Comparison of Routing Metrics for Static Multi-Hop Wireless Networks

Comparison of Routing Metrics for Static Multi-Hop Wireless Networks Richard Draves, Jitendra Padhye and Brian Zill Microsoft Research Presented by Hoang Nguyen CS598JH - Spring 06 Some slides adopted from the authors and from Professor Robin Kravets

Multi-hop Wireless Networks

Routing in Multi-hop Wireless Networks • Mobile Networks • Minimum-Hop • DSR, AODV… • Static Networks • Minimum-Hop vs. More Hops • Link-quality Based Routing • Signal-to-Noise ratio • Packet loss rate • Round-trip-time • Bandwidth Experimental Comparison

Contributions • Design and Implementation of LQSR (Link Quality Source Routing) protocol • LQSR = DSR with link quality metrics • Experimental Comparison of link quality metrics: • Minimum-hop (HOP) • Per-Hop Round Trip Time (RTT) • Per-hop Packet Pair (PktPair) • Expected Transmission (ETX)

Outline • DSR Revisited • Link quality metrics revisited • LQSR (Link Quality Source Routing) • Experimental results • Conclusion

DSR – Route Discovery Y Z S E F B C M L J A G H D K I N Adopted from Professor Robin Kravets’ lectures

DSR – Route Discovery Y Z [S] S E F B C M L J A G H D K I N [X,Y] Represents list of identifiers appended to RREQ Broadcast transmission Adopted from Professor Robin Kravets’ lectures

DSR – Route Discovery Y Z [S,E] S E F B C M L J A G [S,C] H D K I N Node H receives packet RREQ from two neighbors: potential for collision Adopted from Professor Robin Kravets’ lectures

DSR – Route Discovery Y Z S E F [S,E,F] B C M L J A G H D K [S,C,G] I N Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Adopted from Professor Robin Kravets’ lectures

DSR – Route Discovery Y Z S E F [S,E,F,J] B C M L J A G H D K I N [S,C,G,K] Nodes J and K both broadcast RREQ to node D Since nodes J and K are hidden from each other, their transmissions may collide Adopted from Professor Robin Kravets’ lectures

DSR – Route Discovery Y Z S E [S,E,F,J,M] F B C M L J A G H D K I N Node D does not forward RREQ, because node D is the intended targetof the route discovery Adopted from Professor Robin Kravets’ lectures

DSR – Route Reply Y Z RREP [S,E,F,J,D] S E F B C M L J A G H D K I N Adopted from Professor Robin Kravets’ lectures

DSR – Data Delivery Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I N Packet header size grows with route length Adopted from Professor Robin Kravets’ lectures

DSR – Route Caching [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] F B [J,F,E,S] C M L J A G [C,S] H D [G,C,S] K I N Z [P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format) Adopted from Professor Robin Kravets’ lectures

DSR – Route Caching Y [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] F B [J,F,E,S] C M L [G,C,S] J A G [C,S] H D K [K,G,C,S] I N RREP RREQ Z Assume that there is no link between D and Z. Route Reply (RREP) from node K limits flooding of RREQ. In general, the reduction may be less dramatic. Adopted from Professor Robin Kravets’ lectures

DSR – Route Error J sends a route error to S along J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails Nodes hearing RERR update their route cache to remove link J-D RERR [J-D] Z S E F B C M L J A G H D K I N Adopted from Professor Robin Kravets’ lectures

Per-hop Round Trip Time (RTT) • Node periodically pings each of its neighbors • RTT samples are averaged using exponentially weighted moving average • Path with least sum of RTTs is selected

Per-hop Round Trip Time (RTT) • Advantages • Easy to implement • Accounts for link load and bandwidth • Also accounts for link loss rate • 802.11 retransmits lost packets up to 7 times • Lossy links will have higher RTT • Disadvantages • Expensive • Self-interference due to queuing

Per-hop Packet Pair (PktPair) • Node periodically sends two back-to-back probes to each neighbor • First probe is small, second is large • Neighbor measures delay between the arrival of the two probes; reports back to the sender • Sender averages delay samples using low-pass filter • Path with least sum of delays is selected

Per-hop Packet Pair (PktPair) • Advantages • Self-interference due to queuing is not a problem • Implicitly takes load, bandwidth and loss rate into account • Disadvantages • More expensive than RTT

Expected Transmissions (ETX) • Estimate number of times a packet has to be retransmitted on each hop • Each node periodically broadcasts a probe • 802.11 does not retransmit broadcast packets • Probe carries information about probes received from neighbors • Node can calculate loss rate on forward(Pf) and reverse (Pr) link to each neighbor • Select the path with least total ETX

Expected Transmissions (ETX) • Advantages • Low overhead • Explicitly takes loss rate into account • Disadvantages • Loss rate of broadcast probe packets is not the same as loss rate of data packets • Probe packets are smaller than data packets • Broadcast packets are sent at lower data rate • Does not take data rate or link load into account

LQSR • DSR-like • Route Request, Route Reply and Route Error • Link-quality metrics is appended • Link-state-like • Link caching (not Route caching) • Reactive maintenance • On active routes • Proactive maintenance • Periodically floods RouteRequest-like Link Info

Mesh Testbed 23 Laptops running Windows XP. 802.11a cards: mix of Proxim and Netgear. Diameter: 6-7 hops.

Link bandwidths in the testbed • Cards use Autorate • Total node pairs: • 23x22/2 = 253 • 90 pairs have non-zero bandwidth in both directions. Bandwidths vary significantly; lot of asymmetry.

Experiments • Bulk-transfer TCP Flows • Impact of mobility

Experiment 1 • 3-Minute TCP transfer between each node pair • 23 x 22 = 506 pairs • 1 transfer at a time • Long transfers essential for consistent results • For each transfer, record: • Throughput • Number of paths • Path may change during transfer • Average path length • Weighted by fraction of packets along each path

Median Throughput ETX performs best. RTT performs worst.

Why does ETX perform well? ETX performs better by avoiding low-throughput paths.

Impact on Path Lengths Path length is generally higher under ETX.

Why does RTT perform so poorly? RTT suffers heavily from self-interference

Why PktPair gets worse? PktPair suffers from self-interference only on multi-hop paths.

Summary of Experiment 1 • ETX performs well despite ignoring link bandwidth • Self-interference is the main reason behind poor performance of RTT and PktPair.

Experiment 2

Median

Conclusions • ETX metric performs best in static scenarios • RTT performs worst • PacketPair suffers from self-interference on multi-hop paths • Shortest path routing seems to perform best in mobile scenarios

Comparison of Routing Metrics for Static Multi-Hop Wireless Networks