802.3 Straight-through VS Crossover

Posted on 2003-11-05
Last Modified: 2009-02-17

I was just wondering why there were two types of cables.
I mean, I know Router to Router and Computer to Computer need Crossover and the usually the rest need Straight-through but
why is it that way.
Pin 1 and 2 on a computer are meant to send bytes. 3 and 6 to reveive. So a router or switch will listen on 1 and 2 (since
that's where the computers send). And then, they will send on 3 and 6 because that's were computers expect to receive a signal.
But aren't the routers, like computers, expect a signal on 3 & 6 and would then send it on 1 and 2?
Is there a reason? Did I miss something?

Question by:lopo42

Expert Comment

ID: 9690346
Imagine this.  You have two computers directly connected with a cable (no hub or switch in between them) via a straight-through cable.  If computer one transmits some data on pins 1 and 2, the cable would deliver it to pins 1 and 2 on computer two.  However, computer 2 is only listening on pins 3 and 6, so it never receives the transmission.

By crossing the cable over, you are taking the transmit pairs, and plugging them to the receive slots on the other end.  That way, when computer 1 transmits data on pins 1 and 2, they arrive on pins 3 and 6 when it gets to computer 2.

When a hub or switch is thrown in the mix, you don't need to cross the cables.  This is because the hub/switch actually does the cross-over for you itself.

I hope this helps answer your question.

Good Luck!


Author Comment

ID: 9690981

Thanks for your answer.
I didn't make myself clear, my fault.
The point is: Why aren't there only crossover cables!!?

I mean your computer sends on 1&2 and receives on 3&6. Because it sends on 1&2 and it is connected with a STRAIGTH-THROUGH cable to a swith/hub, the hub/switch has to listen on
1&2. But, Why didn't they think: Ok the hub must receive on 3&6 (like a computer) so we need a CROSSOVER cable between a switch and a hub. And therefore only
corsscables would be used because  a  switch/hub would listen (like a PC) on 3&6 and no more on 1&2.
I still don't see a reason....why this has be done like that. I do understand that NOW, because things (hub/switch/computer) work that way, you need those different cables, but why didn't they just use a crossover cable and making a switch listen on pin 3&6 like a computer!?

Hope this is clearer...


Expert Comment

ID: 9691231
OK.  Let's think this through.  If the switch doesn't do any cross-over functions for you, and both computers use cross-over cables, then you end up with a straight-through cable anyway.  For instance, Computer 1 sends a packet via pins 1 and 2.  The cable crosses it and delivers it to the switch on pins 3 and 6.  The switch delivers the signal to pins 3 and 6 on cable 2.  It is crossed again and is delivered to the transmit pins (1 and 2) on Computer 2.  Obviously, that's no good since the signal needs to arrive on pins 3 and 6.

So, our choices are to either only use 1 cross-over cable in the connection, or to cross the signals at the switch.  If we went the "use only 1 cross-over cable per connection" way, how would we handle 3 computers that need to talk to each other?  2 computers are going to have like cabling (either straight-through or crossed), so those two computers will never be able to talk to each other.  That's no good.

If you cross the signals at the switch, then why use cross-over cables at all?  Cross-over cable, then cross-over at the switch, then cross-over again.  Kind of complicated.  Not to mention that cross-over cables are probably harder to produce than straight-through cables, and are conceptually harder to teach to new people.  The simplest design is the use straight-through cables that will work with any standard (e.g. Ethernet and T1) and have the switches and hubs cross the connections for you.

Anyway, I hope that clears some things up.  Let me know if you need any clarification or have any questions.  =)

-- Matt

Author Comment

ID: 9691427
Hi again :)

Well, yes, it's your last point that I had in mind. Switches/hub have to crossover no matter what.... but yes the only reasons that I see, NOT TO DO IT LIKE THAT, is like you said
1) conceptually maybe it's harder to understand, although I don't find it particularly difficult but it would be easier in real life. For instance, you don't have to figure out if you need a croosover or not. If there are only crossovers, there is no question, you just take it whether you interconnect two switches/ two computer/ or a computer and a switch :)
2) it's maybe cheaper to produce, although I'm not quite convinced, too.

So, I was hoping there was a more subtile reason :)

But, anyway, thank you for your comments!!

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Expert Comment

ID: 9694806

computing and networking is more about conventions than logics: e.g. all your cables have 8 wires, but only use the 4.

Like on the internet: the protocol is 7-bit, so because we mostly use 8-bit data nowadays, everything must be converted in each end, causing a lot of overhead.

Problem is, if a company tries to change these conventions, every-one else gets suspicious and publish a rivaling standard which they claim to be better. Worse: if some government institution suggests standardising things, the companies get really annoyed!

(PC-)computing and internet may seem like a standardised area, but in reality it's an amazing experiment that works despite the anarchy of the corporations. The same corporations who fight any sign of private anarchy with all their force.


LVL 12

Accepted Solution

public earned 20 total points
ID: 9707786
when two crossover cables are connected in series, as when goimg through a patch panel, the result would no longer be a crossover. You would have to count the number of cables patched.
LVL 16

Expert Comment

ID: 10084275
It's not as arbitrary as it somehow sounds on this thread. When you connect a DTE device to a DCE device -- such as a computer to a swtich -- you use a straight thru cable. When you connect 2 DTE devices or 2 DCE devices directly, you use a crossover cable. If all cables were straight through cables you wouldn't be able to connect 2 DTE devices back to back or 2 DCE devices back to back.

Expert Comment

ID: 10337242
Just another point,

Cat5 Patch cables are used for more than just computer networking, Structured cabling systems in offices send the phones through them too, some use different pairs, so if you use a straight through cable at the patch panel end it gurantees what ever you are trying to connect will work rather than having to have different patch cables for different things

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