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IEEE 802.3 and Ethernet
Ethernet is a set of technologies and protocols that are used primarily in LANs. It was first standardized in the 1980s by IEEE 802.3 standard. IEEE 802.3 defines the physical layer and the medium access control (MAC) sub-layer of the data link layer for wired Ethernet networks.
Ethernet is classified into two main categories: classic Ethernet and switched Ethernet. Classic Ethernet is the original form providing data rates between 3 to 10 Mbps, commonly referred to as 10BASE-X variants. Switched Ethernet uses switches to connect stations in the LAN, replacing repeaters and allowing full bandwidth utilization.
IEEE 802.3 Popular Versions
The most significant versions of IEEE 802.3 protocol include:
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IEEE 802.3 (10BASE-5) − The original standard using thick coaxial cable. Connections are made by drilling into the cable to reach the core. Maximum throughput is 10 Mbps with baseband transmission and maximum segment length of 500m.
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IEEE 802.3a (10BASE-2) − Standard for thin coax where cable segments are connected by BNC connectors. The "2" refers to the maximum segment length of approximately 200m (185m precisely).
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IEEE 802.3i (10BASE-T) − Standard for twisted pair using unshielded twisted pair (UTP) copper wires. Further variations include IEEE 802.3u for 100BASE-TX, 100BASE-T4, and 100BASE-FX.
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IEEE 802.3j (10BASE-F) − Standard for Ethernet over fiber optic cables as the transmission medium.
Frame Format Comparison
The frame formats differ slightly between Classic Ethernet and IEEE 802.3:
| Field | Classic Ethernet | IEEE 802.3 | Purpose |
|---|---|---|---|
| Preamble | 8 bytes | 7 bytes | Synchronization and timing |
| Start Frame Delimiter | Part of preamble | 1 byte | Frame start indication |
| Destination Address | 6 bytes | 6 bytes | Target station MAC address |
| Source Address | 6 bytes | 6 bytes | Sender station MAC address |
| Length/Type | Type (2 bytes) | Length (2 bytes) | Protocol type or data length |
| Data + Padding | 46-1500 bytes | 46-1500 bytes | Payload with minimum padding |
| CRC | 4 bytes | 4 bytes | Error detection |
Key Frame Fields
-
Preamble − Provides synchronization pulses for transmission timing
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Start of Frame Delimiter − Contains alternating pattern ending with two consecutive ones
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Address Fields − 6-byte MAC addresses for source and destination stations
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Data Field − Variable length (46-1500 bytes) carrying upper layer data
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CRC − 4-byte cyclic redundancy check for error detection
Conclusion
IEEE 802.3 standardized Ethernet technology, evolving from thick coaxial cables to twisted pair and fiber optic implementations. The frame format provides essential fields for addressing, data transport, and error detection, forming the foundation of modern LAN communications.
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