Standards/Ethernet

Ethernet is standardized as IEEE 802.3. It is used for connecting computers to each other via the Web, and it gets more and more used for connecting embedded systems to the Web.

The most known is Internet Protocol version 4 (IPv4), the fourth revision in the development of the Internet Protocol (IP). Internet Protocol Version 6 (IPv6) is designed to succeed Internet Protocol version 4 (IPv4). IPv6 uses a 128-bit address, whereas IPv4 only uses 32 bits.

Frame structure

An Ethernet frame structure is shown next:

802.3 Ethernet frame structure

Preamble	Start of frame delimiter	MAC destination	MAC source	Ethertype or length	Payload	Frame Check Sequence (FCS)
7 bytes of 1010 1010	1 byte of 1010 1011	6 bytes	6 bytes	2 bytes	46–1500 bytes	4 bytes

The FCS is a 32 bit CRC.

Frames are separated by an interframe gap of 12 bytes.

The maximal frame length, from the MAC addresses to the FCS is of 1518 bytes.

Example: ping request

Frame data

The following data shows the Wireshark display of a ping request frame:

F0 4D A2 33 B7 EF 00 25 64 C2 6F 6A 08 00 45 00
00 3C 31 98 00 00 80 01 CA DA 99 6D 05 E0 99 6D
05 94 08 00 D5 5B 04 00 74 00 61 62 63 64 65 66
67 68 69 6A 6B 6C 6D 6E 6F 70 71 72 73 74 75 76
77 61 62 63 64 65 66 67 68 69

Wireshark does not display the preamble and the FCS. From the frame we read:

• the destination MAC address

F0 4D A2 33 B7 EF

• the source MAC address

                  00 25 64 C2 6F 6A

• the Ethertype (IP)

                                    08 00

• the payload

                                          45 00
00 3C 31 98 00 00 80 01 CA DA 99 6D 05 E0 99 6D
05 94 08 00 D5 5B 04 00 74 00 61 62 63 64 65 66
67 68 69 6A 6B 6C 6D 6E 6F 70 71 72 73 74 75 76
77 61 62 63 64 65 66 67 68 69

MII data sequence

A 100BaseT MII has a 4 bit data interface. The bytes are transferred in a sequence of 2 nibbles, with the least significant nibble first.

The MII data sequence for the preceeding ping request will start with the preamble:

55 55 55 55 55 55 55 5D

followed by the MAC addresses:

0F D4 2A 33 7B FE 00 52 46 2C F6 A6 ...

and so on for the rest of the frame which ends with the last data nibbles:

  ... 86 96

and finish with the FCS:

36 7A AE 28