|Thread Links||Date Links|
|Thread Prev||Thread Next||Thread Index||Date Prev||Date Next||Date Index|
If anyone is interested in the reasoning (and history) behind this problem, please read on…
Telephony – a single pair is a communications channel, multiple communications channels may be carried in a cable. Therefore, the channel that is on (say) pair-A must always be connected to pair-A regardless of the connector type. What leaves one box on pair-A, arrives at the other box on pair-A – if they have the same connector then it will be the same pins. All cables are straight-through.
Multi-lane communications (think back to RS-232 if you like) – a cable is a communications channel with pairs carrying specified (often directional) functions. What leaves one box as “transmit” must arrive in another box as “receive.” If both boxes have the same connector, with the same pinout definition, then all cables with a single gender must be crossover (i.e. male-male or female-female must be crossover, male-female must be straight-through).
When Ethernet defined use of a twisted pair medium, it leverage cabling that was already defined for telephony even though Ethernet defined a multi-lane communication. This posed problems. For that reason, Ethernet defined two variants for the pinout – MDI was meant to be used for end-stations (including routers) and MDI-X was meant to be used by hubs (repeaters & switches). In the odd situations where someone wished to connect two switches together, or wished to connect an end-station directly to a router, then a crossover cable would be required. Eventually, the concept of auto MDI/X was added to the standard as an option but it has since become a basic requirement for any PHY design. There are still some systems in the field that do not support auto-MDI/X but, thankfully, no one ever tries to connect two of them together!
Because of this history, it is not easy to predict where straight-through or crossover connection may be. Almost all equipment works with either and so people do not bother to pay attention.
I hope this helps to put the discussion in context.