GSM Handover Failure: Physical Channel Issues and T313 Timeout

This article explores a common issue: handover failure from UTRAN to GSM, specifically due to physical channel failure. This failure is often followed by a timeout of timer T313. Let’s delve into the scenario.

The handover process is initiated by the UTRAN network sending a “handover command” to the mobile subscriber (UE) on the main DCCH channel.

After the UE receives this command, it starts timer T3103. While the network is sending the handover command and the UE is receiving it, the transmission of signalling messages is generally suspended by the network, except for RR messages essential for the handover or for handling abnormal situations.

Upon receiving the handover command, the mobile releases its existing link layer connections and disconnects the physical channels. It then switches to the channels assigned by the network.

The UE then begins establishing lower layer connections, including channel activation and data link connection establishment. There are four defined procedures for this, and the mobile station must support three of them. The pseudo-synchronization case is optional. A pseudo-synchronized handover will only be commanded to a mobile station that has indicated support for it in its classmark. These cases are:

• Finely synchronized cell case
• Non-synchronized cell case
• Pseudo-synchronised cell case
• Pre-synchronised cell case

If the lower layer connection is established successfully, the mobile station (MS) sends a HANDOVER COMPLETE message. This message, sent on the main DCCH channel, indicates a ‘normal event’ cause to the network.

Upon receiving the HANDOVER COMPLETE message, the network stops timer T3103 and releases the old channels. If requested, the mobile adjusts its transmission based on half the timing advance measured during the process, correcting for the HANDOVER COMMAND message.

Understanding the Failure: Physical Channel Failure and T313

The handover fails when the UE moves from the UTRAN cell to the GSM cell and experiences a physical channel failure, which subsequently leads to the expiry of timer T313.

Here’s how the T313 timer plays a role:

• Timer Start: T313 starts when the UE detects N313 consecutive “out of sync” indications from Layer-1 (the physical layer - PHY).
• Timer Stop: T313 stops when the UE detects N315 consecutive “in sync” indications from Layer-1 (PHY).

Ultimately, Timer T313 will expire when a radio link failure occurs or when a physical channel failure happens. This scenario highlights the importance of a stable physical channel for a successful handover.