 (X.29). full duplex single links. This has been mostly remedied by the 1984 The strength of X.25 lies in the fact that it relies on other standards version of X.25 which caters for multiple links, though X.25 still for interfacing into the lower layers. It resolves the logical and does not cater for multi-point connections. X.25 Packet network Asynchronous host Asynchronous devices Figure 40.10 A packet switch network X.25 and OSI 40/11 2. Poor efficiency with interactive traffic. Because of the way in Application which packets are built up with the data from a single user per packet, X.25 tends to become inefficient with network band- Presentation width if a predominance of single character enquiry traffic is being sent. (Compare this with the statistical multiplexer frame Session with multiple users per frame). However X.25 becomes very efficient for file transfer type applications where the packets can be filled to a maximum. It should be pointed out that in OSI, Transport the layer 4 functionality does include the facility for enveloping multiple channels into a single packet. Network Packet Network Congestion. It is not economically viable for a net- work administration to run a network sized to provide enough bandwidth for the worse case traffic load. Therefore at peak Data Link Data Link times network congestion conditions occur causing existing sessions to slow down and preventing new sessions from start- Physical Physical ing. This is analogous to the public telephone networks i.e. "All lines to Cityville are engaged please try later". 4. Wrong Numbers. Expensive as generally all outgoing calls are billed. 5. Network Variance. Not all networks obey the protocol in the Figure 40.11 X.25 and the OSI model same way. This is due to grey areas in the CCITT X.25 Recom- mendations and to individual manufacturer's interpretations on how to handle exception conditions. The X.25 packet layer provides the majority of the functions 6. Bureaucracy. Most PTTs insist that X.25 products must pass a specified in the OSI