Chapter -2: The Physical layer. - schoolmca - HOMEschoolmca.weebly.com/uploads/4/5/0/8/450802/ccn-chapter-2-physical...Chapter -2: The Physical layer. Slide –21: ... Slide -22: Objectives.

Dr KLG, BVBCET, HUBLI

1

Chapter -2: The Physical layer.

Slide –21: Contents.

Lesson Schedule: Dates. Class No. Portion covered per hour (An estimate). Planned. Engaged

1. The theoretical basis for data communication.

2. Guided transmission media.

3. Wireless transmission media.

4. The public switched telephone network.

5. Continued.

6. Continued.

7. Cable television.


2 Slide -22: Objectives.

The chapter will focus on:

1. How raw bits are transmitted in the network ?

2. What to use to connect to the Internet ? Different transmission media - wired & wireless.

3. How to connect to the internet ? The basics of switching used telephone system.

4. How internet can be accessed over cable television.

Physical Layer. • It defines the Mechanical, Electrical & Timing interfaces to network. • As it concerns with data transmission i.e. transportation of raw bit stream from one machine to other machine.

• Let us study the basis of data transmission. • We will understand that channel puts some limit on how much we can send. • The channels / media are: Guided, Wireless & Satellite.

o 1. Guided: Cooper and Fiber. 2. Wireless: Radio. 3. Satellite. • The widely used communication system in WAN is the telephone system – Fixed, Mobile & Cable TV.

• The communication systems use fiber backbone and different technologies.


3 Slide -23: The theoretical basis for data communication.

• Data can be transmitted on wires by varying some physical property of the signals, like voltage / current.

• This signal (voltage / current) can be represented by a single valued time function f(t). • It is possible to model the behavior of these signals and analyze it mathematically. • In 19th century, Fourier shown that any periodic function with period T can be constructed as a sum of a number of Sine & Cosine signals.

o g(t) = ½ c o Where f = 1 / T is the fundamental frequency.

• This decomposition is called Fourier series analysis. • If component frequencies are known the function g(t) can be reconstructed. • The data signal in the n/w can be handled as periodic signal. • From figure 1 it is clear that, any data can be constructed from the Sine & Cosine components.

• Analysis of the data is comfortable using its components. • How this will help in data communication ? Consider example below. • Transmitting an ASCII character ‘b’ as an 8-bit data / signal. • The transmission line diminishes / attenuates different frequency components by different amount, thus introducing distortion.

• Basically transmission line works as a low pass filter – LPF.


4

b – e Depicts successive approximations of the original signals.

a. Depicts the bit pattern of ‘b’ to be transmitted & first few Fourier / frequency components. RMS amplitudes – vertical lines. These RMS values are proportional to energy transmitted at the corresponding frequency, so are important.

Figure 1


5 • Usually the amplitudes are transmitted from 0 to some fc. fc is cutoff frequency; above this all frequencies are attenuated.

• This frequency range 0 - fc are transmitted without strong attenuation, is called BAND WIDTH of the channel.

• Bandwidth is a physical property of a transmission media. • The b/w of telephone line is restricted to 3100 Hz, even if it can support 1 MHz for short distances. A filter is used to restrict the b/w of the channel which gives system wide efficiency.

• Figure 1 (a – e) also shows the effect of b/w. If b/w is low only fundamental frequency will be transmitted & reconstructed signal is in a. and so on. As the b/w increases the reconstruction become better.

• For voice grade channel with b/w of 3000 Hz & 8-bit data, the number of highest harmonics / component passed is given in the table below i.e. 3000 /(b/8) = 24000 / b.


6 Relation between data rate and harmonics. • When you try to send data at 9600 BPS over voice grade channel, the received signal looks in Fig (c), with 2 harmonics.

• Above 19200 BPS there is no hope signal recovery at the receiver. • However, sophisticated coding / modulation schemes make use of several voltage levels achieve higher data rates. This is the basic of modulation schemes.

• Nyquist realized that even the perfect channel has a finite transmission b/w. • Shannon used the concept and extended it to channel with noise.

Figure 2


7 • Nyquist derived an equation for maximum b/w that can be achieved in noiseless channel. Max data rate = 2 H log2 V bits / sec – H is the b/w. Sampling theorem.

• This shows that, a noiseless voice channel can not transmit binary data exceeding 6000 BPS i.e. 2 x 3000 x log2 (2) = 6000 BPS.

• - • • If noise (random) is present in the channel the situation deteriorates rapidly. • Random thermal noise is always present due to electron moments. • Random noise is measured by the ration of signal power to the noise power called SNR – Signal to Noise Ratio.

• Normally half power point / 3 dB points are specified by manufacturer, which gives half the power (log10 (3) = 0.5).

• Usually it is called decibels (dB) = 10 log10 (S/N). • Shannon proved that the maximum data rate in a noisy channel with b/w H and with signal to noise ratio of S/N is = H log2 (1 + S/N).

• For example a vice grade channel with SNR = 30 dB (typical in telephone system) = 3000 log2 (1 + 30) = 30,000bps.

• It is an upper bound, difficult to achieve in real systems.


8 Transmission media. • As we have seen the main aim of physical layer is to transport raw bit stream from one to other machine.

• Many physical media can be used for the actual transportation. • Each one of them have its own function in terms of B/W, delay, cost, ease of installation, maintenance etc.

B/W Delay Cost Installation ease Maintenance.

• They may be grouped into two main categories: Guided & unguided.


9 Slide -24: Guided transmission media.

• The guided media are Copper wires & Fiber optics cables. • One of the earliest is the magnetic media (floppy, CD, tape etc.). It offers huge bandwidth though difficult to achieve by other means.

• Delay involved is very much, may be minutes / hours / days etc. B/W Delay Cost Installation ease Maintenance

• For many current day applications the delay time must be very low, i.e. online connection is needed.

Twisted pair. • The twisted pair is the oldest, common and still used dominantly for transmission media. • Twisted media (twisted pair) consists of two insulated copper wires of about 1mm thick. • The wires are twisted in helical form, because parallel wires make a finite antenna. Twisting cancels out radiation effects.

• A dominant application is in telephone connection for local loop. • They run for few KM without amplification. • Twisted pair can be used to transmit analog / digital signals. The b/w issues are discussed earlier. B/W of MBPS can be achieved for short distances.

• Because of their low cost & adequate performance, they are widely used and likely to remain so for long.

•


10 CAT3 – Category3 cable: • Twisted pair comes in many varieties, CAT3, CAT5, CAT6, CAT7 etc. • CAT3 consists of 4 twisted pairs covered by a plastic sheath. Normally used in telephone connection, supports b/w up to 16 MHz.

• CAT5 is similar to CAT3 but with moiré twists per inch. Used in computer network, supports b/w up to 100 MHz. Used in LANS.

• CAT6 similar to CAT5 but with more twists per inch, suitable for high speed lines. Used in Computer n/w, supports b/w up to 250 MHz.

