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BY DINESH THAKUR
Conceived in the 1960's, packet switchingis a more recent technology than circuitswitching which addresses a disadvantageof circuit switching: the need to allocateresources for a circuit, thus incurring linkcapacity wastes when no data flows on acircuit.
Packet switching introduces the idea ofcutting data on a flow into packets whichare transmitted over a network withoutany resource being allocated. If no data isavailable at the sender at some pointduring a communication, then no packetis transmitted over the network and noresources are wasted.
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Figure : Datagram Packet Switching. Packets from a given flow are independentand a router can forward two packets from the same flow on two different links.
Since each packet is processed individually by a router, allpackets sent by a host to another host are not guaranteed to usethe same physical links. If the routing algorithm decides tochange the routing tables of the network between the instantstwo packets are sent, then these packets will take different pathsand can even arrive out of order.
In this Figure for instance, packets use two different paths to gofrom User 1 to User 5. Second, on a network topology changesuch as a link failure, the routing protocol will automaticallyrecompute routing tables so as to take the new topology intoaccount and avoid the failed link. As opposed to circuitswitching, no additional traffic engineering algorithm is requiredto reroute traffic.
Virtual circuit packet switching
Figure: Virtual circuit packet switching. All packets from the sameflow use the same virtual circuit.
Virtual circuit packet switching (VCswitching) is a packetswitching technique which merges datagram packet switchingand circuit switching to extract both of their advantages. VCswitching is a variation of datagram packet switching wherepackets flow on socalled logical circuits for which no physicalresources like frequencies or time slots are allocated (see Figure).
Each packet carries a circuit identifier, which is local to a linkand updated by each switch on the path of the packet from itssource to its destination. A virtual circuit is defined by thesequence of the mappings between a link taken by packets andthe circuit identifier packets carry on this link. In VCswitching,routing is performed at circuit establishment time to keep packetforwarding fast.
Other advantages of VCswitching include the traffic engineeringcapability of circuit switching, and the resources usage efficiencyof datagram packet switching. Nevertheless, a main issue of VCSwitched networks is the behavior on a topology change.
As opposed to Datagram Packet Switched networks whichautomatically recompute routing tables on a topology change likea link failure, in VCswitching all virtual circuits that passthrough a failed link are interrupted. Hence, rerouting in VCswitching relies on traffic engineering techniques.
There are a number of important differences between virtualcircuit and datagram networks. The choice strongly impactscomplexity of the different types of node. Use of datagramsbetween intermediate nodes allows relatively simple protocols atthis level, but at the expense of making the end (user) nodesmore complex when endtoend virtual circuit service is desired.
The Internet transmits datagrams between intermediate nodesusing IP. Most Internet users need additional functions such asendtoend error and sequence control to give a reliable service(equivalent to that provided by virtual circuits). This reliabilitymay be provided by the Transmission Control Protocol (TCP),which is used endtoend across the Internet, or by applicationssuch as the trivial file transfer protocol (TFTP) running on top ofthe User Datagram Protocol (UDP)
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Dinesh Thakur is a Technology Columinist and founder of Computer Notes and Technology Motivation. Mail Me At (@Computer
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