The Ethernet Frame Structure and Fields

The basic frame structure is defined in the IEEE 802.3 standard. However several optional formats are being used to extend the protocol’s basic capability. The Early versions of Ethernet were comparatively slow. The latest versions of Ethernet operate at 10 Gigabits per second. This is the fastest version of Ethernet. At the data link layer, the frame structure is almost the same for all speeds of Ethernet.

The Ethernet frame structure adds headers and trailers around the Layer 3 Protocol Data Unit (PDU) to encapsulate the message.  Ethernet-II is the Ethernet frame format uses in TCP/IP networks.  It starts with Preamble which works at the physical layer. Ethernet header contains both Source and Destination Media Access Control address, after which the payload of the frame is present. The last field is CRC (Cyclical Redundancy Checking) which is used to detect the error. The figure below illustrates the frame structure.

The minimum frame size is 64 bytes and the maximum size is 1518 bytes. This includes all bytes from the Destination MAC Address field through the  Frame Check Sequence (FCS) field excluding Preamble field. The Preamble field is not included when describing the size of a frame.

Every less than 64 bytes in length has considered a “runt frame” or “collision fragment” and the receiving stations automatically discard these frames and frames with more than 1500 bytes of data considered “jumbo” or “baby giant frames”. If the frame is less than or greater than above mention size, the receiving device drops and discard the frames.

Preamble and SFD Fields

This preamble field has 7 bytes and Start Frame Delimiter (SFD) field has 1 byte. The Start Frame Delimiter (SFD), also called the start of frame( 1 Byte) and the Preamble field has (7 bytes). Sending and receiving nodes and devices use both fields for synchronization. The first eight bytes of the frame acquire the attention of the receiving nodes. These first few bytes tell the receivers to get ready to receive a new frame.

Destination MAC Address Field

The Destination MAC Address Field has 6-byte, the identifier for the recipient. The address in the frame and the MAC address of the device is compared. If there is a match, the device accepts the frame. The MAC address can be a unicast, multicast or broadcast.

Source MAC Address Field

Source MAC Address, the Media Access Control address of the outgoing network interface card. This 6-byte field identifies the originating device. It must be a unicast address.

Ether Type Field

This field size is 2-byte. This field identifies the upper layer protocol encapsulated in the Ethernet frame. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.

Data Field

This field is containing the original encapsulated data from a higher layer. This field size is 46 – 1500 bytes. All frames must be at least 64 bytes long. If an encapsulated packet is small in size, additional bits called a pad increase the size of the frame to this minimum size.

FCS

The Frame Check Sequence (4 bytes)  detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a cyclic redundancy check (CRC) in the Frame check sequence (FCS) field of the frame. The receiving device receives the frame and generates a cyclic redundancy check (CRC) to look for errors. If the calculations match, no error occurred. Calculations that do not match are an indication that the data has changed, therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.