Ethernet is a protocol on OSI level 2, the data-link layer.
An ethernet package is typically 1500Bytes large(MTU, Maximum Transfer Unit), this has its root in the bus-shaped networks it was designed for. Ethernet uses CSMA/CD (Carrier Sense
Multiple Access with Collision Detect) this means all hosts on the bus listen if the line is free, if they find a free line they can start to send packages. If another host also is sending at the same time the package will collide, resulting in a higher current on the interface. With the package size of 1500 Bytes and a maximum bussegment-lentgh of about 120 meters (I’m
not sure about this) the package will always fill the complete segment. This ensures a collision is allways seen by the sending interface so it can prepare for resending the package.
In the old days ethernet networks were build form coax-cable connecting every node to the bus. Nowadays we have UTP (Unshielded Twisted Pair) cables connecting nodes to a hub or switch, a hub is like a very short bus. A switch is more sophisticated, blocking the bus for all other traffic, thus avoiding the time-consuming collisions.
An ethernet package carrying TCP/IP data looks like.
| Ethernet package | |||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Start Flag | Destination MAC | Source MAC | Data Length | Data | CheckSum | ||||||||||||||||||||
| 8 bytes | 6 bytes | 6 bytes | 2 bytes | 46-1500 bytes | 4 bytes | ||||||||||||||||||||
| Synchronization | Data Link
(ARP) |
Data Link |
|
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