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A null modem cable.

A crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically (DTE-DCE), by a modified cable called a crosslink.[1] Such a distinction between devices was introduced by IBM.

The crossing of wires in a cable or in a connector adaptor allows:

In contrast, a straight-through cable uses direct wiring to connect complementary devices, e.g. a PC to a switch.

Concept

Straight-through cables are used for most applications, but crossover cables are required in others.

In a straight-through cable, pins on one end correspond exactly to the corresponding pins on the other end (pin 1 to pin 1, pin 2 to pin 2, etc.). Using the same wiring scheme at each end yields a straight-through cable (a given color wire connects to a given number pin, the same at both ends). In this case, the terminations are identical, so only one pinout is required.

In a crossover cable, pins do not correspond – some or all of the conductors are swapped at the terminations. For example, if pin 1 on one end goes to pin 2 on the other end, then pin 2 on one end goes to pin 1 on the other end, and the other pins remain unaffected. Such crossover cables are electrically symmetrical, meaning that they work identically regardless of which way you plug them in (if you turn the cable around, it still connects the same pins as before). Using different wiring at each end yields a crossover cable (a given color wire connects to one number pin at one end, and a different number pin at the other).

Examples

An Ethernet crossover cable
Cable requirement for Ethernet link
To
From
MDI MDI-X Auto MDI-X
MDI crossover straight any
MDI-X straight crossover any
Auto MDI-X any any any

Other technologies

Some connection standards use different balanced pairs to transmit data, so crossover cables for them have different configurations to swap the transmit and receive pairs:

Two pairs crossed, two pairs uncrossed
T1 crossover
Pin Connection 1: T568A
Connection 2: T568B
Pins on plug face
pair color pair color
1 2 Pair 2 Tip
white/orange stripe
1 Pair 1 Ring
blue solid
2 2 Pair 2 Ring
orange solid
1 Pair 1 Tip
white/blue stripe
3 3 Pair 3 Tip
white/green stripe
3 Pair 3 Tip
white/green stripe
4 1 Pair 1 Ring
blue solid
2 Pair 2 Tip
white/orange stripe
5 1 Pair 1 Tip
white/blue stripe
2 Pair 2 Ring
orange solid
6 3 Pair 3 Ring
green solid
3 Pair 3 Ring
green solid
7 4 Pair 4 Tip
white/brown stripe
4 Pair 4 Tip
white/brown stripe
8 4 Pair 4 Ring
brown solid
4 Pair 4 Ring
brown solid

See also

References

  1. ^ Dean, Tamara (2010). Network+ Guide to Networks. Delmar. p. 657.