Why aren't ALL CAT-5 cables crossover cables?

[AudioGeek]

I am an audio/video tech and have also done quite a bit of ethernet wiring, but am a newbie to this field, so I apologize in advance for this question.

I understand that if a CAT-5 cable with RJ-45 connectors is wired 568A on BOTH ends (or 568B on both ends), that it is a "straight-thru" cable and therefore what comes into Pin 1 on one end also comes out Pin 1 on the other end, and so on. If it is wired 568A on one end and 568B on the other, it crosses over the Tx and Rx pairs. Therefore, pin 1 on one end will connect to pin 3 on the other, and so on. I think what I'm confused about is the ports on the devices. Please allow me to use the simplest example I can think of, a PC connected to a cable MODEM, to ask my questions, and lets ignore the "low" or "-" part of the signal on the twisted pairs.

Therefore, if pin 1 on an RJ-45 is Tx+, and pin 3 is Rx+, does this mean that the PC expects to RECEIVE data in via pin 3 and TRASMIT data out via pin 1? And is this also true for the cable MODEM as well?

What I know about signal flow comes from the audio world, where "outputs go to inputs." So why doesn't the output from pin 1 on the computer HAVE to be crossed over to the input on pin 3 of the MODEM?

Are the ports on the computer and modem different?

[Aaron Harrison]

Hi

I see where you're going with that... but basically you are correct when you say that 'outputs go to inputs'.

If you have two PCs back to back, you would use a crossover - thereby making outputs go to inputs.

Typically if you have two switches, you would use a crossover (exceptions being switches with auto-MDIX function to swap the rx/tx pairs internally, or switches with a specific uplink port, often with a push button toggle to do the 'non-auto-MDIX').

If you aren't going back to back, then cables are straight through - here the switch Ethernet port will be wired differently so that inputs and outputs are again meeting up.

A quote from google:

"The ports on a hub to which computer systems are attached are called Medium Dependent Interface-Crossed (MDI-X). The crossed designation is derived from the fact that two of the wires within the connection are crossed so that the send signal wire on one device becomes the receive signal of the other. Because the ports are crossed internally, a standard or straight-through cable can be used to connect devices."

Hope this helps

Aaron

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[scottmac]

It kind of decends from the old serial protocols (DCE/DTE). It basically boils down to:

There's two kinds:

1) Things that plug into something (PC, Routers)

2) the Something that things plug into (Switches, Hubs)

When you are plugging a Thing into Something, it's a straight-through ...

When you are plugging a Thing into a Thing, or a Something into a Something, you have to cross the cables to get the inputs and outputs to line up right.

The producer of the Thing or Something have to decide which side of the fence it's on. In the case of Cable Modems (or DSL modems for that matter - as an example) the various manufacturers have fallen on both sides of the fence ... some decided it was a Thing, some decided it was a Something ... some made it work with either ("Auto-MDI/MDIx").

The bottom line, if I understand your role here, is this: All of the cabling installed as permenant infrastructure ("in the wall")is *** ALWAYS *** installed straight-through, EIA/TIA 568a or 568b, it doesn't matter ... but *** ALWAYS *** straight-through.

All cross-overs, conversions, break-outs, and "specials" are done with jumpers / converters / adapters that go from the wall plate to the device.

The permenant infrastructure must be standard and consistent.

Good Luck

Scott

[AudioGeek]

Thanks very much - both this and the previous post have cleared up the confusion. Essentially, a "transmit" will always go to "receive", as I suspected.