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Der-Jiunn Deng 、 Chong-Shuo Fan 、 Chao-Yang Lin Speaker: Chong-Shuo Fan Date:2006/06/26

A QoS Guaranteed Multipolling Scheme for Voice Traffic in IEEE 802.11 Wireless LANs. Der-Jiunn Deng 、 Chong-Shuo Fan 、 Chao-Yang Lin Speaker: Chong-Shuo Fan Date:2006/06/26. Outline. Introduction Improved Approach Simulations Conclusions. 1. Introduction.

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Der-Jiunn Deng 、 Chong-Shuo Fan 、 Chao-Yang Lin Speaker: Chong-Shuo Fan Date:2006/06/26

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  1. A QoS Guaranteed Multipolling Scheme for Voice Traffic in IEEE 802.11 Wireless LANs Der-Jiunn Deng、Chong-Shuo Fan、Chao-Yang Lin Speaker: Chong-Shuo Fan Date:2006/06/26

  2. Outline • Introduction • Improved Approach • Simulations • Conclusions

  3. 1. Introduction • In order to reach a higher Quality of Service (QoS) in network applications, the 802.11e Task Group has deployed a hybrid coordination function (HCF) to improve the original IEEE 802.11 Medium Access Control (MAC) protocol. • The HCF defines two medium access mechanisms, one of which is channel access control. • Nevertheless, choosing the right MAC parameters and QoS mechanism so as to achieve predictable performance remains an unsolved problem

  4. HCF in Controlled Access Mode • HCF operation is similar to the operation of PCF. • HCF can operate in two modes. • Coexisting with EDCF. • Using a contention-free period (CFP).

  5. 2. Improved Approach • For each real-time station S, we Use two variable: • rc: the packet transfer rate • : the maximum amount of jitter (i.e. packet delay variation)

  6. In the BSA of IEEE 802.11 • our AP reserves some of its memory to create token buckets • each representing a real time session that connects two stations, say A and B and generated when A or B enters the WTT state • A packet with a relatively smaller amount of jitter has lower priority

  7. Theorem 1 (1/2) • Let

  8. Theorem 1 (2/2) • If and , i = 2, …,n, then all voice packets of each session can be transmitted within their jitter constraints. • If a packet of the ith session though handoff, satisfies and , where represents the time needed for handoff, this packet will also meet its jitter constraint.

  9. Proof (1/3) • Handoff part • Assume the maximum waiting time of the token, produced by the ith voice source, after handoff from the other BSA is • Our goal • actual waiting time of the packet, say , is less than its required and tolerable jitter , i.e. .,

  10. Proof (2/3) • When i=1 , • The waiting time of the first packet equals its own transmission time (2*SIFS + CFPoll + tp + ACK), therefore, when i =1 this establishes the induction basis. • Assume that our induction hypotheses stands for the (i-1)th voice source, ie. ,

  11. Proof (3/3) • Assume , which means at the time point , all voice sources, from 1 to i-1, will have been multi-polled. Hence, the amount of packets generated between (0, )is , which means the total transmission time will be • From the already known fact , we can derive the following formula: • Since this contradicts our hypothesis, which states that , we obtain , which also stands for the ith voice source.

  12. Theorem 2 • Suppose n voice sources are scheduled in the given priority order. The average waiting time is minimized for voice packets if for all i < j

  13. 2.1 Proposed Scheme

  14. Improvement (1/3) • If accepting the request of a new voice source P in the previous DCF mode, AP will build a new token bucket in its buffer for P, and assign a priority based on P’s tolerated jitter

  15. Improvement (2/3) • Under the PCF mode • the station when polled must wait a period of time, SIFS, before transferring its packet. • When piggyback indicates that the underlying session has not terminated, AP produces a new token every . • However, AP needs SIFS + CFPoll to poll the stations. A station needs SIFS + ACK to respond. • Therefore, in the same connection, the time duration from the removal of T to the production of the next token is - (2*SIFS + CFPoll+ tp +ACK).

  16. Improvement (3/3) • When the underlying session is ready to close, the piggybacking bit = 1, i.e., End-of-file and AP removes the corresponding bucket. • When all buckets are temporarily empty, AP checks if there is enough time to run DCF mode before the next token T arrives. If yes, it sends a CF-End frame to end CFP and enters CP mode. If not, it waits for T

  17. Theorem 3 • Several voice sources with and , i=1,2,3,…,n, are given. • There exists a cycle LCT =L.C.M. (The Least Common Multiple) within which the amount of packets transmitted is . • If two or more packets of different sessions arrive at the same time point, based on Theorem 2 , a session with lower jitter has lower priority. This ensures a minimum total waiting time.

  18. Simulations (1/3)

  19. Simulations (2/3)

  20. Simulations (3/3)

  21. Conclusions • We record the scheduling results in a queue, within which an AP (Access Point) can poll and then enable mobile users to communicate with their opposite sites. • This occurrence can solves the problem that some voice packets do not suit QoS in IEEE 802.11e standard with multi-polling. • During the time-gap in which no voice packets are transmitted, the scheme changes to DCF mode to transfer data packets.

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