Performance Evaluation and Design Improvement of Media Access Control Protocols for Broadband Wireless Local Loop
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1 Performance Evaluation and Design Improvement of Media Access Control Protocols for Broadband Wireless Local Loop Mihir Thaker Masters Thesis Presentation Department of Electrical Engineering and Computer Science University of Kansas July 19, 2 Defense Committee: Dr. James Roberts, Chair Dr. Joseph Evans Dr. Victor Frost
2 Organization Introduction Motivation Performance Evaluation Design Improvements & Re-evaluation Conclusions & Future Work 19 July 2 Mihir Thaker 2
3 Introduction
4 Introduction - Broadband Wireless Local Loop (B-WLL) Global demand for high speed Internet access. Need for a cost effective and viable solution for the last mile problem. Fixed wireless access system addresses this problem. Restrictions eased for MMDS, MDS and U-NII bands. Faster deployment, lower construction, operating and maintenance cost. Deployment as a two way point to multi-point system. 19 July 2 Mihir Thaker 4
5 Introduction - Media Access Control (MAC) Protocols Channel allocation schemes that control the usage of a shared resource and possess desirable performance characteristics. In a wireless system, available bandwidth is a resource shared by a large user population. TDMA, FDMA and CDMA are popular access schemes. Two currently used schemes, Reservation TDMA (R-TDMA), a variation of the TDMA scheme. Multi Frequency Polling (MF-Polling), a variation of the FDMA scheme. Desirable performance characteristics High aggregate throughput Low average queuing delay Support a large user population 19 July 2 Mihir Thaker 5
6 Introduction - Reservation TDMA (R-TDMA TDMA) Slot-on-demand TDMA system. Request can be made by, Contention (Slotted ALOHA with Exponential Backoff) Piggybacking Frame structure is repetitive and is time division duplexed in nature. Contention slots varied according to collision conditions. Higher slot occupancy per frame implies greater frame efficiency. System is thus adaptable to varying traffic conditions. 19 July 2 Mihir Thaker 6
7 Introduction - Multi Frequency Polling (MF-Polling) Symmetric division of available bandwidth. FDM in upstream. Polling effective on each channel when the number of users on the channel exceeds one. Polling cycle time of 3 ms. Inactivity timeouts associated with each user in system. Polling ratio decides the maximum number of users per channel. Exponential backoff with a maximum window size of 124, similar to Ethernet. 19 July 2 Mihir Thaker 7
8 Motivation
9 Motivation Problem How to choose an appropriate MAC protocol? Given a MAC protocol, how can we improve its performance? Solution Performance evaluation based on HTTP and FTP applications for various load conditions. Identified the contention delay component of average queuing delay as a parameter for improvement Design Improvement Maintain the number of contention slots as constant for each frame in the R-TDMA system. Reduction of maximum window size for the MF-Polling system. 19 July 2 Mihir Thaker 9
10 Performance Evaluation
11 Performance Evaluation Test Scenarios Packet Generator Test To measure the performance bound of the protocols. FTP Tests FTP Low Download 1 file/hr, 1, bytes/file FTP High Download 1 files/hr, 1, bytes/file HTTP Tests HTTP Light Browsing 5 pages/hr, 1 objects/page, 12 bytes/object HTTP Heavy Browsing 6 pages/hr, 1 objects/page, 12 bytes/object Medium Load Test FTP Low Download and HTTP Light Browsing Load conditions suggested by OPNET TM.. 19 July 2 Mihir Thaker 11
12 Packet Generator Test Shows the upper bound on the system performance. Aggregate Throughput Comparision Available bandwidth 12.5 MHz, QPSK modulation. Tested using a packet generator with inter-arrival rate marginally greater than the link rate. R-TDMA shows better performance than MF-Polling. However, throughput gradually decreases with increase in the number of users. Throughput (Mbps) R-TDMA MF-Polling 19 July 2 Mihir Thaker 12
13 Packet Generator Test (cont...) Devised a metric which observes the product of number of users and throughput. The MF-Polling system supports a large user population compared to the R-TDMA system. User-Throughut Product (user-mbps) User-Throughput Product Comparison R-TDMA MF-Polling July 2 Mihir Thaker 13
14 FTP Low Download Throughput degradation greater for R-TDMA on account of high collision. Aggregate Throughput Comparison Light load conditions lead to lower throughput for MF-Polling. Throughput (Kbps) R-TDMA MF-Polling 19 July 2 Mihir Thaker 14
15 FTP Low Download (cont...) Greater collision rate leads to steep increase in queuing delay for R-TDMA for a large user population. Queuing delay for MF-Polling is higher than R-TDMA as data transmission is dependent on the polling cycle period. Delay (sec) Average Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 15
