Computer Networks Lecture Notes

CS 2363 / CS 65 – COMPUTER CO MPUTER NETWORKS NETWORK S

UNIT - I

Electrical Electrical and Electron Electroniic s Engi Engineeri neeri ng

UNIT- I Introductio Introductio n to networks – network network architecture – network network performance – Direc Directt link link networks network s – encoding – framing framing – error error detection – transmision transmision – Ethernet Ethernet – Rings Rings – FDDI - Wireles Wireles networks Switched networks – bridges. bridges.

–

Introdu Introductio ction n to Netw Ne tworks orks

De ine Network ? De ine computer networks ?

(May/June ’12 – 2Mark) 2Mark) (Nov/Dec ’11 – 2Mark) 2Mark)

Network

 It is a set of devices connected by communication links.  This device device can be a computer, mobile, printer or o r anything anything else which which is capable capable of sending and receiving information / data. Networking

 Networking is a connecting co nnecting of two two or more compu co mputing ting device devicess together t ogether fo r the purpose purpose of haring data and resources. Purpose of Networking

Computer networks can be used for a variety of purposes, 1. Sharing hardware

In a networked networke d environment, environment , each computer co mputer on a network may access acce ss and use hardwa hardware re resources reso urces on the network, such as printing a document on a shared shared network printer. Sharing files , data, data, and inform information atio n 2. Sharing In a network environment, authorized user may access data and information stored on other computers on the the network. The The capability capability of providing providing access to data data and information info rmation on hared hared storage sto rage device devicess is an important feature of many many networks. networks. 3. Sharing software

Users connected to a network may run application programs on remote computers. 4. Facilitating communications

Using a network, people can communicate efficiently and easily via email, instant mesaging, chat rooms, telephone, video telephone calls, and video conferencing. 5. Security

buii l ding blocks of netwo network rk ? Give the basic bu

1. 2. 3. 4.

(Nov/Dec ’12 – 2Mark) 2Mark)

Computer Computer (Client (Cli ent or Server) Server).. Networking Networki ng Devices (Hub, Bridge, Switc Switch, h, and Router) Route r).. Physical Connectivity. Connectivity. (Wired (Wir ed or Wireles). Wire les). Protocols.

Define Define protocol ? 2Mark) data commu co mmunicati nicati ons. A protocol defines what is Protocol is a set of rules that govern data communicated, how it communi communi cated and when it is communi communi cated. The key elements o protocol are syntax, semantics and timing. P.ANANTH [M.E, MCSE, CCNA], CCNA], Lecturer CSE Page 1

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(M


UNIT - I


Describe network architecture in detail? detail ? architect ure of computer computer network netw ork with Discuss about the architecture with sketches ? With a neat block diagram explain the layered architecture ? Write short notes on network model ?

(Nov (No v/Dec ’11 – 8Mark) 8Mark) (Nov/Dec’12

8Mark) – 8Mark)

(Nov/Dec ’11 – 16Mark) 16Mark) (May/June ’12 – 4Mark) 4Mark)

Network Architecture rchit ecture veffective , s fair sand s robust  A scomputer s network vmust vprovide sgeneral, s cost veffective connectivity among a large number of computers. Designing a network to meet these requirements is no small task.  To deal with this complexity, network designers have developed general blue prints desig n and and i mplementation mplementation usually called c alled network architectures . It guides the design – usually of networks . mos t widely widely refere ref erenced nced architectures, architec tures,  The most - OSI architecture. architect ure. - Inter Internet net architecture. architec ture. Layering and Protocols

servic es provided at the high lay l ayers ers are are i mplemented i n terms of the services The services provided by the low layers layers.

Host-to-host Host-to-host connecti connectivity vity – abstracting abstracting away away the fact that there may may be an arbitrarily arbitraril y complex co mplex network topology between any two hosts. Process-to-process channels – abtracting abtracting away away the fact that the network network occaionally occ aionally loe mesages. features . Layering provides two features  Fir t, sit sdecomposes buii l ding va va vnetwork vinto vmore vmanageab vmanageabll e sdecomposes sthe sproblem sproblem sof s bu components.  Second, it provides a more modular design.

P.ANANTH [M.E, MCSE, CCNA], CCNA], Lecturer CSE Page 2



UNIT - I


The abstract objects that make up the layers of a network system are called protocols. ie) sa sprotocol sproto col sprovides sa scommun sco mmunicatio ication n service service sthat shigher s – slevel slevel sobjects ( such as application application processes proce sses,, or perhaps perhaps higher higher – level level protoc pro tocols ols ) use to exchange exchange messages. different i nterfaces nterfaces. Each Each protocol defines two different Service interfa i nterface ce – defines defines the operation that local object can perform on the protocol. Peer interface interface – form form and and meaning meaning of mesage exchanged exchanged between between protocol proto col peer peer to implement the communication service.

Protocol Hierarchy

     

rul es that that govern g overn network communi communi cation catio n. Protocols Prot ocols are the the set of rul Layer ‘ n’ on one node carries on a conversation with layer ‘ n’ on other node . entiti es comparisi comparising ng the correspond corre spondii ng layers layers on o n diff di fferent erent machine are called The entities peers . ayer and then from physi physical cal The actual data flow is from upper layer to its below l ayer medium to desti nati nation on l ayer. ayer. Betw een each pai pai r of adjacent adjacent l ayers ayers is call ed i nterface nterface . The i nterface nterface defines which primitive operations and services the the lower lower layer layer offers offer s to the upper upper one. l ayers ers and protocol proto col s is cal cal l ed network archit architecture ecture . A set of lay




UNIT - I


ISO ISO /OSI Reference Reference Model

 The Inter Internatio national nal Standard Standard Organiz Organi zation ati on (ISO) (ISO) was one o ne of the fir st organizations organizations to formally define a common way to connect computers. Their architecture, called the Open Systems Interconnection (OSI) architecture.  OSI model is seven lay l ayer er standa s tandard. rd. Principles in defining OSI Layers , ayer should be created where a different different abstracti abstracti on is needed.  A l ayer -defined ined functi function on.  Each layer should perform a wel l -def  The s function vof veach vlayer vshould sbe s chosen vwith van veye vtoward vdefining i nternati nternati onal onal l y standardiz standardized ed protocols. ayer bounda boundari ri es should be chosen to mini mize the the inf i nformatio ormation n fl fl ow across the  The l ayer interfaces.

1. Physical Layer

 The physical layer coordinates the functions required to carry a bit stream over a physical medium. el ectrical, al, mechanical mechanical,, procedural procedural and and  The design issue of physical layer factors are electric functio functiona nall attributes. nterface vand  It sdeals swith sthe s mechanical sand s electrical sspecifications sof sthe s i nterface transmissi on medium.  It also defines the procedures and functions that physical devices and interfaces have have to perform for transmission to occur.




UNIT - I


Fun ctions ctio ns of physical layer 1.

Physical characteristics of interfaces and medium

and the The physical layer defines the characteristics of the interface between the devi ces and transmissi on medium. It also defines the type type of o f transmissi on medium. 2.

