MAC layer Multicast in Wireless Multihop Networks

1. MAC layer Multicast in Wireless Multihop Networks Shweta Jain Samir Das Comsware 2006

2. Overview • Motivation. • Introduction. • Description if IEEE 802.11. • Description of various Multicast MAC protocols. • Description of Multicast MAC. • Performance results. • Conclusion. Comsware 2006

3. Motivation • Many applications in wireless multihop networks need one-to- many (multicast) communication. • Multicast data may need to be transmitted across various hops before it reaches all multicast receivers. • Since wireless links are prone to errors, data may not always be received correctly at the next node along the route. • Error recovery in the upper layer can cause large end to end delays in data delivery. • An efficient and reliable MAC layer multicast protocol can improve the performance of multicast communication. Comsware 2006

4. Introduction • IEEE 802.11 protocol implements positive acknowledgment to provide reliable transmission of unicast data. • The same mechanism can be used to provide reliable multicast support at the MAC layer. • In this paper we present a reliable multicast MAC protocol. • We also evaluate three other Multicast MAC protocols and compare their performance with our approach. • Broadcast based multicast • Unicast based MAC • Multicast MAC. • Our approach– Multicast MAC with cliques. Comsware 2006

5. Description of IEEE 802.11 MAC • Access Mechanism for unicast data transmission – reliable • Access Mechanism for multicast and broadcast data transmission -- unreliable Comsware 2006

6. IEEE 802.11– Unicast transmission • IEEE 802.11 DCF is the basic access mechanism used for reliable unicast. • It uses CSMA/CA and optional Control packet exchange to facilitate medium sharing for unicast data. • Requires positive feedback from the receiver to guarantee data delivery. Comsware 2006

7. IEEE 802.11 IEEE 802.11– Multicast and Broadcast transmission • IEEE 802.11 access mechanism used for multicast and broadcast transmission. • It uses CSMA/CA to facilitate medium sharing for multicast and broadcast data. • Does not implement any feedback mechanism, thus there is no data delivery guarantee. Comsware 2006

8. MAC protocols • Broadcast MAC – Send multicast data as broadcast data (IEEE 802.11 MAC) • Disadvantage: Unreliable transmission. • Unicast MAC– Send a single multicast data packet as multiple unicast packets to each next hop neighbor. • Disadvantage: Large packet delays. • Multicast MAC – Perform multicast RTS/CTS exchange with all nexthop neighbors. • Disadvantages: • Large size of RTS makes it prone to collisions. • The RTS/CTS/Data/ACK exchange may take a time longer than the time for which the two hop neighbors set their virtual carrier sensing mechanism. • Multicast MAC with cliques – Perform multiple multicast RTS/CTS exchange with each nexthop neighbor group. Comsware 2006

9. Multicast MAC • Sender: • Send a single RTS with multiple nexthop addresses. • Wait to receive CTS from each nexthop neighbor. • Send data to those neighbors from which CTS was received. • Retransmit RTS to those neighbors from which CTS was not received. • Receivers: • Send CTS after a fixed delay determined by the order in which the receiver’s address appears in the RTS packet. • Receive Data and send ACK packets after a fixed delay determined by the address order in the data packet. • Others: • Invoke the virtual carrier sensing mechanism to allow collision free RTS/CTS/Data/ACK exchange. Comsware 2006

10. Multicast MAC The wait time for Nth receiver where N is the position index of its address in the RTS Frame— N x SIFS + (N-1) x CTS Duration. Comsware 2006

11. Interference Problem • CTS order is Recv 1, Recv 2 and Recv3. • Recv 2 does not send CTS because it hears the CTS sent by Recv 1 as noise and invokes its virtual carrier sensing mechanism. • Sender has to retransmit RTS to Recv 2. • We group the neighbors into groups such that neighbors that can hear each other form a single group called cliques. • We perform the RTS/CTS exchange mechanism to send data to each group. Comsware 2006

12. Multicast MAC with Cliques • We group neighbors in different cliques and send multicast data to atmost 4 neighbors at a time. • Advantage • The RTS size is small so it is not prone to collisions • The RTS/CTS/Data/ACK exchange completes before the NAV of two hop neighbors and potential interferers expire. Comsware 2006

13. Clique of neighbors Comsware 2006

14. Performance Evaluation • We used network simulator (ns2) version 2.26 for simulation. • We experimented with a grid scenario of size 1500x300 with 100 nodes • We set up traffic connections for 1 multicast sender and vary the number of receivers -10, 20, 30 and 40 multicast receivers. • We run experiments with two-ray ground propagation model and ricean fading models Comsware 2006

15. Two Ray Ground Propagation model • Packet delivery fraction for multiple unicast and broadcast MAC fall when the number of multicast receivers is increased. • Packet delivery fraction of multicast MAC remain steady with increase in the number of multicast receivers. Comsware 2006

16. Two Ray Ground Propagation model • Average per hop delay rises with the number of multicast receivers for unicast MAC. • Delay remains steady for other MAC protocols. • Delay is less for broadcast MAC is low due to the absence of retransmissions in broadcast MAC. Comsware 2006

17. Ricean Fading model • Packet delivery fraction for all MAC protocols fall when the number of multicast receivers is increased. • Packet delivery fraction of multicast MAC fall less steeply than unicast and broadcast MAC protocols. Comsware 2006

18. Ricean Fading model • Average per hop delay rises with the number of multicast receivers for all MAC protocols other than broadcast MAC. • Delay remains steady for broadcast MAC protocol due to the absence of retransmissions. • Delay is lower for unicast MAC protocol due to lower packet delivery fraction. Comsware 2006

19. Conclusion • We have presented an extension of IEEE 802.11 MAC protocol to provide reliable multicast MAC protocol. • We have demonstrated the performance improvement achieved due to our protocol. Comsware 2006