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Providing QoS for symmetrical voice/video traffic

in wireless networks

AdvisorAdvisor：Wei-Yeh Chen

StudentStudent：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.

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Outline

Introduction Wireless access schemes

802.11b-DCF 802.11-SVP 802.11e-EDCA

Wireless Token Network (Time 、 Address) Results Conclusion

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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.

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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.

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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

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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.

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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.

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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

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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.

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Wireless Token Network WTN is a clean sheet design which only incorporates t

he 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.

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The time divisions of WTN

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WTN-Time divisions

A cycle starts with downstream traffic from the access point. This traffic is sent in a continuous manner until eith

er 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.

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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. O

nce addressing is complete the cycle repeats. If no address is available then no addressing takes

place and the cycle repeats.

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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.

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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

均衡器

完整的時間計算片

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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).

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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 win

s the address. If there is not a free address available no ASF is se

nt and the AP commences a downstream division.

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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 8

00Kbps ，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。

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b

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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.