Representation of bits

data must be encoded into electric el ectrical al or optical optical Physical layer encodes the bit stream data signal . 3.

Data rate

Transmission rate-the number of bits sent each second is defined defined by the t he physical physical layer. 4.

5.

6.

7.

Synchroniza Synchroniza tion tio n of bits

Transmis ransmissi sion on rate and recei ving rate rate must must be same . This i done by synchronizing clocks at sender and receiver. Line configuration connecti on of devices to the media. Various line The physical layer is concerned with the connection configurations are point-to-point or multipoint. Physical topology make a network . Various The physical topology defines how devices are connected to make topologies are mesh, star, ring, bus, or hybrid topology. Transmission mode The sphysical layer salso sdefines sthe direction of vtransmission sbetween sbetween tw t wo device devices: s: simplex, si mplex, hal hal f-d f-dup upll ex, or full-duplex.

2. Data Link Layer

frames.  It groups raw bits (zeros (zero s and and ones) o nes) i nto frames. frames from one node to another another node.  The data link layer responsible for transmitti ng frames transmi ssion facil facility ity , to a reliable  The data link layer transforms the physical layer, a raw transmi link .  It makes the physical layer appear error-free to the upper layer .



CS 2363 / CS 65 – COMPUTER NETWORKS

UNIT - I

Electrical and Electronics Engineeri ng

Fun ctions of data link layer: 1.

Framing

The frame received from network layer is divided into manageable data units called frames. 2.

Physical addressing

If frames are to be send to different systems on the network , the data link layer adds a header to the frame to define the sender and/or receiver of the frame. 3.

4.

5.

Flow control

When the rate of the data transmitted and rate of data received by the receiver i not same, some data may be lost. The data link layer impoes a flow control mechanism to avoid overwhelming the receiver. Error control The data link layer uses this mechanism to detect and retransmit damaged or lost frames . Error control is normally achieved through a trailer added to the end of the frame (Frame Check Sequence) . Access control When two or more devices are connected to the same link , data link layer protocols are necessary to determine which device has control over the link at any given time.

3. Network Layer

 The network layer is responsible for the delivery of packets from source to destination, posibly across multiple networks (links).  The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Fun ctions of network layer: 1.

Logical addressing

The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination. The network layer adds a header to the packet coming from the upper layer includes the logical addresses of the sender and receiver. 2.

Routing

Network layer route or switch the packets to their final destination. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 6

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UNIT - I


4. Transport Layer

 Transport layer accept the data from upper layer and split i t up i nto small er units, then send to network layer, and ensure that the piece all arrive correctly at the other end.  The transport layer is responsible for delivery of the entire message from one process to another process.  It treats each one i ndependently , as though each piece belonged to a separate message .  The transport layer ensures that the whole message arrives intact and in order with error control and flow control at the source-to-destination level.

Fun ctions of transport layer: 1.

Service-point addressing (Port addressing)

Computer performs several operations at the same time. Source-to-destination delivery means deli very not only from one computer to the next but al so from a specific process (running program) on one computer to a specific process (running program) on the other . The transport layer header must therefore include a type of address called a service-point address (or port addres). 2.

Segmentation and reassembly

A message is divided into transmittable segments , with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving sat sthe s destination sand sto s identify vand vreplace vpackets sthat swere s lost vin transmission. 3.

Connection control

The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the tranport layer at the destination machine . A connection ori ented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated. 4.

Flow control

Transport layer performs end to end flow control while data link layer performs it acros the link. 5.

Error control

Error control at thi layer is performed process-to-process rather than acros a ingle link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, l oss, or duplication). Error correction is uually achieved through retransmission. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 7



UNIT - I


5. Session Layer

 It allows different machines to establish sessi ons between them.  The session layer is the network dialog controller.  It establi shes, maintains, and synchronizes the interaction between communicating systems.

Fun ctions of Session layer: 1.

Dialog control

The session layer allows two systems to enter into a dialog. Communication between two processes to take place in either half duplex or full-duple mode. 2.

Synchronization

The session layer allows a process to add checkpoints , or synchronization points , to a stream of data. 6. Presentation Layer

 The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems .  The presentation layer is responsible for translation, compression , and encryption.

Fun ctions of presentation layer: 1.

Translation

The information must be changed to bit streams before being transmitted. Because different computers use di fferent encoding systems , the presentation l ayer is responsibl e for i nteroperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the recei ving machine changes the common format into its recei ver-dependent format. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 8


CS 2363 / CS 65 – COMPUTER NETWORKS 2.

UNIT - I


Encryption

To carry sensitive information , a system must be able to ensure privacy. Encryption means that the sender transforms the ori gi nal information to another form and sends the resulting mesage out over the network. Decryption reverses the original process to transform the message back to its original form. 3.

Compression

Data compresion reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio , and s ideo. 7. Application Layer

 The application layer enables the user to access the network .  It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.  The application layer is responsible for providing services to the user .

Fun ctions of Application layer: 1.

Network virtual terminal

A network virtual terminal is a software version of a physical terminal , and it allows a user to log on to a remote host . The remote host believes it is communicating with one of its own terminals and allows the user to log on. 2.

File Transfer, Access and Management (FTAM)

This application allows a user to access fil es in a remote host (to make changes or read data), to retri eve files from a remote computer for use in the local computer , and to manage or control fil es in a remote computer locally. 3.

Mail services

This application provides the basis for e-mail forwarding and storage. (X.400) 4.

Directory services

Directory services include access for global information and distributed database.

P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 9

(X.500)



UNIT - I


Summary of Layers

Name the seven layers of the OSI model? What is the main function of a Physical Layer? What is the prime function of a Physical Layer? Define Packet? State main function of transport layer? Give two services offered by sesion layer?


(Apr/May’05 – 2mark) (May/Jun’11-2Mark (Nov/Dec’04-2Mar (Nov/Dec’11 – 2Mark) (Apr/May’05 – 2Mark) (Nov/Dec ’04 – 2Mar



UNIT - I


TCP/IP reference model

 TCP/IP stands for Transmission Control Protocol / Internet Protocol .  TCP/IP reference model is a set of protocol s that allow communication across multi ple networks. (Figure: TCP/IP reference model)

(Figure : Protocols and networks in the TCP/IP model initially)

Host-to-Network Layer

 This layer is same as physical and data link layer of OSI model.  This layer also called network interface l ayer .  Host to Network layer cannot define any protocol. Internet Layer

 Internet layer is to permit hosts to send packets i nto any network and have them travel independently to the destination (potentially on a different network).  They may even arrive in a different order than they were sent , in which case it is the job of higher layers to rearrange them , if in-order delivery is desired.  The internet layer defines an official packet format and protocol called IP (Internet Protocol).  The job of the internet layer is to deli ver IP packets where they are supposed to go . Packet routing is clearly the major isue here, as is avoi ding congestion. Transport Layer

 Transport layer is designed to all ow peer entities on the source and destination hosts to carry on a conversation.  Two end-to-end transport protocols are TCP and UDP. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 11



