GSM Operation

GSM Operation:

Call Routing:

-Call Originating from MS

-Call termination to MS

Outgoing Call:

1                    - MS sends dialed number to BSS 

2                    - BSS sends dialed number to MSC

3, 4                - MSC checks VLR if MS is allowed the requested service. If so, MSC asks BSS to allocate resources for call.

5                    - MSC routes the call to GMSC

6                    - GMSC routes the call to local exchange of called user

7, 8, 9, 10      - Answer back (ring back) tone is routed from called user to MS via GMSC, MSC and BSS

Incoming Call:

1                    - Calling a GSM subscribers

2                    - Forwarding call to GSMC

3                    - Signal Setup to HLR

4, 5                - Request MSRN from VLR

6                    - Forward responsible MSC to GMSC

7                    - Forward Call to current MSC

8, 9                - Get current status of MS

10, 11            - Paging of MS

12, 13            - MS answers

14, 15            - Security checks

16, 17            - Set up connection

Handover:

Handover (or, in North America, handoff) occurs in both digital and analogue mobile networks when a mobile station (mobile phone or PDA) with an active data or voice call moves between cells. It is initiated by the network when signal strength drops so low that better signal quality or signal strength can be gained by switching to a new cell.

Between 1 and 2              – Inter BTS / Intra BSC

Between 1 and 3              – Inter BSC/ Intra MSC

Between 1 and 4              – Inter MSC