: - Remote bridges are mainly used where the network is remotely segmented.

The segments are physically far apart is different building, etc. such bridges required several transmission medium to connect two points.

Advantages of latest bridges: - Suitable for CSMA/CD and token bus. Self learning:- requires low manual configuration. No hardware changes required. No software changes required.


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DATA COMMUNICATION AND NETWORKING Ready to install.

Disadvantages of transparent / latest bridges:- Transparent bridges don’t support loop path.

4. Switches : - switch is a device, functionality is similar to bridges means that switches are used to connect two devices of networks. Switches operate of data link layer of OSI model. Switches filter the request as will as control floating of frames.

5. Hubs : - It multiple incoming connections need to be connected with multiple outgoing connections then a HUB is required. Is data communication a HUB is used as a central point where data arrives from one or more directions and is forwarded out one or more other directions. HUBs are multi port repeaters. They operate at the physical layer of OSI model. HUBs are used to provide a physical star topology.

6. Routers : - Routers are both hardware and software. The purpose of a router is to connect nodes across and inter network. Routers work under network layer of OSI model. A router contains routing table as well as routing protocols. Router uses address i.e. the combination of network numbers and node address. There are several protocols available to use with routers. Some of there are: -(i) Routing Information Protocol (RIP) – This routing protocol is use by several network operating

systems for routing the packets from source to destination. Each router maintains a table of each destination and the file table is frequently updated by routing information protocol broadcast message. They also maintain the total number of hop in routing table to select the shortest path to the destination.

(ii) Exterior Gateway Routing Protocol (ERGP) – This is the enhance form of RIP. It also manages there problems. There are frequently arising with RIP. It works like default routing protocol across the internet. It uses up to five conditions to select the best route.

(iii) Open Shortest Path First (OSPF) – It might be possible that there are several path exist between one router to another but main responsibility of this protocol is to select shortest path they use DISKSTRA’S Algorithm. According to that algorithm a graph is build of the subnet. Router is represented as and each of the line is represented communication line. Shortest path is selected according to path length.

7. Gateways: - Gateways is used to connect totally dissimilar networks. They have capability to perform protocol conversion for all seven layers of the OSI model. The common use of this device is to connect a LAN and mainframe computer. Gateways changing protocol, transmitting packet between two different systems.

X.25 Gateways – These gateways are becoming popular because of thee evolution of enterprise networks and WAN. A PC on the remote sites LAN function as a gateway and runs gateway software. Remote LAN can also communicate with mainframe. X.25 was established as a recommendation of the ITU (International Telecommunication Union), an organization that recommends standards for international telephone services. X.25 has been adopted by public data networks and became especially popular in Europe.

In the X.25 view a network operates much like a telephone system. X.25 network is assumed to consist of complex packet switches that contain the intelligence needed to route packets.

Comparison between Routers and Gateways: -Routers and Gateways can manipulate the packets being transmitted. In the case of a router,

that manipulation may be simply some determination of where the packet comes from and where the packets are to go. But an intelligent decision is being made. In a gateway, the decision may be a little more complex because a gateway can perform more functions. It only not perform the router’s function but also converts the message from one packet format to another.Virtual LANs: -

Virtual LAN (VLAN) is a networking technology that allows networks to be segmented logically without having to be physically rewired.

Traditionally, each department in a building used to have its own local area network (LAN). These LANs were created using hubs, and these hubs were connected to a main Ethernet switch in the main room of the building. However, broadcasts sent by any host were received by all hosts on the network, even if all of the hosts do not need to receive them. Also, if the organization of the departments changes, the hubs must be rewired to reflect the new topology of the network.

To overcome these problems, many Ethernet switches nowadays support Virtual LAN (VLAN) technologies. All hubs are replaced by VLAN switches. The network administrator creates virtual network segments whose logical topology is independent of the physical topology of the wiring. Each station is assigned a VLAN identification number (ID), and stations with the same VLAN ID can act and function as though they are all on the same physical network segment.

Broadcasts sent by one host are received only by hosts with the same VLAN ID. The assignment of VLAN IDs is done at the port level on the switches themselves and can be managed remotely using


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DATA COMMUNICATION AND NETWORKINGnetwork management software. Moving a host from one department to another department only requires the assignment of a different VLAN ID to the port on the switch to which the host is connected. No rewiring is needed.

The main advantage of using VLAN technologies is that users can be grouped together according to their need for network communication, regardless of their actual physical locations.

The only disadvantage is that additional configuration is required to set up and establish the VLANs. VLAN techniques are also typically used in Asynchronous Transfer Mode (ATM) networks to partition the network into smaller segments.

