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.
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
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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.
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TEI
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is the address used to access different
physical entities such as an individual TRX for radio signaling.
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Two data link types are defined for each TEI. The data
link types and their corresponding SAPI are:
SAPI=0
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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.
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SAPI=62
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is used for the Operations & Maintenance
Link (OML). This link is used for supporting network TRXC management
procedures.
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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.
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