Packect Switching Nw[Notes]

7/28/2019 Packect Switching Nw[Notes]

1/8

Network Services and Internal Network OperationThe main function of a n/w is to transfer info among the users that are connected to the

n/w or inter n/w. In the fig given below, the transfer may involve a single block of info or asequence of blocks that are temporally related. In case of a single block of info, we areinterested in having the block to be delivered correctly to the destination and in the delayexperienced in traversing he n/w.

In case of a sequence of blocks, we are interested not only in receiving the blocks correctlyand in right sequence but also in delivering a relatively unimpaired temporal relation.

The fig given below is transport protocol that operates end to end across a n/w. Thetransport layer processes at the end systems accept messages from their previous layer andtransfer these messages by xchanging segments end to end across the n/w. The fig shows theinterface at which the n/w service is visible to the transport layer. The n/w service is nothing

but the transport layer and the manner in which the n/w operates to provide the service isirrelevant.

t 0

t 1

Network

Endsystem

Physicallayer

Data linklayer

Physicallayer

Data linklayer Endsystem

Networklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Transportlayer

Transportlayer

Messages Messages

Segments

Networkservice

Networkservice


2/8

The n/w service can either be connection-oriented or connectionless. A connectionlessservice is very simple, with only two basic interactions b/w the transport layer and the n/wlayer and n/w layer that are

i) A request to the n/w that it sends a packet and an indication from the n/w that a packet has arrived.

ii) The user can request transmission of a packet at any time, and does not need toinform the n/w layer that the user intends to transmit info ahead of time.

A connectionless service puts total responsibility for error control, sequencing and flowcontrol on the end-system transport layer. The n/w service can be connection-oriented. Inconnection-oriented the transport layer cannot request transmission of info until a connectionhas been setup. The essential pts here are that the n/w layer must be informed abt the new

flow i.e. abt to be applied to the n/w and that the n/w layer maintains state info abt theflows it is handling . A connection-release procedure may also be required to terminate theconnection. It is clear that providing connection-oriented service entails greater complexitythan connectionless service in the n/w layer.

The services offered by n/w layer are:1. Best effort connectionless service.2. Low-delay connectionless service.3. Connection-oriented reliable stream service4. Connection oriented transfer of packets with delay bandwidth guarantees.

Internal Operations of Network The internal operation of n/w is connectionless if packets are transferred within the n/w

as datagrams, so each packet is routed independently. Consequently packets may follow diff paths from a to b and so may arrive out of order. The internal operation of a n/w is connection-oriented iff packets follow VC(virtual circuit) that have been established from a source to adestination. Thus to provide communications b/w a & b, routing to set up a VC is done onceand thereafter packets are simply forwarded along the established path. If resources arereserved during connection setup, then bandwidth, delay and loss guarantees can be provided.

The fact that n/w offers both services but does not dictate hoe the n/w must operateinternally. The approach suggested by the end to end argument keeps the n/w service as simpleas possible while adding complexity at the edge only as required. This strategy fits very wellwith the need to grow n/ws to very large scale.

A third category of func can be implemented either at the edge or inside the n/w. for eg,while congestion takes place inside the n/w, the remedy involves reducing input flows at theedge of the n/w.


3/8

Congestion control has been implemented in the transport layer and in the n/w layer.

Another set of function is concerned with making the n/w service independent of theunderlying transmissionsystems.Like a different transmission system i.e optical verses wirelessmay have different limits..The n/w layer may be called upon to carry out segmentation insidethe n/w .Alternatively the n/w could send error msg to the sending edge requesting that the

packet size be reduced .PACKET N/W TOPOLOGY

The user access packet in different way as a multiplexer as shown in the figure wherethe packets from a number of users share a transmission line.Here a single transmission line isshared in the access to wide area packet switching n/w.The multiplexer combines the typically

bursty flows the individual computers in to aggregated flows that make efficient use of the

transmission line .The different application within a single computer can generate multiplesimultaneous flows to different destinations .From logical point of view the link can be viewedas carrying either a single aggregated flow or a number separate packet flows.

LAn is another type of accessing method ,which provide the access to packet switchingn/w in many environment.As shown in the figure computer are connected to a sharedtransmission medium .transmssion are broad cast to all computers in the n/w.Each computer is

1

13 3 21 2 2

3 2 11 2 2

1

21

Medium

A B

3 2 11 2 2

1

C

2 1

2 1

2 14 1 2 3 4

Endsystem

Endsystem

Networ k1

2

Physical layer entityData link layer entity

3 Network layer entity

3 Network layer entityTransport layer entity4


4/8

identified by unique physical address and so each station listens for nits address to receivetransmission.Broadcast and multicast transmission are easily provided in this environment.

LAN allow the sharing of resources such as printers,databases and s/w among smallcommunity of users.

Multiple LANs in an organization are interconnected into campus n/w with a structureshown in the fig.LANs for a large group of users such as department are interconnected in anextended LAN through the use of LAN switches,identified by s in the fig

RR

RR

S

SS

s

ss

s

ss

s

ss

s

R

s

R

Backbone

To Internet or wide area

network

OrganizationServers

DepartmentalServer

Gateway

Campus Network

Only outgoingpackets leaveLAN throughrouter

High-speedcampusbackbone netconnects deptrouters

Servers haveredundantconnectivity tobackbone

Resources such as servers and databases that are primarily of use to this department arekept within the sub n/w .This approach reduces delays in accessing the resources and containsthe level of traffic that leaves the sub n/w.Each sub n/w has access to the rest of theorganization through a Router (R) that access the campus backbone n/w .

