Providing QoS for symmetrical voice/video traffic in wireless networks

1. Providing QoS for symmetrical voice/video trafficin wireless networks Advisor：Wei-Yeh Chen Student：王璽農 Wyatt J., Habibi D., Ahmad I. and Zen H., “Providing QoS for symmetrical voice/video traffic in wireless networks”, ICON 2007, 15th IEEE International Conference, vol.10, 2007, pp. 312 -317.

2. Outline • Introduction • Wireless access schemes • 802.11b-DCF • 802.11-SVP • 802.11e-EDCA • Wireless Token Network (Time、Address) • Results • Conclusion

3. Introduction • VoIP and VIoIP are becoming very popular and widespread. These types of real-time services produce streams that are almost symmetrical in nature. • To achieve acceptable QoS a real-time stream requires a certain throughput, delay, and jitter, with the aim being a high throughput that is highly consistent for each node.

4. A new token passing MAC developed by our group, called WTN has been designed with QoS guarantees in mind. • WTN supports a higher number of VoIP clients compared to 802.11e due to its higher channel efficiency, it also provides superior QoS for bidirectional streams.

5. 補充 • 802.11e定義：在802.11無線區域網路上提供QoS所需的機制及運作方式。 • PCF是基於DCF分散式的既有機制所制定出。 • 原始的802.11標準中制定了PCF集中式協調功能，以輪詢的方式實現免競爭傳輸，但未考慮到QoS的需求，因此實用不高。 • DCF=分散式→通道必須競爭 • PCF=集中式→競爭時段、免競爭時段

6. 802.11b-DCF • DCF was originally designed to allow quick, easy and robust access to a wireless channel without complicated addressing or queuing techniques. • When faced with traffic of differing priority DCF does not differentiate between a high priority packet and a low priority packet. • Throughput on average in a saturated network running 802.11 DCF MACs is equal for all nodes if they all have the same traffic pattern.

7. 802.11-SVP • SVP is a modification of 802.11 which specifies that the back-off time for high priority packets should be set to zero. • SVN also specifies that high priority packets should either be put at the head of the queue or put in a separate queue completely. • Both these methods are designed to give priority access to packets which contain high priority data and allows them to access the network in a timely manner at the expense of causing more collisions.

8. 補充 • EDCA=加強型分散式通道存取，改良原始的DCF模式，配合使用QoS訊框格式，依照網路資料流的種類優先競爭通道存取權之功能，而非原始平等的競爭模式，使得即時性服務的資料能優先被傳輸。 • AC_BK → AC_BE → AC_VI → AC_VO (Access Category=存取類別) (Background) (Best Effort) (Video) (Voice) 高優先

9. 802.11e-EDCA • Traffic at each node is differentiated into up to eight queues. Each of the queues has a different arbitrary interframe space (AIFS) and a different minimum contention window time. • EDCA guarantees bandwidth for high priority traffic very well, whilst still maintaining connectivity for low priority traffic.

10. Wireless Token Network • WTN is a clean sheet design which only incorporates the overheads that are absolutely necessary to provide good throughput and QoS. • WTN is a TDM token passing network with separate address negotiation. • Due to the TDM nature of the upstream and downstream traffic at the access point. • WTN provides a dedicated portion of network access time to the access point’s traffic.

11. The time divisions of WTN

12. WTN-Time divisions • A cycle starts with downstream traffic from the access point. • This traffic is sent in a continuous manner until either the access point runs out of traffic or 40 ms expires. • where the AP must compete like any other station to have access to the channel. • This allows more symmetrical traffic patterns without a bottleneck at the AP for received traffic.

13. If a client runs out of traffic a small empty packet is sent to indicate that it is relinquishing the token. • After the upstream traffic time division is complete the access point checks to see if a free address is available. • If an address is available addressing takes place. Once addressing is complete the cycle repeats. • If no address is available then no addressing takes place and the cycle repeats.

14. WTN-Upstream Time Division • During the upstream sequence each client embeds information about the change in its queue lengths in the data frames that are being sent. • This information is stored in the management list.

15. Ccur(i)=client 時間片 • Cuncorrected(i)=client裸時間片值 • Cprev(i)=client上一週期的時間片 • Cyc=上游時間 • β=上一週期的比例 • γi(j)=佇列縮放比例 • Qi=佇列訊息載體 • j=佇列數量 完整的時間計算片 均衡器

16. WTN-Addressing • Since WTN is tightly controlled and a node wanting to send traffic can only do so when handed a token, an addressing time has been set aside to allow unassociated nodes in the network to associate with an AP. • At the end of the upstream time division the AP checks for a free address and if one is found it sends an Address Send Frame (ASF).

17. Unassociated nodes then calculate a random backoff slot in which to transmit an Address Reply Frame (ARF). • The first reply without error received at the AP wins the address. • If there is not a free address available no ASF is sent and the AP commences a downstream division.

18. Results • 一個單一接入點有4個client。這4個client，由雙向流量組成，每個方向每20毫秒由500bytes組成。 • 500bytes=80bytes的audio+420bytes的video。 • 這是一個非常低比率的視頻，想像成每個客戶端相當於一個3G手機，訊框速率和分辨率遠低於正常水平。 • 每一個即時流量大約是200Kbps，接入點的總額約是800Kbps，每個client約是200Kbps。Data傳輸率定為2Mbps。 • 客戶端1有一個FTP接入點，這是用來顯示每個區段如何區分流量。

20. b

23. Conclusion • WTN has been designed with this provision in mind and thus it is capable of servicing these streams if they fall within the bounds of the network throughput. • WTN also achieves a low standard deviation of throughput and high differentiation of traffic.