Chapter – 9Wide area network (wan)

Introduction: - WAN (Wide Area Network) is a digital communication system which interconnects different sites, computer installations and user terminals, and may also enable LANs to communicate with each other. This type of network may be developed to operate nationwide or worldwide. The transmission media used in WANs are normally public systems such as telephone lines, microwave and satellite links.

WAN is used to interconnect LANs which may be at opposite sides of a country or located around the globe.

Wide area networks (WANs) combine the continuous error detection and correction techniques included in synchronous communications with robust network problem determination and data routing to form powerful backbones that ensure high-quality, reliable service for end users.Network Using Wan and Network Services: -

Many wide area network services are emerging these days due to the increasing demand of corporate business houses and public and private sectors for these services at the lowest possible cost.

Users are demanding Wide area network accesses that offer support for transmission of data, video, imaging, fax and voice. The primary driving forces of increased capacity and sophistication for wide area network services are:(i) Host to terminal connection - A terminal is an I/O device, consisting of a keyboard and a monitor

and the host is a back-end processing computer. Hosts and terminals may be located in different locations. Hosts can be connected to the different terminals through local area network connections or through remote dial-up connections. User's commands are typically entered through a terminal. This information is transmitted to a host computer (generally Mainframe computer) over an Ethernet or Token Ring local area network connection. The mainfram processes the input and sends the output over the network to the terminal monitor. Thus, application runs in the host and the terminal does user interfacing function.

Terminals can be of two types: Local Terminals: directly connected to the host via a serial or LAN connection. Remote Terminals: connected to the host via a phone line with a modem at both the

ends.(ii) LAN to LAN connection - Wide area networking may be used for communicating with devices that

reside beyond one's local LAN. For the communication to take place, the two LANs must be in the same WAN. Routers can be used to connect LANs that employ similar protocols.

When two dissimilar LANs are to be connected, tunnels and gateways are made use of.A gateway is any device that is capable of interconnecting networks with dissimilar routing

protocols.WAN links can be grouped into the following three main categories:

Circuit-switched services - A temporary switched circuit established through the telecommunications system for the duration of the communication session. When the connection is terminated, the carrier's switches are freed up for other uses. Examples are modems and dial-up Integrated Services Digital Network (ISDN) connections.

Leased lines - These are dedicated connections that establish permanent switched circuit that is always ready to carry network traffic.

Leased lines are very expensive because then are dedicated to the customer even when they are not in use.

Packet-switched services - These are dedicated or dial-up connections to a public packet-switching network such as X.25, a public frame-relay network, or even a virtual private network (VPN). Intermediary switches send data packets along the best route possible by using the logical address of the destination node, which is contained in the packet header.

(iii) Remote LAN connection - Remote access to a LAN can be either through dial-up connection using a modem or through a leased line. Remote access to the office LAN gives the employees and/or customers access to the following services:

File and print services Client/Server applications such as database applications


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DATA COMMUNICATION AND NETWORKING Applications for remote network administration

Programs such as PcAnywhere control the network-access remotely. However, since remote connection is mainly made using a slow modem, network-access control is often slow and jerky. But it provides high security, saves on hardware and licensing costs, and is simple to implement on a network.

Remote LAN connection allows users to access file, print and other services of the company from remote locations.

Router Concepts: - Routing is the function of the network layer of the OSI model. Routing means finding route or

the next hop for a packet. A device called router does the routing function. It uses a table called routing table to find the route to the packet's final destination. Routing tables contain information about the potential paths that a data packet should take to travel through the internet work and reach its destination.

To view the internal routing table of a computer running Microsoft Windows XP, Windows 2000 or Windows NT, type route print at the command prompt.

Router has to read the header of each packet that arrives and extract the destination address of the packet. The router then sends the packet out on the appropriate transmission path based on a calculation of the optimum route to that destination.

Routing involves two main functions. These are:1. Forwarding Function - When packets need to be sent to a host or hosts on another network, they

are forwarded to a router that is connected to that particular local network. The router to which the packet is forwarded will then check its routing tables to determine the path the packet should take. Packets are usually sent along the path with the lowest cost value or metric. The routing metric mainly includes the following:

Hop Count: The number of intermediate routers between a given network and the local router.

Latency: The time delay in processing a packet through the router or over a given route. Congestion: The length of the packet queue at the incoming port of the router. Load: The processor use at the router or the number of packets per second that it is cur-

rently processing Bandwidth: The available capacity of a route to support network traffic; decreases as

network traffic increases M Reliability: The relative amount of downtime that a particular router might experience

because of malfunctions. Maximum Transmission Unit (MTU): The largest packet size that the router can forward

without needing to fragment the packet.2. Filtering Function - Filtering is the process of controlling the flow of packets based on attributes

such as source Filtering is done to protect the network from unauthorized traffic. Network administrators can create rules for filtering out unwanted packets. A packet that satisfies all the rules is allowed to be transmitted, while a packet that violates any of the rules is dropped.

