MC0075 SEM3 SMU 2011

MC0075 – Computer NetworksBook ID: B0813 & B0814

Set-1

1. Discuss the advantages and disadvantages of synchronous and asynchronous transmission.

Ans:1

Synchronous & Asynchronous transmission:

Synchronous Transmission:

Synchronous is any type of communication in which the parties communicating are "live" or present in the same space and time. A chat room where both parties must be at their computer, connected to the Internet, and using software to communicate in the chat room protocols is a synchronous method of communication. E-mail is an example of an asynchronous mode of communication where one party can send a note to another person and the recipient need not be online to receive the e-mail. Synchronous mode of transmissions are illustrated in figure 3.11

Figure :Synchronous and Asynchronous Transmissions

The two ends of a link are synchronized, by carrying the transmitter’s clock information along with data. Bytes are transmitted continuously, if there are gaps then inserts idle bytes as padding

Advantage:

· This reduces overhead bits

· It overcomes the two main deficiencies of the asynchronous method, that of inefficiency and lack of error detection.

Disadvantage:

· For correct operation the receiver must start to sample the line at the correct instant

Application:

· Used in high speed transmission example: HDLC

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Asynchronous transmission:

Asynchronous refers to processes that proceed independently of each other until one process needs to "interrupt" the other process with a request. Using the client- server model, the server handles many asynchronous requests from its many clients. The client is often able to proceed with other work or must wait on the service requested from the server.

Figure : Asynchronous Transmissions

Asynchronous mode of transmissions is illustrated in figure 3.12. Here a Start and Stop signal is necessary before and after the character. Start signal is of same length as information bit. Stop signal is usually 1, 1.5 or 2 times the length of the information signal

Advantage:

· The character is self contained & Transmitter and receiver need not be synchronized

· Transmitting and receiving clocks are independent of each other

Disadvantage:

· Overhead of start and stop bits

· False recognition of these bits due to noise on the channel

Application:

· If channel is reliable, then suitable for high speed else low speed transmission

· Most common use is in the ASCII terminals

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2. Describe the ISO-OSI reference model and discuss the importance of every layer.

Ans:2

The OSI Reference Model:

This reference model is proposed by International standard organization (ISO) as a a first step towards standardization of the protocols used in various layers in 1983 by Day and Zimmermann. This model is called Open system Interconnection (OSI) reference model. It is referred OSI as it deals with connection open systems. That is the systems are open for communication with other systems. It consists of seven layers.

Layers of OSI Model :

The principles that were applied to arrive at 7 layers:

1. A layer should be created where a different level of abstraction is needed.

2. Each layer should perform a well defined task.

3. The function of each layer should define internationally standardized protocols

4. Layer boundaries should be chosen to minimize the information flow across the interface.

5. The number of layers should not be high or too small.

Figure : ISO - OSI Reference Model

The ISO-OSI reference model is as shown in figure 2.5. As such this model is not a network architecture as it does not specify exact services and protocols. It just tells what each layer should do and where it lies. The bottom most layer is referred as physical layer. ISO has produced standards for each layers and are published separately.



Each layer of the ISO-OSI reference model are discussed below:

1. Physical Layer

This layer is the bottom most layer that is concerned with transmitting raw bits over the communication channel (physical medium). The design issues have to do with making sure that when one side sends a 1 bit, it is received by other side as a 1 bit, and not as a 0 bit. It performs direct transmission of logical information that is digital bit streams into physical phenomena in the form of electronic pulses. Modulators/demodulators are used at this layer. The design issue here largely deals with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below this physical layer.

In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:

· Establishment and termination of a connection to a communications medium.

· Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.

· Modulation, is a technique of conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link.

Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

2. Data Link Layer

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. That is it makes sure that the message indeed reach the other end without corruption or without signal distortion and noise. It accomplishes this task by having the sender break the input data up into the frames called data frames. The DLL of transmitter, then transmits the frames sequentially, and processes acknowledgement frames sent back by the receiver. After processing acknowledgement frame, may be the transmitter needs to re-transmit a copy of the frame. So therefore the DLL at receiver is required to detect duplications of frames.

The best known example of this is Ethernet. This layer manages the interaction of devices with a shared medium. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media


Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from the physical layer into logical chunks of data, known as frames.

This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.

