bsc – base station controller

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BSC

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–Block diagram

BSC Architecture Switch MatrixBSC 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 functionalityInternal 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.

The transfer of payload.

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.

Abis TCE

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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 A-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 depends largely on the predicted traffic load.

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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 intra-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

main tasks of the central module.

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Connection to OMC

Central module is the connected to the OMC.

Every BSS is supervised and managed by an OMC via the BSC.

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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.

TRAU is not used for data connections.

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TRAU - Site location

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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. Transcoding and Rate Adaptation functionality.

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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 assignment of a TCH by the BSC.

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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

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