Packet filtering can be implemented in the following two ways: Static Filtering - In Static Filtering, ports are configured as either permanently open or

permanently closed. For example, to deny outside packets access the company's intranet server on port 80, one can configure the router to block all incoming packets directed towards port 80.

Dynamic Filtering - In Dynamic Filtering, selected ports can be opened for authorized access and closed for others. These ports are opened at the start of a legitimate session and then closed at the end of the session to secure the port against unauthorized attempts. One can configure rules in the router to read the incoming packets, dynamically open the two ports to allow a session to be started, monitor the flow of packets to ensure that no attempt is made to hijack the session by an unauthorized user and close the randomly assigned ports when the session ends.

Routing Method: - There are two routing method: Static Routing - In static routing method, routing tables are manually configured by the network

administrator. Static routing is generally used in smaller networks that contain only a smaller number of routers or where security is a major concern. Routers that use static routing are called static routers. Each static router must be configured and maintained separately because static routers do not exchange routing information with each other.

The routing table must contain a route for every network in the internet work. It is more immune to any hampering by hackers since the network administrator controls the configuration of the routing table.

Dynamic Routing - Dynamic Routing is a routing mechanism which is handled by a routing protocol, such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF) Protocol


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DATA COMMUNICATION AND NETWORKINGetc. These protocols dynamically exchange routing information among routers on an internet work. Routers that use this method are called dynamic routers.

A routing protocol is installed on each Dynamic Router. The routers periodically exchange their routing information so that if the internet work is reconfigured or a router goes down, the routing tables of each router are modified accordingly.

Dynamic routers are less secure because routing tables can be hampered by hackers. If the network is reconfigured or a router goes down, it takes time for this information to propagate between the various routers on the network. Routing protocols also create additional network traffic.

Local Routing: - Local routing is the routing of a packet within a particular network. When a packet arrives at a

router and the IP address indicates that the packet belongs to this network, the router sends an ARP message to find out the physical address of the destination node. The process is explained in the following Section.1. ARP (Address Resolution Protocol) Table - ARP is a network layer protocol concerned with mapping

node names to IP addresses. It matches logical and physical device addresses.On a typical physical network, such as a LAN, each device on a link is identified by a physical

or station address usually imprinted on the network interface card (NIC). ARP maintains tables of name to-address mappings, known as ARP table, and can send out packets for searching the address if a desired name or address is not currently in its table. Physical addresses have local jurisdiction and can be changed easily. For example, if the NIC on a particular machine fails, the physical address changes. The IP addresses, on the other hand, have universal jurisdiction and cannot be changed. ARP is used to find the physical address to the node when its Internet address is known.

Any time a host, or a router, needs to find the physical address of another host on its network, it formats an ARP query packet that includes the IP address and broadcasts it over the network.Reverse Address Resolution Protocol (RARP) - RARP (Reverse Address Resolution Protocol) performs the same function as ARP does but in reverse, that is given a physical address, it determines the IP address.

2. Distributed Routing - In Distributed routing, each router periodically exchanges its routing information with each of its neighbors. For example, let us assume that the hop count is taken as a metric-for-routing in a certain network. Once every T milli seconds, a router A sends to its neighbors a list of hop counts (to each destination). It also receives a similar list from its neighbors. Suppose a neighbor B has sent a routing table with Bi as the hop count required to reach node i. If node A knows that the hop count to reach node B is m, it knows that it can reach node i via B in m + Bi hop counts. By performing similar calculation for all the routing tables that arrive, a node can find out the best estimate and correspondingly modify its routing table.

3. Hierarchical Routing - In Hierarchical routing, a network is divided into sub-networks, with the router in a sub-network knowing only about the nodes within its subnetwork and being ignorant about the nodes in other sub-networks. This frees the routers from keeping information about all the nodes in a network, which would have, as networks grow in size, required large memory, enormous CPU time and huge bandwidth.

4. Distance-Vector Protocol - A distance vector protocol periodically broadcasts the complete routing tables across the internet work. Routing tables are calculated on the basis of the number of hops required to reach the destination network. Other routing metrics such as traffic load, bandwidth available, latency or Maximum Transmission Unit (MTU) are not used in calculating routing tables.

5. Link-State Protocol - Link-State Protocol is more efficient in terms of network overhead than the Distance-Vector Protocol. An example of a protocol that uses Link-State algorithm is the OSPF (Open Shortest Path First) protocol.