• CAT7 similar to CAT6 but with more twists per inch, suitable for high speed lines. Used in Computer n/w, supports b/w up to 600 MHz.

• All these pairs are called UTP cable – (Unshielded Twisted Pair). • IBM has introduced STP cable – Shielded Twisted Pair, which are bulky & expensive. Not used outside IBM installations.

(a) Category 3 UTP. (b) Category 5 UTP. Figure 3


11 Coaxial Cable. • It is also a commonly used media like UTP. • It has better shielding than UPT, so can span more distance at higher speeds. • There are two kinds: 50 Ω cable & 75 Ω cable. • 75 Ω cable is used in analog transmission & cable TV and becoming more important with the advent of Internet over cable.

• 50 Ω cable is used in digital transmission since long. • The structure of coaxial cable is shown below. • Modern cables have b/w close to 1 GHz. • Earlier Coax was widely used in long distance lines, now replaced by Fiber. • Still coax is widely used in cable TV & MAN.

Figure 4


12 Fiber Optics. • Computer speeds have improved from 4.77 MHz in 1981 to 2.0 GHz around 2000. Gain factor of 20 per decade.

• The communication speeds have improved from 56 KBPS to 1 GBPS in the same period. Gain factor of 125 per decade.

• The error rates have fallen form 10-5 per bit to almost zero. • But, in present Fiber technology the b/w in excess of 50,000 GHz is achievable. • Although 10 GHZ fibers are available, we can’t use them as the technology to convert electrical to optical & visa versa is available. 100 GHz speed in the fiber is achieved in the labs.

• In the race was won by communication, the infinite b/w availability is no more a problem. • In this aspect computers are hopelessly slow. Let us see how fiber transmission technology works. • The optical system has 3 key components: Light source, Transmission media, the Detector. o A pulse of light indicates ‘1’ and absence a ‘0’. o The transmission media is an ultra thin fiber glass. o The detector generates an electrical pulse when light falls on it.


13 • Unidirectional data transmission system can be implemented by attaching a light source to one end of the fiber and attaching detector at the other end.

• The electrical signals are converted to light pulses (by light source) transmits these light pulses in the fiber and received and converted to electrical signal (detector).

• For a practical transmission system of fiber: o The core refractive index must be greater than cladding. o The incident angle must be greater than critical angle. o Under these conditions the light ray does not escape into air, but is guided through the core, Figure 5.

Figure: a. Light incidence at different angles. b. Total internal reflection. • In fig-b, only one ray of light is shown, but many different rays are incident at different angles. Each ray produces one mode. A fiber having this property is called multimode fiber. The core diameter is between 50 - 100 µm.

Figure 5


14 • If the core diameter is reduced to a few wavelengths 5 - 10 µm, light almost travels in a straight line, the fiber is called single mode fiber.

• Single mode fiber is costly compared to multimode & supports up to 5o GBPS & up to 100 KM without repeaters.

Transmission of light. • As voltage / current / power is attenuated in the wire, the light is also attenuated in fiber.

• The attenuation depends on the wavelength of the light. The Fig shows attenuation characteristics of the fiber.

• It shows 3 important bands of use centered around 0.85, 1.33, 1.55 µm in IR range. • 0.85 µm has higher attenuation, but lasers & electronics can be made from Gallium arsenide & has almost 20,000 GHz b/w.

• 1.33, 1.55 µm bands have lower attenuation and have around 30,000 GHz b/w. Tejas n/w, B’lore.

• The electrical pulses disperse (deteriorate) when passed through a communication channel.

• Similarly, Light pulses sent across in fiber spread out in length as they propagate. • This spreading is called chromatic dispersion. The spread is dependent on source wavelength.


15 Fiber Cable. • Fibers are similar to coax, it has CORE & a CLADDING. • The core refractive index is higher than cladding. • The fibers can also be connected like other wires. There are three different ways:

o They can terminate in connectors. Usually there is loss of 10-20 % light. o They can be spliced mechanically & inserted in sleeves. Usually 10 % loss. o Fibers can be fused / melted. This also has a small attenuation loss.

Figure 6


16 • Two kinds of sources used are LED & laser. Sources are used for signaling. • Fibers are used in LANs and use 2 types of interfaces: Active & Passive interfaces.

Figure 7


17

Active interface. Passive Interface.

Comparison of Copper & Fiber Medium. Advantages. Disadvantages.

• B/W is very high. • Used in high end n/w. • Low attenuation, repeaters at > 50 km. • No electromagnetic noise interference. • No corrosion. • Very less size & weight, 100 kg /km compared to 800kg / km.

• Secure, difficult to tap.

• Skills required is more. • Fibers can be damaged easily. • Inherently unidirectional, requires a pair for operation.

• Fiber interfacing costly.

Conclusions: The future is in fiber.

Figure 8


18 Slide -25: Wireless transmission media.

Basics • There are users / mobile users who want to be all the time on-line with their Laptops, Notebooks, and Palmtops etc.

• For all these users wireless is the answer. • Wireless along with giving connectivity to the net, provides other important applications. • Some believe that in future fiber & wireless common will consolidate. Electromagnetic Spectrum. • James clerk Maxwell, in 1865, predicted that electromagnetic waves can propagate through space / vacuum, these waves are created due to electron moment in the conductor.

• Heinrich Hertz, in 1887, first observed this. The number of oscillations of the wave per second is called its frequency & measured in Hz in honor.

• The distance between two maxima / minima is called the wavelength, λ = c /f, c is the speed of light = 3x 108 m / sec and is constant.

• An antenna of appropriate size (λ or λ/2) broadcast / receive electromagnetic waves efficiently, when attached to an electrical circuit. All wireless communication is based on this principle.

• In vacuum the light travels at a speed of c = 3x 108 m / sec.


19 • In other media its speed is reduced, it is around 2/3 c in copper or fiber & is frequency dependent.

• The fundamental relation is c = λf. • A rule of thumb is, if λ is in meters and f is in MHz, gives λf = 300 is a constant.

o For f = 300 MHz, λ = 3 Mts. o For f = 2500 MHz, λ = 0.12 Mts. o For f = 5000 MHz, λ = 0.06 Mts.

• The electromagnetic spectrum that can be used for transmission of information by modulation (AM, FM, PM etc.) is shown, along with their ITU names, LF, HF etc.

• UV, X-Ray, Gamma rays are not suitable for transmission even though they have higher b/ws, because: o They are hard to produce. o They do not propagate well through buildings. o They are dangerous to living things.

Figure 9


20 Bandwidth. • The amount of information that can be carried is related to its b/w. • Currently it is possible to encode few bits / Hz at low frequency, may be 8 bits at higher frequency. o Ex . A coax with 750 MHz b/w can carry up to 750 x 8 = 3600 MBPS.

