Mobile Originated Call Flow

Request Access
The MS sends a Channel Request (CHAN_REQ) message on the RACH. The BSS responds with a radio resource assignment (IMM_ASS_CMD) on the AGCH. The MS sends a Service Request (CM_SERV_REQ) message to the BSS on the SDCCH.


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. The BSS will send a Service Accept (CM_SERV_ACC) message letting the MS know that the service request was received and processed.
Once authenticated, the BSS orders the MS to switch to cipher mode with the CIPH_MOD_CMD message.


Initial Call Setup
The MS will immediately switch to cipher mode and send a Cipher Mode Complete (CIPH_MOD_COM) message.
The MS then sends a Call Setup (SETUP) message to the BSS. The message includes the address information (MSISDN) of the called party.
The BSS assigns a TCH to the MS by sending an Assignment Command (ASS_CMD) message. This message includes which Transceiver (TRX) and which Time Slot (TS) to use.
The BSS does not actually assign a TCH to the MS until the MSC sends a Call Proceeding (CALL_PROC) message to the BSS indicating that the IAM has been sent.
The MS immediately switches to the assigned TCH. The MS sends an Assignment Complete (ASS_COM) message back to the BTS on the FACCH.
Remember that a FACCH is not a separate channel; it is simply a stolen time slot from the TCH that is used for signaling data instead of voice traffic.


Call Setup
The MSC sends an Initial Address Message (IAM) to the GMSC. The IAM contains the MSISDN of the called party as the MS dialed it.
The MSC will also send a Call Proceeding (CALL_PROC) message down to the BSS and this is when the BSS would assign a TCH to the MS, as described in step 10 above.
Based on the dialed number, the GMSC decides where to route the IAM within the PSTN.
The PSTN will continue to route the IAM until it reaches the correct Switching Center and the call routing is complete. The PSTN will then establish the call circuit and send an Address Complete Message (ACM) back to the GMSC.
The GMSC then forwards the ACM back to the responsible MSC indicating that the call circuit has been established


Call Establishment
Once the called party answers the phone, the PSTN will send an Answer message to the MSC. The MSC forwards this to the MS in a Connection (CON) message.
Once the MS receives the CON message, it switches over to voice and begins the call. All voice traffic occurs on the assigned TCH.


Call Termination
When either the caller or the called party hangs up, the call will be disconnected. Either party can initiate the disconnect. In this example, the MS initiates the disconnect. The MS sends a Disconnect (DISC) message to the BTS on the FACCH.
The BSS forwards the DISC to the MSC. Once the MSC receives the DISC message, it sends a Release (REL) message through the GMSC to the PSTN as well as down through the BSS to the MS.
The MS responds by sending a Release Complete (REL_COM) message to the BSS on the FACCH. The BSS forwards the REL_COM message up to the MSC. Once the MSC receives the REL_COM message the call is considered ended from the call control perspective.
Although the call has ended, the BSS still has a TCH allocated to the MS. The MSC sends a Channel Release (CHAN_REL) message to the BSS. The BSS forwards the CHAN_REL message to the MS.
The MS responds with a DISC (LAPDm) message and returns to an idle mode. The BSS reallocates the channel for other call or releases the TRX.Once the MSC receives the ACM, it sends an ALERT message to the MS indicating that the call is going through. The BSS sends the ALERT message on the FACCH. Once the MS receives the ALERT, it will generate the ringing sound in the earpiece. The BSS sends an alerting message the subscriber will hear the line ringing.