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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionIntroduction, OSI, TCP/IP and Other Networks Models, Network Topologies WAN, LAN, MAN.Examples of Networks: Novell Networks, Arpanet, Internet

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Computer Network interconnected collection of autonomous computers

Interconnected computers that are able to exchange informationAutonomous if no computer can forcibly start, stop or control another one

Distributed System it is a virtual uniprocessor

Allocation of jobs to processors and files to disks, movement of files betweenwhere they are needed, and all other system functions are automatic in distributedsystems. (Transparent not visible to the user)

Where as in CNs the transmission between systems is controlled explicit ly by the user

Goals of Computer networks

1. Resource SharingTo make all programs, equipment and data available to anyone in the

network2. High reliability

By having alternative sources of supply3. Saving Money

Network of small computers can replace larger ones (like Mainframes)and there by the expenditure can be reduced

4. CommunicationLike e-mails, video conference...etc.--------------------------------------------------------------------------------------------------------------------------

What is the big deal about computer networks?- Allows two or more computers to communicate- Benefitso Programs can be shared - software packages can be installed onto the file server and accessed by allindividual workstations at the same time. This reduces cost, maintenance and makes upgrades easier.o You can access your work from any workstation on the network. Very handy if you have to change computer every time you go to a different classroom.o Data can be shared by all users at the same time. Many people can access or update the information held on adatabase at the same time. Thus information is up to date and accurate.o Users can communicate with others on the network by sending messages and sharing files.o Individual workstations do not need a printer; one high quality printer can now be shared by everyone, thuscutting costs.o Networks provide security. A user must have the correct Password and User ID in order to be able to accessthe information on the network.o Private areas on the network can be set up that allows each user to store their personal files. The only other

person who can access these files is the 'system administrator' who looks after the network.

- Disadvantageso Networks can be expensive to set up. They often involve taking up floors and ceilings to lay hundreds of feetof cableso The File Server needs to be a powerful computer, which often means that it is expensive.o Networks are vulnerable to security problems. Hackers, disgruntled employees or even competitors might tryto break into the system to read or damage crucial information. Much effort is spent preventing unauthorizedaccess to data and software.o If the main File Server breaks down, then the whole system becomes useless and no one can carry onworking.o Because networks are often complicated, they need expensive expert staff to look after them.o As the number of users increase on the network, the performance of the system can be affected and thingsstart to slow down.o Malware, such as worms, easily spread on computer networks and not on stand alone computers.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionClient Server Model

Client machine Servermachine

Client processServer process

Request

Reply

Packet a short form of message that travels in a network

Data address

Network HardwareComputer Networks

Based on type of transmission technology

Broadcast N/Ws Point-to-Point N/Ws

Broadcasting

Sending a message to all the nodes (computers) in a network

To broadcast a message the address field in the packet must contain some specialcode.

Data AddressSpecial code

xxx . xxx . xxx . 255 (for Directedbroadcasting)

255.255.255.255(forLimited broadcasting)

Directed broadcasting the message is broadcasted to all the nodes in someother network

Limited broadcasting the message is broadcasted to all the nodes in the samenetwork

Multicasting

Sending a message to some group of nodes in a network

Point-to-Point Networks

In this type of networks there is a need for many connections between every pairof computers. And the message is subjected to only one destination.

Intermediate nodes

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionIntermediate nodes must be able toselect the best route among theavailable using some RoutingAlgorithms

Computer Networks

Based on scale of the network

LAN MAN WAN

LAN

MAN

WAN Internet

Topology the arrangement of nodes in a network

Some possible topologies are

Bus Mesh

Ring Star

Tree Irregular

LAN ( Local Area Network )

LANs cover small geographical areas like a room/campus

Generally LANs runs at 10-100 Mbps

Possible topologies for LAN are Bus and RingBus only one master is allowed to transmit data at a time

To resolve conflicts in a network we need some control that can be eithercentralized or distributed .

Eg: - Ethernet (IEEE 802.6)

It is a Bus based broadcast network that uses a decentralizedcontrol and runs at 10-100 Mbps. In Ethernet any node can transmit dataat anytime. So there is every chance for collisions. If a collision occurs

the nodes involved in transmission waits for sometime and retransmit thesame data.

