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pick1e
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I have always wondered why there are "crossover" and "straight through" network cables!  I was trying to find some information on the net, but can only find info on how to use them.  Maybe somebody here knows?

I know the basic concept, like for going between two NIC's you need the "send" on one end to end up at the "receive" on the other end, hence the crossover, but why not just wire the ports on the hardware (switches, etc.) just like a NIC?  Then you would only ever need crossover cables, right?  There must be some reason for this-
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Commented:
You have it exactly correct.  The hardware themselves have a recieve and send right on them as well.  So they basically do the crossover for you internally by recieving on one side and sending on the other.

Commented:
Straight through cables are much more simple to make since you don't have to worry about anything but making sure both ends are the same.  Since connections between workstations and hubs are much more common, so I think it was a simplicity thing.  Also, if hubs were made as you say, in order to connect hubs you would still need a straight through cable to go hub to hub (or hub to switch or switch to switch).  Granted, hubs often have crossover ports making different cables uneccessary, but not all of them have this feature and switches rarely (if ever) do.  Therefore, different cables are always needed.  As long as you have to know the difference either way, you might as well make the majority easier to make.
deroodeSystems Administrator

Commented:
I believe this stems from the early days of RJ-45 cabling. When UTP did not exist but RJ-45 connectors were used they used cable that was sort of flat cable. 8 conductors lying side by side in a plastic or rubber flat 'tube'. You could just strip the outer plastic, stick the 8 conductors in the RJ45 socket and crimp.

So basically NIC's and HUB's are different because of the easy straight through flat cable method.

Later with higher network speeds (10Mbit) UTP was invented. In this cable conductors are twisted around each other in 4 pairs. Since Send and Receive each have to connect to a pair you get the odd coloring scheme with 1-3 same color and 2-6 same color.

I cannot tell you why the colors in UTP are chosen as is (probably arbitrary)
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Steve JenningsSr Manager Cloud Networking Ops

Commented:
Wait a minute . . . If you made "hubs like nics" you'd use cross-over cables everywhere . . . except when daisy-chaining hubs or switches or connecting two nics back to back, then you'd need a straight-thru cable.



Commented:
Steve - that's exactly what I said.

Steve JenningsSr Manager Cloud Networking Ops

Commented:
Oops, I wasn't quite finished. Pick1e, look up DCE and DTE instead of straight thru and crossover. Data Terminating Equipment and Data Circuit terminating Equipment have vastly different functions and maybe a good explanation of these two terms will give you a better understanding of why the two exist.
Steve JenningsSr Manager Cloud Networking Ops

Commented:
I rarely read what you've said . . . I just sort of skip right past it.

Commented:
Typical Cowboys fan...
Steve JenningsSr Manager Cloud Networking Ops

Commented:
Hey . . . we still have a shot at the Super Bowl . . . if every other team in the NFC suddenly ceases to exist!
Ok, the wires are in pairs.  From the telecom color code the first pair is White/blue and Blue/white.  White is a Tip color and Blue is a Ring color.  Again from old Telecom days when the operators used phono plugs with the tip at the end and the ring on the outside. The second pair is White/orange and Orange/white, amd the third pair is the Green pair - you guessed it: White/green and Green/white.

For 10Base-T and 100Base-Tx we actually only use the second and third pairs, that would be the Orange and Green pairs.  If the switch transmits on the Orange pair then the NIC must receive on the same Orange pair; therefore, it must transmit on the Green pair.  That's simple enough.

If you want two of the same type devices (PC to PC or Switch to Swtich) to talk to each other you can't just connect the transmit to the transmit and the receive to the receive so you have to cross over the transmit to the receive...

Now to answer the last question.  If you made all the ports the same how many cables would get accidently connected between switches.  How many loops would be made by accident?  How many networks would be taken down by the sloppy technician?

I'm glad it takes a crossover cable to do that.

