tailieunhanh - 802.11® Wireless Networks: The Definitive Guide phần 3

tốc độ cao hơn lớp vật lý sử dụng thời gian khe cắm ngắn hơn. Trạm chọn một khe cắm ngẫu nhiên và chờ đợi cho khe cắm trước khi cố gắng truy cập các phương tiện, tất cả các khe cắm được lựa chọn đều có khả năng. Khi một số trạm đang cố gắng để truyền tải, | tyĩEi J__ỊI Element ID 3 Length 1 jurieni clunnei Traffic Indication Map TIM Access points buffer frames for mobile stations sleeping in low-power mode. Periodically the access point attempts to deliver buffered frames to sleeping stations. A practical reason for this arrangement is that much more power is required to power up a transmitter than to simply turn on a receiver. The designers of envisioned battery-powered mobile stations the decision to have buffered frames delivered to stations periodically was a way to extend battery life for low-power devices. Part of this operation is to send the Traffic Indication Map TIM information element Figure 4-36 to the network to indicate which stations have buffered traffic waiting to be picked up. Figure 4-36. Traffic Indication Map information element ÍỊpíK _ 1Ỉ _I_I__22__ Element ID 4 Length CEP Count CEP Retied 1 Cf p Ma Duntkm 1 Ị Li p Dur Semein mg The meat of the traffic indication map is the virtual bitmap a logical structure composed of 2 008 bits. Each bit is tied to the Association ID. When traffic is buffered for that Association ID the bit is 1. If no traffic is buffered the bit tied to the Association ID is 0. Four fields make up the body of the TIM information element DTIM Count This one-byte field is the number of Beacons that will be transmitted before the next DTIM frame. DTIM frames indicate that buffered broadcast and multicast frames will be delivered shortly. Not all Beacon frames are DTIM frames. DTIM Period This one-byte field indicates the number of Beacon intervals between DTIM frames. Zero is reserved and is not used. The DTIM count cycles through from the period down to 0. Bitmap Control and Partial Virtual Bitmap The Bitmap Control field is divided into two subfields. Bit 0 is used for the traffic indication status of Association ID 0 which is reserved for multicast traffic. The remaining seven .