On a physical layer Ethernet packet consists of a preamble and a start frame delimiter (SFD). It ends with frame check sequence (FCS), which is 32-bit cyclic redundancy check (CRC). On a link layer it has Ethernet frame with a header and a payload. The header consists of 6-octet destination and source MAC addresses, followed by an optional IEEE 802.1q 4-octet tag (2-octet Tag Protocol Identifier - 0x8100 and 2-octet VLAN number) and a mandatory 2-octet !EtherType field. The maximum size of the Ethernet frame, called the maximum transmission unit (MTU) is 1,500 bytes. Gigabit Ethernet and higher speed standards support larger MTUs, called jumbo frames. The minimum size is 60 octets, not counting the 4-octet FCS.
If !EtherType is equal 1,500 or less, then it indicates the length of the frame. Also, if in this case the payload starts with 0xAAAA, it is the SNAP (Subnetwork Access Protocol) type frame. If the payloads starts with 0xFFFF, it is the Novell type frame. For any other start value, it is the LLC (Logical Link Control) frame.
If !EtherType is 1,536 or higher, it is the Ethernet II type frame and indicates which higher level protocol is encapsulated in the payload. For example, 0x0800 is used for an IPv4 protocol and 0x0806 for ARP. The length of the frame is then determined by the inter-packet gap in combination with the valid FCS. Transmitters are required to send a minimum of 12 octets of idle line state before transmitting the next packet.
On the media the most significant octet is transmitted first, however, within the octet the least significant bit is transmitted first.
Maximum efficiency is achieved with largest allowed payload size (MTU = 1,500) and is:
1,500 / 1,538 = 97.53%
for untagged frames, since the packet size is maximum 1500 octet payload + 8 octet preamble + 14 octet header + 4 octet trailer + minimum inter-packet gap corresponding to 12 octets = 1538 octets. The maximum efficiency for VLAN traffic is:
1,500 / 1,542 = 97.28%
The throughput may be calculated from the efficiency:
Throughput = Efficiency × Network bit rate