08.563GPPBSC-BTS Layer 2SpecificationTS
The following information categories are supported by the procedures of this layer 2 Recommendation:
– signalling (including Short Message Service (SMS) information);
– operation and maintenance;
– layer 2 management.
For each of these categories the BSC may have one or more layer 2 links to every TRX and BCF.
The signalling links over the A-bis interface are addressed to the different units by Terminal Endpoint Identifiers (TEI).
The same unit will normally have more than one functional entity. The logical links between different functional entities are identified by functional addresses, the Service Access Points Identifier (SAPI).
Figure 1 shows the architectural model with different layer 2 links.
A number of logical links may be multiplexed on each physical link. The same layer 2 logical link may not be distributed over more than one physical link.
Figure 1: An example of Logical Layer 2 links supported by three permanent
and one semipermanent links