GSM Handover Causes | GSM Handover Triggers

This page describes GSM Handover basics. It mentions causes of handover in GSM or triggers of handover in GSM network.

Handover or handoff refers to transfer of mobile connection from one resource (i.e. Base station, BS) to another BS without disconnecting the ongoing voice or data call.

In GSM, mobile continuously monitors signal quality and signal power level of serving/neighbour Base Stations (BSs). It then sends the "measurement report" to serving BS. Based on the report BS initiates handover of mobile connection to its strongest neighbour BS. After new traffic channels are assigned with new serving BS, all the previous traffic channels are released.

gsm network

The figure-1 depicts GSM network architecture. Other than the above, handover in GSM is triggered due to following reasons.
• Uplink and downlink signal levels.
• Uplink and downlink quality levels.
• Speed of mobile subscribers
• Distance between mobile and serving cell
• Traffic (i.e. number of mobiles per cell)
• Due to maintenance of cell

GSM Handover Causes | GSM Handover Triggers

Let us understand each of these causes or triggers of GSM handover.
Level Triggered Handover: The received signal level is used as trigger point for the handover. This occurs when neighbour cell signal level becomes more than serving cell to the value of about 3 to 6 dB.
Intracell Handover: When the assigned time slot/frequency is interfered then handover is initiated after assigning new channel or TSL. This handover is triggered due to poor Rx-QUAL and not due to received signal level.
Quality triggered Handover: This handover is triggered due to level of Rx-QUAL. Typical trigger level of Rx-QUAL is greater than 3. If the quality goes below this level,
Distance triggered Handover: The maximum cell size is set for the triggering of handover. If distance of mobile from cell exceeds this set distance handover is initiated.
Speed triggered Handover: This is used to limit use of cells for slow moving mobile handsets.
Traffic triggered Handover: In this type, certain traffic level is set for cell to initiate handover between cells. This is needed to offload traffic from overloaded serving cell to nearby cells.
Maintenance Handover: During this type, all the calls are pushed to adjacent cells for maintenance purpose of serving cell. After all the calls are transferred to adjacent cells, the serving cell is switched off for maintenance purpose.

GSM Handover from Network Side

Following are the three types of handover observed in GSM on network side.
• Intra-BSC handover: This handover type is serviced between cells served by same BSC.
• Inter-BSC handover: This handover type is serviced between cells served by different BSCs.
• Inter-MSC handover: This handover type is serviced between cells served by different MSCs.

HANDOVER or Handoff related links

Following are the handovers usually found in various wireless technologies/standards such as femtocell, WLAN, Mobile WiMAX, UMTS, LTE etc.
Handover basics and types
IRAT Handover
Femtocell Handover
Soft handover vs Softer Handover
LTE to WLAN and WLAN to LTE Handover
UMTS to WLAN and WLAN to UMTS handover
Mobile WiMAX Handover as per IEEE 802.16e standard

RF and Wireless Terminologies