16 FTP High Download Gradual rise in throughput for R-TDMA on account of reservation effect. 5 Aggregate Throughput Comparison Large timeout value leads to lower contention for MF-Polling for higher number of users and thus performs better. Throughput (Kbps) R-TDMA MF-Polling 19 July 2 Mihir Thaker 16
17 FTP High Download (cont...) Greater collision rate and variable number of slots leads to steep increase in queuing delay for R-TDMA for large number of users in the system. Queuing delay for MF-Polling improves on account of reduction in contention delay and performs better as compared to the previous test case. Delay (sec) Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 17
18 HTTP Light Browsing Larger amount of data, hence higher frame efficiency and continued reservation for the R-TDMA system. MF-Polling throughput limited by the associated polling cycle time. Throughput (Kbps) Aggregate Throughput Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 18
19 HTTP Light Browsing (cont...) Reservation effect leads to lower contention and lower queuing delay values for R-TDMA. MF-Polling performance hampered on account of the large values of polling cycle time. Delay (sec) Average Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 19
20 HTTP Heavy Browsing Larger amount of data, hence higher frame efficiency and continued reservation for the R-TDMA system. 1 Aggregate Throughput Comparison MF-Polling throughput limited by the associated polling cycle time. Throughput (Kbps) R-TDMA MF-Polling 19 July 2 Mihir Thaker 2
21 HTTP Heavy Browsing (cont...) Reservation effect in R-TDMA leads to lower contention. However, prolonged reservation leads to high queuing delay comparable to MF-Polling. Delay (sec) Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 21
22 Medium Load Combination of FTP Low Download and HTTP Light Browsing. MF-Polling throughput performance stable over a large range of users and hence can support a large user population. Throughput (Kbps) Aggregate Throughput Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 22
23 Medium Load (cont...) MF-Polling queuing delay suffers on account of large contention delay and polling cycle time..5 Queuing Delay Comparison Continuous data on account of HTTP traffic aids R-TDMA to maintain reservation and thus has lower queuing delay values. Delay (sec) R-TDMA MF-Polling July 2 Mihir Thaker 23
24 Proposed Design Improvement
25 Proposed Design Improvement - Parameter Selection Throughput directly affected by the queuing delay. Queuing Delay composed of Contention Delay Delay on account of system architecture. No architectural changes required for improving the performance of the contention mechanism. 19 July 2 Mihir Thaker 25
26 Proposed Design Improvement - Reservation TDMA Varying number of slots cause mismatch between selection of frame for request transmission and number of contention slots available for that frame. Keep number of contention slots fixed to its maximum possible value. Reduces randomness as contention is dependent upon the frame that a user selects for request transmission. 19 July 2 Mihir Thaker 26
27 Proposed Design Improvement - Multi Frequency Polling Retransmission of request depends upon the available window size and polling cycle time. Contention delay can be controlled by reducing the maximum window size to a value such that, The original maximum contention delay is not exceeded by the maximum number of retransmits for the reduced contention window. Value for maximum contention window size reduced from 124 to 32. Retransmit Attempts - Reduced Window Size Maximum Retransmits # Retransmits 19 July 2 Mihir Thaker 27
28 Performance Re-evaluation evaluation
29 FTP Low Download R-TDMA Aggregate Throughput Comparison MF-Polling Aggregate Throughput Comparison 2 1 Throughput (Kbps) Without Improvement With Improvement Throughput (Kbps) Without Improvement With Improvement R-TDMA Queuing Delay Comparison MF-Polling Queuing Delay Comaprison Delay (sec) Without Improvement With Improvement Delay (sec) Without Improvement With Improvement July 2 Mihir Thaker 29
30 FTP Low Download (cont...) Throughput improved on account of reduction in queuing delay for both the protocols. Throughput (Kbps) Aggregate Throughput Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 3
31 FTP Low Download (cont...) Improved queuing delay performance for both the protocols. However, R- TDMA still performs better than MF- Polling. Delay (sec) Average Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 31
