Mobile Terminated Call Flow

Route Establishment to find the MSC/VLR

The calling party dials the MSISDN for the mobile subscriber. The PSTN identifies the network (PLMN) that the dialed MSISDN belongs to and will locate a GMSC for that network. The PSTN sends an Initial Address message to the GMSC.
The GMSC forwards the MSISDN to the HLR and requests routing information for it. 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 then contacts the servicing MSC/VLR and asks it to assign a Mobile Station Routing Number (MSRN) to the call.

The MSC/VLR allocates the MSRN and forwards it to the HLR.Note: It is important to remember that the MSC/VLR assigns a MSRN to the call not to the MS itself.
The HLR forwards the MSRN as well as routing information for the servicing MSC/VLR to the GMSC.
The GMSC sends an Initial Addressing message 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.

Paging the Mobile Station

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.

Initial Setup
The MS receives the Page Request (PAG_REQ) on the PCH. The MS recognizes that the page is intended for it, based on a TMSI or an IMSI.

The MS sends a Channel Request (CHAN_REQ) message on the RACH.

The BSS responds on the AGCH by sending an Immediate Assignment (IMM ASS) message which assigns an SDCCH to the MS. At this point, the network does not know that the MS is the one that it is paging, it only knows that this MS wants access to the network.
The MS immediately switches to the assigned SDCCH and sends a Paging Response (PAG_RES) message on the SDCCH. This lets the network know that the MS is responding to its page.

Authentication
Before the network will provide any services to the MS, the network will require the MS to authenticate itself. The BSS sends an Authentication Request (AUTH_REQ) message to the MS. The RAND serves as the "challenge" for authentication.
The MS calculates the proper SRES based on the RAND that was given and sends the SRES to the BSS in an Authentication Response (AUTH_RESP) message.
The BSS verifies the SRES. If the SRES is correct then the MS is authenticated and allowed access to the network.
Once the MSC/VLR has authenticated the MS, it will order the BSS and MS to switch to cipher mode using the CIPH_MOD_CMD message. Once the MS in encryption mode, the VLR will normally assign a new TMSI to the MS.

Establishing a Channel
Once the MS is authenticated and in encryption mode, The MSC sends a Setup Message to the BSS, the BSS forwards the SETUP message to the MS on the assigned SDCCH.the assigned SDCCH. The SETUP message may include the Calling Line Identification Presentation (CLIP), which is essentially caller ID.
The MS responds by sending a Call Confirmed (CALL_CON) message; which indicates that the MS is able to establish the requested connection. The BSS relays the message up to the MSC.

Call Setup
The BSS then sends an Assignment Command (ASS_CMD) message to the MS on the assigned SDCCH. The ASS_CMD message assigns a Traffic Channel (TCH) to the MS.
The MS immediately switches to the TCH and responds with an Assignment Complete (ASS_COM) message on the FACCH. The MS begins ringing once it has established the TCH. Remember that all signaling that occurs on the traffic channel actually occurs on a FACCH, which is a time slot that is stolen from the TCH and used for signaling. The MS sends an ALERT message to the MSC on the FACCH. The BSS forwards the ALERT message through the PSTN to the calling party and the caller hears the line ringing.

Call Establishment
Once the user answers the call (by pressing the send button), the MS will send a Connect CON message to the MSC. The Connect message is forwarded back to the caller's switch to activate the call.
The MSC sends a Connect Acknowledge CON_ACK message to the MS and the call is established.

Call Disconnect
Disconnect happens the same way as for any other call. In this example, the calling party initiates the disconnect.
When the calling party hangs up, the calling party's switch initiates a Release (REL) message. The message is forwarded to the serving MSC, which is then forwarded to the BSS.
The BSS will send a Disconnect (DISC) message to the MS on the FACCH.
The MS confirms release of the call by sending a Release (REL) message on the FACCH, which is forwarded to the MSC.
The MSC sends e Release Complete (REL_COM) message through the BSS to the MS. As far as call control (CC) is concerned, the connection has been terminated.
The MS still has a TCH assigned to it, so the BSS sends a Channel Release (CHAN_REL) message to the MS. This releases the radio resource on the Air Interface.
The MS responds be sending a final Disconnect message and returns to idle.