Mobile Originated-MO call flow in GSM
This article covers MO MT call flow in GSM. This page covers mobile originated (MO) call flow between Mobile(UE) and network. It covers messages exchanged between Layer 3 entities(RR,MM,CC) at both side. It include channels(RACH,AGCH,SDCCH,FACCH,TCH) used at layer 1 to carry these messages over the air. This article assumes that initial frequency and time synchronization is done between UE and Network as described in GSM tutorial in tutorial section.
• As described in the figure above, before RACH is sent by mobile(UE) mobile is synchronized with network(BTS) both time and frequency wise.
• It means it has tuned frequency as per FCCH and time as per SCH burst. Information here in this FCCH is all zeros which produces continuous sine wave of about 67.7 KHz above the RF carrier centre frequency, This helps mobile(UE) synchronize with the GSM Base station, READ MORE.
• SCH carry frame number and BSIC (Base Station Identity Code) which helps Mobile synchronize with GSM frame structure as well as helps in identification of the Base station in the GSM network, READ MORE.
• It has received and decoded SIs(System Information) from the received BCCH, READ MORE. From the decoded system information of BCCH,mobile station comes to know where it has to transmit CCCH(RACH) and where it has to listen for CCCH(carrying PCH,AGCH).
• RACH is used in mobile originated call while PCH is used in mobile terminated call at the start.
As described in GSM protocol stack, messages flow between both mobile and network at various layers(layer 3,layer 2,layer 1(physical layer). The message flow is self explanatory to establish the circuit switched mobile originated(MO) call in GSM.
Mobile Originated Call Release
The figure above mentions messages exchanged between mobile and network for call release.
MT Call flow in gsm
Refer Mobile Terminated Call.