mb0035 computer networks

COMPUTER NETWORKS MB 0035

Set 1

Name: Rajesh Kumar

Roll number: 520932980

Learning centre: 03036

Subject: MB 0035- COMPUTER NETWORKS

Assignment No.: Set 1

Date of submission at learning centre:

MB0035 – Computer Networks

ASSIGNMENTS

Subject code: MB0035

(4 credits)

Set 1

Marks 60

SUBJECT NAME: COMPUTER NETWORKS

Note: Each Question carries 10 marks

Q1. Explain all design issues for several layers in Computer. What is connection – oriented and connectionless service?

Ans: Design issues for the layers: The various key design issues are present in several layers in computer networks. The important design issues are:

1. Addressing: Mechanism for identifying senders and receivers, on the network need some form of addressing. There are multiple processes running on one machine. Some means is needed for a process on one machine to specify with whom it wants to communicate.

2. Error Control: There may be erroneous transmission due to several problems during communication. These are due to problem in communication circuits, physical medium, due to thermal noise and interference. Many error detecting and error correcting codes are known, but both ends of the connection must agree on which one being used. In addition, the receiver must have some mechanism of telling the sender which messages have been received correctly and which has not.

3. Flow control: If there is a fast sender at one end sending data to a slow receiver, then there must be flow control mechanism to control the loss of data by slow receivers. There are several mechanisms used for flow control such as increasing buffer size at receivers, slow down the fast sender, and so on. Some process will not be in position to accept arbitrarily long messages. Then, there must be some mechanism to disassembling, transmitting and then reassembling messages.

4. Multiplexing / demultiplexing: If the data has to be transmitted on transmission media separately, it is inconvenient or expensive to setup separate connection for each pair of communicating processes. So, multiplexing is needed in the physical layer at sender end and demultiplexing is need at the receiver end.

5. Routing: When data has to be transmitted from source to destination, there may be multiple paths between them. An optimized (shortest) route must be chosen. This decision is made on the basis of several routing algorithms, which chooses optimized route to the destination.

1.4.3 Connection Oriented and Connectionless Services

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Layers can offer two types of services namely connection oriented service and connectionless service.

Connection oriented service: The service user first establishes a connection, uses the connection and then releases the connection. Once the connection is established between source and destination, the path is fixed. The data transmission takes place through this path established. The order of the messages sent will be same at the receiver end. Services are reliable and there is no loss of data. Most of the time, reliable service provides acknowledgement is an overhead and adds delay.

Connectionless Services: In this type of services, no connection is established between source and destination. Here there is no fixed path. Therefore, the messages must carry full destination address and each one of these messages are sent independent of each other. Messages sent will not be delivered at the destination in the same order. Thus, grouping and ordering is required at the receiver end, and the services are not reliable. There is no acknowledgement confirmation from the receiver. Unreliable connectionless service is often called datagram service, which does not return an acknowledgement to the sender. In some cases, establishing a connection to send one short messages is needed. But reliability is required, and then acknowledgement datagram service can be used for these applications.

Another service is the request-reply service. In this type of service, the sender transmits a single datagram containing a request from the client side. Then at the other end, server reply will contain the answer. Request-reply is commonly used to implement communication in the client-server model.

1.4.4 Service Primitives

Primitives are operations. Service is specified as a set of primitives available to a user process. The primitives for connection oriented service are different from those of connectionless service.

There are five service primitives for connection service. They are:

1. LISTEN – Block waiting for an incoming connection.

2. CONNECT – Establish connection with peer on other side.

3. RECEIVE – Block waiting for an incoming message.

4. SEND – Send a message to the peer.

5. DISCONNECT – Terminate a connection from the peer.

The above primitives are used for illustrating connection oriented services interactions.

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Figure 1.9: connection oriented services interactions

Q2. Discuss OSI Reference model.

Ans: The OSI model is based on a proposal developed by the International Standards Organization as a first step towards international standardization of the protocols used in the various layers. The model is called the ISO – (International Standard Organization – Open Systems Interconnection) Reference Model because it deals with connecting open systems – that is, systems that follow the standard are open for communication with other systems, irrespective of a manufacturer.

Its main objectives were to:

· Allow manufacturers of different systems to interconnect equipment through a standard interfaces.

· Allow software and hardware to integrate well and be portable on different systems.

The OSI model has seven layers shown in Figure. The principles that were applied to arrive at the seven layers are as follows:

1. Each layer should perform a well-defined function.

2. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.

3. The layer boundaries should be chosen to minimize the information flow across the interfaces.

The set of rules for communication between entities in a layer is called protocol for that layer.

