Design of Fair Scheduling for WLANs Huei-Wen Ferng ( 馮輝文 ), Ph.D. Associate Professor Department of Computer Science and Information Engineering (CSIE) Nation Taiwan University of Science and Technology (NTUST) Wireless Communications and Networking Engineering (WCANE) Lab URL:

Outline Introduction Design I - DERR Design II – EDDRR-BI and E-DERR-BI Conclusions

Introduction IEEE /IEEE e  Popular wireless LAN standards  But some drawbacks exist. QoS guarantee Fairness

Introduction - DCF

Introduction – Enhance DCF (EDCF) or EDCA Multiple queues of different priorities is used to take care of different QoS requirements.

Design I Design of Distributed Elastic Round Robin (DERR)

Related Work Distributed Fair Scheduling (DFS)  adjusts backoff intervals to implement fairness. Distributed Weighted Fair Queuing (DWFQ)  adjusts contention windows to fulfill fairness. Both DFS and DWFQ have poor performance in throughput and delay due to latent collisions although better fairness can be achieved. Distributed Deficit Round Robin (DDRR)  is designed based on Deficit Round Robin (DRR)  is a distributed fair scheduling scheme.  employs the concept of deficit count to achieve fairness.  is a collision-free scheme.  employs a mapping between deficit count and the inter frame space (IFS)

Main Ideas of DDRR

Main Ideas Allowance  previously employed by elastic round robin (ERR)  The minimum amount of data allowed to be transmitted by a host.  Allowance is adjustable and smaller than the amount of data sent. Classification  Hosts with different QoS requirements are classified into different classes. Weight  defined according to QoS requirements

How to Define Allowance? The excess amount: where and are the excess amount and the total amount of transmitted frames The allowance is then calculated using i: class j: host

Why? is larger for a host with higher throughput and vice versa. Thus, is proportional to the desired throughput specified by host j within class i The excess amount during the previous transmission is deducted for the sake of fairness.

A Mapping Function Constant is used to make fall within PIFS and DIFS specified by the IEEE standard rand is a random number which results in different values of IFS to avoid possible collisions A host with shortest IFS gets the right to transmit frames until the total length of frames exceeds the value of allowance.

Definitions of Weights Weight of class i is defined by Where K E is a pre-specified constant and T E,i (in seconds) is a time period.

Numerical Results and Discussions Ad hoc mode under transmission rate of 2 Mbps No RTS/CTS 8 hosts exist in the system Frame generation rate: 110 kbps data rate: 100 kbps Frame length: 1000 bytes Other parameters:

Throughput and Delay Throughput Delay

Fairness Index Fairness index The difference between DERR and ideal: 0.032, DDRR and ideal: 0.071

Throughput/weight (DDRR) Std. deviation: 0.018

Throughput/weight (DERR) Std. deviation:

Concluding Remarks A fair scheduling mechanism, i.e., DERR is proposed and discussed. DERR avoids collisions through a random mapping between allowance and IFS. DERR offers a better fair sharing algorithm than DDRR.

Design II Design of EDDRR-BI and EDERR-BI

Goal Designing two scheduling mechanisms  Enhanced distributed deficit round robin with backoff interval (EDDRR-BI) and  Enhanced distributed elastic round robin with backoff interval (EDERR-BI)

Main Ideas These two scheduling mechanisms  should consider both priority and fairness,  are designed based on DDRR and DERR,  are targeted for IEEE e.

Design of EDDRR-BI DDRR vs. EDDRR-BI  Both employ the concept of the deficit count.  DDRR defines one type of deficit count, while EDERR-BI defines three.  No backoff procedure is involved in DDRR, but EDDRR-BI has the backoff procedure.  Cross-layer design is further considered in EDDRR-BI.

Design of EDDRR-BI For the ith flow of access category l in station j at time t, DC (t) is defined to denote the deficit count. When no data is transmitted, j l, i l = a denotes audio, l = v denotes video, l = d denotes data

Design of EDDRR-BI When a frame for the ith flow of access category l in station j is transmitted at time t. the frame size is then deducted from the deficit count, i.e., Temporary backoff interval Backoff interval

Design of EDERR-BI Unlike EDDRR-BI, EDERR-BI adapts allowance, which is an elastic and adjustable amount of traffic data allowed for transmission, to govern the kernel of scheduling.

