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metricsReferences :-

1. D. De Couto, D. Aguayo, J. Bicket, R. Morris, AHigh Throughput Path Metric for Multi-HopWireless Routing, ACM Mobicom Conference,(September 2003)

2. R. Draves, J. Padhye, B. Zill, Comparisons ofroutingmetrics for static multi-hop wireless networks, in:ACM Annual Conference of the Special Interest

Groupon Data Communication (SIGCOMM), August2004,pp. 133?44

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Link-quality Metrics

Hop count shortest path in terms ofhops

Per-Hop Round trip time round tripdelay seen by unicast probesbetween neighbouring nodes

Per-Hop Packet pair delay delaybetween a pair of back-to-backprobes to a neighbouring node

Expected Transmission count lossrate of broadcast ackets between

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Hop Count

Advantage

Simplicity

Least overhead Disadvantage

Does not take packet loss or bandwidth

into account

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Per-hop RTT

Advantage

Measures queueing delay if node is busy

Measures delay due to channelcontention

Takes into account delay due to lossylinks

Avoid highly loaded or lossy links

Disadvantage

Load-dependent route instability

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Per-hop Packet pair delay

Advantage

Channel contention

Also considers the link bandwidth Disadvantage

Not completely immune to self-

interference

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ETX

Considers the packet loss ratios inboth directions of each wireless link.

Accounts for :- Wide range of link loss ratios

Links with asymmetric loss ratios

Interference between successive hops ofmulti-hop paths

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The metric

ETX = 1/ (Df * Dr)

Df forward delivery ratio

Dr reverse delivery ratio Assumptions

Link-layer retransmissions

Fixed transmit power level Points to consider

Does not reflect how busy a link is

Unfair load-handling leads to incorrect results

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Deliver ratio from X to Y

= (# packets received by Y) / ( # packetssent by X)

Loss ratio = 1 (Delivery ratio)

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Hop Count vs Best path

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ETX vs Hop-countthroughput

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ETX throughput for largepackets

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ETX throughput with hightransmit power

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Accuracy of linkmeasurements

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Suggested improvements

Predictions of loss ratios for differentpacket sizes

Handling of networks with links thatrun at variety of bit-rates

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Long-lived TCP connections

Throughput comparison

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No. of paths per TCPtransfer

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Median path length

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Impact of path-length on

the throughput

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HOP

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ETX

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RTT

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PktPair

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Variability of TCP throughput

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Conclusion

ETX fairs very well with stationarynodes in a number of aspects ascompared to other link-quality

metrics RTT & PktPair perform poorly

because they are load-sensitive &

hence subject to self-interference