GSM Mobile Originated (MO) Call Flow Explained

This tutorial section covers the Mobile Originated (MO) call flow in GSM. This page explains the messages exchanged between the Mobile (UE) and the network during an MO call.

It details the messages exchanged between Layer 3 entities (RR, MM, CC) on both sides. It also includes the 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 already established between the UE and the Network, as described in the GSM tutorial.

• As depicted in the figure above, before the RACH is sent by the mobile (UE), the mobile is synchronized with the network (BTS) both in terms of time and frequency.
• This means it has tuned its frequency according to the FCCH and its timing according to the SCH burst. The FCCH information consists of all zeros, which generates a continuous sine wave approximately 67.7 KHz above the RF carrier center frequency. This assists the mobile (UE) in synchronizing with the GSM Base station.
• The SCH carries the frame number and BSIC (Base Station Identity Code), which helps the mobile synchronize with the GSM frame structure and identify the Base station within the GSM network.
• The mobile has received and decoded SIs (System Information) from the received BCCH.

Based on the decoded system information from the BCCH, the mobile station learns where to transmit CCCH (RACH) and where to listen for CCCH (carrying PCH, AGCH). RACH is used in mobile originated calls, while PCH is used in mobile terminated calls at the start.

As described in the GSM protocol stack, messages flow between both the mobile and the network at various layers (Layer 3, Layer 2, Layer 1 - physical layer). The message flow is self explanatory in establishing the circuit-switched mobile originated (MO) call in GSM.

Mobile Originated Call Release

The figure above outlines the messages exchanged between the mobile and the network for call release.