IEEE Wireless Communication Magazine Design and Performance of an Enhanced IEEE802.11 MAC Protocol...

IEEE Wireless Communication Magazine

Design and Performance of an Enhanced IEEE802.11 MAC

Protocol for Multihop Coverage Extension

Frank H.P. Fitzek, Diego Angelini, Gianlucca Mazzini, and Michele Zorzi

Universita di Ferrara, Italy

IEEE Wireless Communication Magazine: Design and Performance of an Enhanced IEEE802.11 MAC Protocol for Multihop Coverage Extension

2

Overview

Motivation and Problem

Frequency Usage of IEEE802.11a

New MAC Protocol

Simulation Results

Conclusion


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Motivation for Multi Hop Networks

Multi hop networks are a viable option for installation of a network where it is missing/incomplete or as a complement to existing cellular systems.Example: 3G/WLAN couplingCoverage extension is attractive to reduce installation costs. In large WLAN hotspots ¾ of the total costs are caused by the fixed infrastructure.


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Motivation for Multi Hop Networks

€

Time

Return of Investment

Cost without multi hop

Cost with multi hop


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Problems for multi-hop networks with IEEE802.11 WLAN

Many researchers report about the problem of IEEE802.11b in multi-hop networksReason: RTS /CTS exchange blocks ongoing communicationExample: Simple line communication

DC E FCTSCTSBRTSRTS

A


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Problems for multi-hop networks with IEEE802.11 WLAN

Simple example shows the throughput degradation for multi-hop network operation (6 terminals are blocked while one packet is transmitted)Well documented and reported for IEEE802.11b even for more sophisticated network topologiesThe bandwidth is not use efficiently IEEE802.11a offers higher transmission rates, but the bandwidth inefficiency remains the sameThe problem resides in the MAC design


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Data Rate versus Range IEEE802.11a


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5GHz ISM Frequency Bands


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Channel Structure IEEE802.11a


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Possible Approaches For Spectrum Usage

Divide collision domain into smaller groups: assign all wireless terminals randomly to different channels – RANDOM APPROACH

reduce the RTS/CTS problemlow connectivity (N channels result in connectivity level of 1/N)routing over different channels is difficult

All terminals use the same channel - STATIC APPROACHInefficiency in the bandwidth usage (N channels result in a bandwidth efficiency of 1/N)full connectivity

Communicate the usage of the assignment of the channels to all wireless nodes – DYNAMIC APPROACH

full connectivitySolution need!!!


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Proposed MAC Scheme

Use of a common signaling channelOnly for RTS/CTS exchange4 way handshake with probingData and ack on dedicated channel

Sender Receiver

signaling channel

dedicated channel

RTS1 CTS1 RTS2CTS2

probe data S2R ack

RTS1RTS2probe data S2RCTS1CTS2ack

NAVchannel

NAVchannel


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Proposed MAC Scheme with Collisions

Sender 1 Receiver 1

signaling channel

dedicated channel

RTS1 CTS1 RTS2CTS2

probe data S2R ack

RTS1 CTS1 RTS2CTS2

probe data S2R

Sender 2 Receiver 2RTS1 CTS1

probe


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First Simulation Results

Based on a MATLAB model by D. Angelini, University of Ferrara Only MAC performance No real multi hopPackets are generated in a CBR fashion and send randomly to one of the neighborsPacket length 4kbyte (reservation phase)Surface area 30x30m²We distinguish two different approach

static approachdynamic approach


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Throughout for the dynamic, static and 802.11b standard approach


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Intermediate Discussion

Dynamic approach yields better results than the static or the IEEE802.11b approachBut, the dynamic approach uses 8 channels, while the static approach is using only one.Fairness of the comparison!Improvement of the dynamic scheme

Collisions may occur due to missing information from the signaling channel if terminals are transmitting on a dedicated channel. Solution: 2nd W-NICDo we need the probing packet?


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Possible Improvements

No probing packet assuming that the modulation and coding settings can be derived from the signaling channelRate controlled hand-shake New calculation of the congestion window


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Impact of the number of dedicated channels for one and two W-NICs

1 Wireless NIC

2 Wireless NIC

Larger number of dedicated channels help to improve throughput, but gain decreases for each additional channel (bottleneck of signaling channel)


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Multi-Hop Performance

Real Multi-Hop Scenario Model based on C++ Implementation by F. Bertocchi, University of Ferrara 60 wireless terminalsSender chooses receiver randomly for one packetShortest path routing for multi hop communicationMetric:

Packet Delivery: Ratio of send and received packetsTransmission Delay: Delay between sender and receiver

Three approaches under investigationStatic ChoiceRandom ChoiceDynamic Choice (Our MAC)


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Multi Hop Performance – Packet Delivery


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Multi Hop Performance – Transmission Delay

IEEE Wireless Communication Magazine

Thanks for your [email protected]@fitzek.net


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Further Reading

F.H.P. Fitzek, P. Seeling, M. Reisslein, and M. Zorzi, "Visualisation Tool for Ad Hoc Networks - ViTAN", IEEE Network Magazine, software tools for networking, p. 9, Vol.17, No. 4, July/August 2003

F.H.P. Fitzek, D. Angelini, G. Mazzini, and M. Zorzi, "Design and Performance of an Enhanced IEEE802.11 MAC Protocol for Ad Hoc Networks" , In Proceedings IEEE VTC 2003, Wireless Communications: 3G and Beyond Symposium, October 6-9, 2003, Orlando, Florida, USA

F.H.P. Fitzek, G. Schulte, and M. Zorzi, "Connectivity, Multi Hop, and Upper Bound Capacity Calculation for Multi-Rate Terminals in Ad-Hoc Networks“, In Proceedings of Wireless World Research Forum 9, (WWRF9), July 1-2, 2003, Zürich, Switzerland

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