IEEE 802.3 Frame Format

Preamble The alternating pattern of ones and zeros tells receiving stations that a frame is coming (Ethernet or IEEE 802.3). The Ethernet frame includes an additional byte that is the equivalent of the Start of Frame (SOF) field specified in the IEEE 802.3 frame.

Start-of-Frame (SOF) The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame-reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet.

Destination and Source Addresses The first 3 bytes of the addresses are specified by the IEEE on a vendor-dependent basis. The last 3 bytes are specified by the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single-node) address. The destination address can be unicast, multicast (group), or broadcast (all nodes).

Length (IEEE 802.3) The length indicates the number of bytes of data that follows this field.

Data (IEEE 802.3) After physical-layer and link-layer processing is complete, the data is sent to an upper-layer protocol, which must be defined within the data portion of the frame, if at all. If data in the frame is insufficient to fill the frame to its minimum 64-byte size, padding bytes are inserted to ensure at least a 64-byte frame.

Frame Check Sequence (FCS) This sequence contains a 4-byte cyclic redundancy check (CRC) value, which is created by the sending device and is recalculated by the receiving device to check for damaged frames.

IEEE 802.2 LLC Header

 

Destination Service Access Point (DSAP) - a seven bit address with an eighth bit indicating if it is a specific address (0) or a broadcast address (1). The DSAP is not a station or MAC address; it designates the service control point where the message should be routed.

Source Service Access Point (SSAP) - also a seven bit address; the eighth bit is used to determine if the massage is a command (0) or a response (1). Like the DSAP, the SSAP designates a control point from which the message originated.

Control - Can be 8 or 16 bits long. Lenght is indicated by the first two bits. The 16 bit fields are used to exchange sequence numbers; the 8 bit fields are used for unsequenced information.

Data - the frame's payload.