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I got this cable with a homeplug AV 200 device - its labelled cat 5e

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But looking at the plug end has me rather dubious about it.

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(There's only 4 strands there, not the standard 8 - and missing pins, so I suspect these cables are designed to do so. The wire jacket is also surprisingly loose).

Another angle. This isn't a standard connector, no, someone has went and made a non standard connector specifically for this

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I figured the cable was crap so I cut into it - some of you were wondering if they had merely left out the additional pairs - and the answer is no - its basically half a cat 5 cable.

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I'm currently using it for some legacy equipment that runs on fast ethernet and the plug it came with and it seems to work. However, I'm curious - what're the downsides of such a cable? What do I need to look out for? In what situations would this cat-5 cable be unsuitable for use in anything other than a ethernet cat o'nine tails?

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So long as you're using it with equipment that only uses 4 wires, it should be fine. –  Daniel R Hicks Jan 14 at 1:39
    
I've never actually seen a ethernet socket with just 4 contacts, so thats a tricky thing to work out. It hasn't flaked out on me yet, but its the strangest ethernet cable I've ever seen. –  Journeyman Geek Jan 14 at 6:05
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+1 nice question. GigE and beyond is common enough now that the fact only 2 pairs are required for Fast Ethernet is no longer "common" knowledge. –  Phil Jan 14 at 6:53
    
J - Well, devices that require more than the 4 wires are pretty rare. Only SOME of the high-speed 1000megabit stuff, and few folks have any need for that kind of speed. –  Daniel R Hicks Jan 14 at 11:52
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@pauska: the connector won't hold more than 4 strands of copper - the rest of it is filled in. Every stage of design and construction of this cable were obviously fuelled by copious amounts of illegal substances. –  Journeyman Geek Jan 15 at 1:49
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Standard 100 base ethernet only uses two pairs so this works... what is a little worrisome is it appears someone did it on the cheap and saved a couple of cents by omitting the other pairs. That said, 1000 base ethernet may use all four pairs and this cable can't handle that.

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To add to this - apparently fast ethernet needs 1 2 3 and 6 connected - which is what we see here. I'm going to have to gank one of these cables for testing on my gig-e gear. I'm curious to whether it'll be detected as a inferior cable and the network will throttle itself down. –  Journeyman Geek Jan 14 at 6:38
    
I'm working on my own answer on this - you're theoretically correct, but running some speed tests find some interesting findings I'd like to share. –  Journeyman Geek Jan 14 at 11:53
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I ended up doing some tests myself, and the results were... frankly appalling.

TLDR: Throw out the cable.

Now on to some science.

To address some concerns that were stated in the comments - The initial testing was done on a undamaged cable - I only cut it open after the speed tests. I've repeated the tests with an identical cable from the same kit, and have found the same results. I've also repeated the tests with the 'proper' cat 5 cables with 4 other cables.

I tested this by connecting two systems to the same switch, and swapping out the cable on one of them. I also used the free version of lan speed test to do some 'quick' performance testing . I'm running the test from my laptop to to a share on a 3tb 7200 rpm drive on my desktop. I'm testing the cables at this point, not overall bandwidth, under conditions reasonably similar to realworld situations for me.

I was thinking of doing a direct system to system speed test as well, without the bridge, but looking at my results, that would be a clear waste of time.

Lets start off with the most obvious things.

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The 4 stranded cable isn't capable of gigabit ethernet (no surprise there). In fact, It can't even do fast ethernet.

The speed test took a while to complete... in fact, I had dinner in the time it was running and total time was about 2 hours to write and about as long to read.

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Its pretty close to plain vanilla 10baseTX speeds by the looks of things.

Less obviously? No IPV6. I have no idea what's with that. I've had enough oddness with legacy gear that I'm not even going to touch on this.

I actually tested 2 random 'to spec' cat5 cables from my spares bin. Both have UL marks, and had similar enough performance that I'm only going to post one set of results. These detect as 'proper' gig-e cables. Both were freebies from routers, so its not like I'm comparing apples and oranges

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Gigabit Ethernet Sweetness!

And have performance thats pretty reasonable - taking into account various overheads like the test writing to a file share

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Yes, thats about 200 seconds total vs about 4 hours. And Ipv6 worKs perfectly

Even with the devices it came with - which could easily reach up to 80-100Mbps, the cable might have fallen short. Its clearly not cat 5 e as labelled, and in my specific use, its clearly unacceptably slow. If I wasn't planning on documenting this, I'd probably have stopped the test for the other cable sooner.

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It could also be that your gigabit hardware falls back to 10mbps operation even if 100mbps would be fine. Or that when you cut into the cable to check for the wires you damaged it. You'd need a cable tester to be sure if it meets 100-BaseTX specs... but I agree with your conclusion to junk it; surely not worth the expense of cable testing gear. –  derobert Jan 15 at 21:24
    
I tested it before cutting into it - I have another one of these somewhere, and I figured I could just use this as hookup cable for other projects. –  Journeyman Geek Jan 15 at 23:54
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Most of the time, these cables still include the other 2 pairs in the cable itself, but they don't crimp them. So you can make them real Ethernet cables with the proper tools.

I assume they do this so they can sell "Gigabit" Ethernet cables at a higher price.

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This was free with a piece of equipment, and no, the cables are completely missing. I'd assume its a way to cut cost, but I'd think that the cost of buying special, crippled ethernet cables in bulk is more than buying normal uncrippled ethernet cables –  Journeyman Geek Jan 14 at 1:49
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@JourneymanGeek If you manufacture on a large enough scale, then you do save a bit - copper is expensive. The more typical cost-cutting method is to use thinner wires - which presents a problem over any real length. Actually, if they used properly specced wires for the pairs that do exist, I think I'd prefer this method. Of course, a fully to-spec cable would be much better... –  Bob Jan 14 at 3:15
    
@Bob Not including the full 4 pairs is violating the Cat5e standard. It's not exactly common to see this. –  pauska Jan 14 at 14:07
    
These could be cheap-o patch cables for "unified cabling", as a good portion of those carry phone signals... –  NickW Jan 14 at 14:51
    
@pauska Not from a reputable manufacturer, no. But once you buy cheap imports from China, all bets are off. –  Bob Jan 14 at 14:55
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Cat5 is a standard defining the physical layout and type of conductor not the number of conductors. Things distinctive to cat5 are things like the twist, length, and size of the conductors inside the cable.

Two pairs is all that is needed for 100mbit Ethernet. Your poor performance is probably the result of a bad crimp not that the cable is inherently bad. You can also cause bad performance by doing things like cutting the sheathing on the cable and then changing the physical orientation of the conductors. 1g+ ethernet requires all four pairs.

Cat5e is an improvement to the Cat5 standard which specifies additional testing and improved quality control. A Cat5e cable is not necessarily better than a Cat5 cable.

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Well, I didn't assume this was out of spec initially, and certainly not until testing. That said, the standards I am looking at (ANSI/TIA/EUA 568B) explicitly refer to "4 pair 100 ohm UTP category 5e". I do think testing cables for transfer speed, and basic functionality is something I need to do with freebie cables like this in future. –  Journeyman Geek Jan 15 at 9:37
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