RJ-45, Cat 5 and connecting devices

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This isn't actually a question i've been asked, but just something i thought about myself...

I was reading through the notes on how a NIC will use pins 1 and 2 to transmit data and 3 and 6 to recieve data. And how another device like a switch uses pins 1 and 2 to receive data and 3 and 6 to send data.

With a straight through cable (pin 1 connected to pin 1 on the other side etc...) this works perfectly.

However when 2 NICs / switches want to communicate with each other directly you have to use a crossover cable (pin 1 connected to pin 6, pin 2 connected to pin 3 ... or something).


My question is, why don't all devices use pins 1 and 2 to send and 3 and 6 to receive. That way all devices can be connected together to communicate using just a crossover cable?

Wouldn't that be a bit simpler and save having to have 2 different types of wire?

I'm sure i'll get told why not or i've made a stupid mistake... but cheers in advance

 

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Interesting thought.

I have no idea why this is so off hand. I suspect a forage through the 802.3 archives might turn something up.

If I find anything I'll report.

I may be some time....

Harry.

 

well, all we'd end up doing is swapping the roles of crossover and straight through cable. So what would be the point ? As it is switches are 'transparent' as it were and therefore pass a signal through that can be read by another nic so we crossover internally

 

This article explains it quite well:

"The cross-over (x-over) CAT5 UTP cable has to be one of the most used cables after the classic straight-thru cable. The x-over cable allows us to connect two computers without needing a hub or switch. If you recall, the hub does the x-over for you internally, so you only need to use a straight thru cable from the PC to the hub. Since now we don't have a hub, we need to manually do the x-over.

Why do we need an x-over ?

When sending or receiving data between two devices, e.g computers, one will be sending while the other receives. All this is done via the network cable and if you look at a network cable you will notice that it contains multiple cables. Some of these cables are used to send data, while others are used to receive data and this is exactly what we take into account when creating an x-over cable. We basically connect the TX (transmit) of one end to the RX (receive) of the other !"

So from what I gather the sending devices 'sending' cable pin has to be 'crossed over' to the recieving devices 'recieving' pin. When going through a hub or a switch the crossing over happens there.

However if you are connecting 2 like to like devices you need to manually cross over the send/recieve cables to each devices corresponding pins.

Hope this helps.

 

Very good - but it doesn't quite address the original question!

Harry.

 

If all devices spoke on pins 1 and 2 and listened on pins 3 and 6 then we would ONLY need crossover:

Computer to computer, switch to switch, computer to switch, switch to computer etc...

It's probably just the way it's always been, and to change it would have caused to much confusion and changing devices.

 

er no.. diagram it...
direct connections between 2 devices would be ok, but you're forgetting computer to computer thru a switch....

 

Surely Surfer_Rosa is right Clyde? If switches used the same pin config as a PC then you would need a crossover cable to connect the two. Wouldn't you?

 

er.. hell, its early.. yes, 2 crossovers could, in theory work as the switch also crosses over but...

anyhow, its not the pin config for the device as such, its the cable..

tx cannot go to tx so we either crossover the cable or crossover internally in the switch.

We choose to crossover internally and use the simpler straight thru cable.

using 3 crossovers would probably bugger the EMI resistance of the cable as well as being unnecessarily complicated.

and so far as I know all devices do use pins 13 and 26 for transmit and receive.. its the cables that differ...

 

Er - no. Auto-MDI will interchange the TX and RX pins.

Harry.

 

ok, had forgotten about that.. but then again that was an innovation after the fact, designed to simplify things.

If we go back to basics and look at a hub, it makes sense that the crossover is in the hub and straight through cables connect to it..

anyhow, what does it really matter once we know what cables to use!

 

Right - I've been doing a bit of digging and have come up with an answer of some sort. Note that I like questions like this - it makes me think about fundamentals and I end up learning something.

The answer can be summed up in 3 phrases:
"Ancient history"
"Backwards compatability"
"One size never fits all"

This site has an excelent potted history of the cabling system we have now. The important point here is that RJ45 and the 4 pair cable system was used for telephony long before 10Base-T arrived. That useage needs a straight-through style of cable.

The 802.3 standard specificaly says in 14.1.3.1 (g) Make use of telephone twisted-pair building wiring and telephony wiring practices as an objective. It would appear that AT&T 258A style wiring was in extensive use by then. Details of the wiring standards can be found here .

Finaly - the point of using just one type of cable - there are two types of crossover cable in existance, so that idea is unfortunately not going to fly! In some wiring systems pins 4 and 5 are used for telephony, allowing one cable to carry both. You don't want to cross that with 7 and 8, the remaining pair, which is why the popular cross-over cable only crosses 2 pairs.

However, 1000Base-T requires all 4 pairs to be crossed for a cross-over, and I have actualy seen such cables!

Whew! Does this make some sense as to why things are as they are?

Harry.

 

nice one harry... explains things well.. glad someone had the wherewithall to do that! I kew there was a reason for things being as they were *G*

*smacks self on head re telephone networks*

 

thats because 100Base-T only uses 2 pair, where as 1000TX requires all four pair, hence crossing only the 100Base-T TX and RX pins would not provide the desired result and bugger everything up

 

Cheers for the responces guys.

I initially thought that my way was simpler (and from the outside i guess it does) but i can see it would make things a bit more complicated. Computer-switch-computer would crossover 3 times

...and yeah, i suspected something like using telephone lines origonally had a part to play.

cheers again