The data link layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as TCP.

3. Network Layer

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme values are chosen by the network engineer. The addressing scheme is hierarchical.

The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it’s easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks that carry sacks of mail to other post offices or airports, airplanes that carry airmail between major cities, trucks that distribute mail sacks in a city, and carriers that take a letter to its destinations. Think of fragmentation as splitting a large document into smaller envelopes for shipping, or, in the case of the network layer, splitting an application or transport record into packets.

The major tasks of network layer are listed

· It controls routes for individual message through the actual topology.

· Finds the best route.

· Finds alternate routes.

· It accomplishes buffering and deadlock handling.


4. Transport Layer

The Transport layer provides transparent transfer of data between end users, providing reliable data transfer while relieving the upper layers of it. The transport layer controls the reliability of a given link through flow control, segmentation/de-segmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP).

The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic Presentation services that can be read by the addressee only.

Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM’s SNA or Novell’s IPX over an IP network, or end-to-end encryption with IP security (IP sec). While Generic Routing Encapsulation (GRE) might seem to be a network layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint.

The major tasks of Transport layer are listed below:

· It locates the other party

· It creates a transport pipe between both end-users.

· It breaks the message into packets and reassembles them at the destination.

· It applies flow control to the packet stream.

5. Session Layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session check pointing and recovery, which is not usually used in the Internet protocols suite.

The major tasks of session layer are listed

· It is responsible for the relation between two end-users.


· It maintains the integrity and controls the data exchanged between the end-users.

· The end-users are aware of each other when the relation is established (synchronization).

· It uses naming and addressing to identify a particular user.

· It makes sure that the lower layer guarantees delivering the message (flow control).

6. Presentation Layer

The Presentation layer transforms the data to provide a standard interface for the Application layer. MIME encoding, data encryption and similar manipulation of the presentation are done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

The major tasks of presentation layer are listed below:

· It translates the language used by the application layer.

· It makes the users as independent as possible, and then they can concentrate on conversation.

7. Application Layer (end users)

The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to the users and performs common application services for the application processes. It also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces.

The common application services sub layer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements). Above the common application service sub layer are functions meaningful to user application programs, such as messaging (X.400), directory (X.500), file transfer (FTAM), virtual terminal (VTAM), and batch job manipulation (JTAM).


3. Explain the following with respect to Data Communications:

A) Fourier analysis B) Band limited signals

C) Maximum data rate of a channel

Ans:3

1. Fourier analysis:

In 19th century, the French mathematician Fourier proved that any periodic function of time g (t) with period T can be constructed by summing a number of cosines and sines.

(3.1)

Where f=1/T is the fundamental frequency, and are the sine and cosine amplitudes of the nth harmonics. Such decomposition is called a Fourier series.

2. Band limited signals

Consider the signal given in figure 3.1(a). Figure shows the signal that is the ASCII representation of the character ‘b’ which consists of the bit pattern ‘01100010’ along with its harmonics.

Figure 3.1: (a) A binary signal (b-e) Successive approximation of original signal

Any transmission facility cannot pass all the harmonics and hence few of the harmonics are diminished and distorted. The bandwidth is restricted to low frequencies consisting of 1, 2, 4,






and 8 harmonics and then transmitted. Figure 3.1(b) to 3.1(e) shows the spectra and reconstructed functions for these band-limited signals.

Limiting the bandwidth limits the data rate even for perfect channels. However complex coding schemes that use several voltage levels do exist and can achieve higher data rates.

3. Maximum data rate of a channel

In 1924, H. Nyquist realized the existence of the fundamental limit and derived the equation expressing the maximum data for a finite bandwidth noiseless channel. In 1948, Claude Shannon carried Nyquist work further and extended it to the case of a channel subject to random noise.

In communications, it is not really the amount of noise that concerns us, but rather the amount of noise compared to the level of the desired signal. That is, it is the ratio of signal to noise power that is important, rather than the noise power alone. This Signal-to-Noise Ratio (SNR), usually expressed in decibel (dB), is one of the most important specifications of any communication system. The decibel is a logarithmic unit used for comparisons of power levels or voltage levels. In order to understand the implication of dB, it is important to know that a sound level of zero dB corresponds to the threshold of hearing, which is the smallest sound that can be heard. A normal speech conversation would measure about 60 dB.

