Cat 6 max cable length

[Ropey]

I know the standard is measured for a max cable length of 100m. My question is what exactly do you lose if you go slightly over this - say 120m? When should you use a repeater, and at what point would fibre become a better option?

[Graham L]

The standard says 100 m. If a professional installer follows the standard, there is a very good chance that the system will work correctly.
Therefore, to avoid arguments, a professional will insist on a repeater if there is a longer run, to protect himself.

If you are doing it yourself, the simplest (and cheapest) thing is to try it. If it works, reliably, (as indicated by "no" lost packets, "acceptable" collisions) that's fine. If it doesn't, you need a repeater.

The main problem is loss of signal strength ("attenuation"). Beyond a certain point, the tranceivers don't work properly. Some manufacturers boast about extra sensitivity so you can use longer cable runs.

One other problem can be collisions. If two hosts start transmitting at the "same" time, the collison detection has to operate within a certain number of bits. The time for a signal to get from one end of the segment to the other is what determines the maximum length.

The standard protects you. A system installed in full accordance will (usually ) work. If you stretch the limits, it might not. Optical fibre is expensive. You would need stronger arguments than one long cable run to justify it.

Remember that a hub/switch does not need to be at one "end" of the system. It can be in the middle, which gives you a span of 200 m. Have you got a big house? ;-)

[Ropey]

Thanks Graham
(That was extemely quick - do you have PressF1 on-line all the time?)

Not a big home - that's a cardboard box 8-) . A school with a 32 PC classroom at about 110m from the server. It's currently cabled with a (repeaterless) run of Cat 5, but we suspect performance issues are caused by the cabling. It seems to me that we might get away with a straight run of Cat 6, and still get considerable performance gains over the Cat 5. The admin guy wants to move the server (at the expense of other classrooms), but I think cabling improvements are a better solution.

My understanding is that a hub doesn't boost the signal, but a repeater does. Why doesn't the hub cause the same loss of signal (or even twice the loss since you're degrading from both ends)?

Perry Rowe

[Graham L]

I think all hubs are repeaters. Switches have to be. Thinking about it, you might not find a twisted pair repeater. All I have seen are for coaxial cables.

Quick idea: can you borrow a hub and put it in halfway along the run. (Someone must have one --- replaced with a switch, etc). You might need to cut the cable and crimp connectors on it to test this ... but it would be cheaper than 120m of Cat6.

[-=JM=-]

hub = multi-port repeater

[wuppo]

As Graham L points out, the propagation time along the cable, and hence the collision detection time sets the effective "maximum segment length". Installing CAT6 won't improve this, if the CAT5 is installed correctly.

As the cable distance between two nodes increases, the time taken for a collision to be recognised (and acted on) increases, thereby lowering the overall performance (hence the 100m "limit").

How are the 32 Pcs connected to the 'long run'? If they are connected through a switch, and the other end of the cable terminates at a switch, you basically have only two nodes on a full duplex circuit and won't get collisions.

Try connecting only one PC to the end of the cable and run some throughput tests; if the speed is there, the cable is OK.

[Stumped Badly]

Not all hubs are good repeaters.
There are generally 2 catagories of hubs.

Active: - or sometimes called multiport repater
In active hubs, which all of today’s hubs are, the signal received from one port is regenerated (amplified) and retransmitted to the other ports on the hub. An active hub therefore performs the function of a repeater.

Passive: Simply makes an electrical connection for the cables and doesn't "clean up" the signal before sending it on, so will be useless for what you want to do.
These are older units and are usually non powered.

I think your situation would be best served with a switch, as a hub will broadcast data to all 32 of your nodes, regardless of whether they want it or not, a switch will only send/recieve the info to the particular workstation, therefore cutting down on traffic across the network keeping plenty of bandwidth clear for other users coming online.
Although a 32 node setup won't cause cat6 many problems, in terms of bandwidth it's always nice to have extra in case of future upgrades or a change in type of dataflow. eg. a lot of multimedia work.
As a good quality switch is not far off the cost of a hub, it would make sense to go for a switch.

[Stumped Badly]

Oops, should also have said, as the others have, the 100m limit is there for a reason & you shouldn't try & extend it.

[Ropey]

Thanks all for your prompt and informative replies. I think I can now make a sensible decision based on all of this.