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.

Contents

Overview

Crossover cable pinouts

Other networking technologies

Automatic crossover NICs

Networks created using crossover cables

External links

Overview

The 10BASE-T and 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 (Gigabit crossover) 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.

Crossover cable pinouts

Two pairs crossed, two pairs uncrossed
10baseT/100baseTX crossover ( shown as T568A )

Pin	Connection 1 pair	Connection 2 pair	Connection 1	Connection 2	Pins on plug face (jack is reversed)
1	3	2	Pair 3 Tip white/green stripe	Pair 2 Tip white/orange stripe
2	3	2	Pair 3 Ring green solid	Pair 2 Ring orange solid
3	2	3	Pair 2 Tip white/orange stripe	Pair 3 Tip white/green stripe
4	1	1	Pair 1 Ring blue solid	Pair 1 Ring blue solid
5	1	1	Pair 1 Tip white/blue stripe	Pair 1 Tip white/blue stripe
6	2	3	Pair 2 Ring orange solid	Pair 3 Ring green solid
7	4	4	Pair 4 Tip white/brown stripe	Pair 4 Tip white/brown stripe
8	4	4	Pair 4 Ring brown solid	Pair 4 Ring brown solid

Certain equipment or installations, including those in which phone and/or power are mixed with data in the same cable, may require that the "non-data" pairs 1 and 4 (pins 4, 5, 7 and 8) remain un-crossed.
When a crossover cable is used to connect two routers or switches, some units, especially older ones, will work with two-pairs-crossed cables or four-pairs-crossed cables, but not both. Prudent technicians keep both kinds of crossover cables on hand.

'Gigabit crossover'
All four pairs crossed
10base-T/100base-TX/1000base-TX/T4 crossover (shown as T568B)

Pin	Connection 1 pair	Connection 2 pair	Connection 1	Connection 2	Pins on plug face (jack is reversed)
1	2	3	Pair 2 Tip white/orange stripe	Pair 3 Tip white/green stripe
2	2	3	Pair 2 Ring orange solid	Pair 3 Ring green solid
3	3	2	Pair 3 Tip white/green stripe	Pair 2 Tip white/orange stripe
4	1	4	Pair 1 Ring blue solid	Pair 4 Tip white/brown stripe
5	1	4	Pair 1 Tip white/blue stripe	Pair 4 Ring brown solid
6	3	2	Pair 3 Ring green solid	Pair 2 Ring orange solid
7	4	1	Pair 4 Tip white/brown stripe	Pair 1 Ring blue solid
8	4	1	Pair 4 Ring brown solid	Pair 1 Tip white/blue stripe

It really does not matter if your Ethernet cables are wired as T568A or T568B, just so long as both ends follow the same wiring format. In other words it is just as valid to make a four-pair crossover using T568A, or a two pair crossover using T568B, as it is to wire them the way shown here.
Typical commercially available "pre-wired" cables can, (and often do), follow either format depending on who made them.
What this means is that you may discover that one manufacturer's cables are wired one way and another's the other way, yet both are "correct" and will work.
In either case, T568A or T568B, a normal (un-crossed) cable will have 'both' ends wired according to the layout in the first connections column.

Other networking technologies

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 02 and 04, so a crossover cable for these devices swaps those pairs (pins 01, 02, 07, and 08 are connected to 07, 08, 01, and 02 respectively).

Automatic crossover NICs

Almost all 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.

Networks created using crossover cables

Example crossover network configuration

	Machine 1	Machine 2
IP	192.168.0.1	192.168.0.2
Subnet Mask	255.255.255.0

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. In this network configuration, a default gateway is not used and can be unspecified.

External links

★ How to wire Ethernet Cables, with pinouts for 10-1000BASE-TX/T4 as well as general explanatory text on the various standards, POE (Power Over Ethernet), and other useful information.

★ Crossover cable pinout (10/100base-TX)

★ Pinouts For 100BASE-TX and 1000BASE-T

★ Pinouts For 100BASE-TX and text on 1000BASE-T

★ LAN Wiring on 10base-T, 100base-TX/T4, 1000base-T from Zytrax.com

★ What are Straight and Crossover cable