08.523GPPBase Station Controller - Base Tranceiver Station (BSC-BTS) Interface - Interface PrinciplesTS
6.1 Communication channels
The A-bis interface is considered to have two types of communication channels (see figure 6.1):
– traffic channel (SDC) at 8, 16 or 64 kbit/s carrying speech or data of one radio traffic channel (Bm or Lm channel),
– signalling channels (SCH) at 16, 32 or 64 kbit/s, carrying signalling information (both BSC-MS and BSC-BTS signalling).
Figure 6.1/08.52: Communication Channel Types
6.2 Signalling links
The addressing of TRXs and BCF is made using separate TEIs for each TRX and BCF.
Three logical links are defined for each TEI:
RSL: Radio Signalling Link used for supporting traffic management procedures (MS to network communication). One link per TRX.
OML: Operations and Maintenance Link used for supporting network management procedures (transferring operations and maintenance messages). One link per TRX and BCF.
L2ML: Layer 2 Management Link used for transferring layer 2 management messages to TRX or BCF. One link per TRX and BCF.
A logical diagram for the architecture of the signalling links is given in figure 6.2/08.52.
Only point to point signalling links are used.
SMS messages are also carried on the signalling links.
Figure 6.2/08.52: Logical L2 links of A-bis interface
6.3 Signalling model
A signalling model for the A-bis interface and its signalling environment can be found in figure 6.3.
CM and MM messages are not interpreted by BSC or BTS. Over the A interface they are tranferred using DTAP (Direct Transfer Application Part) and over the A-bis interface they are transferred as transparent messages.
RR messages are mapped to BSSAP (BSS Application Part) in BSC. In BTS, most of them are handled as transparent messages. However, some of them have to be interpreted by BTS (e.g. random access, start ciphering, paging). The BTSM (BTS Management) entities contain procedures for handling these messages and also other procedures for managing the BTS as defined in Technical Specification 3GPP TS 08.58. In BTS there is a mapping between BTSM and the relevant RR messages over the radio interface (RR’).
The Layer 2 protocol over the A-bis interface is based on LAPD. L2 addressing is made to TRX (or BCF) using the TEI of LAPD. Different L2 links are used for traffic management messages (RSL, Radio Signalling Link), network management messages (OML, Operation & Maintenance Link) and L2 management messages (L2ML, Layer 2 Management Link).
A model of Layer 3 for the A-bis interface can be found in figure 6.4.
For traffic management, two types of signalling messages have been defined:
Transparent Messages:Messages which are forwarded by BTS without interpretation or changes.
Non-Transparent Messages:Messages which are sent only between BSC and BTS and which BTS is acting upon or which are the results of BTS actions.
In addition, the messages have been grouped into four main groups: Radio Link Layer Management, Dedicated Channel Management, Common Channel Management and TRX Management messages.
Discrimination between these types and groups is based on the Message Discriminator which is sent as the first octet in all messages. Transparent and non-transparent messages are discriminated by a transparancy flag (T-bit) in the Message Discriminator. Transparent messages are merely forwarded to L2 on the radio interface.
In order to address the relevant radio channel, a Channel Number element is included to support the distribution of messages to relevant physical channels on the TRX. A Link Identifier element supports the distribution on logical links/channels on the radio interface (compare the DLCI element of the A interface, 3GPP TS 08.06).
Figure 6.4/08.52: L3 model
Annex A (informative):
Version for Release 99
Update to 3GPP TS style. Editorial corrections to reference clause. IPR clause removed.