ived acknowledgement and even the appearance of (Figure 54.10). It is most associated with expensive international false frames due to electrical interference. To help identify one rather than national links. 54/10 Packet switching X.25 single Routeing, relaying, packet sequencing and flow control are also link procedure functions of the Network Layer, plus providing services to the Transport Layer and higher layers, such as addressing and data transfer. Multilink To make a normal telephone call, it is necessary to know the number at the other end. In the same way, Network Layer connec- tions rely on a system of reference that provides a unique address to locate each device attached to the network. Two methods predomi- nate in providing end point addressing for PSN attached devices. Depending on the size and complexity of the network, each device Internal network protocol can be given a unique address that remains valid throughout the network. From anywhere on the network, the device can be con- tacted via that address and the address remains the same at all times. Alternatively, the route taken to reach a device is used to build up the address, so that the address differs according to the route taken X.25 multilink procedure and, naturally, also depends on the starting point on the network. A device's address is an outline of the route the data will take to reach In Figure 54.11 the network links are numbered from one to nine. Figure 54.10 Multilink procedure within a PSN An end point device at A sending data to end point device B could Although packets carried along the same virtual circuit may travel on different physical paths, to the Network Layer, these different physical links appear as one logical circuit. MLP manages any number of Single Link Procedures, each of which is an orthodox LAPB circuit. It carries out resequencing for all received frames Switch Switch over different physical links, adding a multilink control field and an information field to each frame MLP offers several advantages: Switch Swit