Distributed Fair Scheduling in a Wireless LAN

Gautam Kulkarni

EE206A (Spring 2001)

Nitin Vaidya, Paramvir Bahl and Seema Gupta

(appeared in Mobicom 2000 Boston, MA)

Introduction

• Requirements of a scheduling discipline:– Ease of implementation

– Fairness and protection

– Performance bounds

– Ease of admission control (if needed)

• With fair scheduling bandwidth for a flow weight

• 802.11 MAC is not fair• How to introduce fairness in wireless LANs ?

Fair Queueing

• “Ideal” scheduling discipline – Generalized Processor Sharing (GPS)

• All fair queueing disciplines try to emulate GPS• Traditional GPS-like disciplines centralized in design• Previous work on fairness in distributed MAC protocols:

– Limited in scope – provide equal bandwidth share (e.g. MACAW)– Suffer in the presence of location-dependent errors

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Fair Scheduling

• Distributed Fair Scheduling (DFS) – new protocol for fair scheduling

• A distributed algorithm derived from the Distributed Coordination Function (DCF) in 802.11

• Emulation of Self-Clocked Fair Queueing (SCFQ) in a distributed manner

• Scheduler maintains a “virtual clock” to keep track of packets to be serviced

SCFQ

• Main idea:Start tag of packet

Finish tag of packet

V(0) = 0. Virtual time = finish tag of packet in service

Transmit packet with smallest finish tagPackets stamped on reaching the head of the queue

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802.11 Distributed Coordination Function

• CSMA/CA• Node i chooses backoff interval = Bi slots• Bi uniformly distributed in [0, cw] where cw = size of contention

window• Decrement Bi

• Is Bi == 0 ?– Yes: Send RTS

• Receive CTS– No CTS ? Double cw, select new Bi and repeat from start

• Send data• Receive ACK

– No: Decrement Bi

Distributed Fair Scheduling (DFS) Protocol

• Marriage of a distributed version of SCFQ with 802.11 DCF

• Key idea – select backoff interval proportional to the finish tag of the packet to be transmitted

• Each node maintains a local virtual clock vi(t)

Backoff interval = Scaling_Factor * length / weight * random number with mean 1

DFS (contd.)

• Collision handling– To reduce “priority” reversals, a small backoff

interval is chosen after the first collision– Backoff interval increased exponentially on further

collisions

• Potential drawbacks– Can exhibit short-term unfairness– Impact of small weights of backlogged flows

Impact of Small Weights

• Recall: Backoff intervals are being used to compare “length/weight”

• Small weights can lead to high idle times – throughput degradation

• Intuition: Any non-decreasing function of length/weight may be used to obtain backoff intervals

• Need to explore alternate mappings

Alternate Mappings

Chosenbackoffinterval

Scaling_factor * length / weight * random number

Alternate Mappings (contd.)

• Advantage– smaller backoff intervals

– less time wasted in counting down when weights of all backlogged flows are small

• Disadvantage– backoff intervals that are different on a linear scale may

become identical on the compressed scale

– possibility for greater number of collisions

Performance Evaluation

• Using modified ns-2 simulator: 2 Mbps channel• Number of nodes = N• Number of flows = N/2• Odd-numbered nodes are destinations,

even-numbered nodes are sources• Unless otherwise specified:

– flow weight = 1 / number of flows – backlogged flows with packet size 584 bytes (including UDP/IP

headers)

– Scaling_Factor = 0.02

Fairness Index

• Fairness measured as a function of

(throughput T / weight ) for each flow f over an interval of time– Unless specified, the interval is 6 seconds

Throughput/Weight Variation across Flows

Throughput / Weight

Flow destination identifier

Flattercurve

is fairer

DFSis fairer

Throughput-Fairness Tradeoff

Fairness

index

Number of flows

Throughput-Fairness Tradeoff

Aggregatethroughput(all flowscombined)

Number of flows

Scaled 802.11

• Fairness of 802.11 can be improved by using larger backoff intervals

• Is DFS fairer simply because it uses large backoff intervals ?

• Scaled 802.11 = 802.11 which uses backoffinterval range comparable with DFS

Short Term Fairness

Frequency

Number of packets transmitted by a flow (over 0.04 second windows)

Narrowdistribution

is fairer

DFS isfairer

Fairness Versus Sampling Interval Size

Fairness Index

Interval Size

Scaling Factor

• How to select the scaling factor ?– Small number : May result in more collisions– Large number: Larger overhead

Impact of Scaling Factor

Fairness Index

Scaling Factor

six flows with weights 1/2,1/4,1/8,1/16,1/32,1/32

Impact of Scaling Factor

Scaling Factor

Aggregate

Throughput

six flows with weights 1/2,1/4,1/8,1/16,1/32,1/32

Conclusions

• DFS improves fairness compared to 802.11 and Scaled 802.11

• Alternative mappings somewhat beneficial

• No distributed fair scheduling protocol may accurately emulate work-conserving centralized protocols (unless clocks are synchronized)

The Mandatory Critique!

• Need to evaluate the effect of collision resolution mechanisms to maintain priorities

• Selection of scaling factor could be adaptive

• Actually, a very good paper!

The End

Acknowledgements: I have borrowed some slides from Prof. Vaidya’s webpage.