Ring some arbitration mechanism is required in ring type networks

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10m Room100m Building1Km Campus10Km City100Km Country1000Km Continent10000Km Whole world

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionEg: - IBM Token Ring (IEEE 802.3)

Runs at 4 and 16 Mbps

Allocating communication channel to a system can be done in two ways:

1. Static Allocation2. Dynamic Allocation

Static allocation divides time into slots & runs a round robin algorithmBut if a system has nothing to transfer time network will be in idle statethroughout that time slot. Channel capacity will get wasted by this.

Dynamic allocation it can be either centralized or decentralized

MAN ( Metropolitan Area Network ) Almost similar to LAN but a special standard is defined for MANs

IEEE 802.6 DQDB ( Distributed Queue Dual Bus )Bus A

1 2 3 4

Bus B

DQDB consists of two unidirectional buses to which all the nodes areconnected. Each bus has a head-end system that initiates transmission activity.

Traffic to the right uses the upper bus. Traffic to the left uses the lower bus.

WAN ( Wide Area Network ) WAN contains a collection of hosts/end systems connected by communication

subnet/subnet .Subnet is a collection of switching elements (routers) that connects two or more

transmission lines (channels/circuits/trunks).

Channel

Subnet the collection of routers and communication lines that moved packetsfrom the source host to destination host

Some routers copy the message they received and pass it to the next node only

when the output line is free. Subnets that use this type of routers are called as Point-to-Point/Store and Forward/Packet Switched subnets.Internetwork (internet)

A collection of interconnected networks Common form of internet is a collection of LANs connected by a WANWe use Gateways/Routers/Brouters/Bridges to connect 2 or more networks

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction

Network Software To reduce design complexity network software is organized as layers. Each layer

offers some services to the higher layers. For this we need some rules and conventionscalled as protocol.

Protocol an agreement between the communicating parties on how communicationis to proceed

Interface defines which services the lower layer offering to its upper oneProtocol stack collection protocols used in all the layersNetwork Architecture set of layers + protocols

At source system when data is passing down each layer adds some headerinformation which used by the same layer in the destination side. Somelayers fragment the data for sophisticated transmission.

At destination system when data is passing up to the higher layers headers areremoved and data is reassembled.

Design issues for the layers:1. Mechanism to identify senders and receivers2. Type of communication3. Mechanism to determine logical channels and their priorities4. Error control5. Sequencing6. Flow control (to synchronize between fast sender and a slow receiver)7. Disassembling & reassembling8. Choosing a route

Types of communication

Simplex data can be transmitted in only one way

Half Duplex data can be transmitted in both the ways but only one way at a time

Full Duplex both the ways at a time

Entity active elements present in each layer. Entities can be either hardware (anintelligent I/O chip) or software (a process) entities.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionPeer entity entities present in same layer but on different machines

SAP point where services are available. Each SAP has a unique address foridentification.

To exchange information two layers must have an agreement between them. That

agreement is done by passing an IDU through SAP.IDU ICI + SDU

SDU passed to the peer entity & then to layer n+1

ICI it is not part of data but passed to the lower layers to do some job

SDU is fragmented and header is added to it PDU

PDU header is used by peer entities to carryout peer protocol

A service is said to be reliable if there is no data lose. Reliability can be achieved byusing acknowledgements to the data sent. But acknowledgements may result in extraoverhead and delay in transmission.

Connection Oriented Service

Eg: - telephone system

Connection is established, used and then released Order of packets at both the sender and receiver sides is same Highly reliable The variations in connection oriented service are

Message sequenceMessage boundaries are defined (eg:- transmitting a book (sequence of pages))

Byte streamMessage boundaries are not necessary and data is sent as a sequenceof bytes (eg:- remote login)

Unreliable connection oriented serviceAcknowledgement is not necessary (eg:- digitized voice no problemeven though some data (voice) is lost but delays(because of acknowledgements) should be avoided)

Connectionless Service

Eg:- postal system

Less reliable Datagram service unreliable (no ack.) connectionless service (eg:- e-mail) Acknowledged datagram service connectionless service with acknowledgements

(eg:- registered letter) Request Reply service request is sent as one datagram and the answer is a reply

for the request (eg:- database request)

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Service Primitives

Service is defined as set of operations called primitives.

