Priority Queuing : Achieving Flow
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Priority Queuing : Achieving Flow 'Fairness' in Wireless Networks. I. One hop. 0. 3. 1. 2. 2. 1. 2. 0. Two hop. 5. 4. T2. T1. I. Routing Packets. Queue 0. Own Packets. Queue 1. Others’ Packets. Queue 2. Q1. Q1.

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The difference in TCP and UDP results is due to the flow control mechanism of TCP.

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The difference in tcp and udp results is due to the flow control mechanism of tcp

Priority Queuing : Achieving Flow 'Fairness' in Wireless Networks

I

One hop

0

3

1

2

2

1

2

0

Two hop

5

4

T2

T1

I

Routing

Packets

Queue 0

Own

Packets

Queue 1

Others’

Packets

Queue 2

Q1

Q1

Q1

Q2

Q2

Q2

0

2

ROUTING

ROUTING

ROUTING

2

1

3

3

Internet

Figure1

In a wireless mesh network, routers are connected wirelessly as shown in Figure 1.

3

Flows

1

Traffic from end-users travel through different routes as shown above. The different lengths and contention along each route affects performance of the flows. Routes that service multiple flows are places where prioritization strategies can be utilized to adjust performance.

T

I

0

2

  • Typical Queuing Methods

  • First-In-First-Out (FIFO)

    • Packets serviced depending on arrival time.

  • Strict Priority

    • Packets placed into different queues according to some criteria (type, source…)

    • Service queue A, unless it’s empty. Then service queue B, and so on.

  • Weighted Fair

    • Packets placed into different queues according to some criteria (type, source…)

    • Service queue i for xi fraction of the time where

    • and T = total number of queues.

Priority Queuing

Achieving Flow 'Fairness' in Wireless Networks

Thomas Shen, University of Illinois-UC and Dr. K.C. Wang, Clemson University

Our Priority Strategy

Figure8

UDP flows are 200Kbps CBR traffic

Quad Chain

  • Use a combination of strict priority and weighted fair queuing

  • Motivation

    • Wireless networks of many different topologies are in use today for anytime, anywhere access

    • Multiple user accesses contend for network resources

      • Contention is primarily arbitrated in medium access control (MAC) and network scheduling

      • At each individual node, the network layer allocates the share of transmission time given by the MAC layer to different flows

    • To control QoS of contending flows, the project exploits priority queuing methods for network scheduling

MAC layer

If packets exist

Queue 0

Threshold 1

Threshold 2

else

Pitfalls

TCP throughput for multi-hop traffic on the quad chain and small mesh were terrible

Lack of MAC access prevents packets from being sent

With few packets, queuing method has no effect

IEEE 802.11 MAC protocol is not efficient for multi-hop networks as documented in literature

If packets exist

Queue 1

Probability 1-p

Figure2

  • Categorize packets based on packet type and source.

If packets exist

Queue 2

Probability p

Figure3

  • Service routing packets first, since routes needs to be established before other packets can reach their intended destination

For example, the packet assignment for the triple chain UDP scenario is shown in Figure4.

Figure9

Figure4

Thresholds T1 and T2 were both varied. T2 controls Flow 1 and 2. At T2 = 0.5 , UDP flows 1 and 2 got the same end-to-end rate.

  • Simulation Setup

  • 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

  • Performance Metrics

    • Calculate end-to-end throughput for TCP

    • Calculate end-to-end success rate for UDP

  • Assumed error-free transmission

  • Link rate: 1Mbps

  • Five trials each

Simulations

Threshold is set equal to the probability of servicing others’ packets before your own.

Small Mesh

Figure10

Figure11

Triple Chain

Threshold

UDP flows are 200Kbps CBR traffic

Figure5

Threshold changed for all intermediate nodes.

The difference in TCP and UDP results is due to the flow control mechanism of TCP.

UDP flows are 100Kbps CBR traffic

Threshold change allocated bandwidth between Flow 2,4 and 5. Other flows were unaffected due to lack of serious contention.

  • Future Work

  • Implement different priority assignment strategies

    • Identify potential objectives to guide priority assignment

      • Ensure throughput regardless of route length by categorizing according to hops

      • Ensure throughput of certain users by categorizing according to source

      • Ensure throughput of certain applications by categorizing accord to packet type.

    • Devise a performance criteria to evaluate fairness

  • 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

SURE 2005

Figure7

Figure6


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