Ethernet Basics LAN Local Area Networks
Ethernet is, today, the de-facto hardware standard for local area networks. Ethernet (Version 2) and the very similar IEEE 802.3 standard define the physical and link layers of carrier sense multiple access/collision detection (CSMA/CD) LANs. In CSMA/CD LANs, all stations can access the network at any time. Before sending data, each station must "listen" to the network to see if it is already in use. Data is sent only if the station doesn't "hear" any data being sent. Collision, is a situation where two stations detect silence on the network and send data at the same time. To overcome collision problems, Ethernet hardware is equipped with collision detection sensors. Whenever a collision is detected, the colliding data is ignored, and the station(s) that originally sent the data, will resend it.
Illustration of an Ethernet Bus
Ethernet and IEEE 802.3 frames are similar.
Both begin with an alternating pattern of ones and zeros (the Preamble), that tells the receiving stations that a frame is coming. Following the preamble, are the address of the destination station and the address of the source (sending) station. In Ethernet frames, the 2-byte field following the source address is a type field, used to identify the data. In IEEE 802.3 frames, the 2-byte field following the source address is a length field, which indicates the number of bytes of data that follow this field and precede the frame check sequence (FCS) field. Following the type/length field is the actual data contained in the frame, followed by a 4-byte FCS field containing a cyclic redundancy check (CRC) value. The CRC is created by the sending device and recalculated by the receiving device to check for damage that might have occurred to the frame in transit.
Frame Format
Ethernet and IEEE 802.3 Frame Formats
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