In
the
OPEN SYSTEMS INTERCONNECTION communications model,the session layer sometimes referred to as the
“port layer” manages the setting up and taking down of associations between
two communicating ends point called a connection.
The
session layer going by its name alone is suggestive of its functions.It simply facilitates sessions between the 7
layers of the OSI model.The session layer allows
users on different machines/computers establish sessions between them.A session allows ordinary data transport, as does
the transport layer,but it also provides some
enhanced services useful in some applications.
A
session for example might be used to allow a user to log into a remote
time-sharing system or to transfer a file between two computers. One of the
services of the session layer is to manage dialogue control. Sessions can
allow traffic to go in both directions at the same time, or in only one
direction at a time. If traffic can only go one way at a time, then the
session layer can keep track of whose turn it is to transmit the session
similar to flow control/windowing function offered to data in the
data link
layer. A related session service is token management.
For
some protocols, it is essential that both sides do not attempt the same
operation at the same time. To manage these activities, the session layer
provides tokens that can be exchanged and consequently, only the side holding
the token is allowed to perform the critical operation.
Another
session service is synchronization. Consider the problems that would occur
when trying to do a two-hour file transfer between two machines on a network
with a 1-hour mean time between crashes. After each transfer was aborted, the
whole transfer would have to start over again and probably fail again with
the next network crash. 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 sent after the last
checkpoint has to be repeated.
Communications/tasks
are performed in the session layer by using certain
protocols. The
OSI-session layer protocol (OSI-SP) provides session management i.e. the
opening and closing of sessions- for further reading go to
www.javvin.com/protocollSOsession.html.
Cisco Networking Simplified by Paula
Del Maggiora and Jim Doherty. Printed in July 2004
MCSE Networking Essentials for
Dummies by Robert Aschermann.
Operating Systems incorporating UNIX
and Windows 4th Edition by Colin Ritchie, published by Continuum
in 2003.
Operating Systems International
Edition Third Edition, by Gary Nutt published in 2004
FOR LINKS TO OTHER RELATED SITES, CLICK
BELOW PLS
www.ics.uci.edu/~dan/pubs/DataCompression.html
www2.rad.com/networks/1994/osi/present.htm
en.wikipedia.org/wiki/Presentation_layer