GSM Originating Call Flow

A Mobile User calling a Land Line Subscriber.

1. MS after dialing a number & pressing SEND key, sends Channel Request(Chan_Req) message on RACH to ask for a signaling channel (Radio Resources). [RACH - Random Access Channel]
   
2. The BSC allocates a Traffic Channel(TCH) using AGCH. TCH allocation assigns a specific Frequency & a Timeslot on that frequency. [AGCH - Access Grant Channel]
   
3. The MS sends a call setup request through SDCCH, to the MSC/VLR. [SDCCH - slow dedicated control channel]. Over SDCCH, all signaling takes place. This includes: marking the MS status as active in the VLR
4. Then comes Authentication Procedure which includes Ciphering (The channel is ciphered so as to protect the call), Equipment Identification, etc.
   
5. The MSC/VLR instructs the BSC to allocate an Idle TCH (this message contains the dialed digits and other information needed for call establishment). The BTS and MS are told to tune to the TCH. 
6. The MSC allocates a voice circuit on one the digital trunks between the MSC and the BSS.
7. MSC informs the BSS about the allocated voice circuit. The call is also switched from signaling to voice.
8. The BSS notifies the Mobile about the changeover to voice mode.
9. The MSC routes the call and sends the call towards the called subscriber.
10. The PSTN indicates to the MSC that it has received all the digits and the called subscriber is being rung.
11. The MSC informs the mobile that the called subscriber is being alerted via a ring.
12. The called subscriber answers the call.

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