Some basic primitives are:

Primitive Meaning ParametersRequest Entity wants the service to do some

workMachine to request, type of service desired and maximummessage size to be used on theconnection

Indication Entity is to be informed about anevent

Callers identity, type of servicedesired and proposed packetsize

Response Entity wants to respond to an eventConfirm The response to an earlier request

has came back

Example of a connection oriented service with eight primitives

CONNECT.Request - Request a connection to be establishedCONNECT.Indication

- Signal the called party Confirmedservice

CONNECT.Response

- Used by the callee to accept/reject calls

CONNECT.Confirm - Tell the caller whether the call was accepted

DATA.Request - Request that data be sentDATA.Indication - Signal the arrival of dataUnconfirmed services

DISCONNECT.Request

- Request that a connection be released

DISCONNECT.Indication

- Signal the peer about the request

Service a set of primitives that a layer provides to the layer above it

Protocol set of rules governing the format and meaning of the frames, packets or messages that are exchanged by the peer entities with in a layer

A Reference model is the set of rules to be followed before constructing a new network

ISO OSI Reference ModelISO International Standard Organization

OSI Open Systems Interconnection

Open systems systems those are open for communication

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Physical L ayer

It mainly deals with transmitting raw bits over communication channel

Design issues are:

How a bit should be represented? What is the transmission type? How to establish a connection? How many pins the network connector has & their purpose?

Data Link Layer

Error control Flow control Framing Access control

Network Layer

Routing Congestion control Addressing

Transport Layer

Segmentation & Reassembling Flow control

This is a pure end-to-end layer

If there are many connections to a single host transport header helps us to distinguishamong themSession Layer

Establish sessions Token management Synchronization (like adding checkpoints to downloading files)

Presentation Layer

Concerns with syntax and semantics of the information

Encoding Compression Encryption

Application Layer

Transferring data between incompatible systems

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction UID User Interface Design

Contains majority of protocols

Work stations, Servers,Gateways

All the 7 layers are present

Hub Only Physical layer

Switch, Bridge Physical layer + Data link layerRouter Physical layer + Data link layer + Network layer

Physical Layer This layer is the lowest layer in the OSI model. It helps in the transmission of data between twomachines that are communicating through a physical medium, which can be optical fibres,copper wire or wireless etc. The following are the main functions of the physical layer:

1. Hardware Specification: The details of the physical cables, network interface cards,wireless radios, etc are a part of this layer.

Coaxial Cable Hybrid Cable Wireless Card Network Card

2. Encoding and Signaling: How are the bits encoded in the medium is also decided bythis layer. For example, on the copper wire medium, we can use different voltage levelsfor a certain time interval to represent '0' and '1'. We may use +5mV for 1nsec torepresent '1' and -5mV for 1nsec to represent '0'. All the issues of modulation is dealt within this layer. eg, we may use Binary phase shift keying for the representation of '1' and '0'rather than using different voltage levels if we have to transfer in RF waves.

Binary Phase Shift Keying

3. Data Transmission and Reception: The transfer of each bit of data is the responsibilityof this layer. This layer assures the transmission of each bit with a high probability . Thetransmission of the bits is not completely reliable as there is no error correction in thislayer.

4. Topology and Network Design: The network design is the integral part of the physicallayer. Which part of the network is the router going to be placed, where the switches willbe used, where we will put the hubs, how many machines is each switch going to handle,what server is going to be placed where, and many such concerns are to be taken care of by the physical layer. The various kinds of net topologies that we decide to use may bering, bus, star or a hybrid of these topologies depending on our requirements.

Network Topologies

A network topology is the basic design of a computer network. It is very much like a map of aroad. It details how key network components such as nodes and links are interconnected. Anetwork's topology is comparable to the blueprints of a new home in which components such asthe electrical system, heating and air conditioning system, and plumbing are integrated into the

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionoverall design. Taken from the Greek work "Topos" meaning "Place," Topology, in relation tonetworking, describes the configuration of the network; including the location of the workstationsand wiring connections. Basically it provides a definition of the components of a Local AreaNetwork (LAN). A topology, which is a pattern of interconnections among nodes, influences anetwork's cost and performance. There are three primary types of network topologies which referto the physical and logical layout of the Network cabling. They are:

1. Star Topology: All devices connected with a Star setup communicate through a centralHub by cable segments. Signals are transmitted and received through the Hub. It is thesimplest and the oldest and all the telephone switches are based on this. In a startopology, each network device has a home run of cabling back to a network hub, givingeach device a separate connection to the network. So, there can be multiple connectionsin parallel.