If an arbitrary signal is passed through the Low pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H samples per second. Sampling the line faster than 2H per second is pointless. If the signal consists of V discrete levels, then Nyquist theorem states that, for a noiseless channel

Maximum data rate = 2H.log2 (V) bits per second. (3.2)

For a noisy channel with bandwidth is again H, knowing signal to noise ratio S/N, the maximum data rate according to Shannon is given as

Maximum data rate = H.log2 (1+S/N) bits per second. (3.3)


4. Explain the following concepts of Internetworking:

A) Internet architecture B) Protocols and Significance for

internetworking

C) Internet layering model

Ans:4

Internet Architecture: B1-226, B2-56

The Internet is a worldwide, publicly accessible network of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government networks, which together carry various information and services, such as electronic mail, online chat, file transfer, and the interlinked web pages and other documents of the World Wide Web.

How are networks interconnected to form an internetwork? The answer has two parts. Physically, two networks can only be connected by a computer that attaches both of them. But just a physical connection cannot provide interconnection where information can be exchanged as there is no guarantee that the computer will cooperate with other machines that wish to communicate.

Internet is not restricted in size. Internets exist that contain a few networks and internets also exist that contain thousands of networks. Similarly the number of computers attached to each network in an internet can vary. Some networks have no computers attached, while others have hundreds.

To have a viable internet, we need a special computer that is willing to transfer packets from one network to another. Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routers.

Fig. : Overview of Internet

Figure 1.1 illustrates the overview of internet architecture. Let us assume a client calls his/her Internet Service provider (ISP) over a dial up telephone line. The modem is a card within a PC that converts the digital signals the computer produces to analog signals that can pass over



telephone system. These signals are transferred to the ISP’s point of presence (POP), where they are removed from the telephone system and injected into the ISP’s regional network. From this point onwards, the system is fully digital and packet switched.

The ISP’s regional network consists of interconnected routers in the various cities the ISP serves. If the packet is destined for a host served directly by the ISP, then the packet is delivered to the host. Otherwise, it is handed over to the ISP’s backbone operator.

At the top we have companies like AT&T, Sprint, that are major backbone operators. They operate large international backbone networks with thousands of routers connected by high bandwidth optical fibers. Large companies run server farms often connect directly to the backbone. Backbone operators encourage this direct connection by renting space called Carrier hotels.

If a packet given to the backbone is destined for an ISP or company served by the backbone, it is sent to the closet router and handed off there. Many backbones of varying sizes exist in the world. In such cases the packet may have to be sent to a competing backbone. To allow packets to hop between backbones, all major backbones connect at the network access point (NAP). Basically a NAP is a room full of routers, at least one per backbone. A LAN in a room connects all these routers, so packets can be forwarded from any backbone to any other backbone. In addition to being interconnected at NAP’s, the larger backbones have numerous direct connections between their routers, a technique known as private peering.

An organization uses single router to connect its entire network. There are two reasons

· Because the CPU and memory in a router are used to process each packet, the processor in one router is insufficient.

· Redundancy improves internet reliability. Protocol software continuously monitors internet connections and instructs the routers to send traffic along alternative paths when a network or router fails.

Hence when planning an internet, an organization must choose a design that meets its need for reliability, capacity, and cost. In particular, the exact details of the expected traffic, the organization’s reliability requirements, internet topology, that often depends on the bandwidth of the physical networks and finally the cost of available router hardware.

Protocols and Significance for Internetworking


1 Protocols for internetworking

Many protocols have been used for use in an internet. One suite known as The TCP/IP internet protocol stands out most widely used for internets. Most networking professional simply refer this protocol as TCP/IP. Work on the transmission control protocol (TCP) began in the 1970’s. The U.S military funded the research in TCP/IP and internetworking through the Advanced Research Projects Agency in short known as ARPA.

2 Significance of internetworking and TCP/IP

Internetworking has become one of the important technique in the modern networking. Internet technology has revolutionized the computer communication. The TCP/IP technology has made possible a global Internet, which reaches millions of schools, commercial organizations, government and military etc around the world.

