1	Multicasting in Mobile Ad-Hoc Networks: Achieving High Packet Delivery Ratios Thomas Kunz Systems and Computer Engineering Carleton University http://kunz-pc.sce.carleton.ca/ tkunz@sce.carleton.ca
2	Mobile Ad Hoc Networks Infrastructure-less, may need to traverse multiple wireless links to reach a destination
3	Mobile Ad Hoc Networks Mobility causes route changes
4	Why Ad Hoc Networks ? Ease of deployment Speed of deployment Decreased dependence on infrastructure
5	Many Applications Personal area networking cell phone, laptop, ear phone, wrist watch Military environments soldiers, tanks, planes Civilian environments taxi cab network meeting rooms sports stadiums boats, small aircraft Emergency operations search-and-rescue policing and fire fighting
6	Motivation Many applications for ad hoc networks require one-to-many and many-to-many communication Multicast protocols are intended to efficiently support such communication patterns Multicasting well researched in fixed networks (i.e., the Internet), building efficient distribution structures (typically a multicast tree) Ad hoc networks: dynamic topology makes it harder to maintain distribution structure with low overhead
7	Motivation (cont.) MANET specific protocols are being proposed MAODV: multicast extensions for AODV, establishes shared tree ODMRP: new multicast protocol, based on per-source mesh ADMR: completely on-demand, per-source tree Goals: Study multicasting protocols Develop a protocol that achieves high packet delivery ratio with low overhead
8	The Problem of High Packet Delivery Ratios Results published in literature: Multicast protocols perform poorly (packet delivery ratio below 90%) as network topology changes more often (nodes move with higher speed and/or pause less) Multicast protocols also often do not scale well with number of multicast senders and/or number of multicast receivers Broadcast protocols can be beneficial under high mobility and/or large group sizes Quite a bit of work on efficient broadcast protocols, rather than simplistic flooding approach, as broadcasting control messages inherent part of many routing protocols
9	Simulation Environment NS2 simulations to validate literature results Explored 3 protocols: FLOOD: simple broadcast protocol BCAST: broadcast protocol that reduces packet retransmissions (do not retransmit of there is no new neighbor that does not yet know about the data packet) ODMRP: mesh-based multicast protocol The “usual” simulation parameters: area of 1500m x 300m, 50 nodes, 802.11 MAC at 2 Mbps, at low or high mobility. 1, 2, 5, or 10 senders 10, 20, 30, 40, or 50 receivers Each sender sends a 256 byte packet every 500 ms Performance Metrics: Packet Delivery Ratio and Packet Latency
10	Simulation Results
11	“Reliable” Protocol: Key Design Alternatives Reliability Mechanism? Forward Error Correction: Overhead with each packet, design often based on worst-case assumptions Retransmissions: detect packet loss and recover Ack-based Nack-based Which Protocol Layer? Transport Layer Routing Layer Flow Control, Security, etc. (not considered)
12	Reliable BCAST Each node keeps cache of recently transmitted packets (FIFO, small) Each node, upon receiving packet X from sender S, checks whether it received packet X-1 from that sender If not, broadcast retransmission request (NACK) to 1-hop neighbors Neighbors listen to overhear other retransmission and cancels theirs First set of experiments revealed that under high traffic load, too many NACKs were issued, flooding the network and resulting in overall worse performance (70% PDR for 10 sender scenarios) Added feature: NACK throttle
13	Performance Results
14	Conclusions and Future Work Proposed protocol achieves over 99% PDR for relatively small number of multicast senders As ad hoc networks tend to experience temporary partitions, achieving 100% PDR is not realistic Future work Have protocol parameters derived automatically Increase link and network capacity by modifying MAC Explore the pros and cons of broadcast vs. multicast using other multicast protocols (MAODV, ADMR) Flow control, security, non-uniform traffic