Data Communications Networking Devices - 4th Ed.pdf

2.4 PACKET SWITCHING NETWORKS _______________________________________________ 187from each source routed to a common destination took different paths, they werereassembled into their original order at their destination nodes.Although datagram packet networks permit transmission paths to be dynamicallyaltered to correspond to network conditions and activity, they require a substantialamount of packet switch processing overhead. This is because each switchmust know the state of circuits to other switches to implement appropriate routingalgorithms. While most packet switching networks were originally based upondatagram technology, a majority of those networks were converted to the use ofvirtual circuit packet switching.Virtual circuit packet networksIn a virtual circuit packet network, a ®xed path is established from the dataoriginator to the recipient at the time a call is established. Thereafter, all packets¯ow over the same path, although the packets from two or more devices can sharethe use of circuits between packet switches that form a speci®c virtual circuit.Figure 2.11 illustrates the data ¯ow through a virtual circuit packet network.Note that although a route will be established based upon network activity, onceestablished the route remains ®xed for the duration of the call. Thus, packets will¯ow in sequence through each switch which reduces both the amount of processingrequired to be performed at each switch and delays associated with waiting for outof sequence packets to arrive at a destination node prior to being able to pass anordered sequence of packets to their destination.Figure 2.11 Virtual circuit packet network data ¯ow. In a virtual circuit packet networkpackets are routed in sequence over a ®xed path established at the time a call occursPacket formationAlthough many devices have the ability to directly create packets, most devicesrely upon the use of packet assembler/disassemblers PADs). PADs are essentiallyprotocol conversion devices that accept data from data terminal equipment DTE)supporting one protocol and converting the data into packets based upon the ITUX.25 protocol.