Return Loss error on Fluke

35 pts.
Tags:
Cabling
Network design
Return Loss
Let me first say, I'm a total newbie to networking... Okay, got that out of the way. I've run cat5e 94 feet thru my basement. When I connect each end of the cable to a Fluke DSP-4000 Network Analyzer it tells me that Return Loss failed (RL and RL @ Remote). If I drill into the details on the analyzer it says that pair 3,6 failed and the others all passed. I'm running this from one end of the basement to the other end and up thru a wall. On more than one occasion it crosses next to power lines. However, I don't think there's any way for me to avoid this... Is that what's probably causing the issue? Should the fact that 3,6 failed and the others all passed lead me to some conclusion and course of action? Thanks in advance for any help you all can offer... -- Dave
ASKED: June 30, 2008  2:43 AM
UPDATED: July 12, 2008  7:07 PM

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Besides running by power conduits, are you running past any florescent lights? This will introduce noise on the cable and also impact frequency. Return loss is also a measurement of how effective the cable is at carrying different frequencies. By definition RL: is a measure of the reflections that are caused by the impedance mismatches at all locations along the link and is expressed in decibel (dB). Nominal characteristic impedance of each link segment is 100 ohm You will see lower return loss at lower frequencies and the reverse at higher frequencies… higher loss. Are you testing this at a high frequency for expected 1Gbps throughput or better? What happens if you reduce the test to 100 Mbps? Also, what types of ends have you put on the cable? Have you tried changing out the connectors?

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  • DaveR
    Thanks. I'm not running by any florescent light, but I do cross and run adjacent to power cables in a couple spots. To be honest, I'm not sure what frequency the Fluke was testing at (total newbie, remember :-)), but all I need is 10/100 as it's just a line from my office PC to the router. I just put on normal network ends, but I can try to replace them if you think that might be the issue. What are the odds that the power cables are the issue? And if that's it, are there any effective options to shield the network cable? Would it help if I put a piece of metal or plastic conduit around the network cable at those spots? Thanks again for the help -- I'm swimming in unfamiliar waters here... -- Dave
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  • Labnuke99
    Distance from power cables is always a good thing. Data cabling should run in separate cable trays or along a different path from power lines if possible. I would try replacing the ends of the cable as those sometimes are difficult to get crimped on and the pins may become bent or not in full contact with the cable.
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  • petkoa
    Hi, DaveR I don't think power cable crossing/running in parallel is an issue in your case; if you had more than one pair failing, I'd say you wrongly arranged the cables from different twisted pairs, but with only one failing I'd really listen to Labnuke99 and replace the RJ45-s. According to my experience, power line crossing hardly could be an issue with LAN cabling, but LAN cabling running together with the power line for several meters readily could be (though, I have seen a NIC/switch port wasted on both ends of a LAN cable crossing - across the brick wall - a lightning-conductor...). Good luck (with new RJ-s), Petko
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  • Jmkelly
    In this case, I wouldn't worry about power lines or fluorescent lights. Those generate noise, not return loss. "Return loss" is how much signal power is being reflected back from imperfections in the cable. In practice, the problem is usually a bad punchdown on one or both jacks, or bad jacks. Having return loss on just one pair suggests that that's your problem. It could also be a kink or other flaw in the cable -- let's hope not, or you'll have to re-pull the whole thing. Have you tried swapping the two units of the Fluke DSP from one end to the other? That may give you a clue as to the location of the problem(s). Probably your best bet is to reterminate one or both ends. If reterminating doesn't work, and you don't think there are any flaws in the cable, try using Systimax jacks. Also, make sure you're building the jacks right. Strip the outer jacket off the cable carefully, using the pull string rather than anything with a blade -- nicks on the conductors may cause return loss. Preserve the twist of each pair as you lay it into the jack, and leave as little wire exposed as possible; bring the outer jacket all the way up to the bottom of the jack, as far as you can. Fan out the pairs as evenly as possible (to avoid crosstalk errors). It could also be that the Fluke itself is having problems -- sometimes the test heads and their connections get glitchy. You don't say whether you've run the Fluke on a known-good circuit; you might try that before spending too much time on the "bad" circuit.
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  • Labnuke99
    JMKelly - thanks for your insights. Sounds like you have a lot of experience in this area and I defer to your knowledge. Thanks for sharing.
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  • DaveR
    Thanks for all the help folks -- much appreciated. Here's where I've ended up... I tested the Fluke on another line that works fine and it comes back successful, so I think it's working. I tried to eyeball the run to make sure there are no kinks and I couldn't see any, but there's a few feet in a wall... I replaced the connectors w/standard rj45 jacks w/o using anything sharp to do it. The Fluke still shows return loss failure. However, I was able to plug it into my router and use the line?!? Does that surprise anyone? It didn't work previously. Anyway, I think I'm back up and going for now, though still a bit confused. As a side note, if I decide to try your Systimax suggestion, is that something I can get at any local electronics store (e.g. Bust Buy), or do I need to go to a specialty store for those? Thanks again for all the help -- you guys are great. -- Dave
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  • Jmkelly
    Regarding the router working even though the Fluke complains -- yes, that can happen. Sometimes -- probably fortunately for us cable monkeys! -- on a link of marginal quality, the network equipment is more forgiving than the cable scanner. To get Systimax jacks you'll probably need to go to a trade shop like Graybar or Anixter. Actually, try Google first!
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  • DaveR
    Wonderful -- thanks again!!
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