8.3 DATAGRAM NETWORKS

When a network transports data as datagrams, it first decomposes the data into packets of variable size. The packets are then sent one by one from node to node along a path to the packet destination. Each packet contains a special field that specifies its destination address. Thus, in a datagram network, the nodes store and forward the individual packets, one at a time. The routes taken by successive packets may be different, even if they go from the same source to the same destination. Also, because the packet sizes may be different, as suggested in Figure 8.8, the transmission times of the packets are different, since they are equal to the packet lengths divided by the transmission rate. In our study of datagram networks we begin by describing a model that we will use to formulate the control questions.

Figure 8.8: Datagram networks. Packets of different length are transported in a store-and-forward manner.

8.3.1 Queuing Model

The queuing model of Figure 8.9 can be used by the designer to predict the transmission delays and to design good routing and flow-control algorithms. The top part of the figure shows one node with its components: a receiver converts the optical signal on the incoming fiber into packets. The packets are stored into memory and are then retransmitted on one outgoing fiber.

Figure 8.9: Packet-switching node and queuing model.

The bottom part of the figure shows an abstract representation of the same node. The packets are viewed as "customers"...