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 Satyajit Rai Guides: Prof. Sridhar Iyer ( 01329009 ) Dr. Leena Chandran Wadia January 15, 2003 1 KReSIT, IIT Bombay Satyajit Rai

� � 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. 2 KReSIT, IIT Bombay Satyajit Rai

Hidden and Exposed Node Problems Tx Range B A Tx Range D C Tx Range Tx Range 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. 3 KReSIT, IIT Bombay Satyajit Rai

� � � � Centralized Multihop Scenario Tx Range central Station Inner Stations Boundary Stations Outer Stations 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 . 4 KReSIT, IIT Bombay Satyajit Rai

� � � � Centralized Multihop Scenario Tx Range central Station Inner Stations Boundary Stations Outer Stations 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 . 4-a KReSIT, IIT Bombay Satyajit Rai

� � � 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. 5 KReSIT, IIT Bombay Satyajit Rai

� � � � 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. 6 KReSIT, IIT Bombay Satyajit Rai

IEEE 802.11 DCF and PCF Operation SIFS DIFS RTS Data Source SIFS SIFS CTS ACK Destination DIFS NAV (RTS) CW Other NAV (CTS) NAV (Data) Defer Access Backoff Started DCF Operation CFP Repitition Interval CFP Repitition Interval CFP CP CFP CP B PCF DCF B PCF DCF NAV NAV PCF Operation 7 KReSIT, IIT Bombay Satyajit Rai

� � � � � 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. 8 KReSIT, IIT Bombay Satyajit Rai

� � � Scheduled MAC in Multihop Networks Central station (PC) Stations using PCF Stations using DCF 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. 9 KReSIT, IIT Bombay Satyajit Rai

� � � � 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. 10 KReSIT, IIT Bombay Satyajit Rai

� � Solution: Dual MAC Link Layer Dual MAC DCF PCF MAC MAC 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. 11 KReSIT, IIT Bombay Satyajit Rai

� � � 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. 12 KReSIT, IIT Bombay Satyajit Rai

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; 13 KReSIT, IIT Bombay Satyajit Rai

� � � 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. 14 KReSIT, IIT Bombay Satyajit Rai

Results: Packet Delivery Ratio performance of DCF and Dual MAC at 30 packets/sec 1 dual dcf 0.8 Packet Delivery Ratio 0.6 0.4 0.2 0 10 15 20 25 30 35 40 45 CBR connections 15 KReSIT, IIT Bombay Satyajit Rai

Results: Throughput Performance of DCF and Dual MAC 50000 dual dcf 45000 40000 35000 Throughput (Kbytes) 30000 25000 20000 15000 10000 5000 0 20000 40000 60000 80000 100000 120000 140000 160000 Offered Load (Kbytes) 16 KReSIT, IIT Bombay Satyajit Rai

� � � � � 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. 17 KReSIT, IIT Bombay Satyajit Rai

Thank you Thank You 18 KReSIT, IIT Bombay Satyajit Rai