>> A good Ethernet cable-pair tester can spot such things pretty quickly. > > I disagree. > > *Certainly*, incorrect pair terminations can cause the sort of problems > described, however I haven't yet come across a cable tester which can > identify > that a cable correctly connected from end to end with wires {1..8} <-> {1..8} > is in fact not correctly connected in ethernet 1-2 3-6 4-5 7-8 pairs. > > All cable testers I have so far encountered will check that all wires are > correctly connected end to end and not cross connected etc., but have no clue > whether the wire joining pin 3 at one end to pin 3 at the other end is > twisted > together with the cable from pin 4 or pin 6. > > I would be very interested to find such a tester, if you can point us at one.
Well, I did say a _good_ cable tester, and that means "expensive" :-) I agree, the simple DC-continuity type of cable tester won't catch more than a fraction of the problems. These inexpensive testers ($50-$100 range, usually) can diagnose open wires, or cases where the two ends of the cable are wired differently and the signals don't match up at all. As you correctly point out, they're useless for "mixed pair" problems since these don't show up at all on DC. Electrical detection of such problems has to be done in a high-frequency (signal or pulse) domain. What you need is a tester which has either or both of two capabilities: (1) Crosstalk measurement - excite one pair with a signal, and measure the other pairs/wires to see if some of the signal leaks across onto them. (2) TDR - Time Domain Reflectometry. Excite a pair with a known pulse, and measure the voltage across the pair over time. This lets you measure the actual impedance of the pair all the way to the "far end". A swapped-pair problem will show up pretty quickly as an incorrect (and varying) impedance on the "pair" you are trying to drive. TDR can locate broken wires and sorts - not just the fact that they're there, but how far down the cable they are. A good TDR setup can even let you "see" things like a place where the cable has been bent too sharply or pinched, and the twisted-pair wires have been smooshed out of their proper configuration. As to specifics, a bit of Google-fu turns out the following possibilities. I haven't used any of them myself, but the data sheet summaries of capabilities look promising. The high-end Fluke cable-testers such as the CIQ-100, DSX-5000, and DSX-8000 would probably work - these are described as being able to locate crosstalk faults and impedance faults. Another very interesting device is the PocketEthernet tester, which sells for 199 Euros (less without VAT). It analyzes the wiremap, has TDR capability, and includes a bunch of higher-level diagnostics as well (bit-error-rate, Ethernet link analysis and capabilities- announcement, DHCP, ping tests, bit-error-rate testing, etc.). It uses a tablet or smartphone as its GUI (connected via BlueTooth) and generates a pretty spiffy-looking report in PDF format. I don't know what its sensitivity would be to such problems on a short (e.g. jumper) cable - that'd be a good question to ask the manufacturer. For diagnosing signal integrity problems in a building's installed Ethernet wiring, I'd want to have a device of this sort available. The high-end devices can probably be rented. The PocketEthernet is cheap enough that I'd just buy one if I can to do any work of this sort. There's also another dirt-cheap method for diagnosing bad Ethernet jumpers - substitution. Buy a bunch of known-good CAT-6E (e.g.) jumpers from a reliable vendor, and inspect them. Mark them as "This is a good one". If you have a suspect connection, start swapping cables one at a time - replace each jumper with one of your known-good supply and re-test to see if the problem goes away. If it does, take the cable you removed from service and _immediately_ cut off both ends right at the connector, and then throw the whole thing into the trash, so that no one will _ever_ put it back in service. Do not try to salvage, repair, or re-use a bad jumper - life's too short to try to pinch pennies like that. (If you don't do this, the old cable _will_ come back to haunt you some day.) -- _____________________________________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- Check out the new Asterisk community forum at: https://community.asterisk.org/ New to Asterisk? Start here: https://wiki.asterisk.org/wiki/display/AST/Getting+Started asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users