OLSR

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Metrics

Making sense of LQ ILQ and ETX in OLSR

You will see a table like this in the OLSR status screen on Freifunk

Topology entries

Destination IP Last hop IP LQ ILQ ETX

10.51.1.13 10.51.1.10 1.00 1.00 1.00

10.51.1.20 10.51.1.10 1.00 1.00 1.00

10.51.1.1 10.51.1.13 0.90 1.00 1.11

10.51.1.10 10.51.1.13 1.00 1.00 1.00

10.51.1.12 10.51.1.13 1.00 0.90 1.11

10.51.1.14 10.51.1.13 0.90 1.00 1.11

10.51.1.20 10.51.1.13 1.00 1.00 1.00

10.51.1.13 10.51.1.14 1.00 0.90 1.11

10.51.1.1 10.51.1.20 0.90 0.75 1.49

10.51.1.10 10.51.1.20 1.00 1.00 1.00

10.51.1.12 10.51.1.20 1.00 1.00 1.00

10.51.1.13 10.51.1.20 1.00 1.00 1.00

LQ = The percentage of packets that successfully are received from your neighbor

ILQ or NLQ = The percentage of packets that are successfully sent from yourself to the neighbor

ETX = Expected Trasnmission count = How many trasnmission attempts are required to get packets through = 1 / (LQ*ILQ)

How the ETX metric in OLSR is calculated

The ETX of a link is the predicted number of data transmissions required to send a packet over that link, including retransmissions. The ETX of a route is the sum of the ETX for each link in the route. For example, the ETX of a three-hop route with perfect links is three; the ETX of a one-hop route with a 50% delivery ratio is two.

The ETX of a link is calculated using the forward and reverse delivery ratios of the link. The forward delivery ratio, df , is the measured probability that a data packet successfully arrives at the recipient; the reverse delivery ratio, dr , is the probability that the ACK packet is successfully received. The expected probability that a transmission is successfully received and acknowledged is df x dr .

A sender will retransmit a packet that is not successfully acknowledged. Because each attempt to transmit a packet can be considered a Bernoulli trial, the expected number of transmissions is:

ETX = 1/ (df x dr)


For a full description of how df and dr is actually calculated see MIT's publication site