quence numbers running from with a clear confirmation packet. zero to seven. The maximum window size is therefore seven packets before acknowledgement is needed. Changing the window size in the Network Layer has a similar 54.4.3.1 Logical Channel Numbering effect to changing it in the Data Link Layer, only more so. In a virtual call from device A to B, every packet from A has to pass The network layer can support multiplexing, sustaining many active through packet switches 4, 2 and 1 before reaching B. In other virtual calls simultaneously. For example, it can set up virtual calls words, the packet on this route must pass across four different links. to devices B and C from device A at the same time. As a result, a Each link adds delay (transit time) before the data gets through. mechanism has to exist to identify the relevant virtual call along For each packet a network layer acknowledgement must follow which packets are transmitted and received. down the same tortuous path in the opposite direction to reach A unique Logical Channel Number is therefore assigned to each device A. virtual call and used to refer to that virtual call for the lifetime of the If, for instance, each link in the virtual circuit above imposes a call. Each packet contains the LCN of the logical call to which it delay of 10ms, ignoring for this example any delay within the refers at its start, followed by a packet type field explaining the switches themselves, the four links used between end point devices packet's function and any additional information that may be A and B amount to an imposed delay of 40ms on each packet needed depending on the packet type. travelling one way, and double including the obligatory acknow- For device A to set up a call to B, it assigns a unique LCN to the ledgement. But the delay is proportional to packet length and the virtual call, carried in the call request packet. The LCN indicates to acknowledgement is usually short, unless piggy-backed. packet switch 4 that all other packets transferred between switch