Advantages Network administration and error detection is easier because problem is isolated to

central node Networks runs even if one host fails Expansion becomes easier and scalability of the network increases More suited for larger networks

Disadvantages Broadcasting and multicasting is not easy because some extra functionality needs

to be provided to the central hub If the central node fails, the whole network goes down; thus making the switch

some kind of a bottleneck Installation costs are high because each node needs to be connected to the central

switch

2. Bus Topology: The simplest and one of the most common of all topologies, Bus consistsof a single cable, called a Backbone, that connects all workstations on the network using asingle line. All transmissions must pass through each of the connected devices tocomplete the desired request. Each workstation has its own individual signal that identifiesit and allows for the requested data to be returned to the correct originator. In the BusNetwork, messages are sent in both directions from a single point and are read by thenode (computer or peripheral on the network) identified by the code with the message.Most Local Area Networks (LANs) are Bus Networks because the network will continue tofunction even if one computer is down. This topology works equally well for either peer topeer or client server.

The purpose of the terminators at either end of the network is to stop the signal beingreflected back.

Advantages Broadcasting and multicasting is much simpler Network is redundant in the sense that failure of one node doesn't effect the

network. The other part may still function properly Least expensive since less amount of cabling is required and no network switches

are required Good for smaller networks not requiring higher speeds

Disadvantages Trouble shooting and error detection becomes a problem because, logically, all

nodes are equal Less secure because sniffing is easier Limited in size and speed

3. Ring Topology: All the nodes in a Ring Network are connected in a closed circle of cable.Messages that are transmitted travel around the ring until they reach the computer thatthey are addressed to, the signal being refreshed by each node. In a ring topology, thenetwork signal is passed through each network card of each device and passed on to thenext device. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. There is a very nicefeature that everybody gets a chance to send a packet and it is guaranteed that everynode gets to send a packet in a finite amount of time.

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Advantages Broadcasting and multicasting is simple since you just need to send out one

message Less expensive since less cable footage is required It is guaranteed that each host will be able to transmit within a finite time interval

Very orderly network where every device has access to the token and theopportunity to transmit

Performs better than a star network under heavy network load

Disadvantages Failure of one node brings the whole network down Error detection and network administration becomes difficult Moves, adds and changes of devices can effect the network It is slower than star topology under normal load

Generally, a BUS architecture is preferred over the other topologies - ofcourse, this is a very subjective opinionand the final design depends on the requirements of the network more than anything else. Lately, most networksare shifting towards the STAR topology. Ideally we would like to design networks, which physically resemblethe STAR topology, but behave like BUS or RING topology.

Data Link Layer This layer provides reliable transmission of a packet by using the services of the physical layerwhich transmits bits over the medium in an unreliable fashion. This layer is concerned with :

1. Framing : Breaking input data into frames (typically a few hundred bytes) and caring aboutthe frame boundaries and the size of each frame.

2. Acknowledgment: Sent by the receiving end to inform the source that the frame wasreceived without any error.

3. Sequence Numbering: To acknowledge which frame was received.4. Error Detection : The frames may be damaged, lost or duplicated leading to errors. The

error control is on link to link basis.5. Retransmission: The packet is retransmitted if the source fails to receive acknowledgment.6. Flow Control: Necessary for a fast transmitter to keep pace with a slow receiver.

Data Link Layer

Network LayerIts basic functions are routing and congestion control.Routing: This deals with determining how packets will be routed (transferred) from source todestination. It can be of three types :

Static : Routes are based on static tables that are "wired into" the network and are rarelychanged.

Dynamic : All packets of one application can follow different routes depending upon thetopology of the network, the shortest path and the current network load.

Semi-Dynamic : A route is chosen at the start of each conversation and then all thepackets of the application follow the same route.

Routing

The services provided by the network can be of two types :

Connection less service: Each packet of an application is treated as an independententity. On each packet of the application the destination address is provided and thepacket is routed.

Connection oriented service: Here, first a connection is established and then allpackets of the application follow the same route. To understand the above concept, wecan also draw an analogy from the real life. Connection oriented service is modeled afterthe telephone system. All voice packets go on the same path after the connection isestablished till the connection is hung up. It acts like a tube ; the sender pushes the

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionobjects in at one end and the receiver takes them out in the same order at the other end.Connection less service is modeled after the postal system. Each letter carries thedestination address and is routed independent of all the others. Here, it is possible thatthe letter sent first is delayed so that the second letter reaches the destination before thefirst letter.

Congestion Control: A router can be connected to 4-5 networks. If all the networks send packetat the same time with maximum rate possible then the router may not be able to handle all thepackets and may drop some/all packets. In this context the dropping of the packets should beminimized and the source whose packet was dropped should be informed. The control of suchcongestion is also a function of the network layer. Other issues related with this layer aretransmitting time, delays, jittering.

