A-bis interface [Under GSM]
BTS understands the physical nature of radio access and takes care of managing actual physical communication with MS. Though BTS is the one talking to MS, it is controlled by BSC. For example, MS request a channel (to make a call, send SMS etc.), the decision of granting a channel and the channel number is done by BSC.
On MS side, BTS has LAPDm and GSM Physical layer to talk to MS, on BSC side it uses "(ISDN) D channel" for Physical layer, LAPD for Data Link Layer as shown in diagram below.
DCM - Dedicated Channel Management
With DCM messages, Layer 1 of air interface is managed. For example, when MS request access, the communication related (dedicated) channel that is to be reserved happen through DCM messages.
CCM - Common control Channel Management
CCM messages carry CCCH (PCH or AGCH) information (received from or to be sent to MS). For example, when MS is to be told about assigned dedicated channel, CCM messages carry that data from BSC to BTS. CCM also include broadcast information (to transmitted over BCCH).
RLM - Radio Link Management
RLM concerns Layer 2 of air interface. Also RLM messages (DATA_REQ/DATA_IND) help BTS forward MS Layer 3 messages (CC, MM, and some of RR) to BSC.
TRXM - Transmission Management
TRXM include flow control, error reporting etc.
» BTS-BSC "A-bis" interface - 8.51, 8.52, 8.54 (layer1), 8.56 (layer2), 8.58 (layer3) «
In practice, BTSs and BSC are provided by single vendor and the interface in proprietary in nature. "Most of BTS operations need to be controlled by BSC", "Insufficient clarity in standards in describing L3 of A-bis" are probable reasons for this practice.
We will continue with A interface in next article.
References: GSM book by Mouly and Pautet, GSM Networks book by Heine
© Copyright Samir Amberkar 2010
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