 4 A window size of one means device A only transmits one packet and A for this virtual call must carry the same LCN. every 80ms: a maximum packet rate of 12.5 packets every second. The call request packet is then routed across the network to packet A window size of two makes a dramatic difference. Device A switch 1, the most appropriate to device B. Switch 1 then assigns a sends the first packet then another 10ms later, the minimum delay unique LCN for the for the circuit to device B and sends the call time. (Figure 54.12). It takes 40ms for the first packet to reach B, request packet to B using this new LCN, which means all other for which B sends an acknowledgement. The second packet arrives packets for this virtual call must carry this LCN. 10ms later and is also acknowledged. Acknowledgement of the first Usually, as in the above example, the LCNs at each end of the packet arrives at device A after 80ms, time taken for the round trip. virtual call will be different. Furthermore, the LCN is only signifi- A then sends a third packet, receives acknowledgement of second cant between an end point device and the packet switch to which it packet 10ms after the arrival of the first acknowledgement and is is connected. then able to send a fourth packet into the network. It is important to remember that within the PSN, internal network In other words, device A sends two packets in every 80ms period: links may be subject to different, often enhanced, protocols from 25 packets a second to B, twice the rate of a window size of one. If those affecting the link between a switch and its attached end-point a window size of seven was imposed, device A could send seven devices. packets before the first could feasibly be acknowledged and achieve Just as in a conventional telephone system, a normal subscriber is a packet rate of 87.5 each second. plugged into the network via standard circuitry, whereas sophisti- It may seem odd, then, that the most common window size used cated communications equipment comes into play, from fibre