Ethernet

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     +-------+       +-------+       +-------+       +--

Coax

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 Ohm resistors at both ends, this yields a voltage of (50 / 2) * -.080 = - 2 Volts.

Twisted Pair

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.

Power over Ethernet

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.