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.