Welcome to the CSIR Meraka Institute's "COIN" Blog

Monday, June 20, 2005

How the ETX metric in OLSR is calculated

The ETX of a link is the predicted number of data transmissions required to send a packet over that link, including retransmissions. The ETX of a route is the sum of the ETX for each link in the route. For example, the ETX of a three-hop route with perfect links is three; the ETX of a one-hop route with a 50% delivery ratio is two.

The ETX of a link is calculated using the forward and reverse delivery ratios of the link. The forward delivery ratio, df , is the measured probability that a data packet successfully arrives at the recipient; the reverse delivery ratio, dr , is the probability that the ACK packet is successfully received. The expected probability that a transmission is successfully received and acknowledged is df x dr .

A sender will retransmit a packet that is not successfully acknowledged. Because each attempt to transmit a packet can be considered a Bernoulli trial, the expected number of transmissions is:

ETX = 1/ (df x dr)


For a full description of how df and dr is actually calculated see MIT's publication site

2 comments:

job opportunitya said...

I took pleasure in the site and I will go back!
Surfing online for blogs like this one is worth my
time. Sensational blog.
I want you to look for my plastic surgery minneapolis blog.

job opportunitya said...

Cool blog. I dig your site outline and I plan on
returning again! I just love finding blogs like this
when I have the time.
My celebrity plastic surgery photo blog, is something you need to peep out!