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Traffic Analysis and Design of Wireless IP Networks phần 9

Để thích ứng với xếp hàng công bằng với các mạng không dây, sửa đổi là cần thiết trong cơ chế lập kế hoạch.dễ bị lỗi. Trọng công bằng xếp hàng mô hình WFQ cho phép bất kỳ dòng chảy i được cấp dung lượng kênh trên một khoảng thời gian nhất định t1, t2] để nó giảm thiểu (6.1) từ Chương 6. | Performance Analysis of Cellular IP Networks 289 Figure 9.15 Probability distribution function of packet losses in series at handovers 20- Mbps wireless link bandwidth WFQ scheduling. applied FCFS and WFQscheduling respectively. This is due to the background traffic that reduces the burstiness of the flow. Hence scheduling discipline influences the losses. As one may expect WFQ scheduling results in lower packet loss than FCFS in the case when the flow is multiplexed with other background flows. Packet loss of the VBR flow as a function of time is shown in Figure 9.16. The simulations are performed at different network loads. We notice that a Figure 9.16 Packet loss at handovers of a VBR flow at different traffic load and 2 Mbps wireless link bandwidth. 290 Traffic Analysis and Design of Wireless IP Networks higher network load increases losses as well due to longer queuing time at the network nodes. In the case of soft handover we may have losses or duplicate packets. We can reduce packet losses in the soft handover scheme by semi-soft handover 22 which we described in Chapter 3. Typical semi-soft delay is 100 ms. Without losing generality in our simulations we use single hop between the crossover node and the base stations. In this case we analyze the losses under two different differentiation mechanisms priority mechanism and WFQ. But even in the case when priority is given to VBR packets over the background traffic as shown in Figure 9.17 b we notice the delay peak at each handover due to the additional semi-soft delay. If we compare packet delay of the hard handover shown in Figure 9.17 a to packet delay of the semi-soft handover shown in Figure 9.17 c one may notice a higher packet delay at handovers in the latter case. In this example average packet delay of the VBR flow is 51.31 ms when using semi-soft handover while the delay is 43.62 ms when hard handover is applied mobility parameters are r 0.1 km and v 50 km hr while total traffic load is 90 . 9.5.3 .