GSM Mobile Terminated Call

A Land Line Subsriber calling a Mobile Subscriber.

The main difference between a call to an MS and a call from an MS is that, the exact location of MS is unknown. Hence, the MS must be located using paging before a connection can be established. 

1. The PSTN subscriber dials the MS’s telephone number (MSISDN), the MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. A connection is established to the MS’s home GMSC. The PSTN sends an Initial Address message (IAM) to the GMSC.
2. The GMSC analyzes the MSISDN to find out which HLR, the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR. The HLR looks up the MSISDN and determines the IMSI and the SS7 address for the MSC/VLR that is servicing the MS. The HLR also checks if the service, “call forwarding to C-number” is activated, if so, the call is rerouted by the GMSC to that number.
   
3. The HLR then contacts the servicing MSC/VLR and asks it to assign a MSRN to the call. [MSRN - Mobile Station Routing Number].
4. The MSC/VLR returns an MSRN via HLR to the GMSC.
5. The GMSC sends an Initial Addressing message (IAM) to the servicing MSC/VLR and uses the MSRN to route the call to the MSC/VLR. Once the servicing MSC/VLR receives the call, the MSRN can be released and may be made available for reassignment.
6. The MSC/VLR then orders all of its BSCs and BTSs to page the MS. Since the MSC/VLR does not know exactly which BSC and BTS the MS is monitoring, the page will be sent out across the entire Location Area(LA). 
7. When the MS detects the paging message to the BTS’s in the desired LA. The BTS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area.
8. When the MS detects the paging message, it sends a request on RACH for a SDCCH.
9. The BSC provides a SDCCH, using AGCH.
10. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes: Marking the MS as “active” in the VLR. Authentication procedure (Start ciphering, Equipment identification).
11. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.

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