UNIT - I


 TCP (Transmission Control Protocol ) is a reli able connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It fragments the incoming byte stream into discrete messages and passes each one on to the internet layer . At the destination, the receiving TCP process reassembles the recei ved messages into the output stream. TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver with more mesages than it can handle.  UDP (User Datagram Protocol) i an unreliable, connectionless protocol for applications that do not want TCP's sequencing or flow control and wish to provide their own. It is also widely used for one-shot, client-server-type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting peech or video. Application Layer

 TCP/IP model does not have sessi on or presentation layers.  It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), and electronic mail (SMTP).  The virtual terminal protocol ( TELNET) allows a user on one machine to l og onto a distant machine and work there.  The file transfer protocol ( FTP) provides a way to move data effici ently from one machine to another.  Electronic mail wa originally jut a kind of file transfer , but later a pecialized protocol ( SMTP ) was developed for it.  Many other protocols have been added to these over the years: the Domain Name System (DNS) for mapping host names onto their network addresses , NNTP , the protocol for moving USENET news arti cl es around, and HTTP , the protocol for fetching pages on the World Wide Web, and many others. Perform a comparative study between the ISO/OSI model and TCP/IP reference model? (May/June’12 – 16Mark) Compare and contrast ISO/OSI and TCP/IP reference models? (Nov/Dec’09 – 8Mark) Perform a comparative study between the ISO OSI model and the TCP/IP reference model? (May/June’07 – 12Mark) Compare ISO-OSI model and TCP/IP reference model? (May/June’10 – 2Mark )

Comparison of ISO/OSI and TCP/IP reference model  TCP/IP tands for Transmission Control Protocol / Internet Protocol .  Layers in the TCP/IP protocol uite do not exactly match thoe in the OSI model.  TCP/IP protocol suite was defined as having four l ayers : They are,  Host-to-Network, Internet, Transport, Application  When TCP/IP is compared to OSI,  The Host-to-Network layer is equivalent to the combination of the physical and data link layers .  The Internet layer is equivalent to the network layer .  The application layer in TCP/IP is equivalent to the session, presentation, application layer of the OSI reference model. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 12



UNIT - I


 The first four layers provide physical standards, network i nterfaces, internetworking, and transport functions that correspond to the first four layers of the OSI model.  The three topmost layers in the OSI model , however, are represented in TCP/IP by a singl e layer called the appli cation layer .  The OSI model specifies which functions belong to each of its layers , the layers of the TCP/IP protocol uite contain relatively independent protocols that can be mixed and matched depending on the needs of the system.  At the transport layer , TCP/IP defines three protocol s : Transmission Control Protocol (TCP) , User Datagram Protocol (UDP) , and Stream Control Transmission Protocol (SCTP).  At the network layer , the main protocol defined by TCP/IP is the Internetworking Protocol (IP). 1. Host to Network (Physical and Data Link Layers)

 This layer is same as physical and data link layer of OSI model.  This layer also called network interface l ayer .  Host to Network layer cannot define any protocol. 2. Internet layer (Network Layer)

 At the network layer , TCP/IP supports the Internetworking Protocol .  IP, in turn, use four supporting protocols: ARP, RARP, ICMP, IGMP. o 2.1. Internetworking Protocol (IP)  The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols.  It is an unreliable and connectionless protocol -a best-effort deli very service.  IP transports data in packets called datagrams, each of which i tranported separately.  IP provide the following services, a. Addressing Determine the Route to deliver data to the detination hot. b. Fragmentation Breaking the mesages into pieces if an intervening network cannot

handle a large message. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 13



UNIT - I

Electrical and Electronics Engineering

2.2 Addres Resolution Protocol  The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address .  In a LAN each device on a link is identified by a physical or station address , usually imprinted on the network i nterface card (NIC) .  ARP is used to find the physical address of the node when its Internet address is known. 2.3. Reverse Addres Resolution Protocol:  The Reverse Address Resolution Protocol (RARP) allows a host to discover its Internet address when i t knows only i ts physical address. 2.4. Internet Control Mesage Protocol:  The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender.  ICMP sends query and error reporti ng messages. 2.5. Internet Group Management Protocol:  The Internet Group Management Protocol (IGMP) is used to facilitate the simultaneous transmissi on of a message to a group of recipients. 3. Transport Layer

 The transport layer was represented in TCP/IP by two protocols: TCP , UDP, SCTP o  IP is a host-to-host protocol , meaning that it can deli ver a packet from one physical device to another.  UDP and TCP are tranport level protocol responsible for delivery of a message from a process (running program) to another process.  SCTP has some newer applications. 3.1. User Datagram Protocol:  UDP is a connection ori ented protocol (unreliable)  It is a proces-to-proces protocol that adds only port addresses , checksum error control , and length i nformation to the data from the upper layer. 3.2. Transmission Control Protocol:

 TCP provides transport-layer services to applications.  TCP is a connection ori ented protocol ( reliable) transport protocol.  Connection-oriented means: A connection must be established between both ends of a transmission before either o can transmit data. At the sending end of each transmision, TCP divides a stream of data into smaller o nits called segments. Each segment includes a sequence number for reordering after receipt, together with o an acknowledgment number for the segments received. At the receiving end, TCP collects each datagram as it comes in and reorders the o ransmission based on sequence numbers . P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 14



UNIT - I


3.3. Stream Control Transmission Protocol:  The Stream Control Transmission Protocol (SCTP) provides support for newer applications uch as s oi ce over the Internet .  It is a transport layer protocol that combines the best features of UDP and TCP. 4. Application Layer

 The application layer in TCP/IP is equivalent to the combined sessi on, presentation, and application layers in the OSI model Many protocols are defined at this layer. ( TELNET, SMTP, FTP, DNS) Explain the various factors contributing to the network performance ?

(Apr/May’11

– 8Mark)

Network Performance

 Network performance is measured in two ways : bandwidth (also called throughput) and latency (also called delay).  Bandwidth of a network is given by the number of bits that can be transmitted over the network in a certain period of time.  Latency correspond to how long it takes a message to travel from o ne end of a network to the other. Latency i measured strictly in terms of time . It’ alo called Transit time.  Latency =Propagation + Transmit + Queue  Propagation = Distance/Speed of Light  Transmit = Size/Bandwidth  Distance is the length of the wi re, Speed of Light is the effecti ve speed of l ight over that wire , Size is the size of the packet , and Bandwidth is the bandwidth at which the packet is transmitted. Delay × Bandwi dth Product

 The latency corresponds to the length of the pipe and the bandwidth gives the diameter of the pipe, then the delay × bandwidth product gives the volume of the pipe the number of bits it holds.  The delay × bandwidth product is important to know when constructing high performance networks because it corresponds to how many bits the sender must transmit before the first bit arrives at the receiver.

Data Representation

Information (Data) represented in different form such as text, numbers, images, audio, and video.




UNIT - I


Data Flow

Communication between two devices can be simplex, half-duplex, or full-duplex. 

Simplex o

o

o



One device can transmit data and the other can only receive. Keyboards and monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output . The mplex mode can use the entire capacity of the channel to send data in one direction.

