Providing QoS
for symmetrical voice/video traffic
in 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.
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Outline
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Introduction
Wireless access schemes
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802.11b-DCF
802.11-SVP
802.11e-EDCA
Wireless Token Network (Time、Address)
Results
Conclusion
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Introduction
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VoIP and VIoIP are becoming very popular and
widespread. These types of real-time services
produce streams that are almost symmetrical in
nature.
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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.
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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
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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。
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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
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DCF was originally designed to allow quick, easy
and robust access to a wireless channel without
complicated addressing or queuing techniques.
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When faced with traffic of differing priority DCF
does not differentiate between a high priority
packet and a low priority packet.
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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
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SVP is a modification of 802.11 which specifies
that the back-off time for high priority packets
should be set to zero.
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SVN also specifies that high priority packets
should either be put at the head of the queue or put
in a separate queue completely.
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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)
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高優先
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802.11e-EDCA
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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.
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EDCA guarantees bandwidth for high priority
traffic very well, whilst still maintaining
connectivity for low priority traffic.
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Wireless Token Network
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WTN is a clean sheet design which only incorporates
the overheads that are absolutely necessary to provide
good throughput and QoS.
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WTN is a TDM token passing network with separate
address negotiation.
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Due to the TDM nature of the upstream and
downstream traffic at the access point.
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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
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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.
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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.
Once 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
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During the upstream sequence each client embeds
information about the change in its queue lengths
in the data frames that are being sent.
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This information is stored in the management list.
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完整的時間計算片
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均衡器
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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
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WTN-Addressing
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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.
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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
wins the address.
 If there is not a free address available no ASF is
sent and the AP commences a downstream division.
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Results
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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。
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b
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Conclusion
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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.
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WTN also achieves a low standard deviation of
throughput and high differentiation of traffic.
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