Internetworking: Internetworks are multiple networks that are connected in such a way thatthey act as one large network, connecting multiple office or department networks. Internetworksare connected by networking hardware such as routers, switches, and bridges. Internetworking isa solution born of three networking problems: isolated LANs, duplication of resources, and thelack of a centralized network management system. With connected LANs, companies no longerhave to duplicate programs or resources on each network. This in turn gives way to managing

the network from one central location instead of trying to manage each separate LAN. We shouldbe able to transmit any packet from one network to any other network even if they followdifferent protocols or use different addressing modes.

Inter-Networking

Network Layer does not guarantee that the packet will reach its intended destination. There areno reliability guarantees.

Transport LayerIts functions are :

Multiplexing / Demultiplexing: Normally the transport layer will create distinct networkconnection for each transport connection required by the session layer. The transportlayer may either create multiple network connections (to improve throughput) or it maymultiplex several transport connections onto the same network connection (becausecreating and maintaining networks may be expensive). In the latter case, demultiplexingwill be required at the receiving end. A point to note here is that communication is alwayscarried out between two processes and not between two machines. This is also known asprocess-to-process communication.

Fragmentation and Re-assembly : The data accepted by the transport layer from the

session layer is split up into smaller units (fragmentation) if needed and then passed tothe network layer. Correspondingly, the data provided by the network layer to thetransport layer on the receiving side is re-assembled.

Fragmentation

Reassembly

Types of service : The transport layer also decides the type of service that should beprovided to the session layer. The service may be perfectly reliable, or may be reliable

within certain tolerances or may not be reliable at all. The message may or may not bereceived in the order in which it was sent. The decision regarding the type of service to beprovided is taken at the time when the connection is established.

Error Control : If reliable service is provided then error detection and error recoveryoperations are also performed. It provides error control mechanism on end to end basis.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction Flow Control : A fast host cannot keep pace with a slow one. Hence, this is a mechanism

to regulate the flow of information. Connection Establishment / Release : The transport layer also establishes and

releases the connection across the network. This requires some sort of naming mechanismso that a process on one machine can indicate with whom it wants to communicate.

Session LayerIt deals with the concept of Sessions i.e. when a user logins to a remote server he should be

authenticated before getting access to the files and application programs. Another job of session layer is to establish and maintain sessions. If during the transfer of data between twomachines the session breaks down, it is the session layer which re-establishes the connection. Italso ensures that the data transfer starts from where it breaks keeping it transparent to the enduser. e.g. In case of a session with a database server, this layer introduces check points atvarious places so that in case the connection is broken and reestablished, the transition runningon the database is not lost even if the user has not committed. This activity is calledSynchronization . Another function of this layer is Dialogue Control which determines whoseturn is it to speak in a session. It is useful in video conferencing.

Presentation Layer This layer is concerned with the syntax and semantics of the information transmitted. In order tomake it possible for computers with different data representations to communicate datastructures to be exchanged can be defined in abstract way along with standard encoding. It alsomanages these abstract data structures and allows higher level of data structures to be definedan exchange. It encodes the data in standard agreed way(network format). Suppose there aretwo machines A and B one follows 'Big Endian' and other 'Little Endian' for data representation.

This layer ensures that the data transmitted by one gets converted in the form compatible to

other machine. This layer is concerned with the syntax and semantics of the informationtransmitted. In order to make it possible for computers with different data representations tocommunicate data structures to be exchanged can be defined in abstract way along withstandard encoding. It also manages these abstract data structures and allows higher level of data structures to be defined an exchange. Other functions include compression, encryption etc.

Application Layer The seventh layer contains the application protocols with which the user gains access to thenetwork. The choice of which specific protocols and their associated functions are to be used atthe application level is up to the individual user. Thus the boundary between the presentation

layer and the application layer represents a separation of the protocols imposed by the networkdesigners from those being selected and implemented by the network users. For examplecommonly used protocols are HTTP(for web browsing), FTP(for file transfer) etc.

TCP/IP Reference Model

Internet Layer

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroductionInjects packets into any network and have them travel independently (order is not

taken care off) and rearranging them is the responsibility of the higher layers.

Defines an official packet format (IP)

IP Internet Protocol

Transport Layer

It also defines two official packet formats: TCP and UDP

TCP Transmission Control Protocol

Reliable connection oriented protocol

Based on byte streams. Fragments incoming byte streams and reassembles atdestination.

