How does things work between MODEM and PC on the RJ45 ethernet cable?
I'm assuming you're taking about 100 Mbps Ethernet here, and by modem you mean a cable modem (not an old 56k modems, which probably wouldn't be connected to a computer with an RJ45 cable).

The TX pins are outputs and the RX pins are inputs. The TX pins on one end are connected to the RX pins on the other end. Data to be sent from the computer to the modem would go out of the TX pins on the computer, down the cable, and into the RX pins on the modem. Data to be sent from the modem to the computer would go out of the TX pins on the modem, down the cable, and into the RX pins on the computer. The signals themselves are sent differentially, so whatever is sent on TX+ is sent inverted on TX-, and the receiver looks at the difference between RX+ and RX-.







simulate this circuit &ndash; Schematic created using CircuitLab