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The Abis protocol is the protocol spoken between the BSC (Base Station Controller) and BTS (Base Transceiver Station).

It is specified in the following GSM standards:
  • 3GPP TS 08.51 : BSC-BTS Interface General Aspects
  • 3GPP TS 08.52 : BSC-BTS Interface principles
  • 3GPP TS 08.54 : BSC-BTS Layer 1: Structure of Physical Circuits
  • 3GPP TS 08.56 : BSC-BTS Layer 2: Specification
  • 3GPP TS 08.58 : BSC-BTS Layer 3: Specification
    • This contains the specification of the RSL (Radio Signalling Link) on SAPI 0
  • 3GPP TS 21.21 : Network Management procedures and messages on the A-bis interface
    • This basically describes the OML (Organization and Maintenance Layer) on SAPI 62
  • 3GPP TS 04.08 : Mobile radio interface layer 3 specification
    • Description of the on-air (BTS-MS) layer 3, as carried in "DATA INDICATION" and other Abis messageS

RSL (Radio Signalling Link)

The RSL contains two types of messages
  • transparent messages (forwarded by BTS without interpretation or changes). T-bit in message discriminator 1
  • non-transparent messages (messages sent between BSC and BTS)

furthermore, the messages are grouped into four main groups (see below).

All messages are transmitted using LAPD I frames, except for MEASUREMENT RESULT (UI). T-bit in message discriminator 0

Radio Link Layer Management

All messsages are prefixed by 6-byte header:
  • 1 byte message discriminator
  • 1 byte message type
  • 2 byte channel number
  • 2 byte link identifier
The actual message types are:
  • EST_IND: Link establishment indication (BTS->BSC)
  • EST_REQ: Link establishment request (BSC->BTS)
  • EST_CONF: Link establishment Confirmation (BTS->BSC)
  • REL_IND: Link release indication (BTS->BSC)
  • REL_REQ: Link release request (BSC->BTS)
  • REL_CONF: Link release confirmation (BTS->BSC)
  • DATA_REQ: Transmission of transparent L3 message in acknowledged mode (BSC->BTS)
  • DATA_IND: Reception of transparent L3 message in acknowledged mode (BTS->BSC)
  • UNIT_DATA_REQ: Transmission of transparent L3 message in unacknowledged mode (BSC->BTS)
  • UNIT_DATA_IND: Reception of transparent L3 message in unacknowledged mode (BTS->BSC)
  • ERROR_IND: Link error indication (BTS->BSC)

Dedicated Channel Management

All messsages are prefixed by 4-byte header:
  • 1 byte message discriminator
  • 1 byte message type
  • 2 byte channel number
The actual message types are:
  • CHAN_ACTIV: Channel activation (BSC->BTS)
  • CHAN_ACTIV_ACK: Channel activation acknowledgement (BTS->BSC)
  • CHAN_ACTIV_NACK: Channel activation not acknowledged (BTS->BSC)
  • MODE_MODIFY: Channel mode modify (BSC->BTS)
  • MODE_MODIFY_ACK: Channel mode modify acknowledgement (BTS->BSC)
  • MODE_MODIFY_NACK: Channel mode modify not acknowledged (BTS->BSC)
  • HANDO_DET: Handover detection (BTS->BSC)
  • ENCR_CMD: Start of Encryption (BSC->BTS)
  • MEAS_RES: Measurement result (BTS->BSC)
  • PREPROC_CONFIGURE: Configure pre-processed measurement report (BSC->BTS)
  • PREPROC_MEAS_RES: Preprocessed measurement report (BTS->BSC)
  • DEACT_SACCH: Deactivate SACCH (BSC->BTS)
  • RF_CHAN_REL: Radio channel release (BSC->BTS)
  • RF_CHAN_REL_ACK: Radio channel release ack (BTS->BSC)
  • MS_POWER_CONTROL: MS power control (BSC->BTS)
  • BS_POWER_CONTROL: Transmission power control (BSC->BTS)
  • CONN_FAIL_IND: Connection failure indication (BTS->BSC)
  • PHYS_CONTEXT_REQ: Physical context request (BSC->BTS)
  • PHYS_CONTEXT_CONF: Physical context confirmation (BTS->BSC)
  • SACCH_INFO_MODIFY: SACCH nformation modify (BSC->BTS)
  • TALKER_DET: Talker detection (BTS->BSC)
  • LISTENER_DET: Listener detection (BTS->BSC)
  • REMOTE_CODEC_CONF: Remote codec configuration (BTS->BSC)
  • RTD_REP: Round Trip Delay Report (BTS->BSC)
  • PRE_HANDO_NOTIF: Pre-handover warning (BSC->BTS)
  • CODEC_MOD_REQ: Multirate codec configuration change request (BSC->BTS)
  • CODEC_MOD_ACK: Multirate codec configuration change acknowledge (BTS->BSC)
  • CODEC_MOD_PERF: Multirate codec change performed (BTS->BSC)
  • TFO_REPORT: TFO Report / AMR (BTS->BSC)
  • TFO_MODIFICATION_REQ: TFO Modification Request (BSC->BTS)

Common Channel Management

All messsages are prefixed by 4-byte header:
  • 1 byte message discriminator
  • 1 byte message type
  • 2 byte channel number
The actual message types are:
  • CHAN_RQS: Channel request by MS (BTS->BSC)
  • PAG_CMD: Paging command (BSC->BTS)
  • DELETE_IND: Delete indication (BTS->BSC)
  • CCCH_LOAD_IND: CCCH load indication (BTS->BSC)
  • BCCH_INFO: Broadcast information modify (BSC->BTS)
  • SMS_BROADCAST_REQ: Short Message Cell Broadcast (BSC->BTS)
  • SMS_BROADCAST_CMD: Short Message Cell Broadcast (BSC->BTS)
  • IMM_ASS_CMD: Immediate Assign command (BSC->BTS)
  • NOTIF_CMD: Notification (BSC->BTS)

TRX Management

All messsages are prefixed by 2-byte header:
  • 1 byte message discriminator
  • 1 byte message type
The actual message types are:
  • RF_RES_IND: Radio resource indication (BTS->BSC)
  • SACCH_FILL: SACCH filling information modify (BSC->BTS)
  • OVERLOAD: Flow control (BTS->BSC)
  • ERROR_REPORT: Error reporting (BTS->BSC)