cct swt and pkt swt

8/2/2019 CCT Swt and PKT Swt

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DATA SWITCHING TECHNIQUES

AND

PACKET SWITCHED DATA

NETWORKS


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OBJECTIVES :

Data switching techniques

Circuit switched sub network

Store and forward switched sub network

Packet switching

Virtual circuit routing

Packet switching services


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Sub network Topology

Switching is selection and establishment of a path froma source to a specific destination through the sub-

network. Switching is carried out on specific demandfrom the source. The motivation for resorting toswitched networks arises out of the following two majorrequirements :

* Flexible Topology

Switching provides capability to deliver informationpresented at one access point of the sub-network to avariety of destinations, which can be selected by theusers. Thus, switching provides a flexibleinterconnection topology.

* Resource Sharing

The sub-network resources are available to all theusers of the sub-network. Thus, the interconnectionresources are shared by many users.


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Switching Techniques

At each node, there is need to decide theroute through trunk circuits which finallylead to the exit node. This switchingfunction and other related functions are

carried out at the nodes. There are twobasic techniques employed in the nodesfor switching data to appropriate route :

Circuit switching Store-and-forward switching


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In circuit switched sub-networks, a connectionbetween the two communicating end systems isestablished and then transmission of data takesplace over this connection. The sub-networkconsists of circuit switching nodes which areinterconnected by trunk circuits. A nodeconnects an incoming circuits to an outgoing

trunk circuit. The most common example ofcircuit sub-network is the telephone network.Trunks may be real (metallic pairs, FDMchannels, PCM channels) or "virtual". If they are

virtual, they must be immediately available totheir user whenever information is to betransmitted.


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A connection is built up by connecting the trunk circuitsin tandem up to the exit node. To establish thisconnection, the originating station sends a

CONNECTION ESTABLISHMENT REQUEST withaddress of the destination to the entry node. The entrynode builds up a path by connecting one of the trunkcircuits going in the desired direction to the end system.The address information is transferred to next node

where again cross connections are made. This processis repeated at each intermediate node and finally at theexit node which connects to the destination. The exitnode signals INCOMING CALL INDICATION to it. If thedestination returns CALL ACCEPTANCE signal, the sub-network sends CONNECTION CONFIRMATION to the

call originator. Thus, an end to end connection isestablished. After the confirmation is received, datatransfer can begin on the established connection. Theconnection is bi-directional, i.e. transmission can beeither direction.


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The users have full time ownership ofthe connection till it is released by

them. Note that address of thedestination is specified only onceduring call set up. All subsequent data

blocks are transmitted on the pathalready established


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The sub-network usually does not retainthe information on the end-points of a

connection after it is established.Therefore, if the connection gets broken,the sub-network does not have any

capability to restore the connection. The circuit switched sub-network provides

end-to-end connection for transmission of

data and it does not have any error controlor flow control capabilities


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Note that the connection does not involvelayers-2 (LLC) and two of the sub-networknodes for transfer of user data. Relayingfunction of layer-1 must be utilized fortransmission of the electrical signals. In

other words, the sub-network does notincorporate error and flow control functionsfor the user data. These layer-2 functionsare carried out end-to-end. The signaling

information exchanged during callestablishment and release phases, on theother hand, involves layer-2 and Layer-3.


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Delays in Circuit Switched Sub-network

Connection establishment in circuit

switched sub-networks involves certain setup. It includes cross connectionestablishment delays at each node andconnection request propagation delays.Once the connection is set up, user datatransfer involves only propagation delayand it is constant. There is almost no delay

at the nodes during data transfer phase.Data is transmitted immediately andincrementally as soon as it is presented tothe sub-network.


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During peak hours of traffic, once

a connection is established, thereis no increase in transmissiondelay through the sub-network as

a dedicated transmission pathalways exists. There may be

increased delay in establishmentof the connection as networkresources may not be free.


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Store And Forward Switched Sub-

Networks

Message switching sub-networks.

Packet switching sub-networks.


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In message switching sub-networks,

the complete message is switchedat the sub-network nodes. In packetswitching sub-networks, the

message is first divided into smallerpackets of data and then thesepackets are switched through the

sub-network. The switching modeadopted in both these sub-networktypes.


