bits is bits

You are both correct, it's both AC/DC

Ethernet uses both AC and DC voltages, but in different ways: data is transmitted using a form of AC (alternating current), while power over Ethernet (PoE) uses DC (direct current). Data signals are sent as voltage pulses that alternate in polarity, while PoE supplies consistent DC power, typically around \(48V\), to devices like IP cameras and phones

Data transmission
Ethernet data is a series of electrical signals representing binary 0s and 1s.
These signals are sent as voltage pulses that change direction, which is a form of alternating current (AC)

Power over Ethernet (PoE)    

PoE is a technology that delivers DC power over the same Ethernet cable used for data

It is a low-voltage DC system that avoids the need for separate power outlets near devices

The voltage is in the \(44V\) to \(57V\) DC range, with \(48V\) DC being the most common voltage

The power-sourcing equipment (PSE) sends this DC power to the powered devices (PD). 
 

In a nutshell, copper Ethernet sends 2.5V pulses representing ones and zeroes over twisted-pair cables. 

One wire of the pair carries +2.5V pulses

The other wire carries -2.5V pulses

The pulses are identical in pattern but opposite in voltage, so that if the carrier voltage were altered, say by EMI, the difference between opposite pulses would be unchanged at 5V peak to peak, allowing the network interface to correctly differentiate ones from zeroes regardless of how dirty the signal may be. 

That’s the physical, low-level layer. On top of that come advanced protocols such as TCP with their own error-correction protocols.

Note that none of this applies to fiber. Fiber sends pulses of light through a glass medium where light=1 and dark=0, and these light pulses are completely impervious to EMI.

Here is a very good article on twisted-pair Ethernet. Imo it is required reading for audiophiles who have Ethernet components in their systems - in other words, anyone who streams music - and who want to understand which factors can and can’t affect the sound quality of their systems.

https://www.practicalnetworking.net/stand-alone/ethernet-wiring/

I replaced the ethernet cable between my streamer and switch with SFP optical fibre. As far as I can discern, it made absolutely zero difference to sound quality.

 

That’s in a highly resolving system. Both the Linn Klimax DSM/3 streamer and Melco S100 switch have SFP ports and I used the often recommended Finisar optical modules. 

 

Other changes I have made in the past like adding the Melco switch and upgrading the streamer have been beneficial, but the SFP optical fibre wasn’t. Others using the same streamer have reported improvement from changing to fibre. Maybe the Melco is already providing sufficient isolation.

@newton_john that’s most likely the case. A person would expect the multi-thousand dollar Melco switch to have a completely noise-free output, and therefore the fiber run downstream of the switch would become redundant in terms of noise reduction and yield no sonic benefit.

In regard to network altering bits, https://www.google.com/search?client=firefox-b-1-d&q=can+network+alter+bits&udm=50&fbs=AIIjpHxU7SXXniUZfeShr2fp4giZ1Y6MJ25_tmWITc7uy4KIeoJTKjrFjVxydQWqI2NcOhZVmrJB8DQUK5IzxA2fZbQF4YL5sNSRJGgx0e9Z9AxExzjE4_ynshmXB4KOs3cwRUeWWsKhEolaqKblYfDXcX8o44lrMqH67kppK0pOExd7q2yJDRQQKlpEBHmw6_MVnVB63-D4QfZ_FNhtd2-sQaouA4jq5w&ved=2ahUKEwiZ6cTNlsiQAxVBnokEHf3VMcMQ0NsOegQIAxAB&aep=10&ntc=1&mstk=AUtExfDDuUPdWFfnGdINKi92dM-0CQQv45KQzyN3TxSAjuzkjTDZj4cKP_6bIpma1AUzx6NavD42tl140w4xi3Sgkid8-P-eSrD5v4x2PcZaclIqGC-OhQgO0V6i3YnUcz_miFgRRBn8eDnB6u25ZN4KfXbSRDu5G-M2odA&csuir=1. 

Now perhaps AI full of it.

Now perhaps AI full of it.

@sns 

Yes.