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.