A sub n/w also uses the campus backbone to reach the outside world such as theinternet or other sites belonging to the orgnistion through a gateway router.Depending on thetype of organization the gateway may implement firewall functions to control the traffic that isallowed into and out of the campus n/w .

The routers in the campus n/w are interconnected to for the campus backbone n/wspecified by the mesh of switches designated S..Typically for large organizations such asuniversities these routers are interconnected by using very high speed mean sin terme of giga

bits Ethernet or anATM n/w The routers use the internet protocol which enables them tooperate over various data link and n/w technologies.This approach allows the n/w to adapt tochanges in traffic pattern as well as changes in topology due to the false in equipment.


5/8

Interdomain level

Intradomain level

Autonomoussystem or

domain

Border routers

Border routers

Internet service provider

s

ssLAN

Connecting to Internet ServiceProvider

CampusNetwork

network administeredby single organization

The routers in the campus n/w form a domain or autonomus system.The term domainindicates that the routers run the sme routing protocol.The term autonomus system is used for one or more domains under a single administration.All routing decisions inside the autonomussystem are independent of any other n/w.

DATAGRAMS AND VIRTUAL CIRCUITS :

Datagram packet switching

In Datagram packet-switching it is a packet switching technology by which each packet, now called a

datagram, is treated as a separate entity. Each packet is routed independently through the network.

Therefore packets contain a header with the full information about the destination. The intermediate nodes

examine the header of a packet and select an appropriate link to another node which is nearer to the

destination. In this system, the packets do not follow a pre-established route, and the intermediate nodes

do not require prior knowledge of the routes that will be used.

The individual packets which form a data stream may follow different paths between the source and the

destination. As a result, the packets may arrive at the destination out of order. When this occurs, the

packets will have to be reassembled to form the original message.

Because each packet is switched independently, there is no need for connection setup and no need to

dedicate bandwidth in the form of a circuit.


6/8

Datagram packet switches use a variety of techniques to forward traffic; they are differentiated by how

long it takes the packet to pass through the switch and their ability to filter out corrupted packets.

There are three primary types of datagram packet switches:

Store and forward : buffers data until the entire packet is received and checked for errors. This

prevents corrupted packets from propagating throughout the network but increases switching delay.

Fragment free : filters out most error packets but doesn't necessarily prevent the propagation of

errors throughout the network. It offers faster switching speeds and lower delay than store-and-

forward mode.

Cut through :does not filter errors; it switches packets at the highest throughput, offering the least

forwarding delay.

The most common datagram network is the Internet, which uses the IP network protocol. Applications

which do not require more than a best effort service can be supported by direct use of packets in a

datagram network, using the User Datagram Protocol (UDP) transport protocol. Applications like voice and

video communications and notifying messages to alert a user that she/he has received new email are using

UDP. Applications like e-mail, web browsing and file upload and download need reliable communications,

such as guaranteed delivery, error control and sequence control. This reliability ensures that all the data is

received in the correct order without errors. It is provided by a protocol such as the Transmission Control

Protocol (TCP) or the File Transfer Protocol (FTP).

Virtual circuits

telecommunications and computer networks , a virtual circuit (VC ), synonymous with virtual

connection and virtual channel , is a connection oriented communication service that is delivered by means

of packet mode communication. After a connection or virtual circuit is established between two nodes or

application processes, a bit stream or byte stream may be delivered between the nodes; a virtual circuit protocol

allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.

Virtual circuit communication resembles circuit switching , since both are connection oriented, meaning that in

both cases data is delivered in correct order, and signalling overhead is required during a connection

establishment phase. However, circuit switching provides constant bit rate and latency, while these may vary ina virtual circuit service because of reasons such as:

varying packet queue lengths in the network nodes,

varying bit rate generated by the application,

varying load from other users sharing the same network resources by means of statistical multiplexing ,

etc.
http://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Computer_networkshttp://en.wikipedia.org/wiki/Computer_networkshttp://en.wikipedia.org/wiki/Virtual_channelhttp://en.wikipedia.org/wiki/Connection_orientedhttp://en.wikipedia.org/wiki/Packet_modehttp://en.wikipedia.org/wiki/Packet_modehttp://en.wikipedia.org/wiki/Bit_streamhttp://en.wikipedia.org/wiki/Byte_streamhttp://en.wikipedia.org/wiki/Byte_streamhttp://en.wikipedia.org/wiki/Circuit_switchinghttp://en.wikipedia.org/wiki/Statistical_multiplexinghttp://en.wikipedia.org/wiki/Statistical_multiplexinghttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Computer_networkshttp://en.wikipedia.org/wiki/Virtual_channelhttp://en.wikipedia.org/wiki/Connection_orientedhttp://en.wikipedia.org/wiki/Packet_modehttp://en.wikipedia.org/wiki/Bit_streamhttp://en.wikipedia.org/wiki/Byte_streamhttp://en.wikipedia.org/wiki/Circuit_switchinghttp://en.wikipedia.org/wiki/Statistical_multiplexing


7/8

STRUCTURE OF SWITCH /ROUTER:

Switched Network

AccessNetwork

Switch


8/8

Controller

1

2

3

N

LinecardLinecardLinecard

Linecard

I n t e r c o n n e c t i o n

f a b r i c

LinecardLinecardLinecard

Linecard

1

2

3

N

Input ports Output ports

Data path

Control path(a)

Documents

Packect Switching Nw[Notes]