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Figure 2.1: The OSI Reference Model

The Physical Layer

The Physical layer coordinates the function required to carry a bit (0s and 1s) stream over a physical medium. It defines electrical and mechanical specifications of cables, connectors and signaling options that physically link two nodes on a network.

The Data Link Layer

The main task of the data link layer is to provide error free transmission. It accomplishes this task by having the sender configure the input data into data frames, transmit the frames sequentially, between network devices and process the acknowledgement frames sent back by the intermediate receiver. The data link layer creates and recognizes frame boundaries. This can be accomplished by attaching special bit pattern to the beginning and end of the frame. Since these bit patterns can accidentally occur in the data, special care must be taken to make sure these patterns are not incorrectly interpreted as frame boundaries.

The Network Layer

Whereas the data link layer is responsible for delivery on a hop, the network layer ensures that each packet travels from its sources and destination successfully and efficiently. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are “wired into” the network and rarely changed. They can also be determined at the start of each conversation, for example, a terminal session. Finally, they can be highly dynamic, being determined a new for each packet, to reflect the current network load. When a packet has to travel from one network to another to get its destination, many problems can arise. The addressing used by the second network may be different from the first one. The second network may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.

The Transport Layer

The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all

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arrive correctly at other end. Furthermore, all this must be done efficiently, and in a way that isolates the upper layers from the inevitable changes in the hardware technology. Transport layer provides location and media independent end-to-end data transfer service to session and upper layers.

The Session Layer

The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides enhanced services useful in some applications. A session might by used to allow a user to log into a remote timesharing systems or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic cans only way at a time (analogous to a single railroad track), the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, the session layer provides tokens that can be exchanged. Only the side holding the token may perform the desired operation.

Another session service is synchronization. Consider the problem that might occur when trying to do a 2 hour file transfer between two machines with an one hour mean time between crashes. After each transfer was aborted, the whole transfer would have to start over again and would probably fail again the next time as well. To eliminate this problem, the session layer provides a way to insert markers after the appropriate checkpoints.

The Presentation Layer

Unlike all the lower layers, which are just interested in moving bits reliably from here to there, the presentation layer is concerned with the syntax and semantics of the information transmitted.

A typical example of a presentation service is encoding data in standard agreed upon way. Most user programs do not exchange random binary bit strings, they exchange things such as people’s names, dates, amounts of money and invoices. These items are represented as character strings, integers, floating-point number, and data structures composed of several simpler items. Different computers have different codes for representing character strings (e.g., ASCII and Unicode), integers (e.g., one’s complement and two’s complement), and so on. In order to make it possible for computers with different representations to communicate, the data structure to be exchanged can be defined in an abstract way, along with a standard encoding to be used “on the wire”. The presentation layer manages these abstract data structures and converts from the representation used inside the computer to the network standard representation and back.

The Application Layer

Application layer supports functions that control and supervise OSI application processes such as start/maintain/stop application, allocate/ de allocate OSI resources, accounting, and

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check point and recovering. It also supports remote job execution, file transfer protocol, message transfer and virtual terminal.

Q 3. Describe different types of Data transmission modes.

Ans: The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

Figure 3.3: Data transmission and modes

3.3.1 Serial and Parallel

Serial Transmission

In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices.

The advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces cost of transmission over parallel by roughly a factor of n.

Since communication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-to-parallel). Serial transmission occurs in one of three ways: asynchronous, synchronous, and isochronous.

Parallel Transmission

Binary data, consisting of 1 s and 0 s, may be organized into groups of n bits each. Computers produce and consume data in groups of bits much as we conceive of and use spoken language in the form of words rather than letters. By grouping, we can send data n bits at a time instead of 1. This is called parallel transmission.

The mechanism for parallel transmission is a simple one: Use n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock tick from one device to another.

The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor on n over serial transmission.

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But there is a significant disadvantage: cost. Parallel transmission requires n communication lines just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances.

3.3.2 Simplex, Half duplex and Full duplex

There are three modes of data transmission that correspond to the three types of circuits available. These are:

a) Simplex

b) Half-duplex

c) Full-duplex

Figure 3.4: Different Modes of Data Transmission

Simplex

Simplex communications imply a simple method of communicating, which they are. In simplex communication mode, there is a one-way communication transmission. Television transmission is a good example of simplex communications. The main transmitter sends out a signal (broadcast), but it does not expect a reply as the receiving units cannot issue a reply back to the transmitter. A data collection terminal on a factory floor or a line printer (receive only). Another example of simplex communication is a keyboard attached to a computer because the keyboard can only send data to the computer.