Also handles Flow control

UDP User Datagram Protocol

Unreliable connectionless protocol

Uses client-server/request-reply model

Application Layer

Contains high level protocols

Telnet Virtual terminals (logon to a remote system and workthere)

ftp (file transfer protocol) File transfer

SMTP (Simple Mail TransferProtocol) e-mailsDNS (Domain Name Server) Mapping hostnames to network addresshttp (hyper text transferprotocol)

Fetching in WWW

Host-to-Network Layer

Host is connected to the network here, using some protocol and send IP packets through

the connection

OSI TCP/IP7 layers 5 layersNo definition formulticasting

Multicasting defined

Not flexible FlexibleIdle standards Practical

Public Networks

Research

CooperativeCorporate

Novell NetwareIt is a popular LAN networking package

Netware is more useful when a mainframe is downsized to a network of PCsNetware reference model

Application layer SAP Fileserver

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction Transport layer NCP SPX

Network layer IPX

Data link layer Ethernet Token ring ARC net

Physical layer Ethernet Token ring ARC net

Physical layer & Data link layer chosen from various industry standards

Network layer uses IPX (Internet Packet eXchange) protocol

An unreliable connectionless internetwork protocol

It is similar to IP but uses a 12-byte address

Transport layer uses NCP/SPX/TCP

NCP Network Core Protocol

It is a connection oriented protocol

Along with data transport it offers some other services. Thats whyNCP is called as heart of Netware

SPX Sequenced Packet eXchange

It offers only data transport service

IPX Packet Format

2 2 1 1 12

12Checksum Packet length Transport control Packet type Destination addressSource address Data

Transport control contains the number of networks the packet has traversed

Packet length length of header field + length of data field

Packet type specifies whether it is a control packet or a data packet

Address (12 byte) 32 bits (to identify network) + 48 bits (to identify machine) + 16

bits (to identify socket)SAP Service Advertising Protocol

Using SAP each server in the network broadcasts a message for every 1 minute about theservices offered by it. Special agents present in routers collect these messages and forms adatabase. When ever a client is booted it broadcasts a message asking for its nearest server.Agent in a local router verifies the database and sends back the client its nearest server details.

ARPANETARPA Advanced Research Projects Agency (research arm of the U.S. D epartment o f

Defense)ARPANET is packet switched network consisting of a subnet and host computers

Subnet consists of IMP(Interface Message Processors)s connected by transmission lines

For high reliability, each IMP would be connected to at least two other IMPs

A host could send messages up to 8063bits to its IMP, which would then break these upinto packets of at most 1008 bits and forward them independently toward thedeastination.

Subnet was the first electronic store and forward packet switching network

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In 1980s DNS (Domain Naming System) was created to organize machines into domainsand map hostnames onto IP addresses.

NSF the U.S. National Science Foundation

InternetARPANET + NSFNET Internet

Traditionally, the Internet had 4 main applications:

1. Email2. News3. Remote login4. File transfer

Problems1. An alternative to a LAN is simply a big timesharing system with terminals for all users. Give two

advantages of a client-server system using a LAN.Ans. The LAN model can be grown incrementally. If the LAN is just a long cable. it cannot be brought down by a single failure (if the servers are replicated) It is

probably cheaper. It provides more computing power and better interactiveinterfaces.

2. The performance of a client-server system is influenced by two network factors: thebandwidth of the network (how many bits/sec it can transport) and the latency (howmany seconds it takes for the first bit to get from the client to the server). Give anexample of a network that exhibits high bandwidth and high latency.Then give an example of one with low bandwidth and low latency.Ans. A transcontinental fiber link might have many gigabits/sec of bandwidth,but the latency will also be high due to the speed of light propagation over thousands of kilometers. In contrast, a 56-kbps modem calling a computer inthe same building has low bandwidth and low latency.

3. Besides bandwidth and latency, what other parameter is needed to give a goodcharacterization of the quality of service offered by a network used for digitizedvoice traffic?

Ans: A uniform delivery time is needed for voice, so the amount of jitter in thenetwork is important. This could be expressed as the standard deviation of the delivery time. Having short delay but large variability is actually worse than a somewhat longer delay and low variability.

