Twisted-Pair Sister

The evolution of the Ethernet standard and related products was very significant in my working life. The switch from coaxial cable to twisted-pair wiring with hubs and switches made local area networking easier to install and much more reliable.

On LinkedIn some time ago I told the tale of the strangest 10BaseT problem I worked on.


Part 9: As networking gear and the expertise to deploy it improved, helped by the introduction of 10BaseT, physical layer problems became much less common. Installation issues were moving up the OSI model to layer 2, and I was spending more time helping customers that were installing their own Ethernet bridges. Sometimes our 2E:2E:2E addresses didn’t work right, or at all, especially with inexpensive products. It was a perfectly valid LAA MAC, registered with the IEEE.

One of the system developers had written a small TCP/IP stack for Windows for Workgroups 3.11. By taking advantage of a piece of Novell software called ODINSUP, the stack could communicate with network adapters. My part was to come up with Config.sys and Autoexec.bat configurations that would work reliably for selected 10BaseT adapters from Intel and 3Com. The intended application was an emulator for the company’s proprietary user interface.

A customer that was running PC’s with the emulator over a network was having trouble. The symptom was bizarre — when typed, the lower-case letter “r” wasn’t echoing. Because the PC project was new and experimental, the developer and I got on a plane.

At the hospital we confirmed that, sure enough, the problem was the letter “r” didn’t appear when typed. It didn’t matter if the character was in a field read or a text editor. It wasn’t device-dependent, but I quickly determined that it was host-dependent. A couple systems had the problem, but others didn’t. I looked at the RJ45 plugs going into a 10BaseT hub from the host systems.

In a nice way, I asked, “Where did these cables come from?” Someone in the shop had made them, I was told. They were 4-wire cables, using pins 1,2,3,4. 10BaseT only needs two pairs of wires, but the pairs are 1-2 and 3-6, not 3-4.

I asked the customer if they had any manufactured cables. Some were found and I swapped out the homemade Ethernet cables. That did it. The developer was typing “r” in the emulator and it echoed every time. How that could have been the only observed problem has always been a mystery to me. I called the office and told our boss we had fixed the problem by using r-rated cables.

2 thoughts on “Twisted-Pair Sister”

  1. Yeah, considering the audience, why not get into the devilish details? In practice, spare pairs of solid-core wire in those phone cable bundles didn’t always work well, or at all, for Ethernet. All it took was a less than perfect splice on an S66 punch block. But I’m being picky, and I really admire the Serial Port project.

  2. Fun story. And good overview of some of the history. I was surprised he didn’t go into the difference between “hub” and “switch” devices, and he came close to talking about different twist rates in UTP cable but didn’t quite get there…

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