Mobile Originated-MO call flow in GSM

This tutorial section on GSM basics covers following sub topics:
Main Page  Frame  Burst types  PHY  gsm stack  speech processing  ARFCN  Channel types  GSM Security  Network Entry  Mobile basics 


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.

mobile originated,MO call flow GSM

• 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.

Useful Links

SDCCH Channel     SACCH Channel

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

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.

PS MO MT call flow in gsm

Refer following links for PS call flow in gsm:
GSM downlink PS data call flow
GSM uplink PS data call flow
Circuit Switching vs Packet Switching

RF and Wireless Terminologies