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Introducing
Aurora Plus
Issue 5 - 04/01 1-15
Basic Rate power
Power is provided across the network so that, in the event of
a local mains power failure, a basic telephone service is still
provided.
The OSI 7-Layer Model
The Open Systems Interconnection (OSI) is an agreed,
international standard governing the way systems
communicate. The standard model has seven layers, as
follows:
Layer 1
The physical layer. Transmits bits between the
terminal and the network. It defines
connectors, line coding, transmission rates and
anything else concerning the transfer of bits.
Layer 2
The data link layer. Provides link level control.
Error detection and correction are handled by
assembling the bits into frames. All Layer 2
formats derive from a standard known as High
Level Data Link Control (HDLC).
Layer 3
The network layer. Routes messages to their
destination.
Layer 4
The transport layer. The end-to-end layer
which sets up and maintains connections.
Layer 5
The session layer. Handles the co-ordination
between processes.
Layer 6
The presentation layer provides data
formatting and code conversion.
Layer 7
The application layer. This is the task to be
performed, e.g. remote login and file transfer.
Chap01.P65 30/04/01, 16:26 15