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Store-and-forward mechanism :

Store-and-forward sub-network consists of

store-and-forward nodes interconnected bytrunks. One channel is usually sufficientbetween a pair of nodes. Multiple channels canbe provided to increase reliability. Each nodeis equipped with a storage device wherein allincoming messages are temporarily stored foronward transmission. The basic operation of

store-and-forward service is similar to thetelegram service. A message along withaddress is sent from node to node till itreaches destination.


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The store-and-forward service isunidirectional. After delivery of themessage, the sub-network does not sendback any confirmation to the source. Ifend-system B is required to send anacknowledgement to the message

received from A, the acknowledgement willbe treated as like any other message bythe sub-network and will carry address ofA in its header. It is not so in circuit

switching service which provides an end-to-end connection between twocommunicating devices for communication

in both the directions.


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For node-to-node transfer of the message,the sub-network may employ some error

control mechanism. The message may beappended with error checking bits and if anyerror is detected by the receiving node, it may

request the sending node for re-transmissionof the message. Therefore, the sending nodeis required to keep a copy of the message till

an acknowledgement is received. Once anode has correctly sent a message, itdiscards the copy and thereafter it hasnothing to do with the message.


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Since at each stage of transmission, amessage is stored in a buffer at the

node, each node-to-node transfer isan independent operation. The trunkscan operate at different data rates.

Even the source and sinks of themessages can operate at differentspeeds. It is not so in circuit switching

where the users have to operate atthe same speed.


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Note that in store-and-forward

switching, there is no call establishment

phase. Each block of information istreated as independent entity by the

sub-network and, therefore, each block

of information carries the destinationaddress. Thus, unlike circuit switching

where address was sent only once

during call establishment phase, instore-and-forward switching, address isrepeated on each block of information.


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Delay in Message Delivery

Message delivery time is sum of the following

components : Time required to send the message to the entrynode. It is determined by the transmission datarate and message size. Propagation time to the

entry node is usually negligible. Node delay which includes :

Message processing at each node (timerequired for error checking, routing, etc.).Waiting

time in the queues at each node. Transmission time at each node (determined by

the transmission data rate) and propagation timefor transmission across the trunk.


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Delay in Message Delivery

Total time required to deliver the message is

linear sum of all these components of time asthey occur in a sequential manner. Deliverytime varies from message to messagebecause of random waiting times in queues

and alternate routes between same pair ofentry and exist nodes. Therefore, timerelationship of the messages and theirsequence are not guaranteed in a store-and-

forward sub-network. As traffic increasesthere is increase in message delivery timebecause the queues get longer and theremay be congestion on the route.


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Feature Circuit Switching Store-&-Forward Switching

Connection set-upConnection set-up phaserequiredFinite connections set-up delay

Connection not required to be set-up

Disconnection phase Required Not required

Destination address Specified once Specified on each message

Delivery delay Constant delivery delayDelivery delay is significant andrandom.

Temporal order Temporal order is maintained Temporal order is not maintained.

Time relationshipMessage-to-message timerelationship is maintained.

Time relationship between twomessages is not retained.

Error checking No error checkingSome transit error control ispossible.

Uni/Bi-directional Bi-directional Unidirectional

Message delivery Very high probability of delivery Delivery is not guaranteed.

Busy hourIncreased connection set-updelay.

Increased delivery delay.

Data rate conversionUsually changing data rate isnot feasible.

Data rates at user ends can bedifferent.


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Packet Switching : In message switching, a message is transmitted by

a node to another node after it has been completelyreceived. This results in significant delivery delay.This delay can be reduced by dividing the messageinto smaller chunks of data called packets. Now,each packet can be transmitted by a node to thenext node immediately after it is received. Note thattotal delivery time is not linear sum of allcomponents of delay as there is some overlapping.The total delivery time thus gets reduced. There is

some increase in the processing time at the entryand exit nodes because the message needs to bepartitioned into packets at the entry node andreassembled at the exit node.


