Seasonal affective disorder

If you want a job done right …

Computerworld  |  Shark Tank
Computerworld / IDG

It’s 1984, and pilot fish’s software development unit is getting ready to move into brand-new quarters in a building that’s still under construction. Electricians are already installing the power and telephone wiring for all of the offices and cubicles, so it’s decided to have them install the Ethernet cabling as well.

It being the ’80s (which someone cleverly has dubbed “The LAN Before Time”), the workstations on every desk will have to be networked together using 10Base5 Ethernet — the original thick Ethernet. The backbone bus is made of thick, stiff, shielded coaxial cable. One cable will run the length of the office, a couple of hundred feet, with a few dozen transceivers about every 10 to 15 feet. The transceivers will be in sealed black boxes about the size of a VHS tape cassette (again, the ’80s!), with a type N screw-on coax connector on each end and a 15-pin D connector for the drop cable that will connect to a computer.

Aside from running drop cables through all of the power poles for the cubicles, installing the Ethernet cabling means cutting off appropriate lengths of coax cable to go between successive transceivers, and installing a type N connector on each end of each piece of coax. Then the electricians will place the coax cable and transceivers on top of the ceiling tiles and their supports, and connect everything together. A terminator will be screwed onto each end of the assembled cable.

Soon, all is ready, and the developers move in. And everything works fine.

Until it doesn’t. Fish and crew figure there’s a break in the continuity of the coaxial cable. Sophisticated tools exist that can locate such a break, but they don’t have them.

Says fish, “We used the binary ladder approach: Climb up a stepladder, remove a ceiling tile, unscrew a coax connector, put terminators on both sections, then find out which half of the network is still broken. Repeat this process on the broken half, until you gradually close in on the defective cable section or transceiver. Sometimes a hardware guy would do this, but if they were busy, it was usually my job to track down such problems.”

And sometimes, when they split the coax in two, they found that both halves worked OK. And when they reconnected the two halves, everything worked fine. So the problem had been fixed, but they didn’t know why or how.

That keeps happening for a few weeks, then the problems go away.

Until they come back, several months later. Again, the problem disappears when they go looking for it, so they have no idea what is causing it.

“Finally,” reports fish, “after several rounds of these episodes, one of our hardware guys — call him Fred — decided to spend more time trying to find the cause. He came in one weekend and carefully examined the whole Ethernet backbone. And he finally fixed the problem for good.”

What Fred found was that the electricians were a little sloppy some of the time. When they installed those type N connectors on the pieces of coax, the center pin of the coax connector didn’t always extend quite as far out from its base as it was supposed to. As a result, when those connectors were screwed onto a transceiver, they would make contact, but only barely. Then, when the weather changed in spring and fall and the building’s systems switched between heating and cooling, the result was different patterns of vibrations in the ceiling tile supports — and these vibrations would shake some of those tenuous contacts loose. When fish and friends moved one piece of the coax to search for the problem, that was enough to make the contacts come back together.

To fix the problem, Fred went through the whole network and replaced all of the type N connectors on all of the pieces of coax. A happy ending? Yes, but then the unit was moved again ...

Sharky is affected by the seasons, too. Things can be slow in the summertime, so send me your true tales of IT life at sharky@computerworld.com. You can also subscribe to the Daily Shark Newsletter and read some great old tales in the Sharkives.

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