• So fiber has tremendous scope as it has much larger b/w.

o From c = f λ, differentiate w.r.t. λ - df /dλ = c / λ2, o To get finite difference, we only look at absolute values, o we get ∆f = ∆λ (c / λ2), o i.e. given the width of the wave length band, ∆λ we can compute corresponding frequency band ∆f & the data rate the band can produce.

o Ex for 1.33 µm band, λ= 1.33 x 10-6 & ∆λ = 0.17 x 10-6. o So, ∆f = 30 THz, at 8 bits / Hz the b/w = 240 TBPS. Look at the b/w.

• Most transmission lines use a narrow frequency band (∆f / f << 1) to get best reception (many watts / Hz).

• In some case, a wide band is used with variations: FHSS & DSSS. • FHSS – Frequency Hoping Spread spectrum.

o The frequency hops from one to other hundreds of times / sec.


21 o It is popular for military communication as it makes transmission hard to detect & next to impossible to jam.

o It also provides good resistance to multi-path fading, because the direct signal arrives first. Reflected signal follows longer path & arrive later. By then the frequency at the receiver has changed making them irrelevant, thus eliminating interference between direct & reflected signals.

o This technique is also used in commercial products both in 802.11 & 802.15 – Bluetooth. (Story of Hedy Lammar- German, Film actress, Husband-Gun Hitler, Naked, spread spectrum in pass time).

• DSSS – Direct sequence Spread spectrum. o It spreads he signal over a wide frequency band & is popular in commercial world. o Used in 2G mobile & may become dominant in 3G mobile sets. o It has very good spectral efficiency, noise immunity etc. o WLANs also use DSSS.


22 Radio Transmission. • Radio waves are easy to generate. • Can travel long distances. • Can penetrate buildings easily. • Radio waves are omni directional i.e. travel in all directions with no alignment problems. • So, used widely in out door / indoor applications. • The properties of radio waves are frequency dependent.

o At low frequency, they pass through obstacles well, but the power falls off rapidly with distance from the source roughly as 1/r2 in air.

o At high frequency, waves tend to travel in straight line and reflected from obstacles & absorbed by rain.

• Radio waves, at all frequencies, are subjected to interference from electrical equipments. • For this reason (interference) the frequency bands are licensed by every govt. • The VLF, LF, MF bands are ground waves & can travel up to 1000km.

o Radio transmission uses these bands. • In the HF & VHF bands the ground waves are absorbed by earth.

o Military use these bands widely.


23 Microwave Transmission. • Travel in nearly straight lines (> 100 MHz). • Directing the energy into a narrow beam using parabolic antenna is possible. • Gives better SNR & requires antenna alignment. • Multiple transmitter / receiver at same frequency are possible, because of non interference.

• Earlier to fiber, microwaves were used for long distance telephone transmission, now most of them replaced by fiber.

• Require repeaters approximately at 80 km each on towers of 100 m. • Effect of multi-path still exists and is a serious problem. • The transmission is weather & frequency dependent.

(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere. Figure 10


24 • Problem at higher frequency, at 4 GHz, waves are absorbed by water: during rain the links may have to be shut off.

• Microwaves are widely used for long distance telephone communication, mobile phones, TV etc.

• Right of way is not a problem. • Suitable in congested urban areas where cable laying is costly & cumbersome. Distribution of spectrum: Politics of EMS. • There are national & international agreements to use frequency spectrum. • National governments allocate frequency spectrum for AM, FM, TV, Mobile, Telephone companies, Police, Maritime, Navigation, Military, Government & others. TRAI in India.

• Worldwide ITU-R (International Telecommunication Union) coordinates at global level. • Politically powerful groups compete for spectrum & lot of malpractice happens. • There are various ways of allocating frequency to competitors: discretionary power of Govt officials, lottery, auction etc.

• One more approach, everybody is free to use, put a cap on the power transmitted, so that interference is avoided.


25 • ISM (Industrial Scientific Medical) band is unlicensed users. • To reduce interference the devices using ISM band has to use spread spectrum technique.

• ISM band is used for home gadgets, cordless phones, radio control toys, wireless mouse / keyboard etc.

• The ISM bands used are shown in figure 11.

• 900 MHz band works best, but is over loaded. • 2.4 GHz band is available, but will have interference from microwave ovens, radar instruments.

• 5 ∼ 7 GHz band is new, expensive at present, because new equipment to be developed. 802.11a is already using it.

ISM Band

Figure 11


26 IR & mm waves: Infrared & millimeter waves • Widely used in short range communication. • Used in remote control of TV, VCR, stereo: IR. • The range is limited to few meters. • Relatively directional, cheap & easy to build. • A major drawback is they don’t pass through solid objects. • In general operate near visible light band, behave more and more like light & less like radio.

• Some times this (don’t pass through solid objects) is an advantage, so that you can’t control in another room.

• No Govt. licensing is required. • IR has limited applications in Desktop, Palmtop, printers etc. • IR is not a major player in communication.


27 Light wave communication. • Using Laser. • Many limitations in transmission.

Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

Figure 12


28 Communication Satellites. • A repeater in the sky. • Geostationary satellites used for communication. • Bands used L, S, C, Ku, Ka. • Specialized applications are best suited for satellite communication. • Finally the cost plays the role.

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

The principal satellite bands.

Figure 13


29

(a) Relaying in space. (b) Relaying on the ground.

Satellite as a Hub.

Figure 15 Figure 14


30 Slide -26: The Public Switched Telephone Network (PSTN).

• PSTN was designed long ago with a different goal. The main goal is to transmit human Voice.

• As they are, not suitable for computer to computer communication, but the situation is changing after the introduction of digital technology & Fiber optics.

Why PSTN ? • Now, PSTN has been tied to computer network. That is telephone system is tightly intertwined with WAN. Hence it is necessary to study PSTN system.

• Consider a situation: A system / network in your main office need to be connected to a system / network in the branch office, some 100s of kms away ! o You have distance problem. o You have cost issue. o You have right-of-way issue.

• The solution is you need to use the existing communication facility. • Then the PSTN is tied to your computer network. • When this happens the objectives of PSTN changes. That has happed & PSTN has seen a sea change.

• Let us look at PSTN in some depth.


31 The trouble. • When we work on the LAN, through a dedicated line, the speed of transmission is many MBPS, typically 10-1000 MBPS.

• When a computer is connected through a PSTN line at few KBPS (dial-up line), typically 56 KBPS.

• The speed difference is by a factor of 20,000. • If the dial-up line is replaced by ADSL (Asymmetric Digital subscriber Line) which has a magnitude of 10 times the PSTN basic line, still the difference is big.

• Computer system designers are used to this kind of limitations. • If there is a difference of speed of two systems, they have found out ways to use them efficiently.

• This is covered in basic courses like interfacing devices and many more. Structure of the Telephone system. • In 1876 Alexander Graham Bell invented & patented telephone. • There was a great demand for telephones then. They were sold in pair. • There was single wire between each pair of phones and the earth acted as a return conductor.


32 • Owner has to connect separate wires for each phone. That means, N phone require N connectors. All are hung across the road & overhead.

• Obviously this model was not going work. Figure 16 a. • After seeing all this, Bell former Bell Telephone Company & opened first switching office in Connecticut in 1878.

