Ethernet is a vague question, there are dozens of physical implementations. There are copper (single ended and differential), optical, wireless, etc.
Perhaps you are asking about 10base t (or 100base t) over copper, which is quite common. The voltage on the 10base t cable is 0V or +/- 2.5V. 10base t and 100base t are point to point, so it can be (and is) impedance matched at both ends. Since it is impedance matched, it is (neither or both) a current source or a voltage source. It can be described either as +/-5V with 100 ohms in series or +/-50mA with 100 ohms in parallel. 100base t uses +/-1V, same cable.
10base t uses manchester encoding where the position of the rising and falling edges determine a 0 or 1. 100base t uses an encoding method called "8b10b" where 10-bit values are transmitted that correspond to all possible 8-bit symbols. The permissible 10-bit codes all have five ones and five zeros, allowing AC coupling. Manchester encoding requires 16 transitions per 8 bits, but 8b10b is more efficient in terms of bandwidth, requiring only 10 transitions per 8 bits.
I know of no implementation that used 15V, not even back in the early 1980s when the networks were still experimental.