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)