A patch cable with 2 switched wires is the only way a connection can be made... ??

[Jayce]

Figure this one out...

I like wiring a lot, and I find a lot of interest in troubleshooting network problems. I ran into an interesting one today.

I work at a school district. Today, I was setting up a computer and here the connection said unplugged/plugged back in/unplugged/etc about 5 times. Then magically shut off. Hmm, wtf?

First thought - check the cable. 568-B standard wiring. Looks fine. Put it in my tester... pins 3 and 5 crossed. Bad cable. So I replace it with a cable that checks out fine on my tester.

No connection. "Cable Unplugged."

Second thought - check the jack. I pulled the jack from the wall. 568-B standard punch-down. Looked good to me, but hey... let's try it again. So I re-punch a new jack. Perfect text-book punch-down jack... Just to add to the mix, I took a functioning computer and put it in its place and booted it as well.

No connection. "Cable Unplugged."

Thirdly... check the switch. So I go to the closet and find the switch it's plugged into. Switch looks good. Maybe the port blew? I change the ports. Hmm... no lights, even after 3 full minutes. Okay, try another switch all together. So I plugged it in another Netgear 24port switch that was there.

Nadda.

Okay, let's test the cable. So I bust out my cable tester and the terminator. I put the terminator at one end, the automatic tester at the other. So I'm testing from patch cable to wall jack... wall jack to cable... cable that goes from there to the server closet's patch panel... patch panel to patch cable.

Checked out perfect. All 8 pins.

Wow, I'm confused.

Two different switches.
An entire run of cable that is tested to be perfectly fine - B standard.
Two computers, both in working order... no connection.

In a weird hunch, I put the old cable back in.

BAM. Connected. 100mb/s.

WTF? How the **** can you have a full blown B standard network from a 24 port Netgear switch, all the way up to the main office to the computer, and BAM throw in a patch cable that has pins 3 and 5 crossed and MAGICALLY it works?

I'm not an expert, but I am a technician and I like to figure out problems like this. I look forward to finding problems like this just to see if I can make things work. And granted, things worked out this time, but not in the way I intended.

 

[mikesgroovin]

Re: A patch cable with 2 switched wires is the only way a connection can be made... ?

Well, I'm not sure what kind of feedback you are looking for here. Glad you got it working but from a troubleshooting/diagnostic standpoint, you did great step by step.

Fact is, a cable with pins crossed wont work where it isn't supposed to unless they are crossed on the other side. After all, pins dont have to be laid out (OW - O - GW - BL - BLW - G - BRW - BR) ... they can be mixed up all sort of ways, but just have to be the same on the other side. They also should be in pairs.. so even if your cable is (BLW - BL - BRW - O - OW - BR - GW - G) it will still work, a little better than something that isn't using 1, 2, 3 and 6 with their opposite color pair, but in the end will still work.
Link lights are also extremely unreliable when you have opposite information reading on a meter or a RJ45 tester of some sort. Most devices dont need both pairs or all four pairs to make the link light turn on and then tell the nic driver how fast it is going to be able to use the connection at. Some just need a pair of transmits or a pair of receives to link... pretty scary, but true


If you wanted to waste a lot of time developing it and make a lot of money, I bet this would sell:

An intelligent adapter or intelligent LAN connection.. So, the hardware would respond intelligently on OSI layers 2 and 1 based on what the OS can provide speed information for on the top layers 7, 6 and 5. Then based on an algorithmic formula, tell you EXACTLY how fast the network connection or bytes/packets are transferring at in real time. Mostly used for diagnostics and other web browsing and webhitting tools.. but useful none-the-less

 

[splinter-69]

Re: A patch cable with 2 switched wires is the only way a connection can be made... ?

The patch cable you are using could be a cross-over cable.

 

[Jayce]

Re: A patch cable with 2 switched wires is the only way a connection can be made... ?

The patch cable you are using could be a cross-over cable.

No. I have a crossover cable in my bag. I compared them out of curiosity and they're different. Somebody simply screwed up making the cable... visually, the cable doesn't look bad, because the pins that were crossed are blue/stripe and green/stripe, so it's a white wire with a green stripe + blue stripe. The stripes were lined up perfectly so you could barely see them. So I can see why it was crimped the way it was, but when you look super close, you can see a nick of a blue wire, telling me it's wrong from the other end of the cable, where I can plainly see green.