IEEE 802.3 (Ethernet) MAC frame

Picture 1: IEEE 802.3 (Ethernet) MAC frame


The IEEE 802.3 standard is so called Ethernet LAN network. The MAC frame for this standard is shown on Picture 1. The length of the frame can vary from 64 up to 1518 bytes. The first 7 bytes of the frame are reserved for the field Preamble. The Preamble field consists seven octets of bit array: 10101010. The next field in the frame is SD - Start Delimiter and this field is 1 byte long and it consists the bit array: 10101011. The next two fields in the frame are reserved for the DA - Destination Address and SA - Source Address. Both of these fields can be 2 or 6 bytes long. The first two bits in these fields define the type of the addresses. The meaning of the bits are:

I bit:

> 0 - Single address;
> 1 - Group address;

II bit:

> 0 - Local address;
> 1 - Global address.


The field that follow the address fields in the frame is reserved for the Length of the frame and its 2 bytes long. After this field comes the Information field, then Pad field, and finally, the frame ends with FCS - Frame Check Sequence field, which is 4 bytes long.