“A High-Throughput Path Metric for Multi-Hop Wireless Routing” presents the expected transmission count (ETX) metric for finding high-throughput paths on multi-hop wireless networks. By considering link loss rates, asymmetrical loss ratios between the directions of a link, and interference, the paper argues that ETX substantially outperforms routing protocols based on finding the shortest path (fewest hops) in an ad-hoc wireless network.
There are several shortcomings with using minimum hop count for routing in ad-hoc wireless networks:
- Shortest-path routing protocols essentially assume that either a link works (probe packets are delivered successfully), or it doesn’t. While wireless networks can be roughly classified in this manner, wireless links have a much richer space of performance and loss characteristics than “it works” or “it doesn’t”.
- Minimizing the hop count typically means maximizing the physical distance traveled by each hop, which leads to selecting links with weak signals and high loss rates. As the related RoofNet paper discusses, it is often preferable to utilize a path of short, high-quality links over a single long-distance, low-quality link.
- In a dense ad-hoc network, there may be many different routes with the same number of hops. Deciding between routes on the basis of hop count is therefore not sufficient in any case.
There are problems with two naive alternatives to min-hop-count:
- An improved routing metric could simply choose the path by maximizing the product of the per-link delivery ratios. This is inadequate, however, because it would result in preferring a two-hop route with perfect delivery to a one-hop route with a low loss rate — the latter route is actually better in many cases.
- Choosing links on the basis of end-to-end delay is problematic, because of oscillation: once a route is utilized for traffic, its delay will often increase. This will result in the routing metric choosing another path, whose latency will then increase, causing it to switch back to the first path, and so on.
ETX Metric Design
The ETX metric for a link is the expected number of data transmissions required to send a packet of a certain size over the link. The ETX metric for a path is the sum of the per-link ETX metrics. ETX calculates per-link delivery ratios for both directions of a link (transmit and receive), to take into account the asymmetrical nature of wireless communication. Delivery ratios are calculated by periodic probes of a fixed size (sent once per second by default via broadcast); each node calculates per-link delivery ratios based on the most recent 10 seconds of activity.
ETX assumes that radios have a fixed transmit speed, and that congestion is not a factor. Neither assumption is very realistic: variable speed radios are now pervasive, and congestion might well be a factor in ad-hoc networks with sizable user bases. ETX also assumes that delivery rates are the same regardless of packet size, which is probably inaccurate (e.g. ACKs are small and take less time to transfer).
I was surprised that the paper spent so much time comparing ETX with min-hop-count metrics: it seems quite straightforward that min-hop-count would fair pretty poorly in an ad-hoc wireless network. In that sense, performance comparisons between ETX and min-hop-count are also not that informative: of course ETX will beat min-hop-count, because the latter is simply a terrible choice for this environment. It seems surprising that min-hop-count is truly the best example of prior work they could measure against — why not compare with the naive “maximize product of per-link delivery ratio” scheme, for example?
I didn’t get that much out of this paper: the RoofNet paper already summarizes an improved version of the ETX metric. Because the ETX evaluation compares it with such a pessimal alternative (min-hop-count), I didn’t get much out of the evaluation in this paper.