At first thought it might appear adequate for many types of application in which flow of information is unidirectional. However, in almost all data processing applications, communication in both directions is required. Even for a “one-way” flow of information from a terminal to computer, the system will be designed to allow the computer to signal the terminal that data has been received. Without this capability, the remote used might enter data and never know that it was not received by the other terminal. Hence, simplex circuits are seldom used because a return path is generally needed to send acknowledgement, control or error signals.

Half-duplex

In half-duplex mode, both units communicate over the same medium, but only one unit can send at a time. While one is in send mode, the other unit is in receiving mode. It is like two

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polite people talking to each other – one talks, the other listens, but neither one talks at the same time. Thus, a half duplex line can alternately send and receive data. It requires two wires. This is the most common type of transmission for voice communications because only one person is supposed to speak at a time. It is also used to connect a terminal with a computer. The terminal might transmit data and then the computer responds with an acknowledgement. The transmission of data to and from a hard disk is also done in half duplex mode.

Full – duplex

In a half-duplex system, the line must be “turned around” each time the direction is reversed. This involves a special switching circuit and requires a small amount of time (approximately 150 milliseconds). With high speed capabilities of the computer, this turn-around time is unacceptable in many instances. Also, some applications require simultaneous transmission in both directions. In such cases, a full-duplex system is used that allows information to flow simultaneously in both directions on the transmission path. Use of a full-duplex line improves efficiency as the line turn-around time required in a half-duplex arrangement is eliminated. It requires four wires.

3.3.3 Synchronous and Asynchronous transmission

Synchronous Transmission

In synchronous transmission, the bit stream is combined into longer “frames”, which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one. It is left to the receiver to separate the bit stream into bytes for decoding purpose. In other words, data are transmitted as an unbroken sting of 1s and 0s, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information.

Without gaps and start and stop bits, there is no built-in mechanism to help the receiving device adjust its bits synchronization midstream. Timing becomes very important, therefore, because the accuracy of the received information is completely dependent on the ability of the receiving device to keep an accurate count of the bits as they come in.

The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove at the receiving end, and, by extension, with fewer bits to move across the link, synchronous transmission is faster than asynchronous transmission of data from one computer to another. Byte synchronization is accomplished in the data link layer.

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Asynchronous Transmission

Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed upon patterns. As long as those patterns are followed, the receiving device can retrieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually 8 bits, is sent along the link as a unit. The sending system handles each group independently, relaying it to the link whenever ready, without regard to t timer.

Without synchronization, the receiver cannot use timing to predict when the next group will arrive. To alert the receiver to the arrival of an new group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit. To let the receiver know that the byte is finished, 1 or more additional bits are appended to the end of the byte. These bits, usually 1s, are called stop bits.

By this method, each byte is increased in size to at least 10 bits, of which 8 bits is information and 2 bits or more are signals to the receiver. In addition, the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits.

The start and stop bits and the gap alert the receiver to the beginning and end of the each byte and also it to synchronize with the data stream. This mechanism is called asynchronous because, at the byte level, the sender and receiver do not have to be synchronized. But within each byte, the receiver must still by synchronized with the incoming bit stream.

That is, some synchronization is required, but only for the duration of a single byte. The receiving device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. after n bits, the receiver looks for a stop bit. As soon as it detects the stop bit, it waits until it detects the next start bit.

Figure 3.6: Asynchronous transmission

Isochronous Transmission

In real-time audio and video, in which uneven delays between frames are not acceptable, synchronous transmission fails. For example, TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. If each image is send by using one or more frames, there should be no delays between frames. For this type of application, synchronization between characters is not enough; the entire stream of bits must be synchronized. The isochronous transmission guarantees that the data arrive at a fixed rate.

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Q 4. Define Switching. What is the difference between circuit switching and Packet Switching?

Ans: A network is a set of connected devices. Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One of the better solutions is switching. A switch is network consists of a series of interlinked nodes, called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes are connected to the end systems (computers or telephones). Others are used only for routing. Switched networks are divided, as shown in the figure.

Figure 3.7: Different types of switching techniques

3.4.1 Circuit switching

A circuit switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. It is mainly used for telephones to call from one to one.

