Đang chuẩn bị liên kết để tải về tài liệu:
Scalable voip mobility intedration and deployment- P14

Scalable voip mobility intedration and deployment- P14: 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. | 130 Chapter 5 The formula for the channels to frequencies is 2.407GHz 0.5GHz channel for the 2.4GHz band and the simpler to remember 5GHz 0.5GHz channel for the 5GHz band. The only channels that are in the 2.4GHz band are channels 1-14. Everything else is in the 5GHz band. Therefore channel 36 is 5.18GHz and channel 100 is 5.50GHz. The total number of channels is large but many factors reduce the number that can be practically used. First to note is that the 2.4GHz band where 802.11b and 802.11g run only has three nonoverlapping channels four in Japan to choose from. Unfortunately the eleven channel numbers available in the United States gives the false impression of 11 independent channels and to this day there exist some Wi-Fi deployments that mistakenly use all 11 channels causing an RF nightmare. To avoid overlapping channels adjacent channel selections need to be four channel numbers apart. Therefore channels 1 and 5 do not overlap. In the 2.4GHz band custom usually spreads the channels out even a bit further and using only channels 1 6 and 11 is recommended. The authors of the standard recognized the problem the overlap causes and for the 5GHz band disallowed overlap by preventing devices from using the intermediate channels. Therefore no channels in the 5 GHz band overlap when it comes to 20MHz channels. 40MHz channels are an exception and will be discussed in the section on 802.11n Section 5.5.3. The unlicensed spectrum was originally designed for and still is allocated to other uses besides Wi-Fi. The 2.4GHz band was created to allow in part for microwave ovens to emit radio noise as they operate as it is impractical to completely block their radio emissions. Because that noise prevented being able to provide the protections from interference that licensed bands have the regulatory agencies allowed inventors to experiment providing other services in this band. And so began 802.11. The 5GHz band is in theory more set aside from radiation. Except for the top