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Saturday, May 3, 2008

Mobile Network Abis Interface

The interface between the BSC (base station controller) and BTS (base transceiver station) is called A bis responsible for the following functions between a BSC and a BTS:
  1. Voice-data traffic exchange

  2. Signaling exchange between the BSC and the BTS

  3. Transporting synchronization information from the BSC to the BTS
Physically, an A bis interface is a primiary rate connection of 64Kbits/s or 2048Mbits/s (or 1.5Mbits/s in North America) digital link. On these digital links, bearer of signaling traffic channels of 16 or 64 Kbits/s are established.





The data link layer (layer 2) protocol running over the A bis interface is LAPD. Running on top of the LAPD is the layer 3 BTS management (BTSM) protocol, which include 3 major functions:
  1. Traffic management (radio resource control and connection control) function over the Radio Signaling Link (RSL) (SAPI=0)

  2. Operation and maintenance function over the operation and maintenance link (OML) (SAPI=62)

  3. Layer 2 management message function over the layer 2 management link (L2ML) (SAPI=63)


Amongst these 3 functions, traffic management function is the most important one. In details, this functional block includes

  1. Radio link layer management procedures controlling the establish, modify and release of LAPDm connections over the air interface (Um)

  2. Dedicated channel management procedures controlling the ciphering, channel measurement transportation, transmitter power control, handover detection and dedicated channel modification etc.

  3. Common channel management procedures for transferring channel requests from MSs, paging MSs, CCCH traffic load measurements, BCCH broadcast information modification, channel assignment and cell broadcast short message transmission etc.

  4. TRX management procedures for free traffic channel measurements or flow control.
Over all, A bis layer 3 procedures are
  1. RLM Radio Link Layer Management.

  2. DCM Dedicated Channel Management.

  3. CCM Common Channel Management.

  4. TRX Transceiver Management.

  5. O&M Operating and Maintenance.


Radio Link layer Management messages includes

  1. (01) DATA-REQ: Data request.

  2. (02) DATA-IND: Data indication.

  3. (03) ERROR-IND: Error indication.

  4. (04) IS-REQ: Establish request.

  5. (05) IS-CONF: Establish confirm.

  6. (06) IS-IND: Establish indication.

  7. (07) REL-REQ: Release request.

  8. (08) REL-CONF: Release confirm.

  9. (09) REL-IND: Release indication.

  10. (0a) UNIT-DT-REQ: Unit data request.

  11. (0b) UNIT-DT-IND: Unit data indication.

Dedicated Channel Management messages

  1. (30) BS-POWER-CTRL: Base Station power control.

  2. (21) CHAN-ACT: Channel activation.

  3. (22) CHAN-ACT-ACK: Channel activation acknowledge.

  4. (23) CHAN-ACT-NACK: Channel activation negative acknowledge.

  5. (24) CONN-FAILURE: Connection failure.

  6. (25) DEACTIV-SACCH: Deactivate SACCH.

  7. (26) ENCRYPT-COMM: Encryption command.

  8. (27) HANDOVER-DET: Handover detection.

  9. (28) MEASURE-RES: Measurement result.

  10. (2a) MODE-MOD-ACK: Mode modify acknowledge.

  11. (2b) MODE-MOD-NACK: Mode modify negative acknowledge.

  12. (29) MODE-MOD-REQ: Mode modify request.

  13. (2f) MS-POWER-CTRL: Mobile System power control.

  14. (2d) PHYS-CON-CONF: Physical context confirm.

  15. (2c) PHYS-CON-REQ: Physical context request.

  16. (32) PREP-MEAS-RES: Preprocessed measurement result.

  17. (31) PREPROC-CONF: Pre-processing configuration.

  18. (33) RF-CH-REL-ACK: RF channels release acknowledge.

  19. (2e) RF-CHANN-REL: Radio Frequency channel release.

Common Channel Management messages

  1. (11) BCCH-INFO: BCCH information.

  2. (12) CCCH-LOAD-IND: CCCH load indication.

  3. (13) CHANNEL-RQD: Channel required.

  4. (14) DELETE-IND: Delete indication.

  5. (16) IMM-ASS-COM: Immediate assign command.

  6. (15) PAGING-CMD: Paging command.

  7. (17) SMS-BC-REQ: SMS broadcast request.

TRX Management messages

  1. (19) RF-RES-IND: Radio frequency resource indication.

  2. (1A) SACCH-FILL: SACCH filling.

  3. (1B) OVERLOAD: Overload.

  4. (1C) ERR-REP: Error report.

Operating & Maintenance messages

  1. (80) O&M Message: Operation and maintenance messages.

  2. (40) MMI Transfer: Man/machine interface transfer.

  3. (20) TRAU O&M: Transcoder/transfer rate adapter unit.
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