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BSC – Base Station
Controller
• BSC forms the center of the BSS.
• Connects to many BTSs over the
Abis-interface (depending on the manufacturer) • A small digital exchange with some
mobile-specific extensions.
• BSC was defined with the intention of removing most of the radio-related load from the MSC.
BSC Architecture Switch
Matrix
BSC acts like a small digital exchange and hence has a switch matrix.
Switch Matrix
-Switch traffic channels coming from A interface coming from MSC to the correct Abis interface channels.
Switch matrix –
Takes care of relay functionality Internal Control bus
BSC Architecture -Terminal Control
Elements of the Abis-Interface
• Connection to the BTSs is established via the Abis-terminal control elements (TCEs),
• Provide the control function for a TRX or a BTS from BSC’s central unit.
The number of Abis TCEs that a BSC may contain depends largely on number of BTSs (and on the system manufacturer.)
BSC- A-bis TCE
Major tasks of the Abis-TCEs:
Set up LAPD connections toward the BTS peers. The transfer of signaling data.
BSC – Abis TCE contd.,
• Abis TCEs also may be responsible for the administration of BTS radio resources.
• Performs assignment and release of
signaling and traffic channels over the Abis-interface.
•Also performs evaluation of measurement results from the BTS concerning busy and idle channels, which are relevant for power control and used in making decisions about handovers.
Connections from the Abis TCEs to the A-TCEs are realized by the switch matrix.
On the other side, the PCM connections are achieved by associated transmission
elements.
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A-Interface Terminal Control
Elements
Connection of a BSC to the MSC is established via
the A-TCEs.
BSC is connected to only one MSC.
Large number of TCEs is required to support
A-interface since all the payload and the major part of the signaling data of the entire BSS have to be conveyed over this interface.
The number of necessary signaling channels
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Database
The BSC is the control center of the BSS. BSC must maintain a relatively large
database in which the maintenance status of the whole BSS the quality of the radio resources and terrestrial resources are dynamically administrated.
BSC database contains the complete BTS
operations software for all attached BTSs and all BSS specific information, such as assigned frequencies
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Central Module
Major tasks of the BSC is to decide when a
handover should take place.
BSC will decide BTS handover and
intra-BSC handover without needing the MSC.
For all BSC external handovers, the BSC
needs to involve the MSC.
Handover decision and power control are
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Connection to OMC
Central module is the connected to the
OMC.
Every BSS is supervised and managed
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TRAU- Transcoding and
Adaptation Unit
One of the most interesting functions in
GSM involves the TRAU
Typically is located between the BSC
and the MSC.
Task of the TRAU is to compress or
decompress speech between the MS and the TRAU.
Method used is called regular pulse
excitation–long term prediction (RPE-LTP).
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TRAU
Able to Compress speech from 64 Kbps to
16 Kbps, in the case of a fullrate channel and to 8 Kbps in the case of a halfrate
channel.
The interface between the BSC and the
TRAU is called Ater interface.
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TRAU Site Location
Speech compression is intended mainly to
save resources over the Air-interface, it also is suitable to save line costs when applied on terrestrial links.
Specifications allow for the installation of the
TRAU between the BTS and the BSC.
That requires, however, the use of 64-Kbps
channels between the BSC and the MSC and hence the use of more links.
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The functions performed by BSC can be summarized as below:
Communicates with MSC and BTS. Manages Radio resources for BTS.
Assigns Frequency and time slots to all the
MS in its area.
Handles call setup. Radio power control.
Manages the Handovers.
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Call Set up
Call set up involves the following
processes:
Paging: the BSC sends paging
messages to the BTS’s defined within the desired LA.
Signaling set-up: during call set-up,
the MS connection is transferred to an SDCCH allocated by the BSC.
Assignment of traffic channel: after
SDCCH assignment, the call set-up procedure continues with the
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During a call
The main BSC functions during a call
are:
Dynamic power control in MS and
RBS: the BSC calculates adequate MS
and BTS output power based on the received measurements of the uplink and downlink.
This is sent to the BTS and the MS every
480 ms to maintain good connection quality.
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Locating: This function continuously
evaluates the radio connection to the MS, and, if necessary, suggests a
handover to another cell.
Handover: if the locating function
proposes that a handover take place, the BSC then decides which cell to
handover to and begins the handover process.
Frequency Hopping: two types of
hopping are supported by the BSC as Baseband hopping and synthesizer hopping