TAILIEUCHUNG - emerging wireless multimedia services and technologies phần 6

một phương tiện truyền thông nén dữ liệu cụ thể hoặc đại diện được gọi là loại tải trọng, cộng với một định dạng tải trọng cho đóng gói trong tiếp theo là cho cả hai UE độc lập kích hoạt ngữ cảnh PDP thích hợp GGSNs của họ, | 210 QoS Provision in Wireless Multimedia Networks 5 Assuming a QSTA having Uj active TSs is selected for polling the scheduler calculates the TXOP duration TD in three steps a First for every TS j of QSTAi j 1. n the scheduler calculates TD j as the maximum of i the time required to accommodate the pending traffic as indicated by the queue size of that TS QSj plus any overheads and ii mTDj to ensure that the assigned TXOP will have at least the minimum duration TDj max QS O mTDj . Rj V In the special case where QSj is equal to zero TDj is set equal to the time for the transmission of a Null-Data MSDU to allow QSTAi to update the queue size information for TSij. b TD for QSTA is calculated as the sum of all TDj n TDi TDj. j 1 c Finally TD obtained from is compared with the current TXOP Timer value to ensure conforming traffic TD min TD - T . 6 After the scheduler assigns the TXOP it reduces the respective TXOP timer value accordingly and returns to step 1 T T - TDi. Although the above calculations are considered simple enough to be executed at the beginning of an idle slot it is possible in most cases to execute steps 2-5 in parallel with step 1 while waiting for the channel to become idle provided that the QSTA currently transmitting is not eligible for the next poll . it does not satisfy . The exploitation of queue size information for calculating accurate TXOP durations is particularly effective for both Constant Bit Rate CBR and Variable Bit Rate VBR traffic but introduces some extra delay and increases the transmission overhead percentage. For VBR traffic this seems to be unavoidable since its behaviour cannot be accurately predicted. On the other hand due to its periodic nature CBR traffic has a much more predictable behaviour. An enhanced version of ARROW scheduler can take advantage of this characteristic to reduce overhead and delays. To differentiate the two versions of the algorithm we refer to bas c ARROW and enhanced .

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