4. A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, the designers may put a high-speed line, a medium-speed line, a lowspeed line, or no line.If it takes 100 ms of computer time to generate and inspect each topology, how long will it take toinspect all of them?Ans: Call the routers A, B, C, D, and E. There are ten potential lines: AB, AC, AD,

AE, BC, BD, BE, CD, CE, and DE. Each of these has four possibilities (three

speeds or no line), so the total number of topologies is 4 10 = 1,048,576. At 100ms each, it takes 104,857.6 sec, or slightly more than 29 hours to inspect themall.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction5. A group of 2 n - 1 routers are interconnected in a centralized binary tree, with a router at each tree

node. Router i communicates with router j by sending a message to the root of the tree. The rootthen sends the message back down to j . Derive an approximate expression for the mean number of hops per message for large n , assuming that all router pairs are equally likely.

Ans:

6. A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to accessthe channel at the same time. As a simplistic example, suppose that time is divided into discreteslots, with each of the n hosts attempting to use the channel with probability p during each slot.What fractions of the slots are wasted due to collisions?

7. What are two reasons for using layered protocols?Ans : Among other reasons for using layered protocols, using them leads tobreakingup the design problem into smaller, more manageable pieces, and layeringmeans that protocols can be changed without affecting higher or lower ones.

8. What is the principal difference between connectionless communication and connection orientedcommunication?Ans: Connection-oriented communication has three phases. In the establishment

phase a request is made to set up a connection. Only after this phase has beensuccessfully completed can the data transfer phase be started and datatransported. Then comes the release phase. Connectionless communicationdoes not have these phases. It just sends the data.

9. Two networks each provide reliable connection-oriented service. One of them offers areliable byte stream and the other offers a reliable message stream. Are these identical? If so, whyis the distinction made? If not, give an example of how they differ.Ans: Message and byte streams are different. In a message stream, the network keeps track of message boundaries. In a byte stream, it does not. For example,suppose a process writes 1024 bytes to a connection and then a little later writes another 1024 bytes. The receiver then does a read for 2048 bytes. With amessage stream, the receiver will get two messages, of 1024 byteseach. With a byte stream, the message boundaries do not count and thereceiver will get the full 2048 bytes as a single unit. The fact that there wereoriginally two distinct messages is lost.

10. What does ''negotiation'' mean when discussing network protocols? Give an example.Ans: Negotiation has to do with getting both sides to agree on some parametersor values to be used during the communication. Maximum packet size is oneexample, but there are many others.

11. In some networks, the data link layer handles transmission errors by requesting damaged framesto be retransmitted. If the probability of a frame's being damaged is p , what is the mean numberof transmissions required to send a frame? Assume that acknowledgements are never lost.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction12. If the unit exchanged at the data link level is called a frame and the unit exchanged at the network

level is called a packet, do frames encapsulate packets or do packetsencapsulate frames? Explain your answer.Ans: Frames encapsulate packets. When a packet arrives at the data link layer,the entire thing, header, data, and all, is used as the data field of a frame. Theentire packet is put in an envelope (the frame), so to speak (assuming it fits).

13. A system has an n -layer protocol hierarchy. Applications generate messages of length M bytes. Ateach of the layers, an h -byte header is added. What fraction of the network bandwidth is filled withheaders?

14. List two ways in which the OSI reference model and the TCP/IP reference model are the same. Nowlist two ways in which they differ.Ans: Both models are based on layered protocols. Both have a network,transport, and application layer. In both models, the transport service can

provide a reliable end-to-end byte stream. On the other hand, they differ inseveral ways. The number of layers is different, the TCP/IP does not havesession or presentation layers, OSI does not support internetworking, and OSIhas both connection-oriented and connectionless service in the network layer.

15. The Internet is roughly doubling in size every 18 months. Although no one really knows for sure,one estimate put the number of hosts on it at 100 million in 2001. Use these data to compute theexpected number of Internet hosts in the year 2010. Do you believe this? Explain why or why not.Ans: Doubling every 18 months means a factor of four gain in 3 years. In 9 years,the gain is then 4 3 or 64, leading to 6.4 billion hosts. My intuition says that ismuch too conservative, since by then probably every television in the world and

possibly billions of other appliances will be on home LANs connected to theInternet. The average person in the developed world may have dozens of Internet hosts by then.

16. When a file is transferred between two computers, two acknowledgement strategies are possible.In the first one, the file is chopped up into packets, which are individually acknowledged by thereceiver, but the file transfer as a whole is not acknowledged. In the second one, the packets arenot acknowledged individually, but the entire file is acknowledged when it arrives. Discuss thesetwo approaches.Ans: If the network tends to lose packets, it is better to acknowledge each oneseparately, so the lost packets can be retransmitted. On the other hand, if thenetwork is highly reliable, sending one acknowledgement at the end of theentire transfer saves bandwidth in the normal case (but requires the entire fileto be retransmitted if even a single packet is lost).

