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.