Hello Rob. On Thursday, 04 March, 2021, you wrote > One of my machines was showing 8% packet loss > when pinging the same site as another machine on > the same hub at the same time which was > reporting 0%. ... > What I find odd though is why not 0% or 100%? > Surely the wires inside the cable can only break
"All signals are analogue, even if they're digital." The ethernet cable is a transmission line, carrying high-frequency signals from the transmitter to the receiver. Although 100BaseTx/ 1GBaseT use twisted-pair, it's little different from the coaxial cable connecting a television or radio to its antenna. Without attaching the cable to a time-domain reflectometry scope, it's not possible to say what caused the packet loss. However: - A fracture in the cable or in an RJ-45/conductor joint will block DC, but not the high-frequency AC of the ethernet signal. To the latter, the fracture will look like a low-value capacitor; the signal will be degraded, but not blocked completely. - Ethernet uses forward error correction (FEC). Extra bits are added to the transmitted data stream, adding redundancy to the data. At the receiver, these bits are used to detect corrupted data and, if there are not too many, recover lost bits. This means that ethernet can tolerate a certain amount of interference before you start seeing packet loss at the ICMP layer. Damage to the cable can make it more susceptible to interference - you will not notice until the bit errors exceed the ability of the FEC to correct it. For information on how this sort of problem is measured, have a look at: https://en.wikipedia.org/wiki/Eye_diagram > it wasn't as if the cable was being moved > around - it was stationary. To lose one packet > every 12 or so seems very odd for a cable issue. Even if the cable wasn't being moved deliberately, it was still being influenced by temperature changes. The resulting expansion and contraction of the copper and PVC can cause a dodgy joint to break, or a weak point to fracture. > would the issue have been to do with: > > a) the length of the cable or > b) the quality of the cable or > c) both? Most likely (b). Well-manufactured cables with good strain relief will still fail eventually, but they will last longer. For a short while, I worked for an ISP. A regular task was to check the ethernet interface statistics on the routers and switches for rapidly increasing FEC or CRC errors. That information would allow us to detect dodgy cables and interfaces that were on their last legs. Nick. -- Nick Chalk ................. once a Radio Designer Confidence is failing to understand the problem. -- Please post to: [email protected] Web Interface: https://mailman.lug.org.uk/mailman/listinfo/hampshire LUG URL: http://www.hantslug.org.uk --------------------------------------------------------------
