Achieving Quality of Service in Wireless Networks A simulation comparison of MAC layer protocols. CS444N Presentation By: Priyank Garg Rushabh Doshi Majid Malek Maggie Cheng Russell Greene

Overview MAC (DCF/PCF) Medium access governed by CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). Stations begin decrementing backoff counter DIFS time after medium is idle. Absence of an ACK indicates an error in transmission (possibly due to a collision). The transmitting station executes a binary exponential backoff and attempts re-transmission. PCF allows for a Point Coordinator (typically Access Point) to assign transmit opportunities in a contention free period that precedes the DCF contention period. Round Robin polling is used by Point Coordinator to allocate contention free transmit opportunities to stations. Point Coordinator never knows offered load at stations.

802.11e QoS Enhancements (EDCF/HCF) EDCF defines 8 traffic classes. Various parameters governing backoff can be individually set per traffic class. Medium access is similar to DCF with addition of AIFS. Station cannot begin decrementing backoff timer until after AIFS. Within a node, each traffic class has a dedicated queue. Traffic class queues contend for access to the virtual channel. Frames that gain access to the virtual channel then contend for medium. HCF is analogous to PCF but allows a Hybrid Coordinator to maintain state for nodes and allocate contention free transmit opportunities intelligently. The Hybrid coordinator uses the offered load per traffic class at each station for scheduling.

Scenarios + Rational Scenario 1: Shows how individual parameter variations affect the Video Stream which is higher priority. Scenario 2: A more realistic scenario where all QoS flows must meet their bandwidth requirements. There is an additional 10ms latency bound for audio flows which have highest priority and a 50ms maximum jitter allowance for the video stream which has medium priority. The parameters that can be varied in EDCF are (1) Minimum Contention Window (CWMin) (2) AIFS (CWOffset) (3) Backoff Window Growth Factor (PFactor)

Scenario 1: Varying EDCF Parameters Plots show Latency Cumulative Distribution Function for varying AIFS (CWOffset), Minimum Contention Window (CWmin), and Backoff Window Growth Factor (PFactor). Increasing CWOffset and CWmin of FTP stream greatly improves jitter characteristics of video stream. In Scenario 1, PFactor has minimal impact because only two stations contend for medium.

Scenario 2: Traditional DCF Plots show Bandwidth, Latency Cumulative Distribution Function, and Latency of traffic in Scenario 2 with DCF MAC. Under DCF, all traffic is treated equally, therefore latency and jitter characteristics of priority traffic (Audio/Video) are unacceptable.

Scenario 2: Optimized EDCF Plots show Bandwidth, Latency Cumulative Distribution Function, and Latency of traffic in Scenario 2. The MAC is EDCF with parameters optimized. Requirements of Audio and Video traffic are now met at the expense of lower bandwidth and higher latency for data traffic.

Scenario 2: Bandwidth Comparison Default EDCF vs. Tuned EDCF Tuning EDCF allows Video stream to stably maintain the required bandwidth. Without tuning the parameters, the performance is not acceptable, but the performance requirements can be met within the degrees of freedom provided.

Scenario 3: Overloaded network conditions used to test for benefits of QoS aware MAC

Scenario 3: Comparison Bandwidth DCF/EDCF/HCF Plots compare bandwidth between DCF, EDCF, and HCF running at the MAC layer. The DCF bandwidth plot shows considerable degradation in the video stream, while the EDCF and HCF plots show incremental improvements. Apart from the overall QoS improvements, the net throughput is also improved due to greater efficiency of the medium.

Scenario 3: Comparison Latency Dist DCF/EDCF/HCF Plots compare the latency distribution between DCF, EDCF, and HCF running at the MAC layer. DCF gives unacceptable performance for the real time audio and streaming video traffic. EDCF can be tuned to meet the required bounds but the data traffic suffers. HCF improves high priority stream latencies and jitter with minimal impact on lower priority streams compared to EDCF.

Results and Conclusions Current standard uses DCF/PCF which lacks facilities required for providing QoS. There is no support for prioritized traffic classes and no support for intelligent polling e standard replaces DCF/PCF based MAC with EDCF/HCF combination. The new MAC provides for prioritized traffic classes. HCF provides for intelligent polling and allocation of contention free transmit opportunities. EDCF provides for parameterized contention for the Medium based on CSMA/CA allowing traffic differentiation. EDCF allows for the requirements of high priority traffic classes to be met. However, it is at the expense of performance of lower priority classes. HCF also allows for requirements of higher priority traffic classes to be met, but in a more efficient manner (less impact to lower priority traffic classes) than DCF. This is due to the smarter allocation possible because of centralized load aware scheduling of the Contention Free Period.