Half-Duplex o

o o o



In implex smode sof scommunication, s data vcan vflow vin vone vdirecti on sonl ( nidirectional).

In half-duplex mode of communication each station can both transmit and receive, but not at the same time.

When one device i s sending , the other can only recei ve, and vice versa. The entire capacity of a channel is used by any device transmitting at the time. Walkie-talkies are the example for half-duplex systems.

Full-Duplex o

o o

In full-duplex mode both stations can transmit and recei ve simultaneously.

The capacity of the channel is divided between the two directions. Example is telephone communication system, when two people are communicating by a telephone line, both can talk and listen at the same time.




UNIT - I


Di tinguish between point-to-point links and multi-point links with relevant diagram? s (May/June’12 – 16Mark) Network Classification Computer transmit data by two methods,  Broadcast Network.  Point – to – Point Network.

Broadcast networks: Broadcast network uses single communication channel that is shared by many stations. The data to be transmitted is converted in small packets form. Each packet contains address field of the destination station. It is also posible to send same packets to all stations within a network, it is called as broadcasting. When data packets are sent to a specific group of stations it is called as multicasting. In multicasting data is sent to selec ted group of statio ns multicasting data is sent to selected group of stations multicasting is a selective proces.

Point-to-point networks: Point-to-point networks provide a dedicated link between in any two stations. The data packets are sent from source station to the destination station. Such a transmission is called unicasting.

In point-to-point links, however, it is often the case that two bit streams can be simultaneously transmitted over the link at the same time, one going in each direction. Such a link is said to be full-duplex. A point-to-point link that supports data flowing in only one direction at a time — such a link is called half-duplex — requires that the two nodes connected to the link alternate using it.



CS 2363 / CS 65 – COMPUTER NETWORKS Write the categories of networks ? s What are LAN?

UNIT - I

Electrical and Electronics Engineeri ng (Apr/May’11

– 2Mark)

(Apr/May’05 – 2mar

Categories of Networks Local Area Network (LAN) Wide Area Network (WAN) Metropolitan Area Network (MAN) LAN

 A LAN connects network devices over a rel atively short distance . It interconnects computers in a li mited area such a a home, school , or a computer laboratory.  LANs are capable of transmitting data at very fast rates , much faster than data can be transmitted over a telephone line; but the distances are l imi ted, and there is also a limit on the number of computers that can be attached to a single LAN.  However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves.  Printers, hard disks, programs and others computers can be shared with the help of LAN.

WAN

 A wide area network (WAN) is a large telecommunications network that consists of a col lection of LANs and other networks.  A WAN spans a large geographic area, such as a state, province or country. WANs often connect multi ple small er networks, uch a local area networks (LANs)  The world's most popular WAN is the Internet .  A network device called a router connects LANs to a WAN.  A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership.




UNIT - I


MAN

 A network spanning a physical area larger than a LAN but small er than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation.  A MAN often acts as a high speed network to allow sharing of regional resources (similar to a large LAN). It is also frequently used to provide a shared connection to other networks using a li nk to a WAN.  Examples of metropolitan area networks of various sizes can be found in the metropolitan areas of London, England; Lodz, Poland; and Geneva, Switzerland. Large universities also ometimes use the term to describe their networks. A recent trend is the installation of wireless MANs.




UNIT - I

Electrical and Electronics Engineering

Di cuss the four basic network topologies and give the advantages and disadvantages of each type? (Apr/May’11 – 8Mark) List out the four basic topologies ? s (Apr/May’11 – 2Mark) What are the different network topologies to organize computer networks? (Nov/Dec’04 – 2Mark)

Network Topologies  The sterm s physical vtopology refers sto sthe s way vin vwhich va vnetwork vis vphysically interconnected.  Two or more devices connect to a link; two o r more links form a topology.  The network topology is the geometric representation of the relationship of all the links (medium) and li nking devices (nodes) to one another.  There are four basic topologi es possible: Mesh, Star, Bus, and Ring.

1. Mesh Topology

 In a mesh topology, every device has a dedicated poi nt-to-point li nk to every other device.  The term dedicated means that the link carries traffic only between the two devices it connects.  To find the number of physical l inks in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node.  Node 1 must be connected to n - 1 nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n - 1 nodes.  If each physical l ink allows communication in both directions (duplex mode), we can divide the number of links by 2 . In other words, we can say that in a meh topology, we need n(n -1) /2 duplex-mode links.

Advantages:

1. Dedicated links - Each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. 2. Robust - If one link becomes unusable, it does not affect the entire network. 3. Privacy or security - When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining acces to messages. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 20



UNIT - I


4. Easy fault identification and fault isolation - Point-to-point links between the nodes, so fault identification and fault isolation easy. Disadvantages: 1. Difficulty in installation and reconfiguration - Every device must be connected to every

other device so installation and reconnection are difficult. 2. More Expensive - The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. 2. Star Topology

 In a star topology, each device has a dedicated poi nt-to-point li nk only to a central controller , usually called a hub.  The devices are not directly linked to one another.  In star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another device, it ends the data to the controller, which then relays the data to the other connected device.

Advantages :

1. 2. 3. 4.

5.

A star topology is less expensive than a mesh topology. Easy to install and reconfigure - Each device needs only one link and one I/O port to connect. Robustness - If one link fails, only that link is affected. Easy fault identification and fault isolation – If one link fails that device only does not work in the network all other links are remain active. This is easy fault identification and fault isolation. Additions, moves, and deletions involve only one connection between that device and the hub.

Disadvantages:

1. 2. 3.

If the hub goes down, the whole network is down. Each device requires own cabling segment from a central hub. Cabling more than bus and ring topology but les than mes topology.

3. Bus Topology

 A bus topology, one long cable acts as a backbone to link all the devices in a network.




UNIT - I


 Nodes are connected to the bu cable by drop lines and taps.  A drop line is a connection running between the device and the main cable.  A tap is a connector that splices the main cable to create a contact with the metallic core. Advantages:

1. 2. 3. 4.

Easy to installation. Needs fewer connecti vity devices . Low cost. Bus topology uses less cabling than mesh and star topologies. Disadvantages: 1. Difficult to add new devices . 2. Signal reflection at the taps can cause degradation in quality. 3. Heavy network traffic can slow a performance. 4. Difficult reconnection and fault isolation. 5. In addition, a fault or break i n the bus cable stops all transmission, even between devices on the same side of the problem. . 4. Ring Topology

 In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of i t.  A signal is passed along the ring in one direction, from device to device, until it reaches its destination.  Each device in the ring incorporates a repeater . When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along

Advantages:

1. 2. 3. 4.

Easy to install and reconfigure. Fault i solation i easy. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections . .

Disadvantages:

1. Unidirectional traffic. 2. A break in the ring can be affect entire network . 3. Adding or removing the node disrupts the network .




