Ethernet
Ethernet is the most used networking technology for LANs today. It defines wiring and signaling for the Physical layer of the OSI model. For the Data Link layer, it defines frame formats and protocols.
Ethernet is described as IEEE 802.3 standard. It uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and supports speeds up to 100 Gbps. It can use coaxial, twisted pair and fiber optic cables. Ethernet uses frames to with source and destination MAC addresses to deliver data.NOTE
The term Ethernet LAN refers to a combination of computers, switches, and different kinds of cables that use the Ethernet standard to communicate over the network. It is by far the most popular LAN technology today.
Ethernet frame
We have already learned that encapsulated data defined by the Network Access layer is called an Ethernet frame. An Ethernet frame starts with a header, which contains the source and destination MAC addresses, among other data. The middle part of the frame is the actual data. The frame ends with a field called Frame Check Sequence (FCS).
The Ethernet frame structure is defined in the IEEE 802.3 standard. Here is a graphical representation of an Ethernet frame and a description of each field in the frame:
- Preamble – informs the receiving system that a frame is starting and enables synchronisation.
- SFD (Start Frame Delimiter) – signifies that the Destination MAC Address field begins with the next byte.
- Destination MAC – identifies the receiving system.
- Source MAC – identifies the sending system.
- Type – defines the type of protocol inside the frame, for example IPv4 or IPv6.
- Data and Pad – contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
- FCS (Frame Check Sequence) – contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data.
The FCS field is the only field present in the Ethernet trailer. It allows the receiver to discover whether errors occurred in the frame. Note that Ethernet only detects in-transit corruption of data – it does not attempt to recover a lost frame. Other higher level protocols (e.g. TCP) perform error recovery.