IEEE 802.3 MAC Frame and Address Format
Following is the IEEE 802.3 MAC frame format with the associated fields. The numbers within each field indicate the length of the field in octets or bytes. The IEEE 802.3 frame format is described in Figure 3.2.
Figure 3.2 IEEE 802.3 frame format containing IEEE 802.2 LLC Protocol Data Unit (PDU).
The preamble field allows the hardware to establish the frame timing. The Start of Frame (SOF) is set to a bit pattern of 10101011 to delimit the start of the IEEE 802.3 frame. The destination and source MAC addresses follow the format described in Figure 3.3.
Figure 3.3 MAC address format including I/G Address Bit, U/L Address Bit, and OUI.
There is a minimum and maximum size for an Ethernet frame. The frame size starts at the first octet of the destination MAC address until the last octet of the FCS. The minimum frame size is 64 octets. If the frame is less than the minimum, padding is inserted after the last octet of data and before the FCS to bring the frame up to 64 octets. The maximum frame size is 1,518 octets.
The length field describes the size of the data after the length field and before the FCS. This is also called the transmission unit. There is a Maximum Transmission Unit (MTU) size of 1,500. This transmission unit also includes an IEEE 802.2 LLC Protocol Data Unit (PDU).
The MTU for Ethernet is 1,500 octets. The minimum frame size is 64 octets, and the maximum frame size is 1,518 octets.
LLC provides an interface to the higher levels of the OSI/RM via a Service Access Point (SAP). The SAP is a one-octet field used to direct the frame within the internal software of the station to the appropriate process or application. The LLC PDU contains a destination and source SAP and a one- or two-octet control field.
The FCS is a Cyclic Redundancy Check (CRC) to validate the integrity of the data transmitted.