cross layer optimization for voip over wlan [yuan liu zhu]

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April 21, 2023 Cross-layer optimization for VoIP over WLAN 1April 21, 2023 1

Cross Layer optimization for VoIP over WLAN[Yuan Liu Zhu]

A Presentation for Resilinets group

Sarvesh Kumar Varatharajan

Department of Electrical Engineering & Computer Science

University of [email protected]

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Abstract

• Problems of VoIP over WLAN • Unacceptable voice delays when coexisting with other

applications• Low voice Capacity due to heavy headers from the

above layers

• Cross layer schemes to improve voice capacity • Voice stream enjoys fixed bandwidth.• Voice delay is minimum• Simulation at various PHY rates over various voice codec• Proposed scheme advances throughput of voice stream

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Organization of the paper

• Introduction• Background• Cross Layer Optimizations• Theoretical Analysis• Simulation and Results• Conclusion

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Introduction

• VoIP applications• Delay• Heavy packet headers from upper layers reduces voice

capacity• 802.11 can support only limited number of VoIP

connections• Capacity of G.711 VoIP using CBR model and a 20ms packet interval is only 12 calls[2]• A scheme is proposed where interaction with application

layer reduces delay• Time slot reuse method to increase capacity

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Background

• VoIP Overview• Overview of 802.11 and 802.11e• Related Work

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VoIP overview

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• Variable delay in each hop• End to end delay threshold

Reference[1]

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Overview of 802.11 and 802.11e

• Distributed Coordination function (DCF)• Contention based access• Best effort service• No QoS support• Infeasible option for VoIP

• Point Coordination function (PCF)• When number of polling stations is large then higher the end to

end delay• Poor performance of VoIP

• Service Differentiation based on MAC schemes to classify traffic types with relative priorities

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Overview of 802.11 and 802.11e

• 802.11e addresses the problem of 802.11• Hybrid Coordination Function (HCF)

– Enhanced Distributed Coordinated Access( EDCA )• Significant time delay

– HCF Controlled Channel Access(HCCA)• POLL, ACK required. Additional overhead

Reference[1]

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Related Work

• VoIP over PCF[3][4]• VoIP over DCF[5]• No solutions to improve VoIP performance over HCCA

of 802.11e• Paper proposes schemes to improve VoIP capacity over

HCCA of 802.11e


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Cross Layer Optimization

• Assumption: All nodes of WLAN have fixed and equal PHY rate• Slotted TXOP• Time Slot reuse• Packet-header Compression

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Slotted TXOP

• POLL + ACK reduces VoIP capacity in HCCA• ACK unnecessary for voice• Slotted TXOP

– Each node has fixed PHY rate and same voice codec

Reference[1]

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Slotted TXOP (contd)

• Minimizing time delay

Reference[1]

• Application layer informed about the time slot• Packaging is done before the time slot of the node

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Time-slot reuse

• Voice Activity Detection(VAD)• MAC layer rarely downloads and uploads voice packet at

the same time

Reference[1]

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Time slot reuse(contd)

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Reference[1]

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Packet-header compression

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• Typical VoIP packet at MAC layer 40 –byte IP/UDP/RTP headers while payload only 10 to 30 bytes

• Packet Header Compression(PHC)– Casner’s Algorithm[6]– Most of the fields in the IP,UDP and RTP headers do

not change over the lifetime of RTP session(Represent by fewer bits)

– RTP header fields such as sequence number and time stamp are increased by a constant amount for successive packets

– Differential coding– This algorithm can compress the header to 2 bytes

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Cross Layer Optimization

• Assumption: All nodes of WLAN have fixed and equal PHY rate• Slotted TXOP• Time Slot reuse• Packet-header Compression

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Theoretical Analysis

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• TRST with PHC increases network capacity compared to original TRST

• If psil is large enough the network capacity of VoIP increases compared with STXOP, let alone the original HCCA scheme

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Conclusion

• STXOP utilizes slotted TXOP and discards POLL and ACK to system capacity

• Every voice stream enjoys fixed bandwidth• Interaction with application layer reduces delay• TRST scheme proposed reutilizes silent slots to advance

system performance and reduce CFP duration• Proposed scheme better than HCCA scheme• System capacity better for codec G 723.1

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References

[1]YUAN Wen-peng, LIU Gan, ZHU Guang-xi,Cross-layer optimization for VoIP over WLAN,June 2007 Volume 1,No 1,Computer Technology and Application,ISSN1934-7332,USA

[2]LIN Y.-B. and Chlamtac, I. Wireless and Mobile Network Architectures, New York:Wiley,2001

[3]Chen D., Garg S., Kappes . And M. and Trivedi K. Supporting VBR VoIP traffic in IEEE 802.11 WLAN in PCF mode. Avaya Laboratories, Basking Ridge, NJ Tech. Rep. ALR-2002-026, 2002

[4]Veeraraghavan, M., Cocker, N. and Moors, T. Support of voice services in IEEE 802.11 wireless LANs Proc. INFOCOM ‘01 Apr. 2001, 1:488-497

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References

[5]Baldwin, R.O. Davis IV, N.J .Midkiff, S.F and Raines, R.A. Packetized voice transmission using RT-MAC, a wireless real- time medium access control protocol. Mobile Comput Commun. Rev 2001, 5(3):11-25

[6] Casner,S. and Jacobson, V. Compressing IP/UDP/RTP headers for low speed serial links. Proc. IETF RFC 2508, Feb 1999.

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cross layer optimization for voip over wlan [yuan liu zhu]

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