UNIT - I


5. Hybrid Topology

 H brid network means combination of any two topologies..  For example, we can have a main star topology with each branch connecting several tations in a bus topology

Assume 5 devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device? (May/June’

Total Nodes=5 Total cables=10 Total Ports needed per device=4




UNIT - I


Direct Link Networks

Explain different approaches of encoding in detail? Explain the unipolar and polar encoding schemes with suitable example? Define Encoding?

(Nov/Dec’12

– 8Mark)

(Apr/May ’11 – 6Mark)

(Nov/Dec’11

– 2Mark)

Encoding

Encoding is the proces of putting a sequence characters (letters, numbers, punctuation and certain ymbols) into a specialized format for efficient transmission or storage. Line Coding:

 Line coding is proces of converting bit stream into digital signal . Ex: Bit stream 100110101

Line Coding Schemes:

 The line coding schemes can be categorized into three types i.e. - unipolar, polar and bipolar.

1. Unipolar:

 Unipolar encoding uses only one voltage level . 1's are encoded as positive value and 0's are encoded as zero value.




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2. Polar:  Polar encoding uses two voltage levels i.e. positive and negative . There are four types o

commonly used polar encoding schemes i.e. NRZ, RZ, Manchester, Differential Manchester.

2.a. Non- Return to Zero (NRZ):  In NRZ encoding, binary 1 is transmitted by sending positive voltage level , and a 0 is transmitted by sending a 0 voltage . In NRZ encoding, voltage level stay constant during the time a bit is transmitted. This method is simple to implement.  In NRZ-L (NRZ- Level) encoding the level of signal depends on the type of bit that it represents. In (NRZ- Invert), the signal is inverted if a '1' is encountered. 2.a.i) NRZ-L:  It is Non-Return to Zero Level. In this the level of the signal is dependent upon the state of the bit. In NRZ-L coding, binary 0 bit is represented by positive voltage and bit 1 is represented by negative voltage leve l . For a given 8 bit data tream shown 00110101 , the voltage say at the level positive for the first 2 bits, then goes to the negative level for the next 2 bits, then backup to

positive(+ive), then down to negative(- ive), again down to positive and finally up to negative voltage.

 If the data bit stream contain continues 0's or 1's then receiver receives a continuous constant oltage . It i difficult to identify the total number of bits transmitted by the s ender . Clock ynchronization is required to identify the total number of bits to be transmitted.




UNIT - I


2.a.ii) NRZ-I:  It is non-return to zero. In NRZ-I, the signal is inverted if a 1 is encountered . The amplitude is

also changes between positive and negative voltage. The states is changes only for 1 and for bit 0, no s tate change . It remains in same state.  The figure shows the 8 bit data stream 00110101 . It is also called non return to zero.

2.a.iii) NRZ-S:  It is non return to zero-space. In NRZ-S, the signal is inverted if a 0 is encountered. The states is change only for 0 , for bit 1, no state change . Figure show the 8 bit data stream 00110101.

2.b.Return to Zero (RZ)  In RZ, the data pulses that represent binary 1 return back to the binary 0 level in the middle of the time period allowed for the bit. This means that for eve ry 1, there is a signal transition, which helps the receiver develop synchronization to the incoming data stream. RZ signal requires two ignal changes to encode one bit and therefore occupies more bandwidth.  Return to Zero (RZ) use three values i.e. positive negative and zero. The RZ encoding

provides synchronization information . P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 26



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 Figure shows the encoding waveform for 8 bit data stream 11000101 .

2.c. Biphase: 2.c.i)Manchester Encoding  In this, each bit period has both the high and low voltage values . If the data is a 1, the first half o the bit time period is sent at the positive level, and the second half of the period is at the negative level. For data bit of 0, first a negative signal and then a positive signal, There is a transition which can be used for synchronization. Sometimes this method is called self clocking method. It requires more bandwidth than NRZ.  Manchester Encoding use an inverion at the middle of each bit interval for synchronization and bit representation.  The manchester encoding waveform for the 8 bit data stream 11000101




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2.c.ii)Differential Manchester  In differentia l manchester encoding, a binary 0 is marked by a transition at the beginning of an interval, whereas a 1 is marked by the absence of a transition, In this encoding method, detecting changes is often more reliable, especially when there is a noise in the channel.  Differential Manchester use the inverion at middle of bit internal for synchronization purpose. The bit representation is defined by the inversion or non inversion at the beginning of the bit .  The differentia l manchester encoding for 8 bit data stream 101011100 .

` 3. Bipolar:  Bipolar encoding uses three levels i.e. positive, zero and negative. Zero level represents binary '0'and alternating positive and negative voltages represents binary '1'. An example of bipolar encoding is Alternate Mark Inversion (AMI).



CS 2363 / CS 65 – COMPUTER NETWORKS Define Framing?

UNIT - I

Electrical and Electronics Engineeri ng (May/June’12

– 2Mark)

Framing

Encapsulates datagram into frame, adding header and trailer. Physical address used in frame headers to identify source and destination. 1. Byte-Oriented Protocol (PPP)

Flag (1Byte)It identifies starting and ending of the frame. Address (1Byte)  sIndicates s Destination vAddress . sIf sit scontains s all v1 ’s s(11111111) indicates all stations are accept the frame. Control (1Byte)PPP normally runs in connectionless mode so this field set to 11000000. It indicates unnumbered frames , it doesnot contain sequence numbers and no flow control, error control. Protocol (2Byte)It defines the information of data field. Payload (Variable)  It contains actual data to transfer. Checksum (FCS) It checks all fiel ds in frame. (Frame Check Sequence) 2. Bit-Oriented Protocol (HDLC)

 High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It implements the ARQ mechanisms.  Configurations and Transfer Modes HDLC provides two common transfer modes that can be used in different configurations: Normal response mode (NRM) o Asynchronous balanced mode (ABM). o Normal Response Mode:   

In normal response mode (NRM), the station configuration is unbalanced. We have one primary station and multiple secondary stations. A primary vtation can end commands; a secondary station can only respond. The NRM is used for both point-to-point and multi ple-point li nks,




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Asynchronous Balanced Mode:  

In asynchronous balanced mode (ABM), the configuration is balanced. The link is point-to-point, and each station can function as a primary and a secondary (acting as peers).

Frames: 

  



HDLC defines three types of frames: Information frames (I-frames) o Supervisory frames (S-frames) o Unnumbered frames (U-frames). o Each type of frame serves as an envelope for the transmission of a different type of message. I-frames are ued to transport user data and control i nformation relating to user data (piggybacking). S-frames are ued only to transport control i nformation, primarily data link layer response for flow and error controls. U-frames are reservation for system management . It is intended for managing the link itself.

Frame Format: 



Each frame in HDLC may contain up to six fields, a beginning flag field, an addres field, a control field, an information field, a frame check sequence (FCS) field, and an ending flag field. In multiple-frame transmissions, the ending flag of one frame can serve as the beginning flag of the next frame.



