Scalable voip mobility intedration and deployment- P9: 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. | Voice Over Ethernet 79 portion of Ethernet works rather simply. When the device starts transmitting it watches the receive twisted pair for its own transmission to return to it. If it sees a signal that differs from its transmission more than just by a delay it knows that another device is also transmitting. To prevent wasting time by having both signals clobber each other the transmitter stops transmitting the frame and sends out a jamming signal for a short burst to ensure that the line is dead. The senders then retry their frame up to a certain number of times. If the signal does not get clobbered by the time the frame reaches the end the sender knows that the frame must have made it down the wire safely. If that were all to it however two devices with data to send would never be able to avoid colliding. That is because they would both detect the collision at nearly the same time stopping their transmissions and waiting the mandatory time for the line to come back to quiet and then they would transmit their next frames immediately. The segment would be in constant collision. To avoid that the CSMA portion of Ethernet is used. Carrier sense is the act of detecting that a transmission is already on the line. First the devices check the receive line to make sure that a signal is not coming in already corresponding to a transmission already in progress. If there is a transmission in progress the transmitter waits until it ends. Then it transmits. If a collision occurs CSMA uses the notion of a random backoff. Instead of each device transmitting exactly after a fixed time from the previous frame after the collision each device picks a random integer greater than or equal to zero and less than the maximum backoff for this transmission. They then wait that many of slots each one measured in microseconds before transmitting again. This step reduces the probability that the devices will collide a second time. If the collision occurs again the maximum backoff doubles .
