serving bss the mss is only transmitting receiving data to from one of the serving bss transmitting bs at any given frame the transmitting bs can change from frame to frame depending on the bs allocation scheme therefore different transmitting bss may be used in transmitting different frames although the mss only receiving and transmitting the traffic with one bs in each frame the mss is simultaneously monitoring other bss during the vsho process for example the mss is controlled to process the dl_map message which is a directory of the slot locations within the downlink subframe and ul_map which is a directory of slot locations within the uplink subframe from all serving bss at each frame based on the dl map and ul map messages from other serving bss the mss decides which serving bs is the transmitting bs for the current frame alternatively the switching of transmitting bs can also be done through the mac message and the mss does not need to read dl_map and ul_map from multiple bss the mss monitors the downlink of all serving bss in the active set and determines its preferred transmitting bs based on received carrier to interference plus noise ratio cinr from all serving bs.