CS 2363 / CS 65 – COMPUTER NETWORKS 









UNIT - I


Fl ag fiel d. The flag field of an HDLC frame is an 8-bit sequence with the bit pattern 01111110 that o identifies both the beginning and the end of a frame and serves as a synchronization pattern for the receiver. Address field. The second field of an HDLC frame contains the address of the secondary station. If a o primary station created the frame, it contains a to address . If a secondary creates the frame, it contains a from address . o Control fiel d. The control field is a 1 or 2-byte segment of the frame used for flow and error control . o The interpretation of bits in this field depends on the frame type. o Information field. The information field contains the user's data from the network layer or management o information. Its length can s ary from one network to another. FCS field. The frame check sequence (FCS) is the HDLC error detection fiel d. It can contain o either a 2- or 4-byte ITU-T CRC.

A network with bandwidth of 10Mbps can pas only an average of 12,000 frames per minute with each frame carrying an average of 10,000 bits. What is the throughput of this network? (Apr/May’11 – 2Mark) Throughput= (12000 * 10000) / 60 = 120000000 / 60 = 2000000 = 2 Mbps (It is 1/5 of bandwidth)




UNIT - I

Electrical and Electronics Engineeri ng (Nov/Dec’11

What is error detection? Explain with example?

Error Detection

Process of checking the data unit for the presence of any error. It is simpler than error correction and it is the first step in error correc tion process. Redundancy:

Error detection uses the concept of redundancy, which means adding error bits for detecting errors at the destination. 4 types of redundancy checks,  VRC  LRC  CRC  Checksum VRC:

It’s a parity check. If the total number of 1 ’s become even called even parity check. Similarly total number of 1 ’s becomes odd called odd parity check. ie) even no  0 odd no 1 1110111 1101111 1110010 11 1100 1100100 Ws os rs l d Ans:

s11101110

11011110

11100100

11 1100

1100100

The receiver counts the 1 ’s and (6, 6, 4, 4, 4) the data would be accepted. If any error occur, the receiver knows that the data are LRC (Two-Dimensional parity):

LRC is also called 2D parity check. In this method, a block of bits is organized in a table (rows and columns). We calculate the parity bit for each row and create a new parity column. sWe sthen scalculate sthe sparity sfor seach scolumn sand sadd sa snew sparity srow. In LRC, a block of bits is divided into rows and a redundant row of bits added to the whole block. Data units: 1100111 1011101 0111001

Now the data units are transmitted in row wise manner including the parity bits. 11001111 10111011 01110010 00000110



– 8Mark)


UNIT - I


Internet checksum Algorithm:

In the internet checksum just add up all the words that are transmitted and then transmit the result of that sum. The result is called the checksum. The receiver perform the same calculation on the received data and compares the result with the received checksum. If any transmitted data, including the c hecksum itself, is corrupted, then the results will not match, so the receiver knows that an error occurred. Before transmitting the data the sender follows the step,  The unit is divided into ‘K ’ sections, each of ‘n’ bits.  All sections are added together using 1 ’s complement.  The sum is complemented and becomes the checksum.  The checksum is sent along with the data. After receiving the data,  The unit is divided into k sections, each of ‘n’ bits.  All sections are added together using 1 ’s complement to get the sum  The sum is completed. If the result is ‘0 ’, the data is accepted or rejected. Eg) 16bit 1 0 1 0 1 0 0 1 0 0 1 1 1 0 0 1

11100010 0 0 0 1 1 1 0 1  1 ’s complement. The pattern sent is 10101001 00111001 00011101. Cyclic Redundancy check: (CRC):

In this method, a sequence of redundant bits called CRC. Is appended to the end of the data unit. So that the resulting data unit becomes exactly divisible by a predetermined binary number. At the receiver side the incoming data unit is divided by the same number and if there is no reminder and the data unit is assumed to be error free and therefore accepted, A reminder indicates that the data unit has been damaged in transit and therefore must be rejected. The mesage is represented by the polynomial by using the value of each bit in the mesage as the co-efficient for each term in the polynomial, starting with the most significant bit to represent the highest order term. Eg) 8-bit message bit 10011010 – corresponds to the polynomial, 7 6 5 4 3 2 1 0 M(x) = 1*xs+0*xs+0*xs+1*xs+1*xs+0 *xs+1*xs+0*xs For the purpose of calculating a CRC, a sender and receiver have to agree on a divisor polynomial C(x). C(x) is a polynomial of degree ‘k ’. 3 2 Eg) C(x) = xs+xs+1 Where k = 3 When a sender wishes to transmit a message M(x) that is n+1 bits long, it sent (n+1) mesage plus k bits. We call the complete transmitted message including the redundant bits P(x). P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 33



UNIT - I


If M(x) is exactly divisible by C(x) k  Multiply M(x) by xs, ie, add k zeros at the end of the mesage.  Divide T(x) by C(x) and find the reminder.  Subtract the reminder from T(x). 7 4 1 Consider the message xs+xs+xs+xs or 10011010 Figure: CRC calculation using polynomial long divisor

Figure CRC calculation using shift register

If the bit in the shift register are labeled 0 through k-1, left to right then put an XOR n gate in front of bit n of there is a term xs in the generates polynomial. Table: Common CRC polynomials CRC C(x) 8 2 1 CRC-8 xs+xs+xs+1 10 9 5 4 1 CRC-10 xs+xs+xs+xs+xs+1 12 11 3 CRC-12 xs+xs+xs+xs+1 16 15 2 CRC-16 xs+xs+ xs+1 The recipient divides the received polynomial by e(x) and if the result is 0, concludes that there were no errors. If the result is non-zero, it may be necesary to discard the corrupted mesage and code that enables error correction is called an error correcting code (ECC)




UNIT - I


ie)

Stop – and – Wait ARQ

The sender sends one frame and waits for an acknowledgement before sending the next frame. Figure: Stop and wait

Advantage: Stop – and wait is simplicity is sent. Di advantage: Inefficiency

– Stop

– each

frame is checked and acknowledged before the next frame

and wait is slow.



CS 2363 / CS 65 – COMPUTER NETWORKS Sliding

UNIT - I


Window Protocols

The sender can transmit several frames before needing an acknowledgement. Sliding window introduces an identification scheme based on the size of the window. The frames are numbered modulo-n, which means they are numbered from 0 to n-1. Eg) if n+8, the frames are numbered 0,1,2,3,4,5,6,7,0,1,2,3,4,5,6,7,0,1………. When the receiver sends an ACK, it includes the number of the next frame it expects to receive.

Figure: Sliding Window Sender Window: At the beginning of a transmission, the sender ’s window contains n-1 frames. As frames are ent out, the left boundary of the window moves inward shrinking the size of the window. ie) The sender sliding window shrinks from the left when frames of data are sent. The sliding window of the sender expands to the right when acknowledgements are received.

Receiver window: At the beginning of transmission the receiver window contains not n-1 frames but n-1 spaces for frames. As new frames come in, the size of the receiver window shrinks.

The sliding window of the receiver shrinks from the left when frames of data are received. The liding window of the receiver expands to the right when acknowledgements are sent.