32 FTP High Download R-TDMA Aggregate Throughput Comparison MF-Polling Aggregate Throughput Comparison 6 5 Throughput (Kbps) Without Improvement With Improvement Throughput (Kbps) Without Improvement With Improvement R-TDMA Queuing Delay Comparison MF-Polling Queuing Delay Comparison Delay (sec) Without Improvement With Improvement Delay (sec) Without Improvement With Improvement 19 July 2 Mihir Thaker 32
33 FTP High Download (cont...) Improved throughput and graceful degradation for MF-Polling on account of improved queuing delay performance. Throughput (Kbps) Aggregate Throughput Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 33
34 FTP High Download (cont...) Queuing delay performance for MF-Polling becomes comparable to that of R-TDMA and is better for more number of users in the system Delay (sec) Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 34
35 HTTP Light Browsing R-TDMA Aggregate Throughput Comaprison MF-Polling Aggregate Throughput Comaprison Throughput (Kbps) Without Improvement With Improvement Throughput (Kbps) Without Improvement With Improvement R-TDMA Queuing Delay Comparison MF-Polling Queuing Delay Comparison Delay (sec) Without Improvement With Improvement Delay (sec) Without Improvement With Improvement 19 July 2 Mihir Thaker 35
36 HTTP Light Browsing (cont...) R-TDMA still performs better than MF-Polling and also has a stable range of throughput. 1 Aggregate Throughput Comparison Throughput (Kbps) R-TDMA MF-Polling 19 July 2 Mihir Thaker 36
37 HTTP Light Browsing (cont...) Since there is less improvement in individual queuing delay performance, the overall comparison remains same as the case without improvement. Delay (sec) Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 37
38 HTTP Heavy Browsing R-TDMA Aggregate Throughput Comaprison MF-Polling Aggregate Throughput Comaprison 1 1 Throughput (Kbps) Without Improvement With Improvement Throughput Without Improvement With Improvement R-TDMA Queuing Delay Comparison MF-Polling Queuing Delay Comparison Delay (sec) Without Improvement With Improvement Delay (sec) Without Improvement With Improvement July 2 Mihir Thaker 38
39 HTTP Heavy Browsing (cont...) Improved delay performance affects the throughput performance for Aggregate Throughput Comparison MF-Polling. 1 Reservation factor comes into effect for R-TDMA and hence reduced contention. Throughput (Kbps) R-TDMA MF-Polling July 2 Mihir Thaker 39
40 HTTP Heavy Browsing (cont...) MF-Polling queuing delay comparable to that of R-TDMA. Average Queuing Delay Comparison Delay (sec) R-TDMA MF-Polling 19 July 2 Mihir Thaker 4
41 Medium Load R-TDMA Aggregate Throughput Comparison MF-Polling Aggregate Throughput Comparison 1 5 Throughput (Kbps) Without Improvement With Improvement Throughput (Kbps) Without Improvement With Improvement R-TDMA Queuing Delay Comparison MF-Polling Queuing Delay Comparison.7.5 Delay (sec) Without Improvement With Improvement Delay (sec) Without Improvement With Improvement July 2 Mihir Thaker 41
42 Medium Load (cont...) Improved queuing delay performance for the R-TDMA system improves its throughput and stabilizes it over a large range of users. Throughput (Kbps) Aggregate Throughput Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 42
43 Medium Load (cont...) R-TDMA has lower values of queuing delay than MF-Polling due to reduced contention. Delay (sec) Averaage Queuing Delay Comparison R-TDMA MF-Polling 19 July 2 Mihir Thaker 43
44 Conclusions and Future Work
45 Conclusions and Future Work Conclusions Protocol architecture is critical in deciding the system performance. Thus, system design can be based upon the output parameter of concern. Contention mechanism is as significant as the protocol architecture. Design improvements recommended for applications that have stringent demands on delay values. R-TDMA R-TDMA provides better throughput and delay characteristics for traffic patterns that are continuous in nature. The R-TDMA system is more suited to HTTP traffic than the MF-Polling system. 19 July 2 Mihir Thaker 45
46 Conclusions and Future Work (cont ) MF-Polling The MF-Polling system performs better under light load conditions or for traffic that is bursty in nature. The MF-Polling system is more suited to FTP traffic than the R-TDMA system. The MF-Polling system can support a larger user population, but delivers lower throughput and higher average queuing delay than the R-TDMA system. 19 July 2 Mihir Thaker 46
47 Conclusions and Future Work (cont ) Future Work Synthesize a MAC scheduler for the MF-TDMA system that would take advantage of the lower queuing delays, high throughput and larger supported user population. Modify the contention mechanism that would take into account various types of users present in system. This allows us to develop a fully QoS-aware MAC system. 19 July 2 Mihir Thaker 47
48 Questions?? 19 July 2 Mihir Thaker 48
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