Priority Queuing

Achieving Flow ‘Fairness’

in Wireless Networks

Thomas Shen

Prof. K.C. Wang

SURE 2005

Wireless Mesh Networks

Similar to ad-hoc networks Characteristics

Cheaper deployment Connectivity Redundancy

Current Technologies Hardware

• 802.11b• Proprietary

Software• Proprietary• Open Source

Open Challenges Security No standard yet – 802.11s

Internet

Motivation

Multiple user access causes contention for network access

MAC layer governs individual node access

Network layer governs flows Study network layer queuing

methods Implement packet assignment

to control flows for QoS

802.11 – MAC layer

Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA) Optional RTS/CTS Random Backoff

Priority Assignment Methods

Strict Priority QueuingQueue

Queue 0

Queue 1

Queue 2

Flow 0

Flow 1

Flow 2

If packets in queue

If packets in queue

else

else

FIFO

Queue 0

Queue 1

Queue 2

Flow 0

Flow 1

Flow 2

Probability 0.1

Probability 0.2

Probability 0.7

Priority Assignment Methods

Weighted Fair Queuing

Our Queuing Strategy Enqueue

Service packets with combination of strict priority and weighted fair queuing

Multiple queues Categorize packets

according to type and source

Controllable weights

Queue 0

Queue 1

Queue 2

Routing Packets Own Packets Others’ Packets

Queue 0

Queue 1

Queue 2

If packets exist

MAC layer

If packets exist

If packets exist

else

Probability p

Probability 1- p

Our Queuing Strategy Dequeue Routing packets always serviced first Modify p to change weights

Threshold = probability of choosing others’ packet over own packet

ns-2

The Network Simulator ns-2Event drivenOpen sourceNetwork Animator NAMhttp://www.isi.edu/nsnam/ns/

Simulations

Types of traffic Constant Bit Rate traffic over UDP

• UDP is unreliable, one way traffic. FTP traffic over TCP

• TCP is reliable, two way traffic with flow control. Metrics

Calculate end-to-end throughput for TCP Calculate end-to-end success rate for UDP

Simulation time of 1000s Random starting time between 1~2s Assumed error-free transmission Five trials each

Triple Chain

I

T

0

1

3

2

Nodes in range connected by dashed lines

Triple Chain UDP

200KBps CBR traffic

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Threshold

UD

P E

nd

-to

-en

d S

uc

ce

ss

Ra

te

Flow 0

Flow 1

Flow 2

Flow 3

One hop

Two hopDashed – Original

Solid - Priority

Triple Chain TCP

0

50

100

150

200

250

300

350

400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Threshold

TC

P T

hro

ug

hp

ut

(KB

ps

)

Flow 0

Flow 1

Flow 2

Flow 3

One hop

Two hopDashed – Original

Solid - Priority

Quad Chain

I

T1 T2

0

1

2

Each 200Kbps CBR traffic

Quad Chain UDP Results

Flow 2Original

Flow 0Original

Flow 1Original

Flow 2New

Flow 0New

Flow 1New

Quad Chain UDP Results

Flow 1New

Flow 0New

Flow 2New

Quad Chain TCP

TCP throughput for 3-hop flow was terribleLack of MAC access prevents packets

from being sentWith few packets, queuing method

has no effect 802.11 not efficient for multi-hop

networks as documented in literature

Small Mesh

100Kbps CBR traffic

I

2

2

4 5

0 1 3

Small Mesh UDP Results

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Threshold

UD

P E

nd

-to

-En

d S

ucc

ess

Rat

e

Flow 0

Flow 1

Flow 2

Flow 3

Flow 4

Flow 5

Two hop

One hop

Three hop

Flow 4/5Original

Flow 2Original

Conclusion

Results show throughput is unbalanced using FIFO

Priority queuing allocates bandwidth among flows

In our simulations, thresholds of 0.5 to 0.7 distributed throughput most equally

Future Work

Implement different priority assignment strategies Identify potential objectives to guide priority

assignment• Ensure throughput regardless of route length by

categorizing packets according to number of hops taken• Ensure throughput of certain users by categorizing

packets according to source• Ensure throughput of certain applications by

categorizing according to packet type Static vs. dynamic priority assignment

Devise a performance criteria to evaluate fairness

Acknowledgement

Professor K.C. Wang Professor D. Noneaker Professor X.B. Xu Clemson University NSF

References

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