OSPF-enabled routers, using any algorithm, compute the shortest path between nodes on an internet work. They create a map (or tree) called the link state database describing the topology or structure of the specific network area. Network areas are groups of networks connected using OSPF-enabled routers that all have router interfaces for any of the networks included in the area. Division of the complete network into network areas has the advantage that each router need maintain only link state information about its own area and other areas connected to it, which improves the scalability of OSPF.

The link state database includes cost information in addition to hop information. The link state database is updated if a router goes down or if the structure of the network is reconfigured. Only the updations are exchanged over the network. This is unlike distance-vector protocol which involved transmission of the complete routing tables. Also, unlike RIP-enabled routers, which broadcast routing information every 30 seconds, OSPF broadcasts changes only when they occur.


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DATA COMMUNICATION AND NETWORKINGThese features substantially reduce inter-router network traffic compared to RIP and make OSPF a more efficient protocol than RIP for large internet works.

Communication Protocols Over Wan : - The two commonly used communication protocols over WAN are ATM and X.25.

1. ATM (Asynchronous Transmission Mode) - ATM is a connection-oriented protocol that can work with either permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). ATM networks use maxi-mum bandwidth, while maintaining guaranteed quality of service (QoS).

The two main benefits of ATM are: high transmission speed flexible bandwidth-on-demand capability

ATM uses fixed-size cells for packaging information. Due to the fixed size of cells, ATM connections are predictable and can be easily managed.

ATM technology works primarily at data-link layer of the OSI (Open Systems Interconnection) reference model.

ATM connects devices over a WAN using virtual channels (VCs) and virtual paths (VPs). Virtual channels consist of one or more physical ATM links connected in a series for transmitting data between remote stations. All cells in a given ATM transmission follow the same VC to ensure reliable data transmission. A Virtual Channel exists only as long as data is being transmitted on it.

A Virtual Path (VP) is a collection of VCs having the same source and destination points. VP can be used to group all the traffic to a given destination and send it over the VP.

ATM cells are of size 53 bytes (48 bytes of data payload and 5 bytes of control and routing information). The payload field contains the data which can be either plain text, images voice or video.

ATM dynamically allocates bandwidth. Bandwidth is allocated only in required amounts. For example, when an ATM link is idle, it utilizes no bandwidth, which in turn results in considerable cost savings.

2. X.25 Protocol - X.25 is a packet-switching protocol for wide area network (WAN). The X.25 standard corresponds in functionality to the first three layers of the Open Systems Interconnection (OSI) reference model. X.25 provides the following specifications: Network Layer: The network layer protocol of X.25 is known as Packet Layer Protocol (PLP). It

defines how to address and deliver X.25 packets using permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). This layer is responsible for call-setup and call-termination and for managing transfer of packets.

Data-link Layer: Data-link layer protocol of X.25 is known as Link Access Procedure Balanced (LAPB). It defines framing and error-correction methods. LAPB is derived from the High-level Data Link Control (HDLG) protocol.

Physical Layer: The physical layer interface of X.25 is called X.21bis and it is used to connect computers and terminals (DTE) with data communications equipment (DCE) such as switches.

Chapter – 10Data transmission networks

Telephone Networks: - Telephone networks provide in addition to the Plain Old Telephone Service (POTS) connection, high-speed data transmission services such as Integrated Services Digital Network (ISDN), Frame Relay, T1 lines and Asymmetric Digital Subscriber Line (ADSL). The telephone networks connect the customer premises equipment (CPE) with a similar equipment at the telephone company's central office. This connection can then be used to support the following:

Dial-up remote access solutions using modems or ISDN. High-speed dedicated leased-line access to the Internet for corporate users. Packet-switched services, such as X.25 services, for point-to-point or multipoint wide area net-

work (WAN) connections with other branches of the telephone company.1. Dial-up Telephone Networks - A Dial-up Telephone Network is a network established using a

modem over ordinary phone lines. The phone lines (also known as dial-up lines), used primarily for voice communication, are less expensive to use but they have less available bandwidth.

Companies often use dial-up networking only for occasional, low-bandwidth usage (such as remote access networking) or as a backup for the more costly dedicated or leased lines.

Dial-up networks are shared with all subscribers in the Public Switched Telephone Network (PSTN) domain.

2. Leased Line - Leased line is a permanent direct connection usually bought by corporate business houses for connecting two geographically separate local area networks (LANs). Leased lines are dedicated circuits that the telephone company reserves for the exclusive use of the customer.

In a leased-line network, the customer's LAN is connected by bridges, routers, modems and terminal adapters to the telephone company's central office, which sets up dedicated switches to connect it to the destination LAN. One can also use a leased line to connect to the Internet.


