From the previous section, it is clear that in packet switching, messages are broken into packets and sends one at a time to the network. Routing control decides how the network will handle the stream of packets as it attempts to route them through the network and deliver them to the intended destination. The routing decision is determined in one of two ways. They are
1. Datagram and
Datagram. In datagram, each packet within a stream is independently routed. A routing table stored in the router (switch) specifies the outgoing link for each destination. The table may be static or it may be periodically updated. In the second case, the routing depends on the router's estimate of the shortest path to the destination. Since the estimate may change with time, consecutive packets may be routed over different links. Therefore each packet must-contain bits denoting the source and destination. Thus may be a significant overhead.
Fig. 11.6 shows a simple communication network where the concept of datagram is explained. The circled one are called the switching nodes whose purpose is to provide a switching facility that will move the data from node to node until they reach the destination. The squared one are called the stations. The stations may be computers, terminals, telephones or other
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