Scalable voip mobility intedration and deployment- P25: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | 240 Chapter 6 the root cause is for being out of range of the access point. Again the thresholds required are not typically visible or exposed to the user or administrator. Voice clients tend to be more proactive in the process of scanning. The two methods just described are for when the client has strong evidence that it is departing the range of the access point. However because the scanning process itself can take as long as it does clients may choose to initiate the scan before the client has disconnected. This may sound like the beginnings of a make-before-break handoff scheme but read on to Section where we see that such a scheme does not in fact happen. Clients may chose to start scanning proactively when the signal strength from the access point begins to dip below a predetermined threshold the signal strength itself is usually measured directly for the beacons . Or they may take into account increasing but not yet disruptive losses for data. Or they may add into account observed information about channel conditions such as an increasing noise floor or the encountering of a higher density of competing clients to trigger the scan. In any event the client is attempting to make some sort of preprogrammed expense reward tradeoff. This tradeoff is often related to the problems of handoff as mentioned shortly. Scanning may also happen in the background for no reason at all. This is less common in voice clients where the desire to ensure battery life acts as a deterrent but nevertheless is employed from time to time. The main reason to do this sort of background scanning is to ensure that the client s scanning table is generally not as stale or to serve as a failsafe in case the triggered scanning behavior does not go off as expected. One of the chief problems with determining when to scan is that the client has no way of knowing whether it is moving or how fast it may be moving. A phone held in the hands of a forklift driver can rapidly go from having been