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DATA COMMUNICATION AND NETWORKINGLeased lines are very expensive as compared to dial-up lines. In return, they are always

available, active and secure.The fee for a leased line is dependent on bandwidth and distance. The fee is usually charged

monthly or quarterly.3. X.25 - X.25 was designed as a global standard for a packet-switching network. It was originally

designed to connect remote terminals with mainframe hosts. X.25 was designed when analog telephone transmissions were done on copper wire. Therefore, it has a large overhead of error-correction information, resulting in inefficient use of bandwidth.

Although newer WAN technologies such as Frame Relay, Integrated Services Digital Network (ISDN) and T-carrier services are preferred these days, X.25 networks are widely used in credit card verification, automatic teller machine transactions and other dedicated business and financial uses.

An X.25 network consists of a backbone of X.25 switches that are called packet switching exchanges (PSEs). These switches provide packet-switching services. PSEs connect DCEs at the local facilities of X.25 carriers to the X.25 Public Data Network (PDN). DTEs at customer premises connect to DCEs at X.25 carrier facilities by using a device called a packet assembler/disassembles (PAD).

One DTE (computer terminal) can initiate a communication session with another by dialing its X.21 address and establishing either permanent virtual circuit or switched virtual circuit. Messages are transmitted in the form of packets. Maximum packet sizes range from 64 to 4096 bytes, depending on the system. Packets are routed through the X.25 backbone network by using the ID number (a 12-bit number) of the virtual circuit established for this particular communication session. This ID number is also known as the Logical Channel Identifier (LCI).

Some important Networks and its characteristics:1. Public Switched Telephone Network (PSTN) –

Analog nature of transmission. It has restricted bandwidth. Available at every place. Transmission rate is low. It is voice based communication i.e. Telephones. Modems are required when it is used for data communication.

2. Public Switched Data Network (PSDN) – Several technologies are used with PSDN. Highly reliable. Provides quality of connections. It supports both low and high speed at reasonable cost. It is very popular for connecting public and private systems to implement electronic mail

services.3. Value Added Network (VAN) – In value added services the provider of such services must process

store and manipulate the data i.e. carried on the network. The technique can be used in specific type of business electronic data interchange is one area for value added services in which two trading partners exchange trading documents such as purchase orders, invoices etc

Integrated Services Digital Network (ISDN): - ISDN is enhanced digital Network which integrates voice, video and data services using digital transmission medium and combining both circuit and packet switching technique. Users can use their digital connections to telephone company for transmitting both voice and data over the some medium.

It is an international communications standard for sending voice, video and data over digital telephone line. This system allows data to be transmitted simultaneously across the world using end to end digital connectivity. There are two basic types of ISDN Servers:(i) Basic Rate Interface (BRI) – BRI consists of two 64 kbps B-channels and one 16 kbps D-channel.

It is most appropriate of individual error and small business. To access a customer should have within 5.5 km of the service provider central office. If distance is extended from 5.5 km repeaters are required.

(ii) Primary Rate Interface (PRI) – PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B-channel + one 64kbps channel. The higher capacity service is central site solution for extending application to large no. of remote users.With its BRI and PRI servers ISDN has the flexibility to meet the bandwidth needs of home office

or company head quarter small office can use ISDN BRI to support al of its voice and data communication requirement and users in a very large office can benefits from ISN PRI’s capacity.Advantage of ISDN: -

Speed – Before ISDN modem was used to make computer communications through public telephone network but it has limited bandwidth to transmit data. Actually ISDN is replacement for old telephone services. It is the fact that ISDN services is provided by the some companies


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DATA COMMUNICATION AND NETWORKINGbut the difference that we can get much faster connection for voice data and video through a single line.

Multiple Device – Before ISDN, it was necessary to have a separate line for each device that has to be used simultaneously but by the use of ISDN it became possible to combine several digital data sources over a single line.

Signaling – The signaling mechanism is also different form telephone. It sends a digital packet through a separate channel so connection establishment and called setup time is very fast.

Application of ISDN: - Internet access Telephoning Video conferencing (Synchronization) Education

NARROWBAND ISDN (N-ISDN): - The original ISDN providing data rates of 64 Kbps to 1.544 Mbps is known as Narrowband ISDN (NISDN). When ISDN was originally designed, this range of data rate was sufficient to handle all existing transmission needs. As applications using the telecommunications networks advanced, these rates proved inadequate to support many applications.BROADBAND ISDN (B-ISDN): - To provide for the needs of the next generation technology, an extension of ISDN, called Broadband ISDN (B-ISDN), is under study. B-ISDN provides subscribers to the network with data rates in the range of 600 Mbps, almost 400 times faster than the PRI rate.Broadband ISDN Services - Broadband ISDN provides two types of services:

Interactive - Interactive services are those services which need two-way transfers between either two subscribers or between a subscriber and a service provider.

