Write short note on Call setup of GSM network for mobile-to-mobile call.
GSM (Global System for Mobile Communications), is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by wireless phones.
Originally, the GSM standard was developed to replace the first generation (1G) analog cellular networks, and was described a digital, circuit switched network optimized for full duplex voice telephony. Over time, GSM’s capabilities expanded to include data communications, first by circuit switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS). The radio network consists of a large number of BTSs. Each BTS is given an identity. These BTSs are grouped according to location area, also given an identity. Each MSC/VLR (Mobile Services Switching Center/Visitor Location Register) serves the BTSs in a number of location areas. The GSM phones reports to the network (VLR) when it moves from a BTS in one Location Area to a BTS in another location area. the VLR always knows in which location area the GSM subscriber is located in at any given moment. Accordingly, the HLR always knows which MSC/VLR the GSM subscriber is at as well. Then, the GSM subscriber’s telephone number tells the network which HLR the actual GSM subscriber belongs to. the radio path and the base station network a call request for a GSM subscriber is sent from a mobile phone to MSC/VLR). The MSC/VLR then collects authentication data from HLR. Next, the MSC/VLR requests HLR of the actual location of GSM subscriber, and the call is established.
Different procedures are necessary depending on the initiating and terminating party:
Mobile Originating Call MOC
1. Channel Request: The MS requests for the allocation of a dedicated signalling channel to perform the call setup.
2. After allocation of a signalling channel the request for MOC call setup, included the TMSI (IMSI) and the last LAI, is forwarded to the VLR
3. The VLR requests the AC via HLR for Triples (if necessary).
4. The VLR initiates Authentication, Cipher start, IMEI check (optional) and TMSI Re-allocation (optional).
5. If all this procedures have been successful, MS sends the Setup information (number of requested subscriber and detailed service description) to the MSC.
6. The MSC requests the VLR to check from the subscriber data whether the requested service an number can be handled (or if there are restrictions which do not allow further proceeding of the call setup)
7. If the VLR indicates that the call should be proceeded, the MSC commands the BSC to assign a Traffic Channel (i.e. resources for speech data transmission) to the MS
8. The BSC assigns a Traffic Channel TCH to the MS
9. The MSC sets up the connection to requested number (called party).
Remark: This MOC as well as the MTC described in the following describes only the principles of an MOC / MTC, not the detailed signalling flow.
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