In order to reduce the polling overhead and provide higher QoS for real-time traffic streams, we proposed a new polling scheme, called Adaptive Time-Stamp Polling (ATSP) scheme. In our proposed scheme, it contains three enhancement parts; one is the adaptive time-stamp poll scheduling which is responsible for calculating the next polling time of each traffic stream.
Basically, it calculates by adding the maximum service interval which is registered in the corresponding TSPEC to the current polling time, instead of adding the same service interval for all traffic streams. This method is more flexible and able to reduce the number of unnecessary polling and lessen the jitter deviation.
The second part is the short interval polling function which focuses on the access delay reduction by using the personalized short interval to poll some traffic stream between receiving first QoS data frame and second QoS data frame. During short interval polling periods, it calculates next polling time by just adding the short interval to detect a more accurate polling time. The last part is used to solve talk-spurt and silence alternation problem of voice traffic streams such as VoIP, it detects the silence traffic streams by counting the consecutive QoS-Null replies up to three, and considers them as in the talk-spurt state when receiving QoS data. When it considers that some traffic stream is in silence state, it polls this stream with a longer interval to reduce the overhead.
We use NS-2 tool (version 2.29) with ns-2 802.11 support which contains 802.11e module (both HCCA and EDCA) to simulate our adaptive time-stamp polling scheme and compare it with round-robin polling scheme. The results show that ATSP has significant improvement in terms of throughput, access delay and jitter deviation. The standard deviation of jitter for real-time traffic in ATSP scheme reduces by more than 60% comparing to RR scheme. The average delay in ATSP scheme also decreased more than 50% comparing to RR scheme and the
standard deviation of access delay is very little, this means frames will unlikely experience high access delay with our ATSP scheme. The most important thing is that we would not sacrifice the best-effort transmission for real-time applications. Best-effort traffic would not suffer from starvation when there are more and more QoS traffic streams added to the channel, and the total utilization of channel will be also improved.
The complexity of our ATSP scheme is a big issue because the HC needs to calculate polling time of each traffic stream respectively and observe which traffic stream has been in the silence state or during the short interval polling period. The short interval polling period of the traffic stream is much shorter than the whole real-time transmission and we use simple method to detect silence streams, so the HC won’t spend much time to observe those traffic streams. Truly complex part is to calculate polling time for each traffic stream. The more QoS traffic streams, the more the calculations. However, those calculations are almost simple additions, so we could conclude that ATSP scheme achieves a significant improvement.
In the future, we will improve our silence detection method to make those decisions more reliable, and find a more efficient mechanism about access delay reduction to minimize extra polling. Besides, the Direct Link Setup (DLS) and Block Acknowledge will be taken into account to support a more reliable and effective transmission for real-time applications on wireless LANs.
References
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