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What is Classic Ethernet MAC Sublayer Protocol?
Classic Ethernet is the original form of Ethernet used primarily in LANs, providing data rates between 3 to 10 Mbps. It operates both in the physical layer and in the MAC (Media Access Control) sublayer of the OSI model. In the physical layer, the features of cables and networks are considered, while in the MAC sublayer, the frame formats for Ethernet data transmission are defined.
Classic Ethernet was first standardized in the 1980s as the IEEE 802.3 standard, establishing the foundation for modern Ethernet technologies.
Frame Format of Classic Ethernet
Classic Ethernet frames can be either Ethernet (DIX) or IEEE 802.3 standard format. The frames of both standards are very similar except for one key field difference.
Main Frame Fields
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Preamble − Provides alert and timing pulse for transmission. 8 bytes in Ethernet (DIX), 7 bytes in IEEE 802.3.
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Start of Frame Delimiter (SOF) − A 1-byte field in IEEE 802.3 frames containing an alternating pattern of ones and zeros ending with two ones.
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Destination Address − A 6-byte field containing the physical address of the destination station.
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Source Address − A 6-byte field containing the physical address of the sending station.
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Type/Length − A 2-byte field that differs between standards. In Ethernet (DIX), it's a Type field indicating which upper-layer protocol should receive the frame. In IEEE 802.3, it's a Length field storing the number of bytes in the data field.
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Data − Variable-sized field carrying data from upper layers. Maximum size is 1500 bytes.
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Padding − Added to data to meet the minimum requirement of 46 bytes for the data field.
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CRC (Cyclic Redundancy Check) − A 4-byte field containing error detection information to ensure data integrity.
Key Differences Between Standards
| Field | Ethernet (DIX) | IEEE 802.3 |
|---|---|---|
| Preamble | 8 bytes | 7 bytes |
| Start of Frame | Included in Preamble | Separate 1-byte SOF field |
| Type/Length Field | Type (protocol identifier) | Length (data field size) |
Conclusion
Classic Ethernet MAC sublayer protocol defines the frame structure for data transmission in early Ethernet networks. The two main standards, Ethernet (DIX) and IEEE 802.3, differ primarily in their preamble structure and the Type/Length field, establishing the foundation for modern Ethernet communication.
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