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Sonata
Hand Held ISDN Tester—User Guide
A2-8 427869
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 Physical layer—transmits bits between the
terminal and the network. Defines connectors,
line coding, transmission rates and anything else
concerning the transfer of bits.
Layer 2 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 Network layer—routes messages to their
destination.
Layer 4 Transport layer—the end-to-end layer which sets
up and maintains connections.
Layer 5 Session layer—handles the co-ordination between
processes.
Layer 6 Presentation layer—provides data formatting and
code conversion.
Layer 7 Application layer—this is the task to be
performed, e.g. remote login and file transfer.
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