Skip to main content

Call Setup GSM Flow




GSM Call Flow (GSM Originating Call)

Call Setup GSM Flow
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.


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 :





Study PDF: GSM Originating Call Flow


Popular posts from this blog

Multiplexing and Multiple Access

Multiplexing And Multiple Access : Digital Communications Difference between Multiplexing and Multiple Access: Sr.No. Multiplexing Multiple Access 1 “In telecommunications and  computer networks, multiplexing is a process where multiple analog  message signals or digital data  streams are combined into one   signal over a shared medium.” “In telecommunications and computer networks, a channel access method or multiple access  method allows several terminals connected to the same multipoint physical medium to transmit over it and to share its capacity.” 2 The multiplexed signal is transmitted  over a communication channel,  which may be a physical transmission medium. A channel-access scheme is  based on a multiplex method  that allows several data streams  or signals to share the  same communication channel or  physical media.

Comparison between armature controlled and field controlled DC servo motor

Comparison between armature controlled and field controlled DC servo motor : Field controlled DC servomotor Armature controlled DC servomotor 1.                         Field is excited by control voltage Armature is excited by control voltage 2.                        Armature            current kept constant Field      current kept constant 3.                        Required             low        power amplifiers are simple to design Required             high       power amplifiers 4.                        Efficiency is poor Efficiency is better 5.                        It has large time constant It             has         small      time constant 6.                        Cost is low Cost is high 7.                        It is open loop system It is close loop system Comparison between ar

Difference in between Time division multiplexing (TDM) and Frequency division multiplexing (FDM)

Difference in between TDM and FDM: This page on FDM versus TDM describes  difference between FDM and TDM multiplexing . FDM is the short form of frequency division multiplexing and TDM is the short form of time division multiplexing. In FDM each signal is modulated onto different unique RF carrier frequency and all carrier frequencies are separated significantly so that bandwidth of the signals do not overlap in frequency domain. Figure: Difference in between TDM and FDM