someone please explain crossover cables

[khold]

I am curious about why crossover cables are needed. I understand how they are used, such as how they are needed when directly connecting two computers together with ethernet, but I don't get what the purpose of the "crossover" is for. Any explanation is welcome.

Thanks

 

[Cypher11]

A Crossover cables imitates a hub, any signal that goes into a hub is crossed over at the entry point, thats why without a hub a crossover cable is needed

 

[chiwawa626]

u can make them out of a regular ethernet cable, i did and it worked great for networking 2 computers directly, i use it to transfer crap from my old computer to new one

 

[Freeze]

whenever you have a LAN card (or NIC, whatever you want to call it), there needs to be a way to send and recieve data. Cat5 cable has two wires (actually more, but they aren't really important for this discussion), one for sending and one for recieving. Normally, the don't cross over, ie the 'red' wire is one pin 1 and green is on pin 7 on one end, and they are on the same pins on the other end. If you are doing peer to peer, no hub then the send and recieve wires must swap, red on pin 1, green on pin 7 on one end and red on 7, greenon 1 on the other. Confused yet? To continue, this is necessary because you can't recieve data on the pin you are trying to send through. You will just confuse the card. So it's like driving the wrong way on a one way street. The cross over cable connects the xmit on one card to the recieve on the other and vise-versa. I think that should explain it. If not let me know.

edit: just a quick note about hubs. they automatically do this cross over internally, thus the need for a cross over cable is eliminated.

 

[Soccerman]

heh yeah Freeze has it down, though I don't remember the wire combination for them (the two normal standards for wiring are 802.3a and 802.3b I think).

Actually when running full duplex 100BaseTx ethernet (aka fast ethernet, runs at 100mbps), you DO use 2 other wires, the others I think are to help keep the signal clean (less crosstalk I think). either that, or I'm spouting jibberish!

a straight through cable is good for connecting your NIC in your computer to a hub (or switch, or router) and then connecting that to another NIC. it (a hub, switch, or router) flips some of the wires automatically (again, I don't know the details) so that the send wire goes to the recieve wire on the NIC, and vice versa.

when you connect 2 NIC's together directly, you need to have a cable to flip those wires around.

the reason for this flip over, is so you don't have to have this kind of thing on a NIC instead of on the cable (imagine having to get 2 types of NICs! much more economical to just flip the wire in the cable instead!)

 

[Antisocial Virge]

Helpful link
http://www.duxcw.com/digest/Howto/network/cable/cable1.htm

 

[Freeze]

NOTE: The pins I referenced above are just examples. I don't know exactly what pin goes where. The link Antisocial-Virge provided is pretty good.

There are 8 total wires I belive, 4 of which are used. The other 4 form a "Twisted Pair" which basically is 2 wires twisted around each other to reduce line noise and crosstalk like you said Soccerman.

 

[Spook]

10Mbps doesn't use all 8 wires, but 100Mbps does....

 

[jeans2nd]

Best explanation I've ever heard. I Finally understand !!

 

[Freeze]

Actually according to the link A-V posted. both only use 4 wires.



<< Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard. Quite contrary to what you may read elsewhere, these pins and wires are not used or required to implement 100BASE-TX duplexing--they are just plain wasted. >>



http://www.duxcw.com/digest/Howto/network/cable/cable5.htm

 

[youseelabruin]

i am a little confused, so &quot;cat5&quot; cable comes in two flavors? 10bt and 100bt? i thought all cat5 cable could do either. i just bought a crossover over cable that had the description &quot;10bt&quot;, does that mean thats the max speed on this cat5 cable? i wanted to be able to do 100mbps

 

[Soccerman]

&quot;10Mbps doesn't use all 8 wires, but 100Mbps does.... &quot;

absolutely not. I'm positive of this now. 100mbps half duplex only takes 2 wires, 100mbps Full duplex takes 4 wires..

in half duplex, 1 wire connects both computers, the second wire AFAIK is just to return electricity (complete the circuit). in full duplex, you have 2 wires communicating, and 2 wires returning electriciy (completing the curcuit) the rest of the wires are already in use by reducing cross talk and noise (through the twist).

I don't think the return wire and the transmition wire are twisted around each other, I think the return wire is twisted around it's own seperate wire. same goes with the transmission wire. thus, all 8 wires ARE utilized in their own way.

 

[dszd0g]

Something to keep in mind as far as CAT 5. If you crimp the cable yourself and don't know what your doing it probably won't be CAT5 anymore. The CAT 5 standard is very strict on exact lengths of the cable and how far the ends can be stripped and how it is crimped. I have tested the ones you buy vs. the ones I've made, the ones that are manufactuered on the assembly lines usually hit higher speeds. When I'm really careful about making the cables to spec I can hit the spec fairly closely though, it just takes patience and practice. I used to have a print out of the spec next to my spool, but I haven't had to make cables recently.

 

[Warrenton]

I buy all my cables. Of course I buy them at a wholesaler locally. a 7ft CAT5e straight (568B wired) is $0.87, and crossover is $0.97. Basically I pay the cost of the materials plus a few cents for the overhead, They are perfect to spec.