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 StaticMobile Motivating scenario Community wireless networks (“Mesh Networks”) Battlefield networks Key challenge Improving network capacity Handling mobility, node failures, limited power.

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

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

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

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

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

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

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

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

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

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

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

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

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

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 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 – does not retransmit broadcast packets Probe carries information about probes received from neighbors Node can calculate loss rate on forward (P f ) and reverse (P r ) 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

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

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

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

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

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

Experiments 1.Bulk-transfer TCP Flows 2.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

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

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