Distributive - Distributive services are of simplex communication form which are sent from a service provider to subscribers. The subscriber does not have to transmit a request each time a service is desired. These services can be without or with user control.

Physical Specifications of Broadband ISDN - The Broadband ISDN model is divided into layers which are closely tied to the design of Asynchronous Transmission Mode.

However, the physical aspects of B-ISDN that are not related to ATM include: Access methods - Broadband ISDN has the following three access methods:

Symmetrical (155.520 Mbps)Asymmetrical (155.520 Mbps/622.080 Mbps)

Functional equipment groupings - The functional groupings of equipment in the Broadband ISDN model are the same as those for Narrowband ISDN. However, these equipments are called B-NT1, B-NT2, B-TE1, B-TE2 and B-TA.

Reference points - Broadband ISDN also uses the reference points similar to Narrowband ISDN (R, S, T and U).

Frame Relay: - Frame relay is a service for people who want connection oriented way to move bits from one station to another at regionable speed and low cost. Frame relay can be used with virtual leased line. The customer leases a permanent virtual circuit between two points and can then send frames between them.Congestion Control: - Frame relay employs congestion-notification mechanisms rather than flow control. Since frame relay is implemented on a reliable network media flow control-function is left to higher-layer protocols.

Frame relay uses two congestion-notification mechanisms. These are:(i) FECN (Forward-Explicit Congestion Notification) - The FECN mechanism is initiated when a sender device (DTE) sends frames into the network. If the network is congested, switches (DCE) will set the value of the FECN bit to 1. When the frames reach the destination device, the set FECN bit indicates that the frame experienced congestion in its path from source to destination. The destination device will pass this information to higher-level protocols for processing.(ii) BECN (Backward-Explicit Congestion Notification) - DCE devices (such as switches) set the value of the BECN bit in those frames that are travelling in the opposite direction of frames with their FECN bit set. This informs the DTE that the particular path is congested. It can then pass this information to higher-level protocols for processing.

Frame Relay header also contains a Discard Eligibility (DE) bit. This bit is set to 1 in less-important frames. Thus, in case of congestion, these unimportant frames can be identified and dropped. When the network becomes congested, DCE devices will discard frames with the DE bit set. This reduces the chance of critical data getting deleted during congestion periods.Cell Relay: - Cell relay uses fix sized packets called cell relay. The basic idea behind ATM is to transmit all information in small, fixed size packets called cell. ATM is both a technology and potentially a service. Sometimes the service is called cell relay. The fixed size cell reduces overhead.ATM Technology: -

ATM is used in many networks including both public and private.


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DATA COMMUNICATION AND NETWORKING ATM technology can

be used in existing twisted pair, co-axial cable, and fiber optics network for local area network as well as WAN communication. In other words such technology is compatible with existing network.

ATM technology is also compatible with wireless and satellite communication.

ATM Layered Structure: -The new wide area service is called B-ISDN. It will offer video on demand, Live television from many sources, full motion multimedia e-mail, CD quality music, high speed data transport and many other services. The underline technology that makes B-ISDN possible is called ATM. Following are the different layers of ATM:

(i) Physical Layer – The physical layers deals with physical devices and medium.(ii) ATM Layer – ATM Layer mainly deals with cell and cell transport. It defines the layout of a cell

and tells what the header fields mean. The size of a cell is 53 bytes (5 bytes of header and 48 bytes of data/payload). Because each cell is the same size and small, delay and other problems with multiplexing different sized packets are avoided. Congestion control is also located here. Functionality of this layer is same as the network layer of OSI Model.

(iii) ATM Adaptation Layer – ATM Adaptation Layer is divided in two sub layers:(a) Segmentation and Reassembly (SAR) – This is the lower part of ATM Adaptation Layer. The

SAR sub layer breaks packets into cell on the transmission side and put them back together again at the destination.

(b) CS – This layer is responsible for accepting messages from the application and breaking them into 48 bytes for transmission.

How ATM Protocol works?When a user sends data over the ATM network. The higher level data unit is passed down to the

CS, which prepares the data for the ATM layers. The data is then passed down the SAR sub layer, which divides the data into appropriately sized segment.

These segments are then passed on to the ATM layer, which defines and appropriate cell header for each segment and encapsulate the header and payload into 53 byte cell. The cells are then passed down to the physical layer, which streams the cell and an appropriate place.Benefits of ATM: –

High bandwidth medium with low delay. Meets the requirement of the telephone, cable television, radio and data industries.

Ability to transmit video without creating a glittering or loosing the synchronization of sound and picture.

ATM switches capable of transmitting 20 gigabits of data/second and a shared switch can transmit up to 662 Gbits/sec.

It is extremely fast and provides dynamic bandwidth. ATM is designed for high performance multimedia networking.