CS 2363 / CS 65 – COMPUTER NETWORKS What is Ethernet? Explain in detail? Explain the IEEE 802.3 standard? s

UNIT - I

Electrical and Electronics Engineeri ng (Nov/Dec’11 (Nov/Dec’12

– 8Mark) – 8Mark)

Ethernet  It is the name of a packet – switching LAN technology which was standardized in 1978.  IEEE 802.3 has two categories.  Baseband – base – digital signal.  Broadband – broad – analog signal. Baseband & Broadband:  Category of baseband  10 base 5  10 base 2  10 base-T  10 base-F  Category of broadband  10 base 36 Here, First number  data rate in Mbps. Last number  maximum cable length  type of cable. Letter

Ethernet uses co-axial cable as transport medium. Carrier sense Multiple Access with collision detection (CSMA/CD):  If multiple users have acces to a single line, then signals may overlap, collision occurs. To overcome this problem, we use CSMA/CD mechanism, which determines, whether the line is clear or not, if it is clear, then transmission can be started, or it works until the line is free.  Slot time = round-trip time + time required to send the jam sequence. Maximum Network length: Maximum length = propagation speed of the signal * (slot time)/2. Addressing:




UNIT - I


 NIC provides each station with 6-byte physical addres. Eg) 4A:30:10:21:10:1A 6 byte = 48 bits = 12 hex digits. Type of addresses: Unicast address:  Defines one recipient (one to one) t  If the LSB of the 1sbyte is 0, then it is unicast. 0

Eg: 25:02:13:45:62:08 Multicast address:  Defines a group of addreses (one to many). t  Here, the LSB of the 1sbyte is 1. Eg) 01:13:64:73:82:17 nd If 2s hex digit is even Unicast. nd If 2s hex digit is odd Multicast. Broadcast addres s:  Here, all the bits are 1’s (48 1’s). Electrical specification: Signaling:  Baseband systems use Manchester digital encoding.  10 broad 36 uses differential Phase shift keying. Date rate:  Ethernet supports data rates between 1 to 100 Mbps. Ethernet frame format:

Preamble 7 bytes

SFD 1 byte

De tination Addres 6-bytes

Source addres 6-bytes

Length PDU 2-bytes

Frame data 64-1518 bytes

CRC 4-byte

Preamble: It has 7 bytes of alternating 0’s & 1’s. It is added at the physical layer. Start frame delimiter (SFD): It is considered at the beginning of the frame. Everything that follows it is data. Destination address (DA): Here, the 48 bit destination addres is contained in this field. The NIC of the destination compares it with network addres. If it matches, then it accepts (receiver) the frame. Source Address (SA): The NIC of the sender host inserts the network NIC addres (Source addres) of the sender. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 38



UNIT - I


Length/type of PDU (Protocol Data Unit) : Indicate the number of bytes in the PDU. PDU data unit in the upper sup layer of the data link layer. Frame data: Contains actual data of the frame. CRC: This field contains the error detection information. Generations: 4 generations are there, Ethernet evolution  Standard Ethernet Fast Ethernet Gigabit Ethernet Ten-Gigabit Ethernet Standard or traditional Ethernet: It defines the types of cable. Connections & signals: Characteristics 10 Base-5 Medium Thick coaxial Max segment length 500m Topology Bus Encoding Manchester

10 Base-2 Thick coaxial 200m Bus Manchester

10 Base-T Twisted pair 100m Bus Manchester

10 Base-F Fiber optic 2 Bus Manchester

Switched Ethernet: Here, if we use a hub and if a device sends a data, then the hub will direct the frame to all other devices. If we use a switch, then only the destination. Station will receive the frame. Fast Ethernet: Here, the data rate is increased to 100 Mbps, & the collisio n is reduced. Category:




UNIT - I


Features Medium

100Base-Tx Twisted pair(UTP)

100Base-Tx UTP or STP

100Base-Fx Optical fiber multimode

Maximum Segment length

100 m

100 m

2 km

ar

Star

Topology Encoding

tar 4B/5B

4B/5B

8B/6T

Gigabit Ethernet:

Gigabit Ethernet, with a 1-Gbps (1000 Mbps) data rate is used for connecting fast Ethernet networks. Here, the acces method is the same, but the collision is reduced. Full duplex mode: Here, a central switch is connected to all computers or other switches. Half- duplex mode: Switch can be replaced by a hub. Topology:

Gigabit Ethernet is designed to connect two or more stations. Possible topologies: Point to point:




UNIT - I


Star :

a)Two stars:

Implementations:

Feature Medium

1000Base-Sx Optical fiber

1000Base-Tx Optical fiber

1000Base-Cx STP

1000Base-T UTP

Signal

Short-wave laser

Long-wave laser

Electrical

Electrical

Maximum Segment length

550 m

550 m(multimode) 5000m(single mode)

25m

100m

tar

tar

Topology

tar

tar

Ten- Gigabit Ethernet :  It operates at 10 Gbps. Feature 10 G Base-S Media Short wave 850m multimode

Maximum Length

300m

10 G Base-L Long wave 1310-nm Single mode

10 G BaseExtended 1550-nm Single mode

10km

40km

Token bus:

Local area networks have a direct application in factory automation. In this processing real time processing with minimum delay is needed. Token bus combines features of Ethernet & token ring. It combines the physical configuration of Ethernet (a bus topology) & the collision. Feature of token ring. Token bus is a physical bus that operates as a logical ring using tokens.



CS 2363 / CS 65 – COMPUTER NETWORKS What is FDDI? What is FDDI?

UNIT - I

Electrical and Electronics Engineeri ng (Nov/Dec’11 – 2Mark) (Nov/Dec’12 – 2Mark)

FDDI Reference Model

Fiber Distributed Data Interface is a se t of ANSI protocols for sending digital data over fiber optic cable. FDDI networks are token passing networks , and support data rates of up to 100 Mbps . FDDI networks are typically used as backbones for Wide Area Networks (WAN) .

(Nov/Dec ’11 – 16Mark) With neat diagrams explain the circuit switching and packet switching? Explain different switched networks ? (May/June’12 – 16Mark) With sa suitable sdiagram sexplain sbriefly sabout irtual circuit switching, datagram switching techniques? (Apr/May’11 – 10Mark) Write short notes on Datagram network and Virtual circuit network ? (May/June’12 -8Mark)

Switching      



A network is a set of connected devices. Whenever we have multi ple devices , we have the problem of how to connect them to make one-to-one communication possible. One solution is to make a point-to-point connection between each pair of devices (a mesh topology) or between a central device and every other device (a star topology). A better solution i s swi tching. A swi tched network consi sts of a seri es of interlinked nodes , called switches. Switches are devices capable of creating temporary connections between two or more devices l inked to the swi tch.

The end systems (communicating devices ) are labeled A, B, C, D, and so on, and the switches are labeled I, II, III, IV, and V. Each switch is connected to multi ple links.




UNIT - I


1. Circuit-Switched Ne tworks    

  

A circuit-switched network consists of a set of swi tches connected by physical links. A connection between two stations is a dedicated path made of one or more li nks. Each connection uses only one dedicated channel on each link . Each link is normally divided into ‘ n’ channels by using FDM or TDM.