--

• The company ran a wire to each house from its office. Figure 16b. • To make a call customer would crank the phone to make a sound at Bell’s office. • The operator would then manually connect to other customer using a jumper cable. • This model has a single switch. Figure 16b. --

• The switching offices came-up everywhere in US.

(a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

Figure 16


33 • Little later, people wanted to make long distance calls between cities. • The company began to connect the switching offices. • The original problem returned: to connect every office to every other became unmanageable.

• So, 2nd level of switching offices was invented. • After some time multiple 2nd level offices were needed. Figure 16 c. • Eventually the hierarchy grew to 5 levels. --

• By 1890, telephone systems with 3 major parts were in place: 1. Switching offices. 2. The wires between customer & switching office: A pair of wire. 3. Long distance connection between switching offices.

• In the 100 years, all the above three areas have been improved. • But the Bell telephone system model remains the same. • --

• A simplified description of telephone system thus: o Every telephone has two wires connecting to nearest end office also called local central office. This wire is also called local loop.

o The typical distance is 1-10 km: less in urban & more in rural. o At one stage 80 % of the capital value was spent on copper wire in the local loop.


34 • When a subscriber makes a call to another subscriber on the same end office, the switching mechanism sets-up a direct electrical connection between two local loops & remains so for the duration of the call.

• If the other phone is connected to another end office, the switching procedure is different o Toll offices are used to interconnect end offices in addition to above.

• Details are given in the figure 17:

The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the

modems and codecs.

Figure 17


35 • A variety of transmission media are used: o Cat-3 twisted pair (earlier untwisted, 25 cm apart) is used in Local loop. o Coaxial cables, Microwave, especially Fiber optics, were widely used between switching offices (trunk lines).

• Earlier most of the transmission in telephone system was analog. With the actual voice being transmitted as electrical signals between sender & receiver.

• With technological developments & advent of fiber optics, the digital electronics & computers, all the switches & trunks are now digital.

• But the local loop is still analog technology. • Digital transmission is preferred, especially for long distance calls because of higher reliability. It is also cheaper & easier to maintain.

• --

Summary of Telephone system: • It has 3 major components.

1. Local Loops: Going into each house, Analog twisted pair Provide access to everyone. So it is critical. But unfortunately it is the weakest. 2. Trunks: Digital fiber optics connecting the switching offices The main issue here is how to collect multiple calls together & send them over the same fiber – multiplexing. 3. Switching offices: There are different ways of switching two user / phones.

• Like any systems in the world, telephone systems also have politics. Read more.


36 • As clear from figure 17, the local loop is connected between user and end office. • Trunk line is connected between end office & toll office. • End office & toll office are the switching centers • Local loops & trunk lines are transmission media. • Numbering is an important aspect in telephone systems: End office supports up to 9999 local loops. More details on-line. Example:

836 2 27 3286

City code Service provider code

End office code Local loop code

9 84 50 15746

Mobile service

Service provider code

End office code Local loop code

• Normally the maximum digits are 10 excluding ISD / country codes, it may vary. • If the numbering system changes, that needs to be reprogrammed. • In the current scenario most of the numbers are used up, so mapping schemes have to be introduced or number code length has to be increased.


37 1. Local Loop: Modem, ADSL & wireless • It is most familiar to many / most people, because they use it. • Traditionally local loop has two wires, now known as twisted wire. • Connected from customer premises to end office. • Over 100 years, local loops used analog signaling (pulse mode). May be used for few more years due to high cost of converting to digital.

• The voice transmission in local loop is analog. In Mobile ? • Lots of changes are taking place in local loop with particular emphasis on data communication from home computers.

• Local loops are usually dial-up lines i.e. one needs to dial a number to get connection. This is dial-up service.

• Different types of services are now available: Lased, dedicated etc. • --

Transmitting digital data over analog local loops: • Whenever a computer wishes to send digital data over analog dial-up line, the data must first be converted to analog form.

• This conversion is done bay device called modem. • The data is converted to digital form at the end office for transmission over the trunks. • In the figure 17, for ISP1 the reverse conversion is required. For ISP2 it is not required. • When you want to get connected to network, through ISP (Internet Service Provider) you are dialing a number, which has a machine connected through a modem.


38 • It has a bank of modems (Figure 17 – ISP1). This was normal until 56k modems. • --

Noise affects transmission: • As we are aware, noise affects analog signals more than digital signals, so less errors in digital transmission.

• Transmission lines have 3 major problems: Attenuation, Delay distribution, Noise. • --

Attenuation: It is the loss of energy as the signal propagates & is expressed usually in dB / km. Delay distortion: This loss is a function of frequency. As the signal transmitted consists of many frequency components (Page 5), each frequency component is attenuated by different amount. To make things worse, the different frequencies travel with different speeds in the wire. This is delay distortion. Noise: As we are aware noise is omni present unwanted energy originating from other than source of information.


39 There are 3 major types of noise we come across in channels: 1. Thermal noise is caused by random motion of electronics and can’t be avoided. 2. Cross talk is caused by inductive coupling between two wires close each other, can be avoidable. Hear others voice during conversation. 3. Impulse noise is caused by spikes on the power line or other causes. For digital data this can wipeout one or more bits.

Let us look at Modems at some length • Modem – MODulatorDEModulator • A device that accepts a serial stream of bits as inputs and produces a carrier modulated by one / more of the techniques discussed next is called a MODEM.

• A modem is inserted between (Digital) computer & (Analog) telephone system. • Because of the 3 major problems just discussed, especially the frequency dependency of attenuation and propagation delay, it is desirable not to have wide frequencies in the signal.

• Unfortunately square wave used in digital signals have a wide frequency spectrum & thus subjected to string attenuation & delay distortion.

• These effects make base band (DC) signaling unsuitable except at slow speeds & over short distances.

• To overcome these problems AC signaling is used on telephone lines.


40 • A continuous tone (frequency) in the range of 1000 - 2000 Hz is introduced & is called sine wave carrier.

• Its (Sine wave carrier) amplitude, frequency, or phase can be modulated to transmit information efficiently. o Amplitude modulation (Amplitude Shift Keying – ASK): Two different amplitudes are used to represent ‘0’ & ‘1’ respectively. This can represent 1-bit of information.

o Frequency modulation (Frequency Shift keying – FSK): Two / more different tones are used to represent ‘0’ & ‘1’ respectively. This can represent 1-bit of information.

o Phase modulation (Phase Shift Keying – PSK): The carrier wave is shifted by an angle at uniform space interval. These phase shifts can be used to represent ‘0’ & ‘1’ respectively. Example: 0° or 180° phase shifts to represent ‘0’ & ‘1’ respectively. This can represent 1-bit of information.

A better option is to use 45°, 135°, 225° & 315° to represent 2-bits of information.

Phase shift at the end of every time period, makes it easier for receiver to recognize the boundaries of the time interval.

