MAC frame format

The following illustrates the format of an Ethernet frame as defined in the original IEEE 802.3 standard:

Preamble (7-bytes)

Start Frame Delimiter (1-byte)

Dest. MAC Address (6-bytes)

Source MAC Address (6-bytes)

Length / Type (2-bytes)

MAC Client Data (0-n bytes)

Pad (0-p bytes)

Frame Check Sequence (4-bytes)

Preamble:

A sequences of 56 bits having alternating 1 and 0 values that are used for synchronization. They serve to give components in the network time to detect the presence of a signal, and being reading the signal before the frame data arrives.Each byte will be 0x55

Start Frame Delimiter:

A sequence of 8 bits having the bit configuration 10101011 that indicates the start of the frame.

Destination & Source MAC Addresses:

The Destination MAC Address field identifies the station or stations that are to receive the frame. The Source MAC Address identifies the station that originated the frame. The 802.3 standard permits these address fields to be either 2-bytes or 6-bytes in length, but virtually all Ethernet implementations in existence today use 6-byte addresses. A Destination Address may specify either an "individual address" destined for a single station, or a "multicast address" destined for a group of stations. A Destination Address of all 1 bits refers to all stations on the LAN and is called a "broadcast address".

Length/Type:

If the value of this field is less than or equal to 1500, then the Length/Type field indicates the number of bytes in the subsequent MAC Client Data field. If the value of this field is greater than or equal to 1536, then the Length/Type field indicates the nature of the MAC client protocol (protocol type).

MAC Client Data:

This field contains the data transferred from the source station to the destination station or stations. The maximum size of this field is 1500 bytes. If the size of this field is less than 46 bytes, then use of the subsequent "Pad" field is necessary to bring the frame size up to the minimum length.

Pad:

If necessary, extra data bytes are appended in this field to bring the frame length up to its minimum size. A minimum Ethernet frame size is 64 bytes from the Destination MAC Address field through the Frame Check Sequence.

Frame Check Sequence:

This field contains a 4-byte cyclical redundancy check (CRC) value used for error checking. When a source station assembles a MAC frame, it performs a CRC calculation on all the bits in the frame from the Destination MAC Address through the Pad fields (that is, all fields except the preamble, start frame delimiter, and frame check sequence). The source station stores the value in this field and transmits it as part of the frame. When the frame is received by the destination station, it performs an identical check. If the calculated value does not match the value in this field, the destination station assumes an error has occurred during transmission and discards the frame.