The worldwide demand for internetworking products has affected most companies sell networking technologies. Competition has increased among the companies that sell the hardware and software needed for internetworking. Companies have extended the designs in two ways

· The protocols have adapted to work with many network technologies

· And new features have been adapted that allow the protocols to transfer data across the internets

Internet Layering Model:


Internet uses the TCP/IP reference model. This model is also called as Internet layering model or internet reference model. This model consists of 5 layers as illustrated in figure 1.3.

Fig. 1.3: The five layers of TCP/IP reference model

A goal was of continuing the conversation between source and destination even if transmission went out of operation. The reference model was named after two of its main protocols, TCP (Transmission Control Protocol) and IP (Internet Protocol). The purpose of each layer of TCP/IP is given below:

Layer 1: Physical layer

This layer corresponds to basic network hardware

Layer 2: Network interface

This layer specifies how to organize data into frames and how a computer transfers frames over a network. It interfaces the TCP/IP protocol stack to the physical network.

Layer 3: Internet

This layer specifies the format of packets sent across an internet. It also specifies the mechanism used to forward packets from a computer through one or more routers to the final destination.

Layer 4: Transport

This layer deals with opening and maintaining connections, ensuring that packets are in fact received. The transport layer is the interface between the application layer and the complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conversations.

Layer 5: Network interface



Each protocol of this layer specifies how one application uses an internet.

5. What is the use of IDENTIFIER and SEQUENCE NUMBER fields of echo request and echo reply message? Explain

Ans:5

Echo Request and Echo Reply message format:

The echo request contains an optional data area. The echo reply contains the copy of the data sent in the request message. The format for the echo request and echo reply is as shown in figure 5.2.

Figure 5.2 echo request and echo reply message format

The field OPTIONALDATA is a variable length that contains data to be returned to the original sender. An echo reply always returns exactly the same data as ws to receive in the request. Field IDENTIFIER and SEQUENCE NUMBER are used by the sender to match replies to requests. The value of the TYPE field specifies whether it is echo request when equal to 8 or echo reply when equal to 0.

6. In what conditions is ARP protocol used? Explain



Ans:6

ARP protocol

In computer networking, the Address Resolution Protocol (ARP) is the standard method for finding a host’s hardware address when only its network layer address is known. ARP is primarily used to translate IP addresses to Ethernet MAC addresses. It is also used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11, and for IP over ATM.

ARP is used in four cases of two hosts communicating:

1. When two hosts are on the same network and one desires to send a packet to the other

2. When two hosts are on different networks and must use a gateway/router to reach the other host

3. When a router needs to forward a packet for one host through another router

4. When a router needs to forward a packet from one host to the destination host on the same network

The first case is used when two hosts are on the same physical network. That is, they can directly communicate without going through a router.

The last three cases are the most used over the Internet as two computers on the internet are typically separated by more than 3 hops. Imagine computer A sends a packet to computer D and there are two routers, B & C, between them. Case 2 covers A sending to B; case 3 covers B sending to C; and case 4 covers C sending to D. ARP is defined in RFC 826. It is a current Internet Standard, STD 37.

Set-2


1. List and discuss the service primitives for connection oriented services.

Ans:

Service Primitives:

A service is formally specified by a set of primitives or operations available to the user to access the service. These primitives tell the service to perform some action or report an action taken by the peer entity. The primitives for the connection-oriented service are given in table 2.2.

Table 2.2: Service primitives for a connection oriented service

Communication in a simple client server model using the above service primitives is illustrated in figure 2.3. First the server executes the LISTEN to indicate that is ready to accept incoming connections. The client executes CONNECT (1) to establish the connection with the server. The server now unblocks the listener and sends back an acknowledgement (2). Thus the connection is established.

Figure 2.3: Simple client server model on a connection oriented network

The next step for a server is to executes a RECEIVE (3) to prepare to accept the first request. The arrival of the request packet unblocks the server so that it can process the request. After it has done the work it uses SEND (4) to answer to the client. It all the data transfer is done then it can use DISCONNECT (5) suspending the client. When the server gets this packet, it also issues a DISCONNECT (6) and when it reaches the client, the client process is releases and the connection is broken. In the process packets may get lost, timings may be wrong, many other complex issues.

The Relationship of Services to Protocols




Figure 2.4: Relationship between the service and protocols

A service is a set of primitives that a layer provides to the layer above it. The service defines what operation the layer is prepared to perform on behalf of its users. It says nothing about the implementation of these operations.

