The Ethernet Frame Structure and The Ethernet Frame Fields

The Ethernet Frame Structure

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 PDU to encapsulate the message being sent, as shown in Figure 4.3.  Ethernet II is the Ethernet frame format used in TCP/IP networks.

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

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

The Ethernet Frame Fields

The preamble This field has 8 bytes. Start Frame Delimiter (SFD), also called the start of frame( 1 Byte) and the Preamble field has (7 bytes). This field is used for synchronization between the sending and receiving nodes and devices. These first eight bytes of the frame are used to 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

Destination MAC Address Field has 6-byte, the identifier for the recipient. This address is used by Layer 2. The address in the frame is compared to the MAC address of the device. If there is a match, the device accepts the frame. Can be a unicast, multicast or broadcast address.

Source MAC Address Field

Source MAC Address, the MAC address of the outgoing network card. This 6-byte field identifies originating device. 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 a small packet is encapsulated, additional bits called a pad are used to increase the size of the frame to this minimum size.

FCS

The Frame Check Sequence (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame. The receiving device receives the frame and generates a 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.