Port Economisers & Gigabit adaptors

[cpvh]

Ok, i'll come clean, this is a question relating to work rather than home......

I have successfully used CAT5 Port Economisers* with 10/100 connections in the past, but what happens if you have Gigabit capable kit a both ends?

I know you wouldn't acieve a Gigabit connection, as you'd only have 2 pairs available for each port and Gb requires all 4, but could you successfully use these at 100Mb/s?

*Devices which allow you to make use of the "unused" pairs in a CAT5 cable such as connection with Hills, just used for illustration prposes - lots of other (and probably cheaper sources out there)

[Mike Barnes]

[e-mail]:

I don't use port economiser devices, but I have connected two twin RJ45 wall plates with one CAT5 cable, two pairs per socket, which amounts to the same thing I would think. It works fine at 100mbs with a Gigabit device at each end.

[Andrew Gabriel]

I just tested it between 2 gigabit switches. It takes noticably longer to autonegotiate (long enough that I thought it wasn't going to work), but then it came up at 100Mb. Don't know if it will work in all cases - it would be an edge case for manufacturer testing.

Some systems allow configuration of the auto-negotiation process and can be configured not to work a certain speeds, so it may well be possible to configure it not to work.

Mine look identical, but came from CPC many years ago.

[cpvh]

I imagine they may well be cheaper (couldn't see Hills prices on the website mind).

Thanks for doing the experiment Andrew, and thanks for the other replies.

Chris

[Gordon Henderson]

I have a few "industrial" PCs which have on-board Gb Ethernet ports, but they only bring out 4 pins to the round screw-in connector on the case... They seem to "just work" (at 10/100) with the other 4 pins not connected to anything...

So I imagine the splitters will be just fine.

Gordon

[Theo Markettos]

We have a building flood-wired with cat5 (or 5e or whatever). But the installers cut corners and have only wired two pairs of each link. We have issues in that kit capable of gigabit at both ends thinks that the link is gigabit-capable, when in fact it isn't. We have to set switch ports to

100Mbit because otherwise autonegotiation sometimes fails, and in a fairly obscure way (link lights come one, but no traffic).

Theo

[John Rumm]

Simple, it can't run gigabit without all the wires. Hence it will fallback to 100 Mbit.

Yup, most gig equipment is also 10 and 100 backward compatible.

[Andrew Gabriel]

The issue here is that it probably isn't expected that two Gigabit ports will be connected with only 2 pairs, so it's an edge case, and unlike connecting Gigabit to 10/100Mbit which everyone is going to be testing.

I'm reminded of something similar I stumbled across with SCSI some time back. I had wide SCSI card and device, but only narrow cable, so I fitted wide to narrow adaptors at each end. When the SCSI is initialised, the card asks the device if it's wide, it says yes, and they agree to transfer data in wide mode. Of course, the top

8 bits of every 16 bits are lost and the transfer fails. Fortunately, the scsi driver in this case had the sense to try going back to 8 bit transfers after some number of failed 16 bit attempts, so after a pause whilst it has several failed attempts and does device resets and drops the transfer speed, it eventually starts working (albeit at slowest transfer speed). Another case of an edge-case which on reflection, I would not have been surprised if it hadn't worked at all.

[John Rumm]

Indeed, as you discovered with your test, the auto negotiation takes longer (much as it does with gig equipment on port with fault cabling), but IME it usually gets there in the end. In the case of PC etc then you can often force 100Mb operation if required.

Must admit to never having tried wide to narrow converters (usually preferring to run two separate buses - one wide LVD for internal hard drives etc, and a narrow 10/20 Mb one for external stuff). Having said that, I have always specced decent Adaptec cards with an additional external narrow bus connection as well as the wide.