tailieunhanh - Traffic Analysis and Design of Wireless IP Networks phần 6

WFQ là một thuật toán dịch, được tìm thấy để thành công để lập lịch trình trong các mạng gói có dây. Nó cung cấp đảm bảo băng thông tối thiểu cho từng loại dịch vụ hoặc dòng chảy. Mặt khác, WFQ có phức tạp tính toán cao, đặc biệt là khi cố gắng | Architecture for Mobile IP Networks with Multiple Traffic Classes 175 Vi j e B t 1 t2 W ti t2 Wj ti t2 r rj 0 WFQis a fluid algorithm which is found to be successful for scheduling in wired packet networks. It provides minimum bandwidth guarantees for each service class or flow. On the other hand WFQ has high computational complexity especially when attempting to support a large number of flows on a high-speed link. We may find several different modifications to WFQ. For example class-based WFQ assigns packets to queues based on user-defined packet classification . by using IP ToS bits . Afterwards packets can receive prioritized service based upon user-configured weights assigned to different queues. In the case ofwireless packet networks however WFQfails to provide isolation of different flows. To adapt fair queuing to wireless networks modifications are needed in the scheduling mechanism. Wireless Scheduling We consider a wireless IP network architecture. Each base station schedules the packets in uplink and downlink direction. In the downlink direction logical queues are mapped onto physical buffers in the base station. In the uplink direction the base station maintains a logical queue of all packets that need to be sent while each mobile terminal queues the packets into its own physical buffers. Also it is usually assumed that neighboring cells transmit on different logical channels. The characteristics of the wireless channel that influence the scheduling at the air interface according to 8 are the following The wireless channel capacity is dynamically varying. Channel errors are location-dependent and bursty by nature. There is a contention on the link among multiple mobile terminals. Mobile terminals do not have a notion on the global link state . they do not know which other terminals have packets to transmit . The scheduling must take care of the both directions on the wireless link uplink and downlink. Mobile terminals are often .

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