Ethernet Wiring: Straight-Through/Crossover, and Auto-MDIX

In order for anything in your network to function at all, routers and switches or any other networking device in your LAN should be connected with the proper cabling. Doing this right prevents transmission errors, and allows data to be transmitted faster and at longer distances.

RJ-45 Pinouts

Straight-Through Pinouts

It’s important to understand the difference between crossover and straight-through cabling.  the EIA/TIA standards organization defines the cabling specifications for Ethernet LANs and pinouts for the RJ-45 connections.

To put it simply, Straight through cabling should be used to connect two devices that send and receive data on DIFFERENT pins.  For example a PC NIC transmits on pair 1,2 and receives on pair 3,6…switch ports do the opposite they receive on 1,2 and transmit on 3,6.  This is when you use straight through cabling.

Here is a picture of a straight through pinout

straightthrough

Notice the same pairs of wires are used for Tx and Rx on both ends of the cable.  This is used for connections between two different devices that Tx and Rx on different pins. Here are some examples as to when you would use Straight Through cabling pinouts:

st2

Crossover Pinouts

Crossover cabling as you may have already guessed is used to connect two devices that send and receive data on the SAME pins.  For example if a switch port is connected to another switch ports…both ports are essentially the ‘same’ in that they transmit and send data on the same pins on both sides.  This is when you use crossover cabling.

Here is a picture of a crossover cable pinout:

crossover

Notice that pairs 1,2 and 3,6 are swapped on both ends.  This allows two like devices to communicate across the cable to each other without errors.  Here are some examples on when to use crossover cable pinouts:

co2

Auto-MDIX

Automatic Medium-dependent interface crossover (Auto-MDIX) is a feature cisco supports that determines if the wrong kind of cable is installed.  It detects the wrong cable and causes the port to swap the pair it uses for transmitting and receiving…so even if you make the mistake and use a crossover cable between a switch and a PC, the switch port will compensate for the error by swapping the Tx and Rx pins for you.

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