A protocol is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer. Figure 2.4 illustrates the relationship of services to protocols. Entities use protocols to implement their service primitives. Protocols relate to the packets sent between entities.

2. Describe the following Medium Access Control Sub Layer’s Multiple access protocols:

A) Pure ALOHA or Unslotted ALOHA B) Slotted ALOHA or Impure ALOHA

Ans:

Pure ALOHA or Unslotted ALOHA:

The ALOHA network was created at the University of Hawaii in 1970 under the leadership of Norman Abramson. The Aloha protocol is an OSI layer 2 protocol for LAN networks with broadcast topology.

The first version of the protocol was basic:

· If you have data to send, send the data

· If the message collides with another transmission, try resending later




Figure 7.3: Pure ALOHA

Figure 7.4: Vulnerable period for the node: frame

The Aloha protocol is an OSI layer 2 protocol used for LAN. A user is assumed to be always in two states: typing or waiting. The station transmits a frame and checks the channel to see f it was successful. If so the user sees the reply and continues to type. If the frame transmission is not successful, the user waits and retransmits the frame over and over until it has been successfully sent.

Let the frame time denote the amount of time needed to transmit the standard fixed length frame. We assume the there are infinite users and generate the new frames according Poisson distribution with the mean N frames per frame time.

· So if N>1 the users are generating the frames at higher rate than the channel can handle. Hence all frames will suffer collision.

· Hence the range for N is

0<N<1

· If N>1 there is collisions and hence retransmission frames are also added with the new frames for transmissions.

Let us consider the probability of k transmission attempts per frame time. Here the transmission of frames includes the new frames as well as the frames that are given for retransmission. This total traffic is also poisoned with the mean G per frame time. That is

· At low load: N is approximately =0, there will be few collisions. Hence few retransmissions that is G=N

· At high load: N >>1, many retransmissions and hence G>N.

· Under all loads: throughput S is just the offered load G times the probability of successful transmission P0




The probability that k frames are generated during a given frame time is given by Poisson distribution

So the probability of zero frames is just . The basic throughput calculation follows a Poisson distribution with an average number of arrivals of 2G arrivals per two frame time. Therefore, the lambda parameter in the Poisson distribution becomes 2G.

Hence

Hence the throughput

We get for G = 0.5 resulting in a maximum throughput of 0.184, i.e. 18.4%.

Pure Aloha had a maximum throughput of about 18.4%. This means that about 81.6% of the total available bandwidth was essentially wasted due to losses from packet collisions.

Slotted ALOHA or Impure ALOHA

An improvement to the original Aloha protocol was Slotted Aloha. It is in 1972, Roberts published a method to double the throughput of an pure ALOHA by uses discrete timeslots. His proposal was to divide the time into discrete slots corresponding to one frame time. This approach requires the users to agree to the frame boundaries. To achieve synchronization one special station emits a pip at the start of each interval similar to a clock. Thus the capacity of slotted ALOHA increased to the maximum throughput of 36.8%.

The throughput for pure and slotted ALOHA system is as shown in figure 7.5. A station can send only at the beginning of a timeslot, and thus collisions are reduced. In this case, the average number of aggregate arrivals is G arrivals per 2X seconds. This leverages the lambda parameter to be G. The maximum throughput is reached for G = 1.







Figure 7.5: Throughput versus offered load traffic.

With Slotted Aloha, a centralized clock sent out small clock tick packets to the outlying stations. Outlying stations were only allowed to send their packets immediately after receiving a clock tick. If there is only one station with a packet to send, this guarantees that there will never be a collision for that packet. On the other hand if there are two stations with packets to send, this algorithm guarantees that there will be a collision, and the whole of the slot period up to the next clock tick is wasted. With some mathematics, it is possible to demonstrate that this protocol does improve the overall channel utilization, by reducing the probability of collisions by a half.

It should be noted that Aloha’s characteristics are still not much different from those experienced today by Wi-Fi, and similar contention-based systems that have no carrier sense capability. There is a certain amount of inherent inefficiency in these systems. It is typical to see these types of networks’ throughput break down significantly as the number of users and message burstiness increase. For these reasons, applications which need highly deterministic load behavior often use token-passing schemes (such as token ring) instead of contention systems

For instance ARCNET is very popular in embedded applications. Nonetheless, contention based systems also have significant advantages, including ease of management and speed in initial communication. Slotted Aloha is used on low bandwidth tactical Satellite communications networks by the US Military; subscriber based Satellite communications networks, and contact less RFID technologies.