Each link is divided into n ( n is 3 in the figure) channels by using FDM or TDM. Circuit swi tching take place at the physical layer . The resources needed to be reserved during entire data transfer process . The data is transferred as a continuous flow . There is no addressing involved when data transfer takes place.

Three Phases: 

The actual communication in a circuit-switched network requires three phases: a. Connection Setup. b. Data transfer. c. Connection teardown.




UNIT - I


a. Connection Setup Phase

The end systems are connected through dedicated l ines to the switches , so connection setup means creating dedicated channel s between the swi tches.



b. Data Transfer Phase

After the establishment of the dedicated ci rcuit (channel), the end systems are can transfer data.



c. Teardown Phase

When one of system needs to disconnect , a signal is sent to each switch to rel ease the resources.



Efficiency

The circuit-switched networks are not efficient as the other two types of networks because resources are allocated sduring sthe entire duration sof the connection (resources are dedicated). These resources are unavail able to other connections.





Delay   

In circuit-switched network normally has low effici ency, the delay in this type of network is mi nimal. During data transfer the data are not delayed at each switch; the resources are allocated for the duration of the connection. The total delay is due to the time needed to create the connection, transfer data, and disconnect the circuit.

How will you transmit the packets by using datagram approach? Explain in detail with a neat diagram? (Apr/May’10 – 6 What is datagram? (Nov/Dec’04 – 2Mark) 2.Packet-Switched Networks 2.1. Datagram Networks  

 

 

Datagram is an independent transmission unit in packet switching network . In packet switching, there is no resource allocation for a packet. This means that there is no reserved bandwi dth on the li nks , and there is no scheduled processi ng time for each packet . Resources are all ocated on demand. The allocation is done on a first-come first-served basis. When a swi tch recei ves a packet , no matter what is the source or destination, the packet must wait if there are other packets being processed . In a datagram network, each packet is treated i ndependently of all others. Datagram switching is normally done at the network layer .




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In this example, four packets (or datagram’s) belong to the same message , but may travel different paths to reach their destination. This approach can cause the datagrams of a transmision to arrive at their destination  out of order with different delays between the packets . Packets may alo be lost or dropped because of a lack of resources.  Upper-layer protocol to reorder the datagrams or lost datagrams before passing  them on to the application. The datagram networks are connectionless networks.  There are no setup or teardown phases . Each packet is treated the same by a switch  regardless of its source or destination. 

Routing Table

If there are no setup or teardown phases , how are the packets routed to their destinations in a datagram network? In this type of network, each switch (or packet switch) has a routing table which is  based on the destination address. The s routing vtables sare s dynamic vand sare s updated vperiodically. sThe s destination  addresses and the corresponding forwarding output ports are recorded in the tables . 




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Destination Addres s

Every packet in a datagram network carries a header that contains, among other information, the destination address of the packet. When the switch receives the packet , this destination address is examined; the  routing table is consulted to find the corresponding port through which the packet hould be forwarded. 

Efficiency

The s efficiency sof sa sdatagram snetwork sis s better vthan sthat sof sa s circuit-switched network ; resources are allocated only when there are packets to be transferred.  If a source sends a packet and there is a delay of a few minutes before another packet can be sent , the resources can be reallocated during these minutes for other packets from other sources. 

Delay

There may be greater delay in a datagram network than in a s irtual-circuit network. Although there are no setup and teardown phases , each packet may experience a wait  at a sw itch before i t is forwarded. Figure is example of delay in a datagram network for one single packet .  

The packet travels through two switches . There are three transmission times (3T), three propagation del ays (lopes 3 τ of the line), and two waiti ng times (W + w) we 1 2 ignore the procesing time in each switch. The total delay is  Total delay =3T + 3 _ __+ W1 + W 2 




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2.2. Virtual-Circuit Networks  A s irtual-circuit network is a cross between a circuit-switched network and a datagram network.  It has some characteri sti cs of both. As in a circuit-switched network, there are setup and teardown phases in addition to o the data transfer phase . Resources can be allocated during the setup phase , a in a circuit-switched network , o or on demand, as in a datagram network. As in a datagram network , data are packetized and each packet carries an address o in the header . As in a circuit-switched network , all packets follow the same path established o during the connection. A s irtual-circuit network is normally implemented in the data link layer, while a o circuit-switched network is implemented in the physical layer and a datagram networ in the network layer .

Addressing

In a virtual-circuit network, two types of addressing are involved: global and local (virtual-ci rcuit identifier). 

Global Addressing

A source or a destination needs to have a gl obal address -an addres that can be unique in the scope of the network or internationally if the network is part of an international network. However, we will see that a gl obal address in virtual-circuit networks is used only to  create a virtual-circuit identifier. 

Virtual-Circuit Identifier

The identifier that is actually used for data transfer is called the s irtual-circuit identifier (VCI). A VCI, unlike a gl obal address , is a small number that has only swi tch scope ; it is  used by a frame between two switches. 




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3. Me ss age Switched Networks

 In mesage switching no physical path is established in advance between sender and receiver.  When the sender has a block of data to be sent , it i s stored in the first swi tching office (i.e., router) and then forwarded later, one hop at a time.  Each block is received in its entirety, inspected for errors , and then retransmitted.  A network using this technique is called a store-and-forward network.

 The first electromechanical tel ecommunication systems used message swi tching , namely, for telegrams. [The mesage was punched on paper tape (off-line) at the sending office, and then read in and transmitted over a communication line to the next office along the way, where it was punched out on paper tape. An operator there tore the tape off and read it in on one of the many tape readers, one reader per outgoing trunk. Such a switching office was called a torn tape office. Paper tape is long gone and message switching is not used any more.]




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Write a note on bridges ? What is the main function of a bridge? Dr aw the line diagram of a LAN showing the location of a BRIDGE?

(Nov/Dec’11

– 8Mark)

(May/June’11- 2M (Nov/Dec ’11 – 2Mark)

Bridges

 A bridge operates in both the physical and the data link layer. A bridge extends the maximum distance of network by connecting separate network segment. A network simply passes on all the signals it receives. It reads the address of the entire signal it receives.  A bridge divides the network into two or more networks.  Bridge performs data link functions such as error detection, frame formatting and frame routing.  Figure shows a bridge.

Two effects of bridges,  Raising the bandwidth.  Separating collision Domains. In Bridge Architecture,  

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At each port of bridge, it has physical layer and MAC sublayer. The physical layer and MAC sublayer protocols at each port of bridge match with the protocols of the respective LAN. The MAC sublayer have relay and routing function between them. When a MAC frame is received by the bridge, it examines the destination addres, it reformats the frame as required by the other LAN. The data fields of MAC frame are of no interest to a bridge.

Functions of Bri dge: A bridge performs following basic functions. 1. Frame filtering and forwarding:

When the bridge receives a frame at any of its ports, it takes any of one actions. i. If the destination address is available on the same port by which it is received, the bridge discards the frame. ii. If the destination addres is on different physical port, it forwards the frame onto that port. P.ANANTH [M.E, MCSE, CCNA], Lecturer CSE Page 49


Computer Networks Lecture Notes

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