Figure 3.8: Circuit switching in telephone

In the figure, each office has three incoming lines and three outgoing lines. When call passes through a switching office, a physical connection is established between the line on which the call came in and one of the output lines, as shown by the dotted lines. An important property of circuit switching is the need to set up an end-to-end path before any data can be sent. The elapsed time between the end of dialing and the start of ringing can easily be 10 sec, more on long-distance or international calls. Before data transmissions begin, the destination telephone should give acknowledgement. Once call setup, the only delay for data is the propagation time for the electromagnetic signal, about 5 msec per 1000 km. There is no problem of congestion.

3.4.3 Packet switching

In packet switching, we transfer the messages in terms of small block fixed sizes called packets. In packet switching, there is no path; packets are routed independently by sharing the network at time to time, by following the best path to the destination. Packets can be in order

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to the destination. Packet switching is more fault tolerant than circuit switching. The store-and-forward transmission is used to route to the destination, while storing the packet in the routers main memory. Congestion may occur when more packets are sending from the various hosts.

3.4.4 Comparison of switching techniques

ItemCircuit Switched

Packet Switched

What is send Voice Messages (divided)

Call setup Required Not Required

Dedicated Physical path

Yes No

Each packet follows the same route

Yes No

Packets arrive in order

Yes No

Is a switch crash is fatal

Yes No

Bandwidth availableYes No

Time of possible congestion

At setup time On every packet

Store-and-forward No Yes

Q 5. Classify Guided medium (wired).Compare fiber optics and copper wire.

Ans: Guided Transmission Medium

Guided media, which are those that provide a conduit form one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A single traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals on the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light.

Comparison of fiber optics and copper wire

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Fiber has many advantages over copper wire as a transmission media. These are:

· It can handle much higher band widths than copper. Due to the low attenuation, repeaters are needed only about every 30 km on long lines, versus about every 5 km for copper.

· Fiber is not being affected by the power surges, electromagnetic interference, or power failures. Nor it is affected by corrosive chemicals in the air, making it deal for harsh factory environment.

· Fiber is lighter than copper. One thousand twisted pairs copper cables of 1 km long weight 8000 kg. But two fibers have more capacity and weigh only 100 kg, which greatly reduces the need for expensive mechanical support systems that must be maintained.

· Fibers do not leak light and are quite difficult to tap. This gives them excellent security against potential wire tappers.

· If new routes designed, the fiber is the first choice because of lower installation cost.

Fibers have the following disadvantages over copper wire:

· Fiber is an unfamiliar technology requiring skills most engineers do not have.

· Since optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber.

· Fiber interfaces cost more than electrical interfaces.

· Fibers can be damaged easily by being bent too much.

Q6. What are different types of satellites?

Ans: Communication satellites have some interesting properties that make them attractive for many applications. In its simplest form, a communication satellite can be thought of as a big microwave repeater in the sky. It contains several transponders, each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal.

Basic of Communication Satellites

Fig. 5.2: Satellite Communication System

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5.2.1 Classification of Satellites

Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:

GEO (Geostationary orbit) LEO (Low Earth Orbit) MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit) HEO (Highly Elliptical Orbit) elliptical orbits

Van-Allen-Belts: ionized particles 2000 – 6000 km and 15000 – 30000 km above earth surface.

5.2.2 Satellite Coverage

Fig. 5.3: Communication satellites and their altitude above the earth

Geostationary orbit

Altitude:

ca. 36000 km above earth surface

Coverage:

Ideally suited for continuous, regional coverage using a single satellite. Can also be used equally effectively for global coverage using a minimum of three satellites

Visibility:

Mobile to satellite visibility decreases with increased latitude of the user. Poor Visibility in built-up, urban regions.

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Low Earth orbit

Altitude:

ca. 500 – 1500 km

Coverage:

Multi-satellite constellations of upwards of 30-50 satellites are required for global, continuous coverage. Single satellites can be used in store and forward mode for localized coverage but only appear for short periods of time.

Visibility:

The use of satellite diversity, by which more than one satellite is visible at any given time, can be used to optimize the link. This can be achieved by either selecting the optimum link or combining the reception of two or more links. The higher the guaranteed minimum elevation angle to the user, the more satellites is needed in the constellation.

Medium Earth orbit

Altitude:

ca. 6000 – 20000 km

Coverage:

Multi-satellite constellations of between 10 and 20 satellites are required for global coverage

Visibility:

Good to excellent global visibility, augmented by the use of satellite diversity techniques

Highly elliptical orbit

Altitude:

Apogee: 40 000–50 000 km, Perigee: 1000–20 000 km.

Coverage:

Three or four satellites are needed to provide continuous coverage to a region

Visibility:

Particularly designed to provide high guaranteed elevation angle to satellite for Northern and Southern temperate latitudes.

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