Minimum ethernet cable length?

Today a cow-orker remined us that there is a minimum length for UTP cables. News to me.

Is there a minimum allowable length for cat5 or cat6 cables? Specifically in a 100BaseT with switches, not hubs. If so, what is(are) the reason(s)?

Googling this found that there is either no minimum, or a minimum of 2.5m, 2.5feet, or 3m or 1m.

I found several mentions of microsoft material that gave a 2.5m minimum length. All the supplied links were dead. There was one quote of microsoft material, but no supporting cites.

I did check 802.3, to find that the 2.5m minimum length applies to CSMA/CD networks, per IEEE 802.3, where overly short cables can cause the collision detection to malfunction. This might apply to half-duplex gigabit lines as well. It should not apply to switch based networks which won’t have physical layer collisions.

All the other mentions of minimum length requirements were unsupported by citations, and the reasons given amounted to hand waving by people who new some terms related to transmission lines, but didn’t actually understand what they were saying: “mumble mumble reflections mumble mumble impedance mumble mumble termination mumble”, which is what I got from the cow-orker. Properly terminated transmission lines (which is what UTP cables are) do not exhibit reflections. Due to Z0 and terminating resistor tolerance, connector glitches, etc, I can see that there would be some reflection, but don’t really see that this matters if there is no collision detection at the physical layer, and the amplitude of the reflections would not be related to cable length (though the timing would).

It appears to me that the 2.5m figure was taken out of context, and a myth was born. Possibly widely spread by microsoft.

I never heard of a minimum; the only such length issue I remember was the thickwire cables that had to have taps and terminators at marked intervals that corresponded to (IIRC) the quarterwave of the 10Mhz signal.

You can buy 18" and 12" UTP patch cables. They are very handy in jumper situations and I never heard of any causing problems.

However, it seems patch plugs will deteriorate a signal, and probably a lot of infrastructure has been upgraded without replacing intemediate patch cables or punch-down blocks - which if Cat-5 instead of Cat-5e or Cat-6 might cause problems with new faster hardware.

I’ve got a UTP patch cable that’s an inch long.

OK, I made it myself, so that I could plug a crossosver coupler onto an ordinary cable, with my inch long cable coming out of the other side.

I’ve never heard of a minimum length standard. Certainly this one works with my cable tester and in real-life use.

I have a 6" cable lying around, so, presumably, no, at least from a signal standpoint. From a physical standpoint, 6" is short enough that it’s hard to bend to get both ends to where you want them, so 6" is about the limit.

I think this is where the idea came from.

In the days where thin or thick coax cables were used in Ethernet (in a daisy chain fashion for thin, with daisy-chained transceivers for thick), there was a minimum distance of 10 ft (3 meters) between stations, although I was able to get networks to work with less by accident.

I have a 9" Ethernet cable that came with a router, and it works fine. In a star configuration or with a switch, the minimum length is no longer a limitation, although someone might come along and say it is in theory.

A daisy chain cannot control when a station begins transmitting, so it is subject to collisions. A switch can store and forward data packets and totally avoid collisions.

(Maximum length is a different story.)

Thanks for the sanity check. I’m sure I won’t ever convince O. of it, but good to know this is the hokum I took it for.

If they didn’t have 6 inch cords, then rack systems would be 10 times the mess they are now.

I’m glad **Musicat **looked that up. I remembered have a problem with using coax to connect some racks years ago. It was very intermittant and it took us a long, long time to find the problem. I seem to recall the mimimum distance being six feet given the transformers we were using.