Data Communications Networking Devices - 4th Ed.pdf

186 __________________________________________________________ WIDE AREA NETWORKSas value-added networks due to the extra value they provide. Since almost allpacket switching networks now support one or more optional facilities, the term`value-added network' is essentially synonymous with the term `packet switchingnetwork'.Packet network architectureThere are two major categories of packet networks, with each category based uponthe method by which packets are routed through the network: datagram andvirtual circuit based networks.Datagram packet networksIn a datagram network, each packet is transmitted independently of other packets,with packet switches routing each packet based upon such factors as the traf®ccurrently carried on circuits linking network switches, the error rate on thosecircuits, as well as whether or not speci®c circuits are available for use.Figure 2.10 illustrates an example of the packet ¯ow through a datagram packetnetwork. In this example it was assumed that two devices creating packets havetheir data ¯ow routed through a packet network consisting of four packet switches,with two computers connected to the network. Let us further assume that thepackets labeled A, B, C and D are routed to computer A, while the packets labeledW, X, Y and Z are routed to computer B.As packets are routed through a datagram packet network, each switch examinesthe packet destination address computer A or B) and routes the packet based uponcriteria similar to that previously discussed, which results in the selection of anoptimum route at a given time. Since packets can traverse different paths, packetscan be received at a destination switch out of sequence. Thus, destination switchesmust be capable of having suf®cient memory to store packets until they can besequenced into their appropriate order prior to their delivery to their ultimatedestination. This resequencing would occur at packet switches 3 and 4 in Figure2.10, when data ¯ows towards the computers with the result that, although packetsFigure 2.10 Datagram packet network data ¯ow. In a datagram packet network packetsare routed based upon the activity and availability of circuits connecting packet switches