Chapter – 11Wireless communication

Introduction: - Mobile devices such as laptop, palmtop, mobile, phones etc. are now available at affordable prices. These devices, based on the wireless communication techniques, help us to achieve mobility. Mobility is the ability to access information and services any-me and anywhere. Through


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DATA COMMUNICATION AND NETWORKINGwireless computing, one can access information, on traffic, ticket booking, check account balance, e-mail from one's home, cafeteria, or even from a grocery store.

Wireless communication makes use of spread, spectrum radio, infrared, cellular radio and satellite communication technologies. Since discussing each of them is out of the scope of this book, only the Cellular radio and Satellite communication technologies are discussed in the following sub-sections.Cellular Radio: - Cellular Radio means using a large number of low-power base stations for transmission, each having a limited coverage area.

An area is divided into a number of smaller areas, called cells. Each of these smaller areas is served by its own low-power radio base station. Frequency channels are allocated to these radio stations in such a way that the channels (frequencies) used in one cell can be reused in another cell some distance away.Principle of Operation - The principle behind cellular radio is this: Instead of using one powerful transmitter, many low-power transmitters are placed throughout a coverage area. For example, a high power transmitter in a region can have twelve conversations (channels).

The cellular radio equipment (base station) can communicate with mobiles as long as they are within range. Radio energy weakens over distance, hence, mobiles must remain within a frequency range to be able to communicate, with each other.Cells - A cell is the basic geographic unit of a cellular system. The term cellular comes from the honeycomb shape of the areas into which a coverage area is divided. Cells can be sized according to the population density in a given area. The splitting of cell into smaller cells is known as cell splitting. A group of cells is called a cluster. No frequency is reused within a cluster.Frequency Reuse - Since only a small number of radio channel frequencies are available, there has to be a way to reuse the available channels. The solution adopted was frequency reuse through the cellular concept. Cells are assigned a group of channels that is completely different from that of the neighboring cells. Infact, all the cells in a cluster use different frequencies. The same group of channels can be used in cells in some other cluster, provided that they are far enough from each other, so that their frequencies do not interfere.Handoff - When a mobile user travels from one cell to another, while he is attending a call, he is moving out of range of one radio base station and entering into the range of another base station. Since adjacent cells do not use the same frequency channel, the call must be either dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Dropping the call is not a desirable solution. The second option must be adopted and it is known as handoff.

Handoff occurs when a call is transferred from one radio channel to another as a mobile equip-ment leaves one cell and enters another one.

When a mobile user leaves a cell, the reception becomes weak. At this point, the cell site in use requests a handoff. The Mobile Telephone Network switches the call to a stronger frequency channel in the new cell. This is done without interrupting or alerting the user. The user does not notice the handoff at all.TELEPHONY (GSM): - GSM stands for Global System for Mobile Communication. Various systems had been developed for cellular communication, but there was no standardization. This led to several incompatibility problems. To overcome such problems, GSM standard was adopted in 1982 for cellular communication.

GSM is a globally accepted standard for digital cellular communication. It provides the specifica-tions, that is the function and interface requirements, but does not address the hardware. This is done to allow buying equipments from different vendors.

GSM network is divided into the following three systems:(i) Switching System (SS) - Switching System is responsible for call processing and subscriber related functions. It includes the following functional units:

Home Location Register (HLR): It is a database storing permanent information about subscribers such as the subscriber's address, service profile and activity status.

Visitor Location Register" (VLR): It is another database that stores temporary information about subscribers This information is needed by MSC (Mobile services Switching Centre) to service the visiting subscribers.

Mobile Services Switching Centre (MSC): It controls calls to and from other telephone systems or data systems. It performs such functions as toll ticketing, network interfacing, common channel signaling etc.

Authentication Centre (AUC): It provides authentication services like identifying authorized users and also ensures the confidentiality of each call.

Equipment Identity Register (EIR): It is a database storing information about the identity of mobile equipments. This prevents calls from stolen, unauthorized or defective mobile stations. Often EIR is integrated with the AUC (Authentication Centre).


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DATA COMMUNICATION AND NETWORKING(ii) Base Switching System (BSS) - All radio related functions are performed by BSS. IC consists of Base Station Controllers (BSC) and Base Transceiver Stations (BTS).

Base Station Controllers (BSC) - They provide all the control functions and physical links between the BTS (Base Transceiver Stations) and MSC (Mobile Services Switching Centre). They control handoffs, radio frequency (RF), power levels in base transceiver stations (BTS) etc.

Base Transceiver Stations (BTS) - BTS is the radio equipment (transceiver + antenna) needed to service a cell. They handle the radio interface to the mobile stations. A group of BTSs is con-trolled by a single BSC (Base Station Controller).

