The signalling link protocol over the Abis interface is the LAPD, which is an HDLC (High Level Data Link Control) type link protocol. In addition to the properties from the HDLC protocol, the LAPD includes a management of many simultaneous connections. The LAPD provides a reliable transfer over a direct link between two nodes.
The addressing of signalling packets to a transceiver TRX and other units of a base station is made by using a Terminal Endpoint Identifier (TEI) field of the LAPD frame. Then, each transceiver and other unit has its own TEI. Different logical links may be defined for a TEI value by means of a Service Access Point Identifier (SAPI) field of the LAPD frame. At present, three logical links are defined for each TEI value
Radio Signalling Link (RSL) for supporting traffic management procedures, SAPI=0
Short Message Service, SAPI=3
Operations and Maintenance Link (OML) for supporting network management procedures, SAPI=62
Layer 2 Management Link (L2ML) for layer 2 management messages, SAPI=63.
For a packet data transmission according to the invention, a new logical link and a new corresponding SAPI value are reserved:
Packet Data Link (PDL) for packet data transmission procedures, SAPI=x (e.g. SAPI=5).