The original Ethernet specification was owned by Digital Equipment Corp, Intel, and Xerox..hence the name Ethernet DIX.
Later in the early 1980’s the IEEE standardized Ethernet, defining parts of Layer 1 and some Layer 2 in the 802.3 Media Access Control (MAC) standard, and other parts of Layer 2 in the 802.2 Logical Link Control (LLC) standard.
Later the IEEE realized that the 1 byte Destination Service Access Point (DSAP) field in the 802.2 LLC header was too small. The IEEE then introduced a new frame format with the Sub-Network Access Protocol (SNAP) header after the 802.2 header.
Descriptions of each of the fields in the header:
- Preamble (DIX) -Allows synchronization and signal transitions for proper clocking of the transmitted signal. This header contains 62 alternating 1s and 0s, and ends with a pair of 1s. ex: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011
- Preamble and Start of Frame Delimiter (802.3) – Also used for synchronization and signal transitions for clocking, however 802.3 simply renames the 8byte DIX preamble as a 7byte preamble with a 1byte Start of Frame Delimiter (SFD).
- Type (or Protocol Type) (DIX) – 2 byte field that identifies the type of protocol or protocol header that follows the header. This allows the receiver of the frame to know how to process a received frame.
- Length (802.3) – This field describes the length in bytes of the data preceding the Length field up to the Ethernet trailer. This allows a receiver to know when the end of the frame is received.
- Destination Service Access Point (DSAP) (802.2) – This is a 1 byte protocol type field. the size limitations of a 1 byte field along with other uses of low order bits required the later addition of SNAP headers.
- Source Service Access Point (SSAP) (802.2) – This is a 1 byte protocol tyupe field that describes the upper layer protocol that created the frame.
- Control (802.2) – This is a 1 or 2byte field that provides mechanisms for connectionless and connection oriented operations. Modern protocols generally only use this for connectionless operation with a 1byte value of 0x03
- Organizationally Unique Identifier (OUI) – this is a 3 byte field used to determine the manufacturer of a network device (ie: Ethernet NIC)
- Type (SNAP) – This is a 2byte field using the same values as the DIX type field creating a larger space with increased number of bits and use of the DSAP field.