EtX

ETX, or Expected Transmission Count, is a metric used in wireless networking to evaluate the quality of a communication link. It estimates the number of transmissions required, on average, to successfully deliver a unicast data frame over a link, including any necessary retransmissions. ETX is designed to reflect both the reliability of data packets reaching the next hop and the ability to return acknowledgments, helping routing decisions favor more dependable paths.

The metric was introduced in the context of wireless mesh networks by researchers including De Couto, Aguayo, Bicket, and Morris. It gained widespread use as a simple, practical way to compare alternate routes in networks where link quality varies over time and space.

Calculation and interpretation: For a single link, ETX is defined as 1 divided by the product of two delivery ratios: pf, the forward delivery ratio (probability that a data packet is received by the next hop), and pr, the reverse delivery ratio (probability that an acknowledgment is received back). Thus ETX = 1 / (pf × pr). Lower ETX indicates a more reliable link. For a multi-hop path, the path ETX is typically the sum of the ETX values of the constituent links, so routing algorithms aim to minimize total ETX.

Variations and considerations: ETX does not directly account for bandwidth or delay; it can overemphasize factors such as hop count in some environments. Extensions like ETT (Estimated Transmission Time) incorporate link data rate to reflect airtime cost. ETX measurements depend on probes or observation of traffic, and thus can be influenced by measurement overhead and time-varying conditions. It remains a foundational metric in wireless routing due to its simplicity and interpretability.