ch 1. Load sharing or balancing is an inherent strength within MLP. 2. Improved reliability and resilience against link failure as packets flow via different physical paths. Switch 3. Additional bandwidth to support increased traffic can be added incrementally without affecting or disrupting existing physical connections. Packet switch links over satellite are subject to far longer transit delays than normal, around 250ms one way, making X.25 inefficient Figure 54.11 Addressing example if used with Modulo-8 windows. The sender will be idle for rela- tively long periods waiting for acknowledgements. ISO and the CCITT therefore produced Modulo-128, within take a route along circuit seven to six, six to three, three to one and LAPB, allowing up to 127 frames to remain outstanding before an out to B. From A's viewpoint, B's address would be 7631 along this acknowledgement is needed. Currently available only within some particular route. For C to send data to B, the shortest route would be public PSNs, it is accessed through an extended SABM command. via circuit two followed by one. B's address from C's viewpoint A future development is likely to be some form of hot keying would be just 21. between Modulo 8 and Modulo 128 modes. Known as a route addressing system, a feature of this method is that the address of any end point device will vary according to who 54.4.3 Packet switching at the Network Layer is sending data, in contrast to the global addressing system in which an end point's address remains the same irrespective of wherever it While the Data Link Layer sorted out an error free link between two is being sent data. connected devices, the Network Layer provides communications A typical call set-up in Figure 54.11 would occur as follows. between devices that are not necessarily connected together they Device A establishes a virtual call on a virtual circuit to device B by will have a network in between them. sending a call request packet down its link to packet switch 4. A call Network Layer terminology talks o