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Explain Operating System Interface OSI Layer 5 – Session Layer

At the 5th number in the OSI model comes the session layer.

About Session Layer

– The purpose of this layer is to maintain a control over the connections that exist between the computers in a network.
– It is this layer’s responsibility to create, manage as well as terminate a connection between the remote and the local applications.
– It serves for the following kinds of operations:
a) Full – duplex operations
b) Half – duplex operations
c) Simplex operations

– Its other functions include the following:
a) Establishing the check points
b) Adjournment
c) Termination
d) Restarting the procedures

– This layer was included in the OSI model so that the sessions could be gracefully closed.
– Graceful closure of the sessions is an important property of TCP (transmission control protocol).
– However, recovery and the check-pointing are not so prominent in the IPS (internet protocol suite).
– One characteristic of session layer is that it cannot be implemented in any environment.
– It can only be implemented explicitly in the environments that have applications using the remote procedure calls.
– This is also why the session layer services are meant for only those kinds of application environments.
– The mechanism that controls the opening, closing and management of the sessions between the various application processes of the users (i.e., the semi – permanent dialogue) is a part of the session layer.
– Requests and responses taking place between the users and the applications form the communication sessions.
– The OSI protocol suite session – layer protocol is an example of the protocol operating in this layer.
– This protocol is more commonly known as the ISO 8327 and X.225.
– Whenever there is a connection loss, this protocol tries to recover it.
– In other situations where a connection might not have been used for a long time, is closed and reopened by the session layer protocol.
– In the stream of the messages that are being exchanged, this protocol provides the synchronization points.

– Some other examples of the protocols operating in the session layer are:
a) ZIP or the zone information protocol (apple talk protocol):
The purpose of this protocol is to maintain coordination between the name binding processes.
b) SCP or session control protocol (phase IV session – layer protocol by DECnet)
The response to the service requests from the presentation layer is given by the session layer within the OSI network architecture’s service layering semantics. Then it issues other service requests back to the transport layer.

Services by Session Layer

– Services provided by the session layer can be well divided in to three major categories as mentioned below:
a) Authentication
b) Authorization
c) Session restoration

– Check – pointing and recovery services are included in the third category.
– The streams origination from several different sources are synchronized or combined through these services only.
– An example of web conferencing can be given. Here the audio and the video streams need to be synchronized for avoiding the problems of lip syncing.
– To ensure that the current speaker is the displayed on the screen, floor control is used.
– Another use of these services is in the live TV program telecasts.
– Here, also the audio and video streams need to be synchronized seamlessly.
– Also, these have to be transitioned for avoiding the problems of excessive overlapping and silent airtime.
– Some other protocols in this layer are:
a) Apple talk data stream protocol (ADSP)
b) Internet storage name service or iSNS
c) Layer 2 tunneling protocol or L2TP
d) Password authentication protocol or PAP
e) Point – to – point tunneling protocol or PPTP
f) Remote procedure call protocol or RPC

– The TCP/ IP layer does not concern itself with the session layer since it is not bothered with the OSI model’s transport protocol semantics.
– The transport layer protocols contain the session management of OSI with transport protocols such as SCTP and TCP.
– Sometimes it is also contained in the application layer protocols.
– Layers in TCP/ IP give a description of the operating scopes (such as link, network, application and host – to – host) and not prescriptions of data semantics.

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