The Ethernet frame format is the structure used to encapsulate and transmit data over Ethernet networks. It consists of several fields that provide information about the frame, its source and destination, and the actual data being transmitted. Here’s an overview of the Ethernet frame format:
Preamble: The preamble is a 7-byte (56-bit) pattern of alternating 1s and 0s that alerts the receiving device that a frame is coming and helps synchronize the clocks between sender and receiver.
Start Frame Delimiter (SFD): This 1-byte (8-bit) field marks the end of the preamble and indicates the start of the actual frame data. It has a fixed value of 10101011.
Destination MAC Address: A 6-byte (48-bit) field that specifies the MAC address of the destination device to which the frame is being sent.
Source MAC Address: Another 6-byte (48-bit) field that indicates the MAC address of the sender or source device.
EtherType/Length: This field can serve two purposes:
- In Ethernet II frames, it’s a 2-byte (16-bit) field called “EtherType,” which indicates the type of payload carried in the frame, such as IP, ARP, or VLAN tagging.
- In IEEE 802.3 frames, it’s a 2-byte (16-bit) “Length” field indicating the length of the payload in bytes.
Data: This field contains the actual data being transmitted. It can vary in size, depending on the payload.
Frame Check Sequence (FCS): A 4-byte (32-bit) FCS field contains error-checking information computed by the sender and verified by the receiver to ensure data integrity during transmission.
Here’s how the Ethernet frame format is typically visualized:
Preamble (7 bytes) | SFD (1 byte) | Destination MAC (6 bytes) | Source MAC (6 bytes) | EtherType/Length (2 bytes) | Data | FCS (4 bytes)Note that the Ethernet frame format can vary slightly depending on the specific Ethernet type (Ethernet II or IEEE 802.3) and whether additional headers, such as VLAN headers, are present.
Ethernet frames serve as the basic units of data transmission in Ethernet networks. They allow devices to communicate by encapsulating data within a standardized format that can be understood by all devices connected to the network.