Scalable voip mobility intedration and deployment- P22: 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. | 210 Chapter 6 PSPoll Mechanism ACK TIM Bit for Client Access Point Client Device Beacon .1. Null PM 1 In Power Save Mode Beacon .0. Awake Time PSNonPoll Mechanism ACK TIM Bit for Client Access Point Client Device FeacoTTi Null PM 1 Null Null PM 0 PM 1 Null PM 0 In Power Save Mode FeacoT T Awake Time Figure Wi-Fi Legacy Power Save beacon. That doesn t guarantee that the client will hear the beacon at exactly that time however. Beacons can be delayed if the air is occupied at that time. Furthermore because beacons are sent out as broadcasts the client might just miss the beacon or the beacon can be collided with. If the client does hear the beacon it can then go to sleep so long as no traffic is buffered for it. Clients may also skip beacons. They would do this to save additional battery at the expense of increasing the amount of time the frames would be buffered. Clients usually let the access points know how many beacons they will skip by sending a listen interval in their Association Request messages. A listen interval of 1 means that the client will wake for every beacon a listen interval of 10 means that the client will wake only for every tenth beacon. Be careful however some clients do not follow the listen interval they state waiting either for more or less beacons than they advertise. The client signals that it is going to sleep by using the power management bit in any unicast frame it sends to the access point except for non-Action management frames . The power management bit is in the Frame Control field for the frame. When the client sends a frame Voice Mobility over Wi-Fi 211 with the power management bit set and when it gets an Acknowledgement in response it knows that the access point has heard the client s change of state and can now go to sleep. From this moment on the access point will buffer frames until the client sends any frame to the access point with the power management bit not set. That signals that the client is now
