The principal use of the session notion is:
Device independence- the session makes see-through whether the endpoint is EBCDIC, ASCII, one-line, multi-line, program or terminal.
Abstraction- handles the high level protocols for recovery, related messages and conversations.
Session creation specifies the protocols to be utilized on the session by each participant. One participant possibly an ASCII typewriter and the other participant may be a sophisticated system. In this casing the sophisticated system has a lot of logic to handle the session protocol and errors on the session. Alternatively if the endpoint is an intelligent terminal and if the other endpoint is willing to accept the terminals protocol the session management is rather simple. (Note- The above is the method it is supposed to work. In practice sessions by means of intelligent terminals are extremely complex as well as the programs are much more subtle because intelligent terminals can make such complex mistakes. In general it is much easier to handle a master-slave session than to handle a symmetric session.)
Network manager merely delivers transmission records to endpoints. Therefore it is the responsibility of data communications to ‘know’ about the device characteristics and to control the device. This signifies that DC must implement all the code to provide the terminal appearance. There is a version of this code for every device type (printer, display, typewriter,...), This reasons the DC component to be very big in terms of thousands (K) Lines Of Code (KLOC).
If the network manager defines a usually useful endpoint model then the DC manager can use this model for endpoints that fit the model. This is the explanation for the TYPE1, TYPE2,... (Endpoints) of SNA and justifies the attempts to define a network logical terminal for ARPANET.
Sessions with dedicated terminals as well as peer nodes of the network are automatically (re)established when the system is (re)started. Of course the operator of the terminal should re-establish his identity therefore that security will not be desecrated. Sessions for switched lines are created vigorously as the terminals connect to the system.
When DC produces the session it specifies what protocols are to be used to translate message formats therefore that the session user is not aware of the characteristics of the device at the other end point.
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