There is lots of info in RFCs about TCP/IP and PPP but almost nothing about Ethernet.
Ethernet is Manchester coded;
A `1' is first high and then low, a `0' is the other way around:
| 1 | 0 | 0 | 1 | 0 | 1 | 1 |
0 V ---+ +---+ +-------+ +-------+ +---+
| | | | | | | | |
- 2 V +-------+ +---+ +-------+ +---+ +---
100 Mbps and GBps ethernet don't use manchaster.
An Ethernet packet is has the following structure:
Preamble (clock run in) 62 bits 1,0 SFD (Start of Frame Delimiter 2 bits 1,1 Destination address 6 Bytes Source address 6 Bytes Length 2 Bytes Data 46 ... 1500 Bytes FCS (Frame Check Sequence) 4 Bytes Pre SFD Dest Src Len Data Fcs +------+-----+-----+-----+-----+-------------+-----+ | 62 b | 2 b | 6 B | 6 B | 2 B | 46...1500 B | 4 B | +------+-----+-----+-----+-----+-------------+-----+
A packet is 72 ... 1526 bytes long.
(62 ... 1516 without preamble and SFD).
It takes 58 ... 1221 us.
A packet starts with 10101010 etc;
0 V ---+ +-------+ +-------+ +-------+
| | | | | | |
- 2 V +-------+ +-------+ +-------+ +--
For 10 Mbps Ethernet the period length is 100 ns.
The flank speed is limited to 25 +- 5 ns;
1 0 1 0 1 0 1
0 V --- ------- ------- -------
\ / \ / \ / \
- 2 V ------- ------- ------- --
This means it's a trapezoid shaped pulse;
1 0 0 1 0 1 1
0 V --- --- ------- ------- ---
\ / \ / \ / \ / \
- 2 V ------- --- ------- --- ---
An Ethernet card behaves like a ca - 80 mA current source (or a 80 mA current sink if you like). Since the cable is terminated with 50 Ω resistors at both ends, this yields a voltage of (50 / 2) * -.080 = - 2 Volts.
This signal is slightly differentiated and differential;
+-+ +-+ +-+
---+ | +-----+ | +-----+ | +-----+
| | | | | | |
| +-----+ | +-----+ | +-----+ |
+-+ +-+ +-+ +-
+-+ +-+ +-+ +-
| +-----+ | +-----+ | +-----+ |
| | | | | | |
---+ | +-----+ | +-----+ | +-----+
+-+ +-+ +-+
The signal is fed to the twisted pair via a low pass filter, transformer and common mode coil. The differential voltage is ca 2 ... 2.5 Vpp.
When there is no data the card sends a 100 ns pulse every 16 ms. After two successive pulses the link LED goes on. No signal for 150 ms means the link is down. Fast ethernet uses a more elaborate system.
Since ethernet uses transformers, it is possible to transport power over
ethernet, in the same way ISDN does.
To this end 48 V DC is applied to the centre taps of the RX and TX transformers;
-----+ +--------------- 1
| |
) (
) (
) *-- PWR TX
) (
) (
| |
-----+ +--------------- 2
-----+ +--------------- 3
| |
) (
) (
) *-- PWR RX
) (
) (
| |
-----+ +--------------- 6
Alternatively, power can be supplied over the spare pairs (4-5 and 7-8). Gbps ethernet however, uses all 4 pairs.