• Figure 18 illustrates these techniques:


41

How to achieve higher speeds ? • Increasing sampling rate will not achieve higher speeds as the telephone lines operate up to 3000 Hz, where the maximum sampling rate is 6000 Hz.

• In practice, most modems sample 2400 times / sec and focus on getting more bits / sample.

(a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation

• A device that accepts a serial stream of bits as inputs and produces a carrier modulated by one / more of these techniques or vice versa is called a MODEM.

Figure 18


42 Baud • The number of samples / sec is measured in baud. One sample is sent during each baud. Thus n-baud line transmits n samples /sec.

Example: • A 2400 baud line sends one symbol every 416.667 µsec (Page 5 & 7). • For a symbol of 0 V for logic ‘0’ & 1 V for logic ‘1’, the bit rate is 2400 bps. • If 0, 1, 2, 3 V are used for 4 logical levels, every symbol consists of 2-bits. The line can transmit 2400 samples at a data rate of 4800 bps.

• Similarly with 4 possible phase shifts there are also 2 bits /symbol at a bit rate of 4800 bps. This is technique widely used and called QPSK (Quadrature Phase Shift Keying). Figure 19.

(a) QPSK. (b) QAM-16. (c) QAM-64.

Figure 19


43 There is always some confusion between B/W, Baud, Symbol & Bit rate. Some clarification: • B/W: B/W of medium is the range of frequency that passes through it with minimum attenuation. It is the physical property of the medium (300 – 3300 Hz for telephone line) measured in Hz.

• Baud rate: It is the number of samples transmitted per sec. Each sample send one piece of information i.e. one symbol.

• Symbol rate: The baud rate & the symbol rate is the same. • Bit rate: It is the amount of information sent over the channel and is equal to Number of symbols sent / sec x bits used / symbol = bits / sec. • All advanced modems use a combination of modulation techniques to transmit multiple bits / baud (different amplitudes – dist from origin & phase –fig above).

Fig Amplitudes Phases Levels Bits / Symbol BPS Scheme

A 1 4 4 2 bits /symbol 2400x2 4800 QPSK

B 4 4 16 4 bits /symbol 2400x4 9600 QAM-16

C 16 4 64 6 bits /symbol 2400x6 14400 QAM-64

• QAM– Quadrature Amplitude Modulation. Higher order QAM is also possible. • A figure (Figure 19 or 20) showing the legal combinations of amplitude & phase are called constellation diagrams.


44 • Each high speed modem standard has its own constellation pattern and can talk only to other modems that use the same one, though most modems can emulate all the slower ones.

Noise effect • From constellation pattern, it is observed that even a small amount of noise in amplitude or phase can result in error and result in many bad bits.

• To reduce this error, higher speed modems do error correction by adding extra bits to each sample. The schemes are known as TCM (Trellis coded modulation). o Example: (V.32 Modem, Figure 20). o Uses 32 constellation points to transmit – 4 data bits & 1 parity bit per symbol at 2400 baud to achieve 9600 bps with error correction.

o Constellation diagram was rotated by 45° for engineering regions. How ever it gives the same result.

(a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps.

Figure 20


45 • V.32 bis (QAM128, figure 20).

o This gives 14400 bps speeds. o This is achieved by transmitting 6 data bits & 1 parity bit per symbol at 2400 baud. o The constellation pattern has 128 points. o Fax modems use this speed to transmit pages scanned as bit maps.

• --

• QAM-256 is not used in any standard telephone modem, but used on cable n/w. • V.34 modem runs at 28,800 bps at 2400 baud with 12 bit data / symbol. • V.34 bis uses 14 data bits / symbol at 2400 baud to achieve 33600 bps. • To increase the effective data rate beyond 33.6 kbps, modems compress the data before transmission.

• All most all the modems test the lines before transmitting user data & if the line quality is lacking it cuts back the speed. Usually they take 30 sec for this.

• Thus the effective modem speed observed, can be lower or equal to or higher than specified rating.

• All modems support full duplex mode transmission. • Fibers use simplex mode. So a pair of fibers is normally used. • The reason that standard modem speed stop at 33.6 kbps is because of Shannon limit for telephone system (35 kbps). Going faster than this violate laws of physics.

• Then, are 56 kbps modems possible ? YES.


46 • The theoretical limit of 35 kbps is determined by the average length of the local loop (1-10 km each).

• From figure 17 the call originating from a computer on left to ISP1 goes over 2 local loops of analog signals & each local loop adds noise.

• If one of the loops can be eliminated, the maximum rate can be doubled. • ISP2 does that. ISP2 is connected through digital connection. • As it is with most ISPs now, the maximum data rate can be as high as 70 kbps. • If you want to connect computer between two home users the speed is 33.6 kbps. • --

Why only 56 kbps Modem ? V.90 Modem. • The telephone line is about 4000 Hz wide with guard band. • The sampling rate is thus 8000 samples /sec. • The number of bits / sample is 8 in US. 1-bit is used for control purpose allowing 56000 bits / sec of user data.

• But in Europe all b bits are available to yield 64 kbps. • For international standard 56 kbps is chosen. This modem standard is called V.90. It provides 33.6 kbps for upstream channel & 56 kbps for down stream, because there is more data transfer from ISP to user.


47 V.92 Modem • This is next to V.90 modem. These modems are capable of 48 kbps on the up stream if the channel handles it.

• Speed determination takes half of older modem (30 sec). • They allow an incoming telephone call to interrupt an internet session, provided the line has call waiting service.

Digital Subscriber lines (DSL) • A subscriber line offering digital service. • When telephone industry got a 56 kbps modem, it was job well done. • But at the same time, TV industry was offering speeds up to 10 mbps on shared cable & satellite companies were planning to offer upward 50 mbps.

• But for survival & growth internet access was important to telephone companies. Companies begun to realize this & the only way was to offer new digital service on local loop.

• Companies started broad band service. Broad band is one which offers higher than 56 kbps.

• The first of this broad band service is DSL with different options; Asymmetric DSL (ADSL) was most popular. The standard for this is still under development.


48 Why modems are slow & limited to 56 kbps? • The telephone line is optimized for only voice signal. Data was intended to be transmitted on these lines.

• Local loops terminated in end office have filters (artificial) to attenuate all frequencies outside usable voice range (300 – 3400 Hz). However the B/W is mentioned as 4000 Hz, as the filter cutoff is not sharp. Because of this data was also restricted to this narrow voice band.

• The trick used in xDSL is that, the incoming line is connected to different kind of switch which does not have this filter, and makes the entire B/W of local loop available. The limiting factor now is physics of the local loop.

• Unfortunately, like any other transmission line, local loop performance depends on its length, thickness, & general quality. (Figure 21).

Bandwidth versus distanced over category 3 UTP for DSL.

Figure 21


49 • The xDSL services have been designed with certain goals:

o The service must work over the existing category 3 twisted pair local loop. o They must not affect customers existing telephone / fax machines. o They must be much faster than 56 kbps. o They should always be on with monthly charges.

