Technical data

www.westermo.com Theoretical and general applications 101
Layer 2 – Data link layer
The data link layer for ISDN is specified by ITU Q.920 to Q.923 standards. The D-
channel’s signalling is defined in Q.921. Link Access Procedure – D channel (LAP-D) is
the protocol used in the data link layer. The LAP-D is nearly identical to X.25 LAP-B
and both are based on HDLC. The structure of the frames used by LAP-D are shown
below:
SAPI (Service Access Point Identifier), 6-bits.
C/R (Command/Response) bit that indicates whether the frame is a command
or answer.
EA0 (Address Extension) Bit that is set to indicate the last byte in an address.
TEI (Terminal Endpoint Identifier) 7-bits device identifier (see page 102).
EA1 (Address Extension) bit, same functionality as EA0.
Control (2 bytes)
The control field is used to show the type of frame and command. There are three dif-
ferent types of frames: Information, Control/Monitoring and Unnumbered frames
where the two first-mentioned also contain the sequence numbers (N[r] and N[s]).
Information
Information to the overlying network layer and user data.
CRC (2 bytes)
Cyclic Redundancy 16-bits checksum to detect bit errors in the transfer.
Flag (1 octet)
Final flag always 7E16 (0111 11102).
Flag (1 octet)
Start flag always 7E16 (0111 11102).
Address (2 octets)
Flag Address Control Information CRC Flag
876 54 3 2 1
SAPI (6 bits) C/R EA0
TEI (7 bits) EA1
B A C K