Why Do Similar Devices Use Cross-Over Cables Instead of Straight-Through Ones?

When linking similar devices to one another, you may wonder why one specific type of cable is used rather than another one. With that in mind, today’s SuperUser Q&A post has the answer to a curious reader’s question.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader user576476 wants to know why similar devices use cross-over cables instead of straight-through ones:

Why do similar devices use a cross-over cable instead of a straight-through cable?
Why do similar devices use cross-over cables instead of straight-through ones?

The Answer

SuperUser contributor Eric F has the answer for us:

Definition of a Cross-Over Cable

A cross-over cable is typically used between devices with the same type of interface (i.e. computer to computer, router to router). Ethernet cables are usually made as an A or B-type interface (which simply means how it is wired).

A cross-over cable simply has an A-type on one end and a B-type on the other end.

What is Happening

Basically, what is happening is that the “send” and “receive” are switched so that one of the devices “send” wires goes to the other device’s “receive” wire, and visa versa with the other wire. In reality, the wires are in pairs, so there are two wires for send and two wires for receive.

If you were to use a straight-through cable (where the wires are all-in-line), then a “send” would be going to a “send” and a “receive” to a “receive”, so the devices would not be able to communicate.

Auto MDI-X

Keep in mind that many modern devices use Auto MDI-X, which is a way for a device to automatically switch the wiring method on its own. If either device on the two ends of the Ethernet cable have Auto MDI-X, then it does not matter if you use a cross-over or straight-through cable. Auto MDI-X was introduced in Gigabit Ethernet, so if either of your devices uses Gigabit, such as routers or computers, it has an extremely high chance of having Auto MDI-X already on it.

Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
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