Ethernet: Framing

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.

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