3.Discuss the different types of noise.

Ans:3



Noise:

Noise is a third impairment. It can be define as unwanted energy from sources other than the transmitter. Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable. Consider a signal as shown in figure 3.5, to which a noise shown in figure 3.6, is added may be in the channel.

Figure 3.5: Signal

Figure 3.6: Noise

Figure 3.7: Signal + Noise

At the receiver, the signal is recovered from the received signal and is shown in figure 3.7. That is signals are reconstructed by sampling. Increased data rate implies "shorter" bits with higher sensitivity to noise

Source of Noise

Thermal:

Agitates the electrons in conductors, and is a function of the temperature. It is often referred to as white noise, because it affects uniformly the different frequencies.

· The thermal noise in a bandwidth W is

Where T=temperature, and

k= Boltzmann’s constant = 1.38 10-23 Joules/degrees Kelvin.

· Signal to noise ratio:








Typically measured at the receiver, because it is the point where the noise is to be removed from the signal.

Intermodulation:

Results from interference of different frequencies sharing the same medium. It is caused by a component malfunction or a signal with excessive strength is used. For example, the mixing of signals at frequencies f1 and f2 might produce energy at the frequency f1 + f2 . This derived signal could interfere with an intended signal at frequency f1 + f2 .

Cross talk:

Similarly cross talk is a noise where foreign signal enters the path of the transmitted signal. That is, cross talk is caused due to the inductive coupling between two wires that are close to each other. Sometime when talking on the telephone, you can hear another conversation in the background. That is cross talk.

Impulse:

These are noise owing to irregular disturbances, such as lightning, flawed communication elements. It is a primary source of error in digital data.



4. What is Non-repudiation? Define cryptanalysis.

Ans:4

Non-repudiation: Non-repudiation, or more specifically non-repudiation of origin, is an important aspect of digital signatures. By this property an entity that has signed some information cannot at a later time deny having signed it. Similarly, access to the public key only does not enable a fraudulent party to fake a valid signature.

It deals with signatures. Not denying or reneging. Digital signatures and certificates provide nonrepudiation because they guarantee the authenticity of a document or message. As a result, the sending parties cannot deny that they sent it (they cannot repudiate it). Nonrepudiation can also be used to ensure that an e-mail message was opened. Example: how does one prove that the order was placed by the customer.

Cryptanalysis:

The main constraint on cryptography is the ability of the code to perform the necessary transformation. From the top-secret military files, to the protection of private notes between friends, various entities over the years have found themselves in need of disguises for their transmissions for many different reasons. This practice of disguising or scrambling messages is called encryption.

In cryptography, a digital signature or digital signature scheme is a type of asymmetric cryptography used to simulate the security properties of a signature in digital, rather than written, form. Digital signature schemes normally give two algorithms, one for signing which involves the user’s secret or private key, and one for verifying signatures which involves the user’s public key. The output of the signature process is called the "digital signature."

5. Explain mask-address pair used in update message. Discuss importance of path

attributes.


Ans:

Update message

Figure 7.4 BGP UPDATE message format

The BGP peers after establishing a TCP connection sends OPEN message and acknowledge then. Then use UPDATE message to advertise new destinations that are reachable or withdraw previous advertisement when a destination has become unreachable. The UPDATE message format is as shown in Figure 7.4.

Each UPDATE message is divided into two parts:

1. List of previously advertised destinations that are being withdrawn

2. List of new destinations being advertised.

Fields labeled variable do not have fixed size. Update message contains following fields:

· WITHDRAWN LEN: is a 2-byte that specifies the size of withdrawn destinations field. If no withdrawn destination then its value =0

· WITHDRAWN DESTINATIONS: IP addresses of withdrawn destinations.

· PATH LEN: is similar to WITHDRAWN LEN, but it specifies the length of path attributes that are associated with new destinations being advertised.

· PATH ATTRIBUTES: it gives the additional information of new destinations. It is discussed in detail below

· DESTINATION NETWORKS: new destinations.

