GSM Call Flow (GSM Originating Call)
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| Call Setup GSM Flow |
Call Setup GSM Flow:
• 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 earlier
• 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.
• 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 earlier
• 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 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.
•
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.
•
Assuming, 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 deallocates the channel and
releases the TCH.
Video Tutorial for GSM Originating Call Flow :