When wiring up an equipment rack, the tendency is always to try to be neat and eliminate unncessary cables. This can mean creating some extremely short cables to connect items that sit next to each other. So, here’s the question… how short is too short?
The answer comes down to something called reflection. Reflection is the “unintentional bounceback of a signal as it enters a connection.” Obviously it’s something to be avoided. Think of it this way. If you get a cup, and you go up to one of those self-serve soda machines, there’s always a little splash of soda as it hits the cup when you’re filling it. If you keep your hand away from the stream then it’s ok, otherwise you get soda all over your hand.
Now think of signal being kind of the same. When the signal hits a connection, it’s like the soda hitting the cup. There’s a little splash but it’s pretty confined. If your cable is too short, then the “splash” bounces back up and hits the other connection, and then you’ve got real trouble. A long enough cable helps it dissipate harmlessly.
So how long is long enough? Generally, a 2-foot (60cm) cable is long enough so reflections aren’t a problem. You should avoid cables shorter than that. If you need two pieces of equipment to be closer than that, we recommend making a 3-foot (90cm) cable so that it will be long enough to bend it in a graceful loop no less that 6 inches (15cm) in diameter. Bending a cable more severely than that can cause several problems.
It may end up ruining your idea of a beautiful install but keeping cables over that minimum length will make everything work better.