Over the last 25 years I've had the fortune of working on the Interop trade show. I've held positions from an attendee, to the lead network engineer, and an instructor. Shortly after I started teaching the CCTT training classes, I had the opportunity to troubleshoot a problem during the show setup.
Right after arriving onsite, one of the technicians told me they were having trouble getting one of the links up and running. We started by plugging a device into the far end. It would get a link light, but could not get a DHCP address. We went back to the switch and tested the port. Sure enough, we were able to get an address and get out to the Internet.
The next step in this troubleshooting process was to check the cable. At the time, the Fluke Networks DTX-1800 CableAnalyzer was the tool to use. So, we put the main on one end and the remote on the other. The Cat5e certification test failed. We could see that the Near End Cross Talk (NEXT) exceeded the test limit. The certification test tells there is a problem with NEXT, but not where the NEXT problem is along the cable.
Switching to Single Test, I ran a HDTDX test. This test measures the NEXT along the length of the cable. It was clear that in the middle of this run there was excessive NEXT. Now that we knew the location, it was time to go find the problem. When we got there, we saw the picture below.
In setting up the network, one of the local electricians ran out of cable. So, instead of pulling a new run of the correct length, he decided to extend it by twisting the pairs together. Twisted pair does not mean that you should physically twist the pairs together. And you really shouldn't use gaffer's tape to secure the connections.
We had the staff onsite use the back cable to pull in a new run that didn't include any twisted connections held together with tape. After connecting the device to the far end, we got link and an address. All was good and we moved on to the next problem.
This was a case of someone that knew better doing something that didn't work. He hoped it would, but if he had certified it before turning it over, he would have seen it was a no go. Take the time to at least validate your work, even better yet, certify it. This way you don't get called back and have to do it all over again.