LAPD protocol

All messages sent on the A-bis interface use the LAPD protocol that enables reliable transmission of information. LAPD provides two kinds of transfer modes: Unacknowledged info transfer with no guarantee that the information frame is successfully delivered to the addressee, and acknowledged information transfer, where each signal is acknowledged, and the system makes sure that the frame has reached the destination successfully. Only measurement reports use unacknowledged information transfer.

Frame Structure and Data Links

A flag, 01111110 (h'7E), delimits a frame. The one flag is enough between consecutive frames. The receiving entity looks for the flag 01111110 to synchronize on the start of a frame.

TEI and SAPI are used to access the right entity and right function at the receiving end.

SAPI	is the address used to access different functions, such as TRXC, CF and Layer 2 management procedures, within one physical entity. The CF (Central Function) link is used in RBS 2000 for common management functions for the TG, for example BTS software download.
TEI	is the address used to access different physical entities such as an individual TRX for radio signaling.

Two data link types are defined for each TEI. The data link types and their corresponding SAPI are:

SAPI=0	is used for the Radio Signaling Link (RSL). This link is used for supporting traffic management procedures mainly for circuit switched traffic. Signalling on Packet Data Channels (PDCH) is not carried by the RSL link. One link is required per TRX defined.
SAPI=62	is used for the Operations & Maintenance Link (OML). This link is used for supporting network TRXC management procedures.

The physical entities (TRX) that BSC communicates with at the BTS, via data links, are referred to as Terminal Equipment. A TEI/SAPI pair, unique within each physical connection identifies each data link. Each physical connection can support a number of data links.

Each TRX have one OML and one RSL signaling link. Additionally there is a CF signalling link to the DX function in the RBS2000. These links use the LAPD protocol:

• The CF link is identified by the TEI value (configurable) and SAPI=62.
• The OML link is identified by the TEI value for the TRX and SAPI=62.
• The RSL link is identified by the TEI value for the TRX and SAPI=0.
• The TRX TEI value is defined by the TRX position in the RBS cabinet.

The LAPD concentrator receives messages from several TRXs and sends these messages on one 64 kbit/s Abis time slot to BSC. The LAPD concentrator also receives messages on this Abis time slot from the BSC and distributes them to the TRXs.

Without LAPD Concentration and LAPD Multiplexing each 64 Kbits/s A-bis time slot can support signalling for only one TRX.

With LAPD Concentration each 64 Kbits/s A-bis time slot can support signalling for up to four TRXs. The allocation of bandwidth between the different TRXs sharing a 64 kbit/s A-bis time slot is dynamic: the concentration is implemented as separately addressed messages which are sent over the common path. This means both transmission delays are minimized - LAPD Concentration is superior to LAPD Multiplexing when it comes to delays and thoughput performance.