The previous post talked about autonegotiation. This time I will talk about cables and pinouts and how auto MDIX works. Although I’m not very old I still like to do it the old school way. I don’t rely on auto MDIX, instead I use the right cable. Lets look at a pinout for T568B:
A regular end device like a PC transmits on pin one and two and receives on pin three and six. Although we have four pairs only two are actually used, unless we are using gigabit Ethernet but that is another topic. A device like a switch does the opposite, it receives on pin one and two and sends on three and six. This is why we use a straight through cable. When connecting similar devices like a switch to a switch we need to use a cross over cable since they want to send on the same pins and receive on the same. So when choosing a cable remember that similar devices requires cross over and different devices needs a straight through.
An engineer at HP developed the auto MDIX standard since he was tired of looking for cross over cables. But how does it work?
The NIC expects to receive Fast Link Pulses (FLP) on pins three and six. If it receives FLPs it will know that the configuration is correct. If it doesn’t receive FLP’s it will switch over to MDI-X mode. This is a very simplified view of it, the process involves different timers and a XOR algorithm. If you want to know more check out the IEEE 802.3 specification section 3, clause 40.4.4.