TAILIEUCHUNG - Scalable voip mobility intedration and deployment- P15

Scalable voip mobility intedration and deployment- P15: 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. | 140 Chapter 5 to properly detect the frame. If however the receiver is out of range the receiver will hear garbage. The garbage will not pass the checksum also garbage and so will be discarded. To prevent radios from interpreting noise as a preamble and locking to the wrong data rate for a possibly very long length the frame-specific information has its own checksum bit or bits depending on the radio type. Only on rare occasions will the checksum bit fail and cause a false reception thus there is no concern for real deployments. In summary a receiver then works by first setting its radio to the lowest common denominator the lowest data rate for the radio. If the fixed sequence of a preamble comes in followed by the data rate and length then the radio moves its modem up to the data rate of the frame and tries to gather the number of bits it calculates will be sent from the length given. Once the amount of time necessary for the length of the frame has concluded the radio then resets back to the lowest data rate and starts attempting to receive again. Clear Channel Assessment and Details on Carrier Sense Now that we ve covered the preamble you can begin to understand what the term carrier sense would mean in wireless. The term clear channel assessment CCA represents how a radio determines if the air is clear or occupied. Informally this is referred to as carrier sense. As mentioned previously transmitters are required to listen before they transmit to determine whether someone else is also speaking and thus to help avoid collisions. When listening the receiver has a number of tools to help discover if a transmission is under way. The most basic concept is that of energy detection. A radio can figure out whether there is energy in the channel by using a power meter. This power meter is usually the one responsible for determining the power level often stated as the Receive Signal Strength Indication RSSI of a real signal. When applied to an unoccupied channel the

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