A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols

J. Broch, D. Maltz, D. Johnson, Y-C Hu, J. Jetcheva, "A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols," ACM Mobicom Conference, (October 1998).

One line summary: This paper uses a network simulator that the authors improved by adding relatively realistic physical and spatial models to compare four wireless ad hoc network protocols: DSDV, TORA, DSR, and AODV, that cover a wide range of different design choices.

Summary

This paper compares four wireless ad hoc network routing protocols using detailed simulations. The four protocols compared were (1) Destination-Sequenced Distance Vector using sequence number triggered updates (DSDV-SQ), (2) Temporally Ordered Routing Algorithm (TORA), (3) Dynamic Source Routing (DSR), and (4) Ad Hoc On-Demand Distance Vector using link layer feedback to detect link breakage (AODV-LL). DSDV-SQ is a hop-by-hop distance vector routing protocol that uses periodic broadcast updates and guarantees loop freedom. TORA is a distributed routing protocol that uses a link-reversal algorithm to provide on demand route discovery and runs on top of IMEP. DSR uses on-demand source routing and consists of Route Discovery and Route Maintenance phases. Lastly, AODV-LL is like a combination of DSR and DSDV because it has on-demand Route Discovery and Route Maintenance along with hop-by-hop routing and sequence numbers. These protocols were simulated using ns-2 at varying levels of node mobility and number of senders. The simulator was enhanced to allow for realistic physical layer and propagation modeling as well as node mobility. It used a random waypoint model to simulate mobility and constant bit rate (CBR) sources. The metrics by which the protocols were judged were (1) packet delivery ratio, (2) routing overhead, and (3) path optimality.

The results are presented for each metric. These results are for hosts that move at a speed of 20 m/s when not paused. In terms of the percentage of packets delivered, DSR and AODV-LL deliver between 95% and 100% of packets regardless of offered load and node mobility. DSDV-SQ drops to 70% packet delivery with constant node mobility, which the authors attribute to packets dropped on account of a stale routing table entry. TORA does well until the number of sources sending packets reaches 30 then experiences congestive collapse due to a positive feedback loop. In terms of routing overhead, DSR and AODV-LL have similar curves although AODV-LL has higher overhead when node mobility is near constant. The authors later note that if measured in bytes instead of packets, the overhead of DSR becomes much greater than AODV-LL. The overhead of TORA depends in part on node mobility and is much higher than any of the other protocols. DSDV-SQ has nearly constant overhead independent of node mobility or offered load. In terms of path optimality, both DSDV-SQ and DSR use near optimal routes regardless of node mobility, whereas TORA and AODV-LL use less optimal routes when node mobility is high. Other interesting observations the authors make are that the percentage of packets successfully delivered for broadcast packets is lower than for unicast packets. They also note that early in their experiments they found a serious integration problem with ARP; they found a workaround but note that this problem would have to be addressed in any real implementation running on top of ARP. The authors don’t really conclusively rank the protocols in the end but it is clear from the experiments that DSR is probably the best protocol, followed by AODV-LL and DSDV, with the relative ranking of these two being less clear due to tradeoffs. TORA is obviously the worst protocol in almost every respect.

Critique

Although I tend to think that real-world experiments are always preferable to simulations, I like that the authors improved the network simulator to include a spatial model to simulate host mobility and a relatively realistic physical layer and radio network interface model. I also appreciated the thoroughness of their simulations with respect to their clearly stated metrics. Their section describing their experimental results was considerably easier to follow than most papers and the way they laid out their graphs made it pretty easy to compare the results from the different protocols. I also like that they provided additional, more in-depth explanations for certain observations where they were warranted, for example, their explanation of the congestive collapse of TORA. Their section containing additional observations was nice too. For some reason I am having trouble questioning their assumptions and making criticisms of this paper (nothing immediately jumps out at me), but because they used a simulator and also implemented all the protocols themselves there are clearly going to be a lot of assumptions that underlie their results. I thought they did a pretty good job of clearly stating what all these assumptions were though, so at least they are there for readers to take into account. I liked this paper and I think it’s probably good to keep it in the syllabus.