Link Access Procedure for the D-Channel

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Link Access Procedure for the D-Channel ( LAPD ) is a protocol of the security layer ( Link Access Protocol or Link Access Procedure ) in ISDN . LAPD is a layer 2 protocol in the sense of the OSI layer model and is used to secure data transmission in the signaling channel (D channel) . It corresponds to the function of HDLC (High-Level Data Link Control) .

standardization

LAPD is described in recommendations Q.920 and Q.921 of the ITU-T .

Data format

The basic structure for data transmission using LAPD can be shown roughly as follows

Flag SAPI TEI CF Data FCS (16 bit) Flag
01111110 111111x 1 x 2 1111111x 3 Control panel User data / information Checksum (CRC) 01111110
111111 TEI administration block (Management-SAPI, M-SAPI for short)
000000 Signaling block (Signaling-SAPI, S-SAPI for short)
010000 Package data block (Package-SAPI, P-SAPI for short)
x 1 is the "Command / Response" bit
x 2 is the "Extended Address" bit, it is always set to "0" in the SAPI, indicating that there are still octets for the address field
0000000 plant
01 Dec - 63 Dec non-automatic (standard)
64 Dec - 126 Dec automatic
127 Dec Broadcast (with TEI allocation procedure)
x 3 is the "Extended Address" bit, it is set to "1" and shows the end of the address field (the address field therefore consists of SAPI and TEI)

variants

The LAPDm variant is available for GSM mobile communications . This stands for Link Access Procedure on the Dm Channel.

literature

  • Ray Horak: Telecommunications and Data Communications Handbook. 2nd Edition, Wiley 2008, ISBN 978-0-470-12722-3 .
  • John Cowley: Communications and Networking. Second Edition, Springer Verlag, London / Heidelberg 2012, ISBN 978-1-4471-4356-7 .
  • Martin Werner: Information and Coding. 2nd edition, Vieweg + Teubner Verlag, Wiesbaden 2008, ISBN 978-3-8348-0232-3 .

See also

Web links