GSM Radio Link Failure: Causes and Mechanisms

In this article, we will analyze the scenarios that can lead to a loss of communication link, commonly known as radio link failure. This can occur when the User Equipment (UE, i.e., mobile device) is in the idle state, connected state, or attempting to connect from the idle state to obtain network services.

Radio link failure can happen in various wireless networks such as GSM, CDMA, or LTE. The common scenarios include:

• Failure at the Lower Layer (RF or Physical Layer): This is often due to factors such as higher frequency offset, high timing offset, or poor Carrier-to-Interference (C/I) conditions, leading to CRC (Cyclic Redundancy Check) failures. As a result, the upper layers fail to decode properly, and neither the mobile nor the base station can successfully negotiate the minimum messages required to establish radio link connectivity.

• Data Link Layer Failure: This is specifically defined in the 3GPP GSM specification document version 04.06.

• During Mobility Procedures: This includes channel assignment, cell reselection, or handover procedures, whether from one Radio Access Technology (RAT) to another, or within the same RAT from one base station to another.

• Timer Expiry: Timers are initiated when a response from the other entity isn’t immediately available. This triggers retransmissions, and eventually the timers expire after waiting for a predefined period.

RF and Layer-1 failure is often collectively referred to as lower layer failure.

Mobile Side (UE)

When a radio link failure is detected by the mobile device (UE), the following actions are typically performed:

• Release of signaling links, unless the base station specifies otherwise.
• Deactivation of all channels.
• The mobile indicates the failure of the RR (Radio Resource) connection to the MM (Mobility Management) sublayer, unless instructed otherwise by the base station.

Network Side

In RR connected mode, the network releases the connection using a channel release procedure due to a lower layer failure.

The network starts the GSM timer T3109 and deactivates the SACCH (Slow Associated Control Channel) to stop SACCH transmission. Once a radio link failure occurs, an indication is passed to the upper MM sublayer (Mobility Management sublayer).

When the GSM timer T3109 expires, the base station (i.e., the network) releases the channels and their allocations, making them available for use by other requesting mobile subscribers.

Subsequently, if a mobile subscriber (MS) does not receive SACCH for a certain period, it will completely release the allocated channels.

Summary

There are two main conditions under which a connection is considered terminated, and a radio link is deemed to have failed:

Due to Layer-1 Failure

The most common reason for declaring a radio link failure on Layer 1 is the inability to decode the SACCH a specific number of times, defined by the parameter RADIO_LINK_TIMEOUT. This applies to both the uplink and downlink directions.

For SACCHs that cannot be read, the channel decoder sets the BFI (Bad Frame Indication) flag to 1. The RADIO_LINK_TIMEOUT parameter can be configured per BTS (Base Transceiver Station) and is broadcast in the BCCH (Broadcast Control Channel) / SYS_INFO 3.

When a radio link failure is detected on Layer 1 on a dedicated channel (such as TCH (Traffic Channel) and SDCCH (Standalone Dedicated Control Channel)), the BTS sends a CONN_FAIL message with cause 1 = radio link failure to the BSC (Base Station Controller). The BSC then sends a CLR_REQ message with cause 1 = radio interface failure to the MSC (Mobile Switching Center) to indicate the connection loss and release the resources.

Due to Layer-2 Failure

The criteria for a radio link failure on Layer 2 are similar to those for Layer 1. When an MS or a BTS sends a Layer 2 frame (I-frame, SABME-frame, etc.) as many times as indicated by the parameter N200 without receiving a response, Layer 3 receives an error indication requesting the release of that channel.

An ERR_IND message with cause 1 = timer T200 expired (N200 + 1) times is sent to the BSC to release the channel. The BSC then performs the channel release procedure.

This often results in a radio link failure on Layer 1 because the radio link is so poor that the SACCH can no longer be decoded.