• Initially, ADSL service was offered by dividing the spectrum on the local loop into 3 frequency bands: POTS, Upstream, & Down stream.

• The alternate approach called DMT (Discrete multi Tone). (Figure 22). • 1.1 MHz spectrum on the local loop is divided into 256 independent channels of 4312.5 Hz each. These channels can be use din Simplex, Duplex & Full duplex mode.

Operation of ADSL using discrete multitone modulation.

Figure 22


50 Channel Used for Reasons

0 Voice Basic service of POTS

1-5 NOT USED To avoid Voice & data Interference.

6 - 256 One – upstream control. One - down stream control. 248 for data stream.

To transmit control data. - To transmit user data. Some times few channel used for up& down stream also.

• Normally 80% of the bandwidth is assigned for downstream, as users download more data than upload. The provider can determine channel allocation, but it is asymmetric. This is ADSL (Asymmetric Digital subscriber Line).

• In ADSL: out of 248, 32 are assigned for upstream & rest for down stream. • Some channels can also be made bidirectional to utilize B/W. • ADSL standard allows speeds of as much as 8 mbps downstream & 1 mbps upstream, but typically providers offer a) 512 kbps down stream & 64 kbps upstream as a standard service b) 1 mbps down stream & 256 kbps upstream as a premium service.

• ADSL is similar to V.34 scheme and the line quality is monitored. Depending on the line quality data rates may be adjusted.


51 • A typical ADSL arrangement is shown in figure 23. • From the ADSL scheme, the telephone company must install a NID (Network Interface Device) in the customers place.

• Close to NID a splitter (filter) is connected to separate analog signals (0–4000 Hz) & data.

• The analog signal is rooted to existing telephone & data signal to DASL modem. Actually ADSL modem is a digital signal processor, which supports QAM modems operating in parallel at different frequencies.

Figure 23


52 • Normally ADSL modems are external. Unlike other modems, one end of ADSL modem is connected to PC through Ethernet cable. Internal ADSL modems may also be available.

• However in some cases USB port may also be used. • The other end of wire, at the end office, a corresponding splitter is connected. • The normal voice is sent to voice switch & the signal above 26 KHz is sent to a device called DSLAM (Digital Subscriber Line Access Multiplexer) which has the similar arrangements as ADSL.

• This separation of voice & data systems & ADSL enable the telephone companies to deploy ADSL service.

• One disadvantage with this system is the presence of NID in the customers place. It is costly implementation & requires technician.

• An alternative splitter less design is standardized called G.Lite or G.992.2. A micro filter is inserted in each telephone jack between the telephone / ADSL modem and the wire.

• The micro filter eliminates frequencies above 3400 Hz for telephone & eliminates frequencies below 26 KHz for ADSL modem.

• But still it requires a splitter in the exchange / end office. • As can be seen ADSL is just a physical layer standard. What runs on top of it depends on the carrier. ATM is a good choice as many companies run ATM in the core network.

•


53 Wireless Local Loops (WLL) • Many governments allowed competitive companies to setup telephone companies during & after 1996.

• It was prohibitively expensive for CLEC (Competing Local Exchange Carrier) to compete with ILEC (Incumbent LEC).

• Many CLEC have discovered a cheaper alternative to twisted pair: The WLL. • This looks like mobile phone, but there are 3 crucial differences: o WLL customer is often wants high speed internet connection, equal to ADSL / more. o Some times a directional antenna has to be installed at the customers place. o The user does not move, so there is no problem of had off.

• So an industry has born: Fixed wireless i.e. WLL. • WLL service was started around 1998. • Initially 2.5 GHz frequency was allotted for WLL at a B/W of 198 MHz covering 50 kms range.

• This WLL service called MMDS (Multichannel Multipoint Distribution service) and can be regarded as MAN.

• MMDS has the advantage of well established technology (2.5 GHz) & readily available equipments.

• But this has a disadvantage of B/W of 198 MHz to be shared over a large geographical area by many users.


54 • This low B/W led to the development of LMDS (Local Multipoint Distribution Service) at frequencies 28-31 GHz (US) / 40 GHz (Europe) with 1.3 GHz B/W & 2 – 5 km range.

• It was difficult to build ICs at these frequencies. This problem was solved with the invention of GaAs ICs.

Figure 23a


55 • Like ADSL, LMDS uses Asymmetric B/W allocation. • With an average of 2000 bps per, user, allows for 18000 active users per sector, but the users are limited to 9000 only.

• Figure 23 a shows 4 sectors & can support up to 36000 active users. • Assuming that 1 in 3 users are on-line during peak time, one tower can support 1 lakh users in 5 km radius.

• These calculations help CLEC to offer voice & data service at lower rates. • To encourage WLL, IEEE has defined a standard in April 2002. It is 802.16, wireless MAN.

• The 802.16 is designed to offer digital telephone, Internet access, interconnection of LANs (competitor to telephone companies), TV & Radio broadcast etc.


56 Trunks & Multiplexing • Economies of scale play an important role in telephone system, which is true with any service.

• Example: The cost of connecting two switching offices is same, whether you use fiber (high B/W) or coax (low B/W) (one / more) major cost is in laying cable & maintaining it.

• Hence it is necessary for telephone companies to transmit as high data as possible. • They have developed several multiplexing schemes. There are two basic categories: FDM & TDM.

FDM (Frequency Division Multiplexing) o Here frequency spectrum is divided into frequency bands. o Radio broadcast band is a good example, with different stations operating at different frequencies without interference. o Stations also operate in TDM mode: Program at some time & commercials at some time. o We will look at FDM & WDM (Wavelength Division Multiplexing), a variant of FDM. WDM is used for fiber n/w.

• FDM system is used for voice signals. The filters limit the voice signal to 300- 3100 Hz, but 4000 Hz is allocated to each channel to keep them well separated.

• Each voice channel is raised in frequency by a different amount. • Then they can be combined into a single channel. • The FDM schemes used around the world are standardized.


57 o 12 voice channel of 4000 Hz multiplexed into 60-1-8 KHz (12 x 4000 = 48000). This is called group. 12-60 KHz is used for some other group. o 5 groups make a super group (12 x 5 = 60 voice channels). o 5 / 10 super groups make a master group (5 x 60 = 300 voice channels is a CCITT standard & 10 x 60 = 600 Voice channels is a Bell systems Standard). o Other standards supporting up to 2,30,000 voice channels also exists.

• FDM is still used over copper, but it requires analog circuitry & is not amenable to being done by computers.

• The figure 24 demonstrates the FDM system.

(a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.

Figure 24


58 WDM (Wavelength Division Multiplexing) • Used in fiber channel, a variation of FDM. The basic principle of operation is shown in the figure 25.

• Four fibers are combined each with its energy present at a different wavelength. The combined energy is transmitted on a single shared fiber.

• At the receiving end a splitter separates each channel. Each o/p fiber has filter which filters one wavelength.

