Ethernet Frame Format
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The Ethernet frame is the fundamental unit of data transmission in Ethernet networks. It encapsulates the data to be transmitted along with control information needed for delivery. The Ethernet frame format has evolved over time, with the most commonly used version being based on the IEEE 802.3 standard. Here's a breakdown of the Ethernet frame format:
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Preamble:
The preamble is a sequence of alternating 1s and 0s used for synchronization and timing recovery. It consists of 7 bytes (56 bits) with a pattern of 10101010.
Start Frame Delimiter (SFD):
The SFD is a one-byte (8 bits) field that marks the end of the preamble and the beginning of the frame. It has a fixed value of 10101011.
Destination MAC Address:
The Destination MAC Address field is 6 bytes (48 bits) long and specifies the MAC address of the intended recipient of the frame.
Source MAC Address:
The Source MAC Address field is also 6 bytes (48 bits) long and contains the MAC address of the sender of the frame.
Type Field:
For Ethernet II frames, this field is 2 bytes (16 bits) long and indicates either the length of the data field or the type of protocol encapsulated in the frame. For IEEE 802.3 frames, this field is used to specify the length of the data field.
Data Field:
The Data Field carries the actual payload data to be transmitted. Its size can vary from 46 to 1500 bytes, excluding the Ethernet header and trailer.
Frame Check Sequence (FCS):
The FCS is a 4-byte (32 bits) field used for error checking. It contains a cyclic redundancy check (CRC) value computed over the entire frame (excluding the preamble and SFD).
The Ethernet frame format provides a structured way to transmit data over Ethernet networks reliably. Each field in the frame serves a specific purpose in ensuring that the data is delivered correctly to the intended recipient and can be processed by the receiving device.
The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of 1500 bytes. The term maximum transmission unit (MTU) defines the maximum Layer 3 packet that can be sent over a medium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is the largest IP MTU allowed over an Ethernet.