Design of EDERR-BI Where E (t’) stands for the excess amount for the ith flow of access category l in station j at time t’ j l,i F (t’) is the total amount of traffic data transmitted at time t’ by the ith flow of access category l in station j. j l,i i)Allowance is defined in proportional to the desired throughput. ii)Allowance increases linearly as time goes. iii)The excess amount at the latest time instant should be deducted to enforce fair scheduling.

Design of EDERR-BI Temporary backoff interval Backoff interval

Definitions of Weights For both EDDRR-BI and EDERR-BI, weights should be properly defined so that one can gauge fairness based on weights. We define weights for flows within the same access category l (l = a, v, d) w ([s] is used to denote the same access categories), [s],j l,i

Definitions of Weights Weights for flows within different access category w ([d] is used to stand for different access categories) Weights for different stations w l,i [d],j j

Simulation Results and Discussions We use the well-know network simulator ns-2 as the simulation platform. The desired flow rates for audio, video, data are fixed at 8 KB/s, 128 KB/s, 120 KB/s.

Simulation Results and Discussions To gauge the performance of EDDRR-BI, EDERR- BI, EDCA and EEDCF. Throughput comparison (Video) Fixing the number of stations at 18, we have 8% and 1.5% improvement for both EDDRR-BI and EDERR-BI compared to EDCA and EEDCF. (Data) We have 14% and 41% (28% and 58%) of improvement for both EDDRR-BI and EDERR-BI compared to EDCA and EEDCF.

Simulation Results and Discussions Delay comparison When the number of stations is 18 (Video) mean delay for EDCA (EEDCF) is 84% and 126% (17% and 44%) higher than EDDRR-BI and EDERR-BI. (Data) mean delay for EDCA (EEDCF) is 5% and 8% (9% and 12%) higher than EDDRR-BI and EDERR-BI. Collision rate comparison When the number of stations is 18. We have 22% of improvement for EDDRR-BI and 31% for EDERR-BI compared to EDCA

Simulation Results and Discussions Fairness comparison

Simulation Results and Discussions We have 120% and 7% (97% and 19%) of improvement for EDDRR-BI and 144% and 33% (230% and 98%) of improvement for EDERR-BI compared to EDCA and EEDCF.

Simulation Results and Discussions 1/ (1-fairness index) 50% and 6% (77% and 18%) of improvement for EDDRR-BI and 100% and 41% (192% and 94%) of improvement for EDERR-BI compared to EDCA and EEDCF are obtained.

Concluding Remarks Two fair scheduling mechanisms, i.e., EDDRR-BI and EDERR-BI, which can interplay with QoS requirements in the IEEE e LAN, are proposed and studied. Through simulations, we successfully illustrate that EDDRR- BI and EDERR-BI outperform EDCA and EEDCF in terms of QoS related performance and fairness. EDERR-BI performs better than EDDRR-BI. EDERR-BI is most suggested for use in the IEEE e LAN to achieve better fairness and guaranteed QoS.

Conclusions Two designs of fair scheduling are discussed. Future work?  Actually, we have done the design for EDDRR and EDERR which are not covered in this talk.

Q & A

References Huei-Wen Ferng, Chung-Fan Lee, Jeng-Ji Huang, and Ge-Ming Chiu, “Designing a fair scheduling mechanism for IEEE wireless LANs,” IEEE Commun. Letters, vol. 9, no. 4, pp , April > DERR Huei-Wen Ferng, Chung-Fan Lee, Han-Yu Liau, and Cheng-Ching Peng, “A fair scheduling mechanism for the IEEE e wireless LAN,” in Proc. International Computer Symposium (ICS) 2006, Taipei, Taiwan, December > EEDCF Huei-Wen Ferng, Han-Yu Liau, and Jeng-Ji Huang, “Fair Scheduling Mechanisms with QoS Consideration for the IEEE e Wireless LAN,” in Proc. IEEE VTC 2007-Spring, Dublin, Ireland, April > EDDRR-BI and EDERR-BI

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