Ethernet crossover cable

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Ethernet crossover cable

An Ethernet crossover cable is a type of network cable used to connect computing devices together directly where they would normally be connected via a network switch, hub or router. For example, one would use a crossover cable to directly connect two personal computers via their network adapters.



Overview

100BASE-TX Ethernet standards use one wire pair for transmission in each direction. The Tx+ line from each device connects to the tip conductor and the Tx- line is connected to the ring. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. When a terminal device is connected to a switch or hub, this crossover is done internally in the latter. A standard straight through cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.

One terminal device may be connected directly to another without the use of a switch or hub, but in that case the crossover must be done externally in the cable. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This is a crossover cable. A crossover cable must also be used to connect two internally crossed devices (e.g., two hubs) as the internal crossovers cancel each other out. This can also be accomplished by using a straight through cable in series with a modular crossover adapter.

Because the only difference between the TIA/EIA-568-B T568A and T568B pin/pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one connector following T568A and the other T568B. Such a cable will work for 10BASE-T or 100BASE-TX. 1000BASE-T4 which uses all four pairs requires the other two pairs (1 and 4) to be swapped and also requires the solid/striped within each of those two pairs to be swapped.

Other technologies use different pairs to transmit data, so crossover cables for them have different configurations to swap the transmit and receive pairs:

* Twisted pair Token ring uses T568B pairs 1 and 3 (the same as T568A pairs 1 and 2), so a crossover cable to connect two Token Ring interfaces must swap these pairs, connecting pins 4, 5, 3, and 6 to 3, 6, 4, and 5 respectively.
* A T1 cable uses T568B pairs 1 and 2, so to connect two T1 CSU/DSU devices back-to-back requires a crossover cable that swaps these pairs. Specifically, pins 1, 2, 4, and 5 are connected to 4, 5, 1, and 2 respectively.
* A 56K DDS cable uses T568B pairs 2 and 4, so a crossover cable for these devices swaps those pairs (pins 1, 2, 7, and 8 are connected to 7, 8, 1, and 2 respectively).



Automatic crossover NICs

Many newer Ethernet network interface cards (NICs), switches and hubs automatically apply an internal crossover when necessary. This feature is known by various vendor-specific terms, e.g., Netgear calls it Auto uplink and trade, and other common vendor terms include Auto-MDI/MDI-X, Universal Cable Recognition and Auto Sensing. This eliminates the need for crossover cables, obsoletes the uplink/normal ports and manual selector switches found on many older hubs and switches, and greatly reduces installation errors, especially by non-technical users.

Automatic MDI/MDI-X capability is specified in the 1000BASE-T standard, so straight-through cables will work in almost all cases. But it is optional, so a crossover cable is needed if neither of the connected devices supports it, or the function has been disabled. Unlike the crossover cable described above, with only pairs 2 and 3 swapped, a 1000BASE-T crossover cable also has pairs 1 and 4 swapped.

Networks created using crossover cables

A two-computer network, sometimes called a peer-to-peer network, can be created using a crossover Ethernet cable. Like any other network, each computer needs to be assigned a unique IP address. The other machine, in turn, can act as the default gateway (or router), mirroring the same address.

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