Compressed mask-address pairs:

In the update message many addresses are listed and the size of the update message goes on increasing. BGP uses it to store destination address and the associated mask. A technique, where instead of IP 32-bit address and a 32-bit mask compressed mask-address pair, is used to reduce the size of the update message.



Here BGP encodes the mask into a single octet that precedes the address. The format of this compressed mask-address pair is as shown in figure 7.5.

Figure 7.5 BGP compress format

This single octet LEN specifies the number of bits in the mask, assuming contiguous mask. The address that follows is also compressed and only those octets are covered by mask is included. Depending on the value of LEN the number of octets in the address field is listed in table 7.1. A zero length is used for default router.

Table 7.1

LENNumber of octets in address

Less than 8 19 to 16 217 to 24 325 to 32 4

Path Attribute

Path attributes are factored to reduce the size of the update message.

That is attributes apply to all destinations. If any destinations have different attributes then, they must be advertised in a separate update message. The path attribute (4-octet) contains a list of items and each item is of the form given in figure 7.6(a).

(type, length, value)

Figure 7.6 (a) path attribute item

The type is 2 bytes long. The format of the type field of an item in path attribute is given in figure 7.6(b).

Figure 7.6 (b) BGP 2-octet type field of path attribute

FLAG Bits Description0 Zero for required attribute, set to 1 if optional




1 Set to 1 for transitive, zero for no transitive2 Set to 1 for complete, zero for partial3 Set to 1 for length is 2 octets, zero for one5-7 Unused and must be zero

Figure 7.7 Flag bits of type field of path attribute

Type code Description1 Specify origin of the path information2 List of AS on path to destination3 Next hop to use for destination4 Discriminator used for multiple AS exit points5 Preference used within an AS6 Indication that routes have been aggregated7 ID of AS that aggregates routes

Figure 7.8 Type codes of type field

The values of flag bits and type code of type field of an item in path attribute is given in figure 7.7 and figure 7.8 respectively.

A length field follows type field may be either 1 or 2 bytes long depending on flag bit – 3, which specifies the size of the length field. Then the contents of length field gives the size of the value filed.

Importance of path attributes:

1. Path information allows a receiver to check for routing loops. The sender can specify exact path through AS to the destination. If any AS is listed more than once then there is a routing loop.

2. Path information allows a receiver to implement policy constraints. A receiver can examine the path so that they should not pass through untrusted AS.

3. Path information allows a receiver to know the source of all routes.

6. Explain the following with respect to E-Mail:

A) Architecture B) Header format

C) User agents D) E-mail Services


Ans:

E-Mail

Electronic mail or e-mail, as it is known by its fans became known to the public at large and its use grew exponentially. The first e-mail systems consisted of file transfer protocols, with the convention that the first line of the message contained the recipient’s address. It is a store and forward method of composing, sending, storing, and receiving messages over electronic communication systems. The term “e-mail” applies both to the Internet e-mail system based on the Simple Mail Transfer Protocol (SMTP) and to intranet systems allowing users within one organization to e-mail each other.

Often workgroup collaboration organizations may use the Internet protocols for internal e-mail service. E-mail is often used to deliver bulk unwanted messages, or “spam”, but filter programs exist which can automatically delete most of these. E-mail systems based on RFC 822 are widely used.

1 Architecture :

E-mail system normally consists of two sub systems

1. the user agents

2. the message transfer agents

The user agents allow people to read and send e-mails. The message transfer agents move the messages from source to destination. The user agents are local programs that provide a command based, menu-based, or graphical method for interacting with e-mail system. The message transfer agents are daemons, which are processes that run in background. Their job is to move datagram e-mail through system.

A key idea in e-mail system is the distinction between the envelope and its contents. The envelope encapsulates the message. It contains all the information needed for transporting the message like destinations address, priority, and security level, all of which are distinct from the message itself.

The message transport agents use the envelope for routing. The message inside the envelope consists of two major sections:

The Header:

The header contains control information for the user agents. It is structured into fields such as summary, sender, receiver, and other information about the e-mail.


· Body:

The body is entirely for human recipient. The message itself as unstructured text; sometimes containing a signature block at the end

2 Header format

The header is separated from the body by a blank line. Envelopes and messages are illustrated in figure 8.1.

