Design and Evaluation of IEEE 802.11 based Dual MAC in MANETs - - PowerPoint PPT Presentation

• M. Tech Dissertation

Design and Evaluation of IEEE 802.11 based Dual MAC in MANETs

Guides:

Satyajit Rai

• Prof. Sridhar Iyer

(01329009)

• Dr. Leena Chandran Wadia

January 15, 2003

KReSIT, IIT Bombay Satyajit Rai

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Motivation

• Traffic pattern in most of the Multihop Ad-Hoc Network

applications is centralized.

• The nodes need to send data to one or more preferred
• destinations. For example,

– Military: Soldiers talk to commander. – Disaster management teams: Stations talk to control/management center. – Sensor Networks: Stations feed data to a central database.

KReSIT, IIT Bombay Satyajit Rai

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Hidden and Exposed Node Problems

B C D

Tx Range Tx Range Tx Range Tx Range

A

Hidden Node Problem: If A and C want to transmit to B there will be collision at B, since A and C are hidden to each other. Exposed Node Problem: If B is transmitting to A, C can not transmit to D, since it senses the medium busy.

KReSIT, IIT Bombay Satyajit Rai

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Centralized Multihop Scenario

Tx Range

Outer Stations Inner Stations Boundary Stations

central Station

• The preferred destination is the central station.
• Nodes within one-hop range of central station are inner stations
• Stations at one hop distance are boundary stations.
• Stations beyond one-hop are outer stations.

KReSIT, IIT Bombay Satyajit Rai

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Centralized Multihop Scenario

Tx Range

Outer Stations Inner Stations Boundary Stations

central Station

• The preferred destination is the central station.
• Nodes within one-hop range of central station are inner stations
• Stations at one hop distance are boundary stations.
• Stations beyond one-hop are outer stations.

KReSIT, IIT Bombay Satyajit Rai

4-a

Focus of the Work

• We focus on performance of IEEE 802.11 MAC in centralized

multihop ad hoc networks.

• Suggest design and architecture of a new MAC called Dual

MAC for such a topology.

• Effect of station Mobility is not considered.

KReSIT, IIT Bombay Satyajit Rai

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MAC in Multihop Networks

• Multihop ad-hoc networks require a distributed medium

access mechanism.

• We investigate IEEE 802.11 MAC as it is the most popular

MAC for multihop ad-hoc networks.

• The Distributed Coordination Function (DCF) operation of

IEEE 802.11 MAC provides the distributed medium access mechanism.

• The Point Coordination Function (PCF) operation of IEEE

802.11 MAC provides a polled medium access mechanism, and is used in fixed Wireless LANs.

KReSIT, IIT Bombay Satyajit Rai

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IEEE 802.11 DCF and PCF Operation

Source RTS Data Destination ACK Other CW NAV (RTS) NAV (CTS) NAV (Data) Backoff Started Defer Access

DIFS SIFS SIFS SIFS DIFS

CTS

DCF Operation

B PCF DCF PCF B DCF NAV NAV CFP Repitition Interval CFP Repitition Interval CP CFP CFP CP

PCF Operation

KReSIT, IIT Bombay Satyajit Rai

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Problems with Centralized Scenario

• As the flows converge towards destination, nodes close to the

preferred destination operate in high load condition.

• If the distributed MAC (DCF) is used, lot of stations try to

access medium at the same time.

• This increases the collisions, and the throughput decreases

heavily.

• It is well known that the scheduled MAC protocols perform

better than distributed MAC protocols in high load conditions.

• We suggest using a scheduled MAC in one hop region

around the central station.

KReSIT, IIT Bombay Satyajit Rai

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Scheduled MAC in Multihop Networks

Stations using DCF Stations using PCF

Central station (PC)

• Requirement for a scheduled MAC

– The scheduled MAC must work smoothly with distributed MAC.

• In an IEEE 802.11 Network, DCF is used as distributed MAC.
• PCF mode of 802.11 can serve as scheduled MAC.

KReSIT, IIT Bombay Satyajit Rai

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Problems with PCF as scheduled MAC

• During the Contention Free Period (CFP) of PCF, the NAV

setting of stations around PC prevents them to communicate with outer stations.

• This means that these stations (esp. boundary stations)

become exposed to PC during CFP .

• As there is not RTS/CTS exchange during CFP

, the transmissions from outer stations could collide with the poll of the PC.

• This means that during the CFP

, outer stations become hidden to PC, and vice versa.

KReSIT, IIT Bombay Satyajit Rai

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Solution: Dual MAC

Link Layer PCF MAC MAC Dual MAC DCF

Phy Phy

Wireless Channel

• We equip boundary stations with Dual MAC.
• Dual MAC is a MAC with two independent MACs

– PCF MAC communicates with PC in PCF mode, – DCF MAC communicates with outer stations in DCF mode.

KReSIT, IIT Bombay Satyajit Rai

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Solution: Dual MAC

• However, the DCF MAC communication can still cause

collisions with stations that are using PCF.

• To avoid this we use PCF on one channel and DCF on

another.

• Dual MAC interface handles the data packets to/from

appropriate MAC and channel.

KReSIT, IIT Bombay Satyajit Rai

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Dual MAC: Working

For outgoing packets: if ( packet is broadcast ) { send packet to both PCF MAC and DCF MAC; } else { channel = get_destination_MAC_channel(); if ( channel == DCF_channel ) send packet on DCF MAC; else send packet on PCF MAC; } For incoming packets: hand over the packet to Link Layer;

KReSIT, IIT Bombay Satyajit Rai

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Simulation and Results

• We have implemented the Dual MAC in NS-2.
• Simulation was performed in NS with 50 stations spread

around an area of 1500m x 1500m.

• Simulation setup

– Central station (PC) at 750m x 750m. – 8 stations use Dual MAC, and are 240m away from PC. – 12 stations are inside one-hop boundary of PC, and use PCF. – 29 stations are outside one hop boundary of PC, and use DCF. – Routing protocol is DSDV.

KReSIT, IIT Bombay Satyajit Rai

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Results: Packet Delivery Ratio

0.2 0.4 0.6 0.8 1 10 15 20 25 30 35 40 45 Packet Delivery Ratio CBR connections performance of DCF and Dual MAC at 30 packets/sec dual dcf

KReSIT, IIT Bombay Satyajit Rai

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Results: Throughput

5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 20000 40000 60000 80000 100000 120000 140000 160000 Throughput (Kbytes) Offered Load (Kbytes) Performance of DCF and Dual MAC dual dcf

KReSIT, IIT Bombay Satyajit Rai

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Conclusion and Future Work Conclusion

• Use of Dual MAC gives more throughput per channel than DCF, even with

a small number of Dual Stations.

• The scheduled MAC can be used to increase throughput in a multihop

ad-hoc networks.

• Dual MAC can act as interface between stations using scheduled and

unscheduled MAC.

Future Work

• A station could automatically switch-on PCF seeing high traffic in its

surrounding.

• This would enable small groups of stations forming a cluster in high traffic

areas, and use scheduled MAC to increase throughput.

KReSIT, IIT Bombay Satyajit Rai

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Thank you

Thank You

KReSIT, IIT Bombay Satyajit Rai

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