Figure 25


59 • WDM is similar to FDM with each channel having different frequency (i.e. wavelength,

λf = c or λ = c/f). λ is usually used at high frequency. • The major difference with electrical FDM is that an optical system uses a diffraction grating which is completely passive & highly reliable.

• WDM technology has grown faster than computer technology. • WDM was invented around 1990 and had 8 channels of 25 gbps per channel. • By 1998 systems with 40 channels were available. • By 2001 systems with 96 channels of 10 gbps (960 gbps) were available. This is enough to transmit 30 full movies / sec in MPEG-2 format.

• Currently Systems with 200 channels are already working in Labs. (Tejas n/w). • DWDM (Dense WDM) with 0.1 nm spacing (λf = c =300 i.e. f=300 MHz) are available. • The reason WDM is popular is that the energy on a single fiber is typically only a few GHz wide, because it is impossible to convert between electrical & optical media any faster.

• By running many channels in parallel of differentλ, the aggregate B/W is increased linearly with number of channels, since the B/W of a single fiber is about 25,000 GHz, theoretically 2500 10 gbps channels, even at 1 bit / Hz it is very high rate.

• Another development was in optical amplifiers. Optical amplifiers are required at 1000 km instead of 100 km intervals as earlier.


60 • At optical amplifiers OEO (Optical to Electrical to Optical) conversion happens, actually amplification is done in electrical domain. (Optical amplification-Physics).

• Figure 25 shows fixed λ system, it is also possible to build WDM switching systems. Here filters may be tunable using Fabry-Perot or Mach-Zender interferometers.

• WDM technology wonderful systems, but still there are lot of copper in the telephone system.

TDM (Time division Multiplexing) • FDM is still used, but requires analog circuits, WDM is a wonderful system but still there is lot of copper in the telephone system.

• In contrast TDM can be handled completely in digital electronics; hence it has become more wide spread in recent years.

• TDM can only be used for digital data, since all local loops are analog; ADCs are used at end office to convert all analog signals to digital data before they are sent over trunk lines.

• Now let us see how TDM works


61 How TDM works for digital trunk lines ? • It also applies to computer data as modems also produce analog o/p. • Codecs (COder DECoder) are used to produce a series of 8-bit binary numbers from analog signals at the end office.

• The codec takes / makes 8000 samples / sec – Nyquist rate (i.e. 1 sample / 125 µs) as the maximum telephone line B/W is 4000 Hz.

• After sampling, the sample is quantized (i.e. approximated in discrete levels) & coded using binary bits. This technique is called PCM.

• PCM forms heart of modern telephone system & with 125 µs / sample, all time intervals within telephone system are multiples of125 µs.

• As CCITT was unable to reach an agreement on an international standard for PCM, multiple & incompatible schemes exist in different countries.

• There are two schemes 1. T carrier (US & Japan) 2. E carrier (Europe & Asia).

T1 carrier • Used in US & Japan. • It consists of 24 voice channels multiplexed • The analog data is sampled on a round robin basis and the o/p fed to one codec, instead of 24 separate codecs.

• All the o/ps are merged in the digital line, this way each of the 24 channels insert 8-bits into o/p stream (7 bits for data & 1 bit for control).


62 • This gives 7 x 8000 = 56000 bps data & 8000 bps of signaling info per channel. • The T1 carrier o/p with 1,544 mbps (193 x 125 µs) is shown below.

• 193rd bit is used for frame synchronization, using the pattern 0101010101 . . . normally the receiver keeps checking this bit for synchronization.

• When the receiver gets out of sync, the receiver can scan this for resynchronization. • Analog customers can’t generate this bit pattern, however digital customers can generate.

The T1 carrier (1.544 Mbps).

Figure 26


63 • But not all 24 channels are used, only 23 channels are used for data and last one is used for a special synchronization pattern to allow faster recovery.

• There are two variations of signaling channel 1. Common channel signaling 2. Channel associated signaling. Here a private sub channel is used signaling. Some times it also called out of band signaling.

E1 carrier • CCITT also recommended a PCM carrier at 2.048 mbps called E1. • This carrier has 32 8-bit data samples packed into the basic 125 µs frame. • 30 channels are used for data and 2 for signaling. --

• To reduce the number of bits needed per channel compaction methods are used. • The basic principle used in all is the relative slow changes in analog signal. • One method used DPCM (Differential Pulse Code Modulation). The other is Delta Mod.

Delta Modulation

Figure 27


64 • Further improvements were made in DPCM to extrapolate the previous few values to predict the next values and then encode the difference between actual & predicted signals.

• The Transmitter and receiver use the same predictor, of course. • This scheme is called predictive encoding and further reduces the number of bits transmitted.

Higher levels of multiplexing • Figure 28 higher order schemes

• Similarly for E1 E1 – 2.048 mbps - 32 channels E2 – 8.848 mpbs - 128 channels E3 – 34.304 mbps – 512 channels E4 – 139.264 mbps – 2048 channels E5 – 565.148 mbps – 8192 channels

Figure 28


65 SONET (Synchronous Optical network) / SDH (Synchronous Digital Hierarchy) • In the early days, telephone companies had their own proprietary optical systems. • With all these proprietary / different optical TDM systems, interconnection was a problem.

• SONET standard initiated in 1985 by Bell core & RBOCs research later joined by CCITT and recommended G.707, G.708, G.709 in 1989 and are called SDH.

• Most of the long distance telephone traffic now uses trunk running SONET in Physical layer.

• SONET was designed with 4 goals: o Interconnection of different carriers to n/w. o Unification of US, Europe & Japanese digital systems. o To provide a way to multiplex multiple digital channels (T1, E1 etc.). o To provide support for operations, administration & maintenance (OAM).

• SONET is a synchronous system, controlled by a master clock with an accuracy of about 1 in 109.

• The basic SONET frame is a block of 810 bytes put out every 125 µs. • These 810 bytes are best described as rectangle of bytes (figure 29) 90 cols by 9 rows. • Thus 8 x 810 = 6480 bits are transmitted 8000 times / sec for a data rate of 51.84 mbps.

• This is the basic SONET channel called STS-1 (Synchronous Transport Signal).


66 • All SONET trunks are multiple of STS-1

Figure 29


67 • SONET & SDH standards are given in table below. Switching

Figure 30


68 • So far we have studied transmission system (Local loops & Trunk Lines) which is outside.

• Another important part of telephone system is the switches, which are inside the switching offices.

• Currently two different techniques are used: 1. Circuit switching 2. Packet switching. • Circuit switching is what the telephone systems use hence we study some details. • Packet switching is what the CCN use, but we study this later. Circuit switching • When a call is placed (Voice / Data) the switching equipment within the telephone system seeks out a physical path from source to destination telephone. This technique is called Circuit switching & shown in figure 31 a.

• Each rectangle represents a switching office (end / toll office). • In this example, each office has a 3 x 3 switch with 3 incoming & 3 outgoing lines. • When a call passes through a switching office, a physical connection is established between one of the i/p lines to one of the o/p lines as shown by dotted lines.

