Introducing AuroraPlus
Issue 5 - 04/01 1-17
Layer 2
Layer 2 provides a secure, error-free connection for Layer 3
call control information, by organising the Layer 3 octets
into error checked frames. Layer 2 procedures, based on
HDLC, are known as Link Access Procedure for a D
Channel (LAPD).
A Layer 2 frame is structured as follows:
Flags Indicate the start and end of the frame.
Address A unique value identifying the message type
and the channel to which it refers. Contains
a SAPI (Service Access Point Indentifier) to
identify the service that the frame is
intended for, a TEI (Terminal Endpoint
Indentifier) to identify the terminal, and a
Command/Response bit.
Control Carries information that identifies the
frame.
Information Carries the TEI management protocol or
layer 3 message.
FC Sequence Used for error detection.
Layer 3
Layer 3 routes messages to their destination. The diagram
below shows the structure of the Layer 3 signalling messages.
Chap01.P65 30/04/01, 16:2717
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