Author

Commented:
Hmmm, still not making logical sense to me.

scraig84 and SteveJ - if every port was built the same, why would you need a different cable to go hub to hub or switch to switch?  say every plug sends on 1/2 and receives on 3/6.  Then when a hub sends data to a NIC, it sends on 1/2, receives on 3/6.  If it sends data to another hub, still sends on 1/2, receives on 3/6?

deroode - if it was a holdover of a convention from flat cable, why is the one pair, the pins 3/6 pair, split up like they are?  I could see if the pairs were 1/2 and 7/8 or 3/4 and 5/6... then if you crimp one connector upside down relative to the other, you'd have a crossover, since 1/2 becomes 7/8 and 7/8 becomes 1/2.  But as it is, say you have "send" on the 1/2 pair, if you flip the one connector, you end up at the 7/8 pair, and your 3/6 ends up back at 6/3... so you'd still have to manipulate the wires into their correct pin numbers.

tmichaelt - Could a need for two totally different types of wires, not to mention the wiring of the hardware, be necessary just to protect the sloppy technician?  I mean, isn't that what the guy gets paid for?  Knowing what cable to plug in where? :)

Commented:
Yeah, now that I think about it a bit more, you are probably right.  If they all send and receive on the same pins then all you would need would be crossovers.  I still think it is from a simplicity and ease of creating the cables standpoint.
Steve JenningsSr Manager Cloud Networking Ops

Commented:
The signal has to be "crossed" somewhere . . . if I receive a signal on 3/6 and then send it on 1/2 then the hub logic has to cross the signal. You'd end up having to employ different logic to communicate between an uplink port and a hub port OR between two hub ports.

Commented:
Right.  But his point is that if all ports were wired to send and receive on the same pins - whether they were hub ports, switch ports, or NIC ports, you would always use crossover cables.  The cross would just always happen in the cable.

Personally though, if you ever have to make cables, it is much easier to make straight-through cables than it is to make crossovers.  Less to keep track of.  It's not that hard to remember to cross between hubs.

Author

Commented:
Actually the original question came into my head when I was making cables at work.  I didn't think it was any harder, I mean you have to separate out the wires anyway...  Just instead of putting the orange pair on 1/2 and splitting green to 3/6 on both sides, you just switch the colors at one end...  So if you're making a crossover, you're doing the exact same thing as making a straight through, just swapping the colors.

I think the hardest part for people would be splitting the green into 3/6 for the straight through cables.  Instead of making the scheme orange/orange, green, blue/blue, green, brown/brown, they could have just made it orange/orange, green/green, blue/blue, brown/brown.  Sending on 1/2 and receiving on 7/8.  Then all cables would be made by just flipping the connector at one end so it was brown/brown, blue/blue, green/green, orange/orange at the opposite end.  Then you've only got one type of cable to make.
I still contend that the design comes from the telecom industry.  The Customer equipment has one pin out and the Network provider equipment has the other pin out.  And yes, in my telecom experience as a telco switch tester many cables get plugged in incorrectly.  Rather than always rely on the person, sometimes its better to come up with a system that makes it harder to make mistakes.

Commented:
I certainly don't pretend to know the exact origin.  It very well may be from the telecom industry.  That certainly wouldn't suprise me.

On the difficulty factor - you're right that you still have to pay attention to what you are doing.  Really, none of this is rocket science by any stretch of the imagination.  However, if I have to sit down and make 10 patch cords, I believe my chances for screwing one up is much higher if they are all crossovers.  I can't just start crimping using a single methodology.  I have to pay attention to each end and make sure I am doing the reverse on the other side.  But hey, that's me.  On the flip side, I have never tried to connect two hubs and sat there dumbfounded as to why they are not linking up with a straight-through cable.  Since you need a cross any time you are connecting two like systems, it is pretty simple in my mind to remember.
Steve JenningsSr Manager Cloud Networking Ops
Commented:
The more I think about it, the more I think you are simply hung up on a semantic issue. "Straight thru", "rollover", and "cross over" are rather odd terms when you look at the cables they represent, and when you think only about ethernet (which this post seems centered on) you omit a large portion of the cabled world that includes modems, muxes, DACs, and repeaters. And just because the pin a signal is communicated toward is called transmit (in a crossover environment) isn't it really "receiving"?
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