GSM Handover: Causes and Triggers Explained

This guide delves into the fundamentals of GSM handover, exploring the causes and triggers that prompt these essential network operations.

Introduction to GSM Handover

Handover, also known as handoff, refers to the seamless transfer of a mobile connection from one base station (BS) to another. This process is crucial for maintaining an uninterrupted voice or data call as a mobile user moves within the network.

In the GSM environment, mobile devices constantly monitor the signal quality and power levels of both the serving base station and neighboring base stations. This information is then compiled into a “measurement report” and sent to the serving BS. Based on this report, the BS can initiate a handover of the mobile connection to the most suitable neighboring BS.

Once the new traffic channels are established with the new serving BS, the previously used traffic channels are released.

Figure 1: GSM Network Architecture

Beyond signal strength, several other factors can trigger handover in GSM networks. These include:

• Uplink and downlink signal levels.
• Uplink and downlink quality levels.
• Speed of the mobile subscriber.
• Distance between the mobile device and the serving cell.
• Traffic load (number of mobiles per cell).
• The need for cell maintenance.

Understanding GSM Handover Triggers

Let’s examine each of these triggers in more detail:

1. Level Triggered Handover

This is a fundamental handover trigger based on received signal levels. The handover is initiated when the signal level from a neighboring cell surpasses the serving cell’s signal level by a certain threshold, typically around 3 to 6 dB.

2. Intracell Handover

An intracell handover occurs when the currently assigned time slot or frequency experiences interference. In this scenario, the handover is triggered to reassign the connection to a new channel or time slot within the same cell. Crucially, this handover is triggered by poor Rx-QUAL (receive quality) rather than low received signal level.

3. Quality Triggered Handover

This handover type is directly triggered by the Rx-QUAL level. A typical trigger level for Rx-QUAL is a value greater than 3. If the quality drops below this level, a handover is initiated to maintain call quality.

4. Distance Triggered Handover

To prevent excessively large cell sizes from compromising performance, a maximum cell size can be defined. If the mobile device’s estimated distance from the serving cell exceeds this predefined distance, a handover is initiated.

5. Speed Triggered Handover

This type of handover is used to optimize cell usage based on subscriber speed. By initiating handovers for faster-moving mobile devices, the network can allocate resources more efficiently and prevent slow-moving devices from unnecessarily occupying cells designed for faster transit.

6. Traffic Triggered Handover

Also known as load balancing, traffic-triggered handover is used to manage traffic congestion within a cell. A certain traffic level is set for the cell, and when this threshold is reached, the cell initiates handovers to neighboring cells to offload some of the traffic. This ensures a better quality of service for all users in the area.

7. Maintenance Handover

When a cell requires maintenance, a maintenance handover is performed. During this process, all active calls within the cell are gradually transferred to adjacent cells. Once all calls have been successfully handed over, the serving cell can be safely switched off for maintenance without disrupting ongoing calls.

GSM Handover from the Network Perspective

From the network’s perspective, there are three primary types of handover:

• Intra-BSC handover: This type of handover occurs between cells that are served by the same Base Station Controller (BSC).

• Inter-BSC handover: This type of handover occurs between cells that are served by different BSCs.

• Inter-MSC handover: This type of handover occurs between cells that are served by different Mobile Switching Centers (MSCs). This is the most complex type of handover and is required when a mobile device moves between the areas controlled by different MSCs.