vice B A sends 1st packet A sends 2nd packet 1st packet arrives at B Acknowledgement from B sent 2nd packet arrives at B Acknowledgement from B sent A receives acknowledgement A sends 3rd packet A receives acknowledgement A sends 4th packet 100ms Figure 54.12 Illustration of a window size of two the end device being called. In other circumstances, an average or 1. There is no flow control mechanism in a datagram network compromise window size is chosen to accommodate most calls. comparable to that of a virtual call network. 2. As each datagram packet contains its explicit addresses and the network contains no record of connections, every packet switch Datagram networks en route has to look up the specific address in its routeing table to find out where it should switch that packet. This can prove An alternative to the concept of the Switched Virtual Circuit or slower than having a LCN identify a route, as happens in a virtual call network. virtual call in providing communications between end point devices over a PSN is a datagram network. There is no formal acknowledgement system. In circumstances The phases of call set up, information transfer and call clearance in which errors may be among the data stream due to adverse are not needed in a datagram network. Each packet is simply line quality and so on, a sender cannot be sure the datagram launched into the PSN, leaving it up to the PSN to route the packet packet was successfully received. to its destination. Without call set-up there is no need for LCNs in the Network Layer, but each packet must therefore contain its After much controversy and changes of mind, the CCITT X.25 source and end point destination addresses. 1984 recommendation removed any support for the connectionless Communicating devices in a datagram network are not connected datagram method. For many network managers it is comforting to be able to track data to its destination. to each other as in a virtual circuit, SO the packet switches do not have to keep records of active calls and can there