(Global System for Mobile Communications

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Architecture of the GSM System

GSM-elements & interfaces

System Architecture: Radio Subsystem

Mobile Station (MS)

Mobile Station (MS)

Block Diagram of BTS with TRX

Block Diagram of BTS Contd.
• Transmitter/Receiver Module – The TRX consists of a low-frequency part for digital signal processing and a high-frequency part for GMSK modulation and demodulation. • The operations and maintenance (O&M) module – connected directly to the BSC by means of a specifically assigned O&M channel. – That allows the O&M module to process the commands from the BSC or the MSC directly into the BTS and to report the results – O&M module provides a human-machine interface (HMI), which allows for local control of the BTS.


Block Diagram of BSC

BSC Contd….
• Terminal Control Elements (TCEs)
– Abis-Interface Terminal Control Elements
• Abis-TCEs are to set up LAPD connections toward the BTS peers, the transfer of signaling data, and last—but not least—the transparent transfer of payload.

– A-Interface Terminal Control Elements
• The connection of a BSC to the MSC is established via the A-TCEs. • A-TCEs is setting up and operating the SS7/SCCP connection toward the MSC.

BSC Contd….
• Database (DB)
– maintenance status of the whole BSS, the quality of the radio resources and terrestrial resources, and so on are dynamically administrated – BSC database contains the complete BTS operations software for all attached BTSs and all BSS specific information,such as assigned frequencies.

BSC Contd….
• Central Module
– One of the major tasks of the BSC is to decide when a handover should take place. The BSC may decide on intra-BTS handover and intraBSC handover without needing the MSC. In contrast, for all BSC external handovers, the BSC needs to involve the MSC. – Handover decision and power control are main tasks of the central module.

Transcoding Rate and Adaptation Unit (TRAU)
• The task of the TRAU is to compress or decompress speech between the MS and the TRAU. – called regular pulse excitation–long term prediction (RPE-LTP)

• It is able to compress speech from 64 Kbps to 16 Kbps
– Fullrate channel (net bit rate with fullrate is 13 Kbps) and to 8 Kbps in the case of a halfrate channel (net bit rate with halfrate is 6.5 Kbps)

Possible Sites for TRAU

System Architecture: Network and Switching Subsystem

Home Location Register (HLR)
• Home Location Register manages the mobile subscribers database which stores – subscriber information – part of the mobile location information – International Mobile subscriber Identity – Mobile station ISDN Number • It is the subscriber number commonly used

– VLR address

Visitor Location Register (VLR)
• Dynamically stores subscriber information, needed to handle incoming/outgoing calls, which includes, – Mobile Station Roaming Number
• When a roaming mobile enters an MSC area. This MSC warns the associated VLR of this situation; the mobile enters a registration procedure through which it is assigned a mobile subscriber roaming number (MSRN)

– Temporary Mobile Subscriber Identity, if applicable – The location area in which the mobile has been registered – Data related to supplementary service parameters



The NSS hierarchy


• Authentication Center(s) (AUC)
– Providing the authentication key used for authorizing the subscriber access to the associated GSM PLMN.

• Equipment Identity Register(s) (EIR)
– Handling Mobile Station Equipment Identity

Equipment Identity Register(s) (EIR)

• White list: contains all the approved types of mobile stations • Black list: contains those IMEIs known to be stolen or to be barred for technical reasons • Gray list: allows tracing of the related mobile stations

Numbering Arrangement in GSM
International Mobile Subscriber Identification number (IMSI) It identifies a unique international universal number of a mobile...
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