Link Access Procedure for the D-Channel
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)
- CF: Control Field
- FCS: Frame Check Sequence
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
- Implementation of the link access procedure on the ISDN D-channel (accessed on July 7, 2017)
- TECHNICAL REFERENCE 41459 (accessed July 7, 2017)
- Line protocols and ISO / OSI layer model Bisync protocol (accessed on July 7, 2017)
- GLOSSARY OF NET-WORKING TERMS (accessed July 7, 2017)