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Sonata
Hand Held ISDN Tester—User Guide
A2-10 427869
Primary Rate Layer 1
Layer 1 Frame Structure
Data is transferred across the line in ‘layer 1’ frames. In
Europe, each frame is 256 bits in length and lasts for 125
microseconds. In the U.S., each frame is 193 bits in length and
lasts for 125 microseconds.
The structure of the European frame is shown below:
Layer 2
Layer 2 provides a secure, error-free connection for Layer 3
call control information, by organising the Layer 3 bits 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 terminal to which it refers. Contains a SAPI
to identify the service for which the frame is
intended, a TEI to identify the terminal and a
Command/Response bit.
Control Carries information that identifies the frame.
Information Carries the signalling or layer 3 message.
FC Sequence Used for error detection.
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