Fig. 8.1: E-mail envelopes and messages

The message header fields that are used in an example of figure 8.1.

consists of following fields

· From: The e-mail address, and optionally name, of the sender of the message.

· To: one or more e-mail addresses, and optionally name, of the receiver’s of the message.

· Subject: A brief summary of the contents of the message.

· Date: The local time and date when the message was originally sent.

E-mail system based on RFC 822 contains the message header as shown in figure 8.2. The figure gives the fields along with their meaning.

http://en.wikipedia.org/wiki/Signature_block




Fig. 8.2: Fields used in the RFC 822 message header

The fields in the message header of E-mail system based on RFC 822 related to message transport are given in figure 8.3. The figure gives the fields along with their meaning.

Fig. 8.3: Header Fields related to message Transport

3 User agents:

It is normally a program and sometimes called a mail reader. It accepts a variety of commands for composing, receiving, replying messages as well as manipulating the mail boxes. Some user agents have a fancy menu or icon driven interfaced that require a mouse where as others are one character commands from keyboard. Functionally these are same. Some systems are menu or icon driven but also have keyboard shortcuts.

To send an e-mail, user provides the message, the destination address and possibly some other parameters. Most e-mail system supports mailing lists.

Example: Reading e-mail

When a user is started up, it looks at the user’s mailbox for incoming e-mail before displaying anything on the screen. Then it announces the number of messages in the mailbox or displays a one-line summary of each e-mail and wait for a command.

The display may look something like that is shown in figure 8.4. Each line of the display contains several fields extracted from the envelope or header of the corresponding message. In a simple e-mail system, the choice of fields is built into the program. In more sophisticated system, user can specify which fields are to be displayed by providing a user profile.

Fig. 8.4 an example Display of mailbox

Referring to the display it contains following fields




1. Message number: it is serial number of the message. It can be displayed from the most currently received messages or vice versa.

2. Flags: contains K means the message is not new, A means the message is already read and F means the message has been forwarded to someone else.

3. size of the message: indicates the length of the message

4. source of the message: originator’s address

5. subject: gives a brief summary of what the message is about.

4 E-mail Services

Basic services:

E-mail systems support five basic functions. These basic functions are:

1. Composition:

It refers to the process of creating messages and answers. Any text editor can be used for the body of the message, the system itself can provide assistance with addressing and the numerous header fields attached to each message.

For example: when answering a message, the e-mail system can extract the originator’s address from the incoming e-mail and automatically insert it into the proper place in the reply.

2. Transfer:

It refers to moving messages from the originator to the recipient. This requires establishing a connection to the destination or some intermediate machine, outputting the message, and finally releasing the connection. E-mail does it automatically without bothering the user.

3. Reporting:

It refers to acknowledging or telling the originator what happened to the message. Was the message delivered? Was it rejected? Numerous applications exist in which confirmation of delivery is important and may even have a legal significance. E-mail system is not very reliable.

4. Displaying

The incoming message has to be displayed so that people can read their e-mail. Sometimes conversation is required or a special viewer must be invoked. For example: if message is a postscript file or digitized voice. Simple conversations and formatting are sometimes attempted.

5. Disposition


It is the final step and concerns what the recipient does with the message after receiving it. Possibilities include throwing it away before reading, throwing it away after reading, saving it, and so on. It should be possible to retrieve and reread saved messages, forward them or process them in other ways.

Advanced services:

In addition to these basic services, some e-mail systems provide a variety of advanced features.

· When people move or when they are away for some period of time, they want their e-mail to be forwarded, so the system should do it automatically.

· Most systems allow user to create mailboxes to store incoming e-mails. Commands are needed to create and destroy mailboxes, inspect the contents of mailboxes, insert and delete messages from the mailboxes.

· Corporate managers often need to send messages to each of their subordinates, customers, or suppliers. This gives rise to the idea of mailing list, which is a list of e-mail addresses. When a message is sent to the mailing list, identical copies are delivered to everyone on the list.

· Carbon copies, blind Carbon copies, high priority e-mail, secret e-mail, alternative recipient’s if primary one is not currently available, and the ability for secretaries to read and answer their bosses e-mail.

· E-mail is now widely used within an industry for intra company communication. It allows far-flung employees to cooperate on projects.

MC0075 SEM3 SMU 2011

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