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Another feature of packet switchednetworks which results in reduced

processing time at the nodes is that thepackets are stored in primary memory ofthe node. Messages on the other hand

are required to be stored in secondarymemory because of their size. Accesstime of primary memory is much less

than the secondary memory. In factdelivery time can be so much reducedthat users can have even interactive type

of dialogue.


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The basic packet switching operation of the sub-network is based on store and forward mechanism.

The only motivation for packetisation is to reducethe delivery time. Most of the additional features ofpacket switching have been possible due toreduced delivery time.

Datagrams And Virtual Circuits

There are two approaches for routing the data

packets through a sub-network : Datagram routing

Virtual circuit routing


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In its simplest form, a datagram is a packet ofdata with complete address of a destination.

Datagrams are sent out onto the sub-networkone by one, and the sub-network interpretsthe destination address on each packet ateach stage of switching and tries to deliver it

to the destination independent of otherdatagrams. It is similar to message switchingdescribed above except that size of message

is limited. In virtual circuit approach, packets are

delivered to the destination over a fixed routewhich is established before handover.


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E


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Datagram Routing

Figure shows a sub-network consisting of

five store-and-forward nodes 'A' to 'E'connected by point to point trunk circuits.To send a datagram across the sub-

network, it is first sent down the link fromthe user to the access node. In each node,the controlling program examines thedesignation address on the datagram anduses some algorithm to choose the nextlink to send the datagram towards itsdestination.


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Some of the possible alternatives for decidingthe route of the datagram across the sub-network are :

Send the datagram to one of the trunk circuits atrandom. Though the datagram will eventuallyreach the destination but the method would be

very inefficient. Another similar approach could be send it on the

trunk which has the shortest queue irrespectiveof its destination.

A brute force approach could be to send thedatagram on all the trunks. The datagram wouldreach the destination quickly but large redundant

traffic would be generated in the sub-network.


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A much better approach than thosementioned above is to set up a routing

table at each node. Given an address, thenode can look up the routing table anddecide the next link, e.g. to send adatagram from sender 'S' to destination

'R', the routing table at node 'A' wouldindicate the next link connecting to node'B'. Similarly at node 'B', the routing table

would indicate that the datagram shouldbe sent to node 'D'. The datagram wouldbe next routed to node 'E' and hencefinally to the destination.


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The approach seems to be feasible, butwe have not considered a very important

aspect, updation of routing tables. If a newnode is added to the sub-network, all therouting tables at various nodes need to be

updated.


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Dynamic Routing Suppose nodes A, B, C and D are all connected and have

up-to-date routing tables, i.e. they have entries for nodes A,

B, C and D but there is no entry for node E. Now imaginenode E is added to the sub-network as shown in the figure.The algorithm used for updating the routing tables is thateach node will send the updation information to the othernodes it is connected to. When node E becomes active, itwill update routing table of node D, node D will in turn,

update routing tables of nodes B and C, and node B willupdate routing table of node A. Another piece ofinformation required for the routing table is number of hops,e.g. distance from node B to node E is two hops via node Dand three hops via nodes C and D. so node D will tell B andC one hop distance to E. B and C will note it down as two

hops distance to E via D, adding another hop. C will alsoinform B about E. It will indicate two hops distance to E.Therefore, B will have two alternative routes to E, one oftwo hops via D and the other of three hops via C. Thus, allthe nodes will have updated routing tables.


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Virtual Circuit Routing :

In the virtual circuit approach, a logicalconnection is established through the sub-network before sending any user data (as

done in circuit switched sub-networks).Unlike datagram approach, the nodes donot make a routing decision for each

packet. It is made once for eachconnection at the time of establishing theconnection. Let us understand how it isdone.


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Fig. 4.12 shows a simple network withuser S attached to node A. User S is

identified by its address A-S (usually partof address of a user identifies the node towhich it is attached). Another user R withaddress D-R is attached to node D.


