Which cable to use for GigaEthernet interface

Rahan skrev:

You can use either. If you use a straight cable you should turn on the auto mdix feature on the switch port. If you use a crossed cable make sure all the pairs are crossed not only 2 of the pairs.

1000Bt was designed to run over Cat5 but the Cat5e and Cat6 is ofcourse better.

But since 2001 the cat5e standard replaced the Cat5 standard so any Cat5 cable manufactured after 2001 _should_ be cat5e. (Please do correct me if this is wrong).

-SAto

I've asked in another thread and received no answer: why is this? Given that 1000base-T transmits and receives on all four pairs simultaneously why is crossing pairs even considered? (And it can't be backwards compatibility with earlier crossover cables because a 1000base-T crossover is wired differently from any other cable!)

For some meaning of "better": since GigE should successfully transfer all bits of data over properly constucted Cat5/e cabling it's difficult to know exactly how Cat6 might do better... :-)

Sam

Sam Wilson skrev:

I do not know. Maybe not too many people know and thats why you haven't got an answer.

The Cat5e has FEXT in its requirements, old Cat5 did not. Cat6 has stricter requirements meaning less noise fext/next etc.

All leading to less problems. It depends on the setup if you'd notice any diffirence though. In a "noisy" envirmont the higher graded cables will do better.

-SAto

Hi All,

Thank you very much for your answers !!!

Nice week

Best Regards Rahan

Straight cable. In GigE there is no such concept as a "cross over" cable. Everything is straight through.

CAT5 should work okay (I've got a few GigE links on CAT5) but CAT5e is better.

Chris.

But, since this is a cisco group... there are cases when you do need to make GigE copper cross-over cables when dealing with older Cisco gear's GigE ports that didn't support auto MDI/MDIX.

That's entirely possible, but there aren't many questions too hard for folks here!

Sam

Not so - see for instance , table B-2 and figure B-3.

It's this I'm having trouble with - given the specification of

1000base-T, why would there ever be a crossover cable?

Sam

,

In looking at that reference again it's clear that table B-2 is nonsense (the side 2 pin numbers are wrong and the signal name is wrong for the wrongly labelled pin 2) but figure B-3 might be right.

Sam

Maybe the Cisco parts were developed before the GigE standard stabalized before auto MDI/MDIX was implemented. Maybe Cisco didn't feel like they needed to support it, or the 3rd part company they bought the GBIC parts I've worked with wouldn't support it, but once they started going down that path, they had to follow along.

I don't think its a huge deal overall. There's plenty of odd things in the networking world .

Look at the description of a GigE port and tell me why any kind of MDI/MDIX is needed. AFAICS every port is wired the same and can be connected to any other GigE port with a straight cable.

Sam

Straight Through or "normal" cross over both work. 1000Base-T can do MDI/MDIX by default.

Sam, The reason why "normal" crossover cable will work is that EVERY cat5 (or better) cross-over cable always crossed all eight cables (in pairs). So while only four cables are needed for 100Base-T, very very few cables were made with just four cables.

If you pick up any cross over sold at any retail store (RadioShack, CompUSA, BestBuys etc) you'll see that the pin out on Cisco's website

is how they are built.

If you connect two like ports together the TX and RX lines must be crossed or you end up connecting TX to TX and RX to RX. If one port is MDI and the othe MDI-X the TX and RX pairs are on pins that match up with a straight cable- an MDI port transmits on one set, an MDI-X on the other.

A Gigabit X-Over is backwards compatible, but a old crossover wired for

10/100 usually only crosses 2 pairs of the wire (pins 1+2 and 3+6). In a gigabit crossover all pairs are crossed, because as you say, all pairs are used.

I'm pretty sure I've come across some early Cisco GigE that wouldn't auto MDI-MDIX and had to make a x-over.

Thanks a lot Chris for your message and for the link.

Best Regards Rahan

I have 11 2950T-48 swtiches that do not have auto MDIX and we bought them brand new in April. :-)

Yup. Very true. It gets worse. Ever lookup the command 'auto negotiation' on CCO? It's enough to make your head spin.

What's the difference between MDI and MDIX in 1000base-T? I can't find a definition. I'll ask again, why would there ever need to be any kind of crossover in 1000base-T?

The first part of this is simply not true. Until recently the vast majority of crossover cables we've ever seen (made for 10/100base-T) crossed over the 1-2 and 3-6 pairs but left 4-5 and 7-8 connected straight through. There are many references to this kind of cable online. [1]

I can't find any published pinout for those other suppliers, but the Cisco page is flawed. Table B-2 which claims to show a 10000base-T crossover cable actually shows a straight through cable (with the signal mislabelled on pin 2 side 2). Figure B-3 does show what might be a crossover cable, but that takes me back to my question of why would you bother?

What does MDIX mean on 1000base-T? That an MDIX interface sends and receives databits 0 and 1 on pins pair 3-6 rather than 1-2, bits 2 and 3 on 1-2 rather than 3-6 and similarly for bits 4&5 and 6&7 on pairs 4-5 and 7-8? Why, when if you made all interfaces the same (MDI) they could talk to any other with a straight through cable?

Sam, aka Baffled of Edinburgh

[1] for example , and (but why is the brown/white-brown pair phase-crossed in the first two?)

Not on 1000base-T - all pairs send are bidirectional - there's no TX and RX to crossover. Look it up.

I know what sites like that say, I just don't know why they say it - allowing a crossover cable just makes life more complicated for everyone.

Sam