tailieunhanh - WIMAX, New Developments 2011 Part 13

Tham khảo tài liệu 'wimax, new developments 2011 part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Analyzing the Throughput and QoS Performance of WiMAX Link in an Urban Environment 317 transmit power of the SS so that transmissions can be received by the BS at optimal levels. The test is conducted at a distance of 220m. As can be seen from the figure 7 a 30KB RWIN size results in an unstable low throughput due to packet losses and repeated retransmissions. Fig. 7. Effect of Buffer Overflows During a TCP Transmission over WiMAX link To find the optimal boundaries for RWIN it is observed that the RWIN should be between 85KB default setting in Linux and 256KB. At RWIN 256KB the TCP session throughput not only increases to 10Mbps but shows a stable and constant throughput performance as depicted in figure 8. Fig. 8. Effect of Window Size on TCP Throughput for QAM64 318 WIMAX New Developments QoS Tests To ascertain the degree of QoS support the tests were conducted by sending TCP background traffic using QAM64 modulation through a non real-time connection at full capacity of the bandwidth throughout the 60 seconds test duration. A 12 Mbps UDP traffic is sent in periodic durations of 10 seconds through a real-time connection with a committed bit rate of 5 Mbps and 10 Mbps the result of which is shown in figure 9 a and 9 b respectively. From figure 9 it is seen that the throughput of the TCP data stream is exactly maximal it is occupying the full available bandwidth spectrum in periods where there is no UDP traffic. As soon as the UDP traffic is generated the system will guarantee the required bandwidth as UDP is sent on a link which is assigned a higher QoS priority. This availability of bandwidth to the high priority UDP stream will be made at the cost of hogging the required bandwidth from the TCP session. Thus it is evident from the figure 9 a and 9 b that the TCP background traffic will occupy only that portion of the bandwidth which is not utilized by the UDP traffic and upon greater demand for bandwidth by the real-time traffic the system ensures the .