MAC frame is a data frame. The so-called data frame is a data link layer protocol data unit, which includes three parts: a frame header, a data portion, the end of the frame. Wherein the header and trailer contain control information necessary, such as synchronizing information, address information, error control information; data part contains the data passed down the network layer, such as ip packet.
At the transmitting end, the data link layer to the network layer data encapsulated in a frame have passed down, up and then sent to the link; At the receiving end, the data link layer data frame is received in and taken out to the network layer. Different data link layer protocols corresponding to different frames, thus, a variety of frames, such as PPP frame, MAC frames, etc., which are not the same specific format.
MAC frame header includes three fields. The first two fields are the 6-byte destination address field length and a source address field, a destination address field contains a destination MAC address information, a source address field contains the MAC source address information. The third field is a 2-byte type field, which contains the information on the sign to use what protocol layer, for receiving an end portion of the data received from the MAC frame to the upper layer protocol. For example, when the value of the Type field is 0x0800, the upper layer indicates it is the IP datagram; if Type field is 0x8137, it indicates that the frame is sent over a Novell IPX.
Data portion of the MAC frame is only one field, a length between 46 to 1500 bytes, the network layer information includes data passed down. Frame end MAC frame is only one field is 4 bytes long, contains the information frame check sequence FCS (using a CRC check).