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 |
| Infrastructure-less, may need to traverse multiple wireless links to reach a destination |
| Mobility causes route changes |
| Ease of deployment | |
| Speed of deployment | |
| Decreased dependence on infrastructure |
| 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 | ||
| 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 |
| 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 | ||
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 | ||
| 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 | ||
“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) | |||
| 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 | ||
| 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 | ||