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

Mesh networking

Mesh networking is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network....

 algorithm

Algorithm

In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

s.

History

Douglas S.J. De Couto was the first to describe ETX in his 2004 doctoral dissertation at MIT. Subsequently, it has been implemented in RoofNet

Roofnet

Roofnet is an experimental 802.11b/g mesh network currently under development at the Computer Science and Artificial Intelligence Laboratory of the Massachusetts Institute of Technology...

/Meraki

Meraki

Meraki is a cloud networking company that provides hardware and software for building large scale wired and wireless networks. These networks are used by businesses, schools, and other organizations that need wireless access points, multi-site wired networks, or both. It uses a centralized...

 and OLSR

Optimized link state routing protocol

The Optimized Link State Routing Protocol is an IP routing protocol optimized for mobile ad-hoc networks, which can also be used on other wireless ad-hoc networks. OLSR is a proactive link-state routing protocol, which uses hello and topology control messages to discover and then disseminate...

mesh networking protocols, among others.

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. 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.