Mobile Terminated Call (MTC)
An MTC refers to a call received by the mobile user. When a caller dials a mobile number, the GMSC (Gateway MSC) queries the HLR to locate the mobile subscriber. After locating the serving MSC and BSS, the network pages the MS, performs authentication, and sets up the call.
Steps:
1–2): Calling user dials MS number → Call reaches GMSC via PSTN.
3): GMSC queries HLR for MS location.
4–5): HLR contacts VLR to get routing info.
6): HLR sends routing info to GMSC.
7): GMSC forwards call to the serving MSC.
8–9): MSC checks MS status in VLR.
10): MSC pages BSSs in the area.
11): BSS pages the MS.
12): MS responds to BSS.
13–14): Authentication and setup via MSC & VLR.
15–17): Traffic channel allocated; call established.
Mobile Originated Call (MOC)
An MOC is a call initiated by the mobile user. The MS sends a request to the BSS, which forwards it to the MSC. The MSC then authenticates the user with the VLR and routes the call through the GMSC to reach the PSTN or other network.
Steps:
1): MS sends a request to BSS to make a call.
2): BSS forwards the request to MSC.
3–4): MSC authenticates MS with VLR.
5): Call setup proceeds from MSC to GMSC.
6–7): GMSC routes the call to PSTN (or other networks).
8–9): MSC allocates a channel for the call.
10): BSS connects the call to MS.