17.List two advantages and two disadvantages of having international standards for networkprotocols.Ans: One advantage is that if everyone uses the standard, everyone can talk toeveryone. Another advantage is that widespread use of any standard will give it economies of scale, as with VLSI chips. A disadvantage is that the political compromises necessary to achieve standardization frequently lead to poor standards. Another disadvantage is that once a standard has been widely adopted, it is difficult to change,, even if new and better techniques or methodsare discovered. Also, by the time it has been accepted, it may be obsolete.

18. How long was a bit on the original 802.3 standard in meters? Use a transmission speed of 10 Mbpsand assume the propagation speed in coax is 2/3 the speed of light in vacuum.Ans: The speed of light in coax is about 200,000 km/sec, which is 200

meters/ sec. At 10 Mbps, it takes 0.1 sec to transmit a bit. Thus, the bit lasts 0.1 sec intime, during which it propagates 20 meters. Thus, a bit is 20 meters long here.

19. An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How longdoes it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a10-Mbps Ethernet? Over 100-Mbps Ethernet?Ans: The image is 1024 768 3 bytes or 2,359,296 bytes. This is 18,874,368bits. At 56,000 bits/sec, it takes about 337.042 sec. At 1,000,000 bits/sec, it takes about 18.874 sec. At 10,000,000 bits/sec, it takes about 1.887 sec. At 100,000,000 bits/sec, it takes about 0.189 sec.

(Frequently asked Questions from previous exam papers)*1. (a) Write any four reasons for using layered protocols.(b) List two ways in which the OSI reference model and the TCP/IP reference model arethe same and list in which they differ.

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Computer NetworksComputer Networks Unit-IUnit-I IntroductionIntroduction(c) Which is the principle difference between CO communication and CL communication.

* 2. (a) Explain in detail ISO-OSI reference model.

(b) Write short notes on interface, service and protocol.

3. (a) With a neat diagram, explain the functionality of layers, protocols and inter-faces.4. List two advantages and two disadvantages of having international standardsfor network, Protocols?.

5. (a) Briefly explain about the TCP/IP reference model.(b) Compare and contrast OSI and TCP/IP models.

6. (a) Compare point -to-point channels with broadcast channels along with suitableexamples?(b) A collection of five routers is to be collected in a point-to-point subnet. Between eachpair of routers, the designers may put a high speed line, a medium-speed line, a low-speed line, or no line. If it takes 100ms of computer time to generate and inspect eachtopology, how long will it take to inspect all of them to find the one that best matchesthe expected load?

7. (a) What are the advantages of having layered architecture? Mention the layers of ISO-OSI reference model?(b) What is Internet? Mention some of the applications of Internet?

8. (a) Define the following terms:i. Computer Networkii. Peer processiii. Protocoliv. Interface.(b) Discuss various network applications and goals in detail.

9. (a) Explain problems of the TCP/IP model and protocols?(b) With a neat diagram explain ARPANET design?

10.

11. (a) What does negotiation mean when discussing network protocol. Give anexample.(b) Which of the OSI layers handles each of the followingi. Breaking the transmitted bit stream into frames.ii. Determine which route through subnet to use.

iii. Dialog control and synchronization.(c) What is the main difference between TCP and UDP.

12. (a) Discuss the classification of Networks according to their size?(b) Discuss the various design issues related to the layers in ISO-OSI model?

13. With the help of appropriate examples explain various types of connection orientedand connection less services.

14. Novell Netware looks more like TCP/IP than like OSI. Justify.

15. (a) Distinguish among LAN,MAN,WAN and Internet.(b) What are the two protocols that are defined at the transport layer of the TCP/IPreference model? Mention their applications?

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16.Two networks each provide a reliable connection oriented service. One of them offersa reliable byte stream and other offers a reliable message stream. Are they identical? If so, why is distinction made? If not give an example of how they differ?

17. Give a detailed description of the Novell Netware IPX packet?

18. (a) How would you utilize an existing telephone network for Computer-to-Computerdata communications?(b) Bad Timing is also a problem for OSI reference model. Discuss.

19. (a) Give a detailed description of the Novell Netware reference model.(b) With suitable examples explain simplex, half-duplex & full-duplex communication.

20. Although wireless networking and mobile computing are often related, they are notidentical. Justify the statement.

21. (a) Differentiate between computer network and distributed system?(b) What are the important goals achieved through networking?(c) Explain about the four main applications of the Internet?

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