Handover Processes

Handover is the process of maintaining a user’s active sessions when a mobile terminal changes its connection point to the access network (called ‘‘point of attachment”), for example, a base station or an access point. Depending on the access network that each point of attachment belongs to, the handover can be either horizontal or vertical. 

A horizontal handover takes place between points of attachment supporting the same network technology,for example, between two neighbouring base stations of a cellular network. On the other hand, a vertical handover occurs between points of attachment supporting different network technologies, for example, between an IEEE 802.11 access point and a cellular network base station.

A handover process can be split into three stages: handover decision, radio link transfer and channel assignment. Handover decision involves the selection of the target point of attachment and the time of the handover. Radio link transfer is the task of forming links to the new point of attachment, and channel assignment deals with the allocation of channel resources. VHD algorithms help mobile terminals to choose the best network to connect to among all the available candidates. Here, we only focus on the research efforts and recent developments on improving the efficiency of VHD process. In contrast to horizontal handover decision algorithms which mainly consider RSS as the only decision criterion, for VHD algorithms, criteria such as cost of services, power consumption and velocity of the mobile terminal may need to be taken into consideration to maximize user satisfaction