• In the early days manual switching was used. • Almon B Strowger invented automatic switching in 19th century, shortly after the invention of telephone. Strowger switching was used for over 100 years around the world.

• A highly simplified model of switching is presented in the figure 31.


69 • The paths in between may be copper wire, fiber, microwave etc. & the switches may have 1000s i/p and o/p lines.

• The basic property of circuit switching is that, a dedicated path between source & destination phone exists during call duration.

•

(a) Circuit switching. (b) Packet switching.

Figure 31


70 • An important property of circuit switching is the need to set-up an end-to-end path before data transmission begins as shown in a.

• The time elapsed between end of dialing & start of ringing could be 10 sec or more. • During this interval the system is hunting for a path as shown in a. • Before data transmission begins there should be an ACK from the destination (call accept signal). For many applications the long set-up time is undesirable.

• As the path is reserved between source & destination before transmission, requires less propagation time for data transmission & no congestion later.

• Congestion happens during switching due to lack of switching / trunk capacity. • • An alternative to circuit switching is packet switching shown in b above. It is widely used in networking.


71 Message Switching • This is also an alternative switching strategy and is called message switching. • Here no path is established before hand. • When sender has data to send, data is first stored in the switching office (could be a router) and then forwarded later one hop at a time.

• Each data block is received in its entirety, inspected for errors and then retransmitted in the direction of destination. Figure 32 a.

• A n/w using this technique is called store & forward n/w as mentioned earlier. • This technique was used for telegrams. o The message was punched on paper tape & read in and transmitted. o It was punched out at the receiving office. o This was torn & fed-in again for onward transmission. o These switching offices were called torn tape offices.

• Message switching is more used and hence no further discussion on that.


72

Timing Diagram (a) Circuit switching (b) Message switching (c) Packet switching

Figure 32


73 Packet Switching • Concept similar to message switching is used, but the development is independent of that. • There was no limit on message size in message switching, but the messages were short in telegram.

• This arbitrary size of message requires huge resources in the switching offices. • These long messages can block switches making message switching useless. • To get around these problems packet switching was invented as discussed earlier. • --

• In packet switching an upper limit is placed on the size of the message / block. • Because of this, resources could be managed at the switching offices. • And also there is monopoly by the users & are well suited for interactive traffic. • Further, pipeline transmission of packets in the n/w is possible as shown in figure 32 b & c.

• So the computer n/ws are usually packet switched & occasionally circuit switched and never message switched.


74 Circuit & Packet Switching: A comparison.

No Item Circuit Switching Packet Switching

1 Call Set-up Required Not needed

2 Dedicated Physical Path Established Not required

3 Fault Tolerant More fault tolerant Less

4 Each packet follows the same route Yes No

5 Packet arrive in order Yes No

6 Is a switching crash fatal Yes No

7 B/W available Fixed Dynamic

8 Time of possible congestion AT set-up time For every packet

9 B/W wastage Yes No

10 Store & Forward transmission No Yes

11 Transparency Yes No

12 Charging Per Min Per Packet

• Both circuit & packet switching are important as we use both in computer n/w.

Mobile Telephone • A new generation of telephone system is in use today that is mobile telephone system. • Cordless & Mobile phone system • 1G – Analog Voice, 2G – Digital Voice, GSM, CDMA, 3G-Digital Voice & Data.


75 Slide -27: Cable Television

• Inspite of fixed & Mobile phones playing an important role in future n/ws, there is an alternative available: Cable TV n/w.

• Cable TV n/w serves fixed line access customers & many are using this service. • Cable TV was conceived in the late 1940s to provide better reception in remote areas. • An early cable TV system is shown in figure 33.

• Over the years there was a huge growth in Cable TV & fibers were also used in some countries. This is because of the huge B/W of fiber. The system is shown in figure 34.

• In recent years, many cable operators have decided to get into the Internet access & Telephony business as well. There are technical differences. Cable TV lines are one way compared to two lines in telephone systems. Figure 34.

Figure 33


76

• Another big difference is: a single wire is shred by all houses in cable TV where as in telephone system each house has a private local loop.

• The B/W is shared in cable TV and more the users more is the response and in telephone B/W is individually assigned.

• One way of tackling this problem is to split up long cables and connect to each one directly to fiber node.

HFC System (Hybrid Fiber & Coax) Figure 34


77 Summary

• The physical layer the basic of all networks. Nature imposes two fundamental limits on all channels. 1. Nyquist limit deals with noiseless channels & 2. Shannon limit deals with noisy channels.

• Transmission media can be guided or unguided. Guide media are: Twisted pair, Coaxial cable, Fiber optics. Unguided media are: Radio, Microwave, Infrared, Laser through air. Satellite is an up & coming transmission system.

• A key element in WAN is the telephone system. Its main components are: local loops, trunks, switches. Local loops are analog, twisted pair circuits, which require modems for transmitting digital data. ADSL offers speeds up to 50 mbps by dividing the local loop into many virtual channels and modulating each one separately. Wireless Local loops are new developments, especially LMDS.

• Trunks are digital and can be multiplexed in several ways: FDM, WDM, TDM. Circuit switching & packet switching are important in networking.

• For mobile applications, the fixed telephone system is not suitable. Mobile phones are currently in widespread use for voice and will be used widely for data soon. There are 3 generations of mobile technology: 1G, supports analog voice, 2G, supports digital voice with GSM & CDMA, 3G, and supports digital voice & data with broad band CDMA.

• An alternate system for network access is the cable TV system. This has gradually evolved from coax to HFC (Hybrid Fiber Coax). It offers very high B/W, but the actual available B/W depends on the number of users currently active.


78 Slide -28: Assignments:

Go through Text, Reference books & Presentation material and submit …

1. Solutions for odd numbered problems of text, chapter 2 for ‘A’ Div.

2. Solutions for even numbered problems of text, chapter 2 for ‘B’ Div.

3. A report on the types of media used in the college network, with specification, structure etc.

4. A report on modulation schemes used for different transmission media, with type of media, specifications, BW, mode of connectivity, applications etc.

5. A report on how you are able to connect to the internet from home through PSTN line, with specifications, configuration details, BW available, facilities / services available, type of channel, interface devices used, ISP, cost etc.

6. A report on how you are able to connect to the internet from your mobile phone, with specifications, configuration details, BW available, facilities / services available, type of channel, interface devices used, ISP, cost etc.

7. A report on how you are able to connect to the internet from your cable TV, with specifications, configuration details, BW available, facilities / services available, type of channel, interface devices used, ISP, cost etc.

8. A report on the college network with details. Visit www.bvb.edu – infocell – services & talk to Dr Satish Annigeri.


79 9. A report on how to make straight through & cross over cables, with color codes, pin connections, jacks, tools used etc.

10. A report. Is it possible to connect your computer at home in HUBLI to yours friend computer at Texas, Dallas, USA ? What is the best way ? What needs to be used ? What you need to do ? What is the cost ? Remember you chat with him / talk to him / see him dancing.

*** END ***

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