cture, as illustrated in Figure 13.20, where different cell configurations overlap to provide service to end-users. Such an approach may also use wireless LANs in its overall service provision strategy. Handover An overlay network needs to facilitate a very fast handover process. In general, a mobile device travelling within a cellular system connects to the most suitable base station. Cellular telecommunications systems are designed to facilitate handover of a user from one base station to another as the channel conditions vary. Handover mechanisms have been specified for almost all cellular standards since analogue 1G systems. As illustrated in Figure 13.21, a mobile stops communicating with a base station and starts with another which has a higher Network Topologies, Design and Convergence Figure 13.21 Handover process Figure 13.22 Hard handover in multi-structured cell topologies signal strength. In Figure 13.21, the mobile continues to communicate with both base stations for some time, until the received signal from one is lower than the other by more than a designed margin. This process is known as a soft handover. Some standards allow a mobile device to be connected to only one base station at any one time. The process is known as a hard handover. In 3G and 4G packet-switched broadband telecommunications systems a hard handover process is used. This is mainly due to the difficulties of managing packet transmissions from multiple base stations. As illustrated in Figure 13.21 a mobile station in an overlaid, multi- structured base station system with large- and small-cell topologies, can connect to any of the base stations or access points near it. The decision on which base station to use is based on the cost and rate of transmission as well as network traffic conditions. It is desirable that the most efficient connection is used at any time, and handovers are made as soon as required. However, the delays associated with handover processes make frequent handovers difficult. Furthermore as packets are direct