or example, to allow a user to log in to a remote time-sharing system or to transfer a file between two computers. A session layer allows ordinary data transport, as does the transport layer, but it also provides some enhanced services useful for some applica- tions. One of these services is to manage dialogue control. Sessions can allow traffic in both directions at the same time or in only one direction at a time. If traffic is allowed only one way at a time, the session layer can help by keep- ing track of whose turn it is. The session layer also provides a token manage- ment function and, with the help of this, only the host holding a token may perform critical operations. Another service by the session layer is to support successful transmis- sion of large files. Without this service a single error might destroy the whole file, which would then have to be retransmitted. To eliminate this problem, the session layer provides a way to insert checkpoints into the data stream SO that after a crash only the data after the last checkpoint have to be repeated. An example of the session layer standards is the International Standard IS 8326/8327 (X.215/225 of ITU-T) that defines the connection-oriented session layer service and protocol. In TCP/IP a separate session layer does not Introduction to Telecommunications Network Engineering exist and there all application support functions are integrated into the appli- cation layer, as shown in Figure 6.11. 6.3.3.7 Presentation Layer As we saw, the lower layers deal primarily with the orderly transfer of bits or data from source to destination. The presentation layer is instead concerned with the format of the transmitted information. Each computer may have its own way of representing data internally, SO agreements and conversions are needed to ensure that different computers can understand each other. The job of the presentation layer is to encode the structured data from the computer's internal format into a bit stream suitable for transmission. This may require compress