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Suppose S wishes to exchange data withR. To establish a connection to R, S sends

a CONNECT REQUEST packet to node Aspecifying the destination address D-R. Italso specifies a label N1 to the node. All

the future packets to be sent to R over theconnection, will bear the same label N1.The destination address is not specified

again. In X-25, as we shall see later, N1 iscalled logical channel identifier. Thus,CONNECT REQUEST is "Connect A-S toD-R. Use label N1 for this link"


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Note that S is also requesting the node to uselabel N1. On receipt of this CONNECT REQUEST,node A takes note of the label N1. Node A

examines the destination address specified in theCONNECT REQUEST packet and works out thenext link in the chain leading to the destinationfrom its routing table. For this link to node B, nodeA selects another label N2 which is unique on thislink. Node A writes this label by the side ofprevious label N1. All the future packets of theconnection being established and going betweennode 'A' and node 'B' shall bear label N2. For now,

node 'A' sends a modified CONNECT REQUESTpacket to node 'B.

"Connect A-S to D-R. use label N2 for this link".


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Note that the label has been changed. Onreceipt of this packet, node B works out theroute and forwards the packet to node 'D' using

another label N3. Node 'B' also keeps a recordrelating labels N2 and N3. Node 'D' sends anINDICATION of incoming call to destination 'R'.It does so using still another label N4 which isunique on the link between node 'D' and 'R'. If 'R'is ready to accept the call, it sends itsACCEPTANCE to node 'D'. It uses label N4already being in use for this link of theconnection. Node 'D' sends this ACCEPTANCE

to node 'B' using label N3. Node 'B' towards thisto node 'A' using label N2. Node 'A' finally sendsa CONFIRMATION to 'S' of having established aconnection to the destination. This confirmationbears label N1.


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Thus, in the connection establishmentphase, a route to the destination is

finalized and a confirmation is receivedfrom the destination. The connection is inthe form of tables relating the labels of

data packets. These tables are maintainedat each node (Fig. 4.13). Whenever apacket is received by a node, it looks up in

therein. It also gives to the packet a newlabel which is also indicated in the table.This connection is called virtual because itdoes not physically exist.


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'S' can send data packets now. It will uselabel N1 and 'R' will use label N4 on their

packets. Addresses of the destinations arenot needed in the data packets.

The motivation behind attaching labels isthat on each link, between the nodes orbetween end system and a node, several

connections can be maintainedsimultaneously. This is illustrated in Fig.4.14.


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Node P is operating two connections, onethrough nodes Q, R and S and other through

nodes Q, R and T. For the first connection, thepackets are labeled "1" on link P-Q, "5" on linkQ-R and "7" on link R-S. For the secondconnection, packets are labeled "2" on link P-Q,

"4" on link Q-R and "3" on link T-R. Node O isalso maintaining two connections, one throughQ, R and S and the other through Q, R and T.Note that four connections are simultaneously

working through link Q-R. Packets are identifiedas belonging to a particular connection by theirlabels on each interconnecting link


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Another very important point to be noted is thatin a particular connection all packets always take

the same route which is decided at the time ofestablishing the connection. Therefore, deliverydelay is more or less constant in virtual circuitsapproach. Minor variations could be due to re-

transmission of packets between two nodes ifthere is an error.

As the packets follow the same route, theirsequence is retained across the sub-network.Datagram routing on the other hand usuallydoes not deliver the packets in the sequencethey were transmitted


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At the time of connection set-up, the

nodes allocate some buffer resources fortemporary storage of the packets. Thebuffer size is determined by the maximum"window size" to be used across the sub-network. Window size specifies themaximum number of packets which can besent to a node without seeking its

permission. By pre-allocating buffers likethis, it is possible to avoid deadlocks


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PACKET SWITCHING SERVICES Packet switching services need to be

distinguished from packet switched

routing discussed above. The serviceprovided to the user can be :

Datagram service

Virtual circuit service


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Datagram service is connectionlessservice having the features as described in

the section on datagram routing. Anadditional feature could be sequencing ofdatagrams by the exit nodes. Virtual circuit

service, on the other hand, is a connectionoriented service which can be providedusing either virtual connection routing

approach or datagram routing approach(Fig. 4.15).


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If datagram routing is employed, theinterface that the service provided to theuser is virtual circuit service. CCITT

Recommendation X.25, Frame Relay arefor the interface between DTE and theaccess node for virtual circuit service.


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Thank you, Your patience is,

Great !

cct swt and pkt swt

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