GSM mobile phone network entry procedure

This page on GSM tutorial covers GSM mobile phone network entry or call setup or Power ON procedure, MO/MT call, VAMOS, AMR, MSK and GMSK modulation.

gsm call setup
Fig.6 GSM Call Setup, it depicts basic flow of logical frames between BTS and MS to establish voice/data connection.

Following steps are followed at GSM Mobile phone before you actually start talking or using it for data operations. These are called initial mobile phone procedures when you power ON the phone.

Step-1 : Mobile phone scans for carriers and determine RSSI of all and pass them to upper layer, upper layer decide which carrier/channel has the highest RSSI and mobile will lock on to that carrier. There are two modes here first mode where mobile has prior knowledge of broadcast carriers and the other mode where mobile has no prior knowledge. In the second case mobile has to search for entire band while in the first case as mobile has broadcast carriers known and it will determine RSSI of those carriers only, hence it will complete cell search operation in less time.

Step-2: Once carrier is known it will detect FB(Frequency correction Burst) on that carrier/channel which is a pure sine wave as mentioned above of value 67.7 KHz. Any deviation from this value is determined and this much frequency offset is corrected on LO module by controlling through VCTCXO/VCO/OCXO used in the handset design.

Step-3: After correcting for Frequency offset, now mobile need to lock on to particular time slot on that carrier frequency in the the GSM time domain frame structure. This is done using SB decoding. 25 bits of decoded data of SCH gives reduced frame number(19 bits) and BSIC(6 bits). Reduced frame number will provide very useful information of mobile's physical slot in the entire hyperframe. BSIC is made of BCC(Base Station Color code- 3 bits) and NCC(Network Color code-3 bits). BCC field directly provide training sequence details(26 bits in size).The correlation is performed with known training sequence to determine peak and hence timing offset is determined on the received frame. channel estimation is also performed using this training sequence. Remember SB comes on the same time slot as FB but after 8 time slots duration. This means time multiplexing of logical channels(FB,SB,BCCH,CCCH...) is used on the dedicated physical time slot(TS0 at Broadcast Freq).

Step-4: Once SB is decoded now BCCH will appear on the same allocated physical time slot but after 8 time slot duration. BCCH is decoded which gives useful system informations(SI). Now mobile is camped on the network and it is ready to use voice services by exchanging useful frames/channels based on mobile initiated or mobile terminated call. If GPRS is enabled on the mobile phone, it can use data services provided by operator.


This tutorial on GSM is the concise representation of GSM technology which covers GSM specifications,GSM system basics, network architecture, network elements, frame structure, GSM physical layer i.e. Layer-1, speech processing in GSM, GSM logical and physical channels,mapping between them,VAMOS,AMR,MSK/GMSK and GSM MO/MT call flow.

Useful Links

GSM protocol stack which covers GSM layer 1, layer 2 and layer 3 basics.Read More
GSM Physical layer or GSM layer 1 modules in depth.READ MORE

GSM standard References

3GPP TS 05.02 and other useful documents relating to GSM can be found at following link

GSM Modem manufacturers

Read more on GSM Modem and GPRS Modem Manufacturers/Suppliers.

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