The area of data communications is the slightest understood aspect of DB/DC systems. It should deal with evolving network managers evolving intelligent terminals and generally seems to be in a continuing state of chaos. Don’t feel too bad if you find this section bewildering.
Data communications is accountable for the flow of messages. Messages may perhaps come via telecommunications lines from terminals and from other systems or messages may be generated by processes running within the system. Messages perhaps destined for external endpoints for buffer areas called queues or for executing processes. Data communications externally offers the functions of.
Internally data communications offers:
a) Message transformation that maps ‘external’ messages to as well as from a format palatable to network manager.b) Device control of terminals.c) Message recuperation in the face of transmission errors as well as system errors.
Messages, Sessions as well as Relationship to network manager:
Messages as well as endpoints are the fundamental objects of data communications. A message comprises of a set of records. Records in turn comprises of a set of fields. Messages thus look very much like database sequential sets. Messages are described by message descriptors. Usual unformatted definitions might be a line from typewriter terminal is a one field, one record message. A screen image for a shown is a two field (control and data) one record message. A multi-screen show image is a multi-field multi-record message.
Data communications depends greatly on the network manager provided by the base operating system. ARPANET, DECNET and SNA (embodied in NCP and VTAM) are illustrations of such network managers.
The network manager offers the notion of endpoint that is the smallest addressable network unit. A workstation, a process, a queue and a card reader are every examples of endpoints.
Network manager conveys rather stylized transmission records (TRs).
These are merely byte strings. Network manager makes a best endeavour to deliver these byte strings to their destination. It is the responsibility of data communications to package messages (records and fields) into transmission records as well as then reconstructs the message from transmission records when they arrive at the other end.
The following figure summarizes this- application as well as terminal control programs sees messages via sessions. DC in the host as well as terminal map: these messages into broadcast records that are carried by the network manager.
The three major layers of a session
There are two methods to send messages: A one shot message is able to be sent to an endpoint in a canonical form with a very rigid protocol. Logon messages (session initiation) are frequently of this form. The second manner to send messages is via an established session between the two endpoints. When the session is started certain protocols are agreed to (example messages will be recoverable (or not), session is half or full duplex, …) afterwards messages sent via the session these protocols Sessions
a) Establish the message formats wanted by the sender and receiver.b) Allow sender as well as receiver to validate one another's identity once rather than revalidating each message.c) Permit a set of messages to be related together (see conversations).d) Establish routing, recovery and pacing protocols.
The network OS provides connections between endpoints. A connection must be thought of as a piece of wire that can carry messages blocked (by data communications) into transmission records. Sessions map numerous to one onto connections. At any immediate a session uses a particular connection. However if the connection fails or if an endpoint fails, the session perhaps transparently mapped to a new connection. For illustration if a terminal breaks the operator may move the session to a new terminal. Likewise if a connection breaks an alternate connection may be established. Connections secrete the problems of transmission management (an SNA term):
Transmission control- blocking as well as de-blocking transmission records (TRs), managing TR sequence numbers and first-level retry logic.
Path control or else routing of TRs through the network.
Link control transfering TRs over teleprocessing lines.
Pacing dividing the bandwidth as well as buffer pools of the network among connections.
The data communications component as well as the network manager cooperates in implementing the notion of session.
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