Expected transmission count
The ETX metric, or expected transmission count, is a measure of the quality of a path between two nodes in a wireless packet data network. It is widely utilized in mesh networking algorithms.
History
Douglas S.J. De Couto was the first to describe ETX in his 2004 doctoral dissertation at MIT.[1] Subsequently, it has been implemented in RoofNet/Meraki and OLSR mesh networking protocols, among others.
In the context of the OLSR protocol,[2] a bidirectional link ETX was defined.
Details
ETX is the number of expected transmissions of a packet necessary for it to be received without error at its destination. This number varies from one to infinity. An ETX of one indicates a perfect transmission medium, where an ETX of infinity represents a completely non-functional link. Note that ETX is an expected transmission count for a future event, as opposed to an actual count of a past event. It is hence a real number, and not an integer. For example, if it took 1898 transmissions to transfer 1024 packets without error, the ETX on the link is 1898/1024, or approximately 1.85. Due to varying characteristics of the transmission medium, the number may vary widely.
It is often useful to convert between ETX and the packet error probability [math]\displaystyle{ e_{pt} }[/math]:
[math]\displaystyle{ \mathrm{ETX} = \frac{1}{1 - e_{pt}} }[/math]
An equivalent relation is used for bidirectional links in the context of the OLSR protocol: [math]\displaystyle{ ETX = 1/(NLQ * LQ) }[/math] where NLQ is the Neighbor Link Quality of the link and LQ is its link quality. Thus, [math]\displaystyle{ e_{pt} = 1-NLQ * LQ }[/math].
References
- ↑ De Couto, Douglas S. J.. "High-Throughput Routing for Multi-Hop Wireless Networks". http://pdos.lcs.mit.edu/papers/grid:decouto-phd/thesis.pdf. Retrieved 2008-09-10.
- ↑ "olsr.org". http://www.olsr.org/docs/README-Link-Quality.html.
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