VIPA CP143 TCP/IP, Manual

Page: 21 / 210

CP143
H1 / TCP/IP manual
Communication protocols
Rev. 00/07 2-5
Level 5 Session layer
The session layer provides the links that enable users of different systems to communicate. In
addition to the standard transport services the session layer also provides high-level services that
may be useful for certain applications. One of these services the control of interactive functions. The
session layer administers the flow of data where data can only be transferred in one direction
(similar to the token-management). A session can provide user-access to other systems or it can
transfer a file from one system to another.
Another service of the session layer is that of synchronization. Certain reference points are
superimposed on the data stream. If the network should crash it is only necessary to re-send the data
from the most recent reference point.
Level 6 Presentation layer
The presentation layer executes those functions that are used very often and therefore justify a
general procedure. This level is also responsible for the syntax and semantics of the transferred data.
One example of the services provided by the presentation layer is the co-ordination of data in a
standard and pre-arranged manner. The presentation layer converts the different data structures and
codes supplied by the various computers into a standard network code so that the users of these
computers may communicate.
Level 7 Application layer
The application layer supports a large number of protocols.
This layer provides various file-transfer and data processing functions to the user. The application
layer also adapts editor programs to the requirements of the respective terminal equipment.
Different file processing systems employ different conventions for file names and different methods
to display lines of text, etc. You must remember these properties when you transfer files between
different systems. This function as well as electronic mail, remote task entry and directory inquiries
are also part of the services provided by the application layer.