(iii) Operation and Support System (OSS) - OSS is connected to all the elements of Switching System (SS) and to the Base Station Controller (BSC). It is the functional entity through which the network operator monitors and controls the system. It offers cost-effective support for centralized, regional and local operational and maintenance activities required for a GSM network.GSM Specifications - Following is the list of GSM specifications:

Frequency Range: 1,850 to 1,990 MHz. Duplex Distance: Duplex distance is the distance between the uplink and downlink frequencies.

Duplex distance specified for GSM is 80 MHz. Channel Separation: For GSM, separation between---adjacent carrier frequencies is 200 KHz. Modulation: Modulation in GSM is done through Gaussian minimum shift keying (GMSK). Transmission rate: GSM has a data rate of 270 kbps. Access Method: GSM utilizes the Time Division Multiple Access (TDMA) concept. Coding: GSM uses Linear Predictive Coding (LPC). Speech is encoded at 13 kbps.

VSAT (VERY SMALL APERTURE TERMINAL): - Very Small Aperture Terminals (VSATs) are tiny terminals that have 1-meter antennas and can put out about 1 watt of power. The uplink is generally good for 19.2 kbps, but the downlink is more, often. 512 kbps. In many VSAT systems, the micro-stations do not have enough power to communicate directly with one another. Instead, a special ground station, the hub, with a large high gain antenna is needed to relay traffic between VSATs as shown in Figure 11.7.

In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier. The trade-off is a longer delay in return for having cheaper end-user stations. The delay time or end-to-end transit time is between 250 to 300 cosec (540 m. sec. for a VSAT system with a hub).

Among the options available in the country, VSAT networks have proved to be the most reliable and cost-effective for a small-sized network. Terrestrial based communication links available in India are notoriously unreliable due to various reasons like lack of foreign investment in infrastructure sector, absence of proper planning, non-availability of resources, delay in implementation of liberalization policy etc. The major benefits of VSAT network are:

very simple and easy to install channel availability to the tune of 99.5 per cent, thus leading to greater reliability high throughput and low bit error rate (BER) for data applications and integration of data and voice in one communication medium

Chapter – 12Security and privacy

Network Security: - In a network several users are working together and access for share the common resource that’s why security must be maintained among network users. Following are the major consideration to maintain network security:

Information that is stored in the system only accessed through authorized users. Sharable resources should be available for only authorized members. Unauthorized users should not be able to insert unnecessary information. Unauthorized users should not be able to access the information of the system.

Protection against Unauthorized use: - To prevent the use of information through outsiders, legal users are assigned an identification in the form of User ID with Password.Types of ATTACK: - There are two types of ATTACK:

(i) Passive ATTACK – This is considered as monitoring of transmission. Main purpose of such types of attack is to obtain the information which is being transmitted. Such types of ATTACK are difficult to detect but possible to prevent.

(ii) Active ATTACK – It includes modification of messages, capturing authentification, creation of false messages, obtaining permissions associated with information etc. Active attack cannot beeasily prevented, but it requires protection of all facilities all the time.

Firewall: - When a small network (LAN) connects with internet, it faces several dangerous security threads. To overcome such problem Firewall is built. Firewalls are basically combination of hardwares &


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DATA COMMUNICATION AND NETWORKINGsoftwares that are built using routers, servers and a variety of software. Firewall reduce the speed of a network but protect against several security threads.Benefits of using Firewall –

Protection against services Access to host To implement authentification procedure efficiency Policy inforcement Proper use of login

Characteristics of good Firewall system – Firewall is mainly built to protect our network. A Firewall system has following characteristics to maintain security:

A Firewall system should be able to deny authorized accession of network services. The Firewall should be flexible. The Firewall system most contains advance authentification measures. The Firewall system should implement filtering mechanism.

Firewall Types – There are three types of Firewall:(i) Proxy Firewall – Such types of Firewall which is working as intermediate between users

request and a particular layer (application layer or session layer or transport layer).(ii) Packet Filtering Firewall – Such types of firewalls checks/examines all the incoming packets,

then the system forwards or drops its according to predefined rules.(iii) Stageful Inspection Firewall – Such Firewalls system is considered as a new technology

based firewall system. Such system provides functionality or awareness to application layer without using any proxy.

Virtual Private Network (VPN): - VPN is a private network of an organization built over public connections, protocols and services used in VPN are those of public networks, but it is so built that it would function as a private corporate network.

In VPNs, users enjoy the same security and privacy features as available in a real private network. It gives secure remote access to the corporate network over the Internet.

VPNs use tunneling technologies to allow users to access private network resources through the Internet or any other public network.

Tunneling solutions are typically based on the Microsoft’s Point-to-Point Tunneling protocol (PPTP).


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