A HighThroughput Path Metric for MultiHop Wireless Routing

A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas Couto, Daniel Aguayo, John Bicket, Robert Morris MIT MOBICOM 03

Problem • Min hop-count metric does not choose highest-throughput path available – Assumes links either work or don’t work – Maximize the loss ratio of each hop – Arbitrarily chooses among same length paths – Lossy links hidden by link-layer retransmission – Thresholds to discard lossy links may disconnect network Question – Is there a “better” metric ?

Understanding min-hop metric Testbed 29 PCs with 802. 11 b radios (fixed transmit power) in ‘ad hoc’ mode 2 nd floor 3 rd floor 5 th floor 4 th floor 6 th floor

Understanding min-hop metric Comparison against max-thruput path better Single hop max thruput – 450 pkt/sec

Min-hop Isn’t The Best Throughput of Available Routes Between A Pair of Nodes (23 & 36) • Shortest route isn’t the best • Routes of the same # of hops provide very different throughput

Understanding min-hop metric Link delivery ratios at different powers Forward direction Reverse direction Links with intermediate delivery ratio

Understanding min-hop metric Asymmetric links • At 1 m. W power, 28 out of 124 links have | delivery_fwd – delivery_rev | > 25% • DATA + link-layer ACKs require both directions to work well Inter-hop interference Throughput = 1/2 Throughput = 1/3 Throughput > 1/4

ETX metric Design goals • • • Find high-throughput paths Account for lossy links Account for asymmetric links Account for inter-link interference Independent of network load (don’t incorporate congestion)

ETX metric Definition • ETX – predicted # of data tx required to successfully send a packet over link/path • ETX (link) = 1 / df x dr • ETX (path) = ∑ ETX(link) • ETX (link) measured by broadcasting periodic probe packets • Reverse-delivery ratio piggybacked in forward probe packets

Alternate metrics (related papers) Goal – Working with lossy channel/links 1. Cut-off threshold – Disconnected network 2. Bottleneck link (highest-loss-ratio link) – Losses accumulate 3. Product of link delivery ratio along path – Does not account for inter-hop interference 4. End-to-end delay – Depends on interface queue lengths

Alternate metrics (related papers) Goal - Load-balancing/congestion avoidance 1. Packet delay 2. # of paths thru node (& its neighbors) 3. # of packets in interface queue of node (& its neighbors)

ETX Evaluation Comparison with min-hop + best-route (DSDV) DSDV hop-count DSDV ETX better Best static route found experimentally

ETX Evaluation Comparison with min-hop (higher power) DSDV hop-count DSDV ETX

ETX Evaluation Comparison with min-hop+ best-route (DSR) DSR hop-count DSR ETX Best static route found experimentally

ETX caveats • “Broadcast” has lower priority • Probe size ≠ Data/Ack size – Under-estimates data loss ratios, over-estimates ACK loss ratios • For >= 4 hops, may choose a slower path with fewer hops • ETX assumes all links run at one bit-rate • Link-layer feedback already does a good job for DSR (& DSDV ? )

Conclusion / Thoughts • Proposed new metric to accommodate lossy/asymmetric links • Detailed experiments on real testbed • Favors shorter paths! • Inter-hop interference accounting problematic • Congestion / Link loss separation ?