 e.g. a packet switch Memory Frame handler DCE e.g. a packet switch Packet Frame handler Memory Frame Packet Packet Frame Packet Packet Packet Frame Packet Frame Packet Packet Packet Packet Packet Physical level Link level Network or packet level Figure 54.7 X.25 packet switch recommendation for PSN access layers in contrast to a full OSI gateway that employs all seven service to the level above and receiving a service from any lower layers.(Figure 54.6). level. Corresponding levels on the other side of the link, a level's peer, communicate via a peer protocol, co-ordinating the exchange of information between peer levels across the interface. (Figure Packet switching and the OSI model 54.7). The higher layers proposed by the ISO, through which data passes At the Physical, Data Link and Network layers of the OSI seven before reaching the packet switching layers, have not been fully laid layer model, equipment at customer premises or in the network must out, though their overall aims are well defined. Once at the X.25 share a compatible set of interfaces and signalling conventions to level, communication can be established between an end point comply with the X.25 standard. device and the corresponding X.25 level of the local DCE. Having Each level or layer is responsible for one aspect of the total X.25 passed the local DTE/DCE interface, a conversion to the network based communication. It is functionally independent from the other protocol takes place for transport through intermediate nodes in the layers, interacting with them in clearly defined areas, providing a network. (Figure 54.8). User A (sender application) User B (destination application) User data End-to-end Higher level Higher level Packet Packet Packet Packet level level level level level level level level Physical Physical Physical Physical level level level level PS network Figure 54.8 PSN transportation through the OSI layers 54/8 Packet switching At the destination DCE, conversion back to the X.25 protocol signals to manage the link traffic volu