Ethernet Frame Structure

Overview

This lecture explains the structure and function of Ethernet frames, a fundamental component for understanding networking basics, and highlights how data is reliably transmitted over networks.

Data Packets and Ethernet Frames

• A data packet is any set of binary data sent from one point to another on a network.
• At the Ethernet level, these data packets are specifically called Ethernet frames.
• Ethernet frames present information in a rigid structure, allowing devices to interpret transmitted bits meaningfully.

Structure of an Ethernet Frame

• The frame begins with a preamble, eight bytes (64 bits) long, split into alternating bits for synchronization and an SFD (start frame delimiter).
• The SFD indicates that the preamble has ended and actual frame data follows.
• Next is the destination MAC address (6 bytes), identifying the recipient, and the source MAC address (6 bytes), indicating the sender.
• The EtherType field (2 bytes) specifies the protocol of the frame's payload.
• Sometimes a VLAN header appears in place of EtherType, followed by EtherType; VLANs enable multiple logical networks on one physical network.

VLANs and Payload

• VLAN (Virtual LAN) tags frames to create distinct logical networks for different types of network traffic.
• The data payload, ranging from 46 to 1500 bytes, contains data from higher network layers.

Error Checking: Frame Check Sequence

• The frame ends with a 4-byte frame check sequence, which stores a checksum derived from a CRC (cyclical redundancy check).
• CRC is a mathematical process that ensures data integrity by producing a unique checksum for the frame.
• If the receiving device's CRC doesn't match the frame's, the data is discarded as corrupted.

Data Transmission and Integrity

• Ethernet frames include mechanisms for detecting, but not correcting, transmission errors.
• Higher-level protocols are responsible for handling retransmissions if data is lost or corrupted.

Key Terms & Definitions

• Ethernet Frame — Structured unit of data sent at the Ethernet layer.
• MAC Address — Unique hardware address identifying network devices; 48 bits (6 bytes) long.
• Preamble — Initial part of the frame used for synchronization, 8 bytes long.
• SFD (Start Frame Delimiter) — Byte signaling the end of the preamble and the start of frame contents.
• EtherType — 2-byte field specifying the payload protocol.
• VLAN (Virtual LAN) — Technique for segmenting a physical network into multiple logical networks.
• Payload — Actual data being transmitted, excluding headers.
• Frame Check Sequence — 4-byte field for frame integrity checking using CRC.
• CRC (Cyclical Redundancy Check) — Error-detecting code ensuring data integrity.

Action Items / Next Steps

• Review the structure and fields of Ethernet frames.
• Prepare for a deeper dive into network protocols in the next lesson.