ncue-csie1 A QoS Guaranteed Multipolling Scheme for Voice Traffic in IEEE Wireless LANs Der-Jiunn Deng 、 Chong-Shuo Fan 、 Chao-Yang Lin Speaker: Chong-Shuo Fan Date:2006/06/26

ncue-csie2 Outline 1. Introduction 2. Improved Approach 3. Simulations 4. Conclusions

ncue-csie3 1. Introduction In order to reach a higher Quality of Service (QoS) in network applications, the e Task Group has deployed a hybrid coordination function (HCF) to improve the original IEEE 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

ncue-csie4 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).

ncue-csie5 2. Improved Approach For each real-time station S, we Use two variable: r c : the packet transfer rate : the maximum amount of jitter (i.e. packet delay variation)

ncue-csie6 In the BSA of IEEE 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

ncue-csie7 Theorem 1 (1/2) Let

ncue-csie8 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.

ncue-csie9 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..,

ncue-csie10 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.,

ncue-csie11 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.

ncue-csie12 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

ncue-csie Proposed Scheme

ncue-csie14 Improvement (1/3) 1. 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

ncue-csie15 Improvement (2/3) 2. 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).

ncue-csie16 Improvement (3/3) 3. When the underlying session is ready to close, the piggybacking bit = 1, i.e., End-of- file and AP removes the corresponding bucket. 4. 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

ncue-csie17 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.

ncue-csie18 Simulations (1/3)

ncue-csie19 Simulations (2/3)

ncue-csie20 Simulations (3/3)

ncue-csie21 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 e 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.