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.
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.
ETX is the number of expected transmissions of a packet necessary for it to be received without error at its destination.
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.
An equivalent relation is used for bidirectional links in the context of the OLSR protocol: