IEEE 802.3 Frame Structure

A brief picture of each of the fields

• Preamble: Every frame starts with a preamble of 7 bytes and every byte containing the bit pattern 10101010. Manchester encoding is working here and this enables the receiver's clock to synchronize with the sender's and initialise itself.
• Start of Frame Delimiter: This field have a byte sequence 10101011 represent the start of the frame itself.
• Destination Address: This standard allows two byte and six byte addresses. Note that the two byte addresses are always local addresses while the 6-byte ones can be local or it can be global.

2 Byte Address     - Manually assigned address

6 Byte Address     - Every and every  Ethernet card with globally unique address

• Multicast: Sending to set of stations. This is ensured by setting the first bit in either 2-byte/6-byte addresses to 1.

Broadcast: Sending to all an every stations. This can be done by setting all bits in the address field to 1.All Ethernet interface cards(Nodes) are a part of this group.

• Source Address: Refer to Destination Address. Same holds true over here.
• Length: The Length field tells how many bytes are present in the data field, like from a

Min. of 0 to a max. of 1500. The Data and padding jointly can be from 46bytes to 1500 bytes as the valid frames must be at least sixty four bytes long, thus if data is less than forty six bytes the amount of padding can be found out by length field.

• Data: essentially this field can be break up into two parts - Data (0 to1500 bytes) and Padding (0 to 46 bytes).

Reasons for having a min. length frame:

1.  To prevent a station from completing the transmission of a short frame before the first bit has even reached the far end of the cable, where it might be collide with another frame. Here the point is transmission time ought to be greater than twice the propagation time between two farthest nodes.

Transmission time for frame > 2*propagation time between two farthest nodes

2.  When a transceiver detects a collision, it removes the current frame, which implies that lost bits and pieces of frames appear on the cable all the time. Hence to make a distinction between valid frames from garbage, 802.3 states that the least length of valid frames ought to be sixty bytes (from Destination Address to Frame Checksum).

• Frame Checksum: It is a 32-bit hash code of the data. If some bits are incorrectly received by the destination (due to noise on the wire), the checksum computed by the destination end wouldn't match with the checksum sent and therefore the error will be detected. In this the checksum algorithm is a recurring redundancy checksum (CRC) kind. The checksum includes the message from Destination Address to Data field.

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