To be extra pedantic, it's serial with an "8P8C modular connector", not over RJ45. Sure, the physical connector is the same thing, but the RJ- standards define

To be extra pedantic, it's serial with an "8P8C modular connector", not over RJ45. Sure, the physical connector is the same thing, but the RJ- standards define the uses of the various pins, not just the physical connector. RJ45, RJ49, and RJ61 all use the same "8P8C modular connector". RJ45S is for only one data line, with a programming resistor. RJ49C is an 8P8C carrying ISDN via NT1, RJ61 is four telephone lines on an 8P8C. There's no RJ standard for RS232 or for Ethernet at all! Everyone (including me) still calls it RJ45 because "8P8C modular connector" is far too long a name.