Expected Transmission Count

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 used extensively in mesh networking algorithms.


Douglas S.J. De Couto was the first to describe ETX in his 2004 doctoral dissertation at MIT. [Cite web|url=http://pdos.lcs.mit.edu/papers/grid:decouto-phd/thesis.pdf|title=High-Throughput Routing for Multi-Hop Wireless Networks|last=De Couto|first=Douglas S. J.|accessdate=2008-09-10] Subsequently, it has been implemented in RoofNet/Meraki and OLSR mesh networking protocols, among others.


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. Because ETX is an "expected" transmission count for a future event, as opposed to an actual count of a past event, it represents a probability, and is therefore 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.


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