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GSM Timers Explained: A Comprehensive Overview

gsm timer mobile communication network protocol

This document provides an overview of various timers used in GSM (Global System for Mobile communications) networks. These timers play a crucial role in managing different aspects of the communication process, from radio link maintenance to call setup and packet data transfer.

GSM Timers Overview

Here’s a breakdown of common GSM timers and their functions:

TimerFunction/Value
T100Helps detect the presence of the GSM radio link by checking SACCH (Slow Associated Control Channel) frames every 480 milliseconds. Essentially, it’s a radio link timeout mechanism.
T200Used for retransmission at the data link layer. The value of this timer varies depending on the specific message being transmitted. For FACCH (Fast Associated Control Channel) messages, it’s typically set to 155ms.
T301The “Alerting” or “ringing” timer. This timer limits the amount of time a user has to answer an incoming call. The standard value is 20 seconds.
T3126When this timer expires, the immediate assignment procedure is aborted. The starting procedure is the same as timer T3146 (described below).
T3146Starts after the mobile station (MS) sends the maximum allowed CHANNEL REQUEST messages OR upon receiving an Immediate Assignment Reject message (whichever comes first) during a PS (Packet Switched) call. When T3146 expires, the Packet Access Procedure is aborted. The maximum value is set to 5 seconds.
T3182Starts when the MS transmits the final Uplink data block. If an Uplink packet ACK (Acknowledgement) is received, everything proceeds normally. If the ACK isn’t received and the timer expires and N3102 > 0, the MS releases resources and attempts to re-establish communication on the cell.
T3190Related to Packet DL (Downlink) Assignment on CCCH (Common Control Channel). It starts upon reception of an IMMEDIATE ASSIGNMENT or PDCH (Packet Data Channel) ASSIGNMENT COMMAND message when in dedicated mode. When T3190 expires, the mobile returns to packet idle mode. The value of this timer is typically set to around 5 seconds.
T3192Also called the TBF (Temporary Block Flow) release timer. When T3192 expires, the UE (User Equipment - mobile device) releases TBF-related information and begins scanning the paging channel.
T3124Used to wait for physical handover information from the network. It stops when the Physical Information message is received at the UE. It expires after waiting a typical value of 320ms if the physical information isn’t received.
T3230Starts when the mobile subscriber sends a CM (Connection Management) service request OR a CM Re-Establishment Request. It stops when the mobile subscriber receives a CM service accept, CM service reject, or when it receives a setting need for ciphering mode. If timer T3230 times out, the call is terminated by the mobile subscriber. It has a value of 15 seconds.
T3330RAU (Routing Area Update) request timer. It expires when the default value (typically 15 seconds) is reached. Usually, either a RAU accept or RAU reject message is sent by the network to the UE upon receipt of the RAU request.