Ethernet frame

A data packet that comes from higher layers is entering an Ethernet fabric in Layer 2 (Data Link) which takes care of assembly, and exits through Layer 1 (Physical), which takes care of transmission. That said, an Ethernet frame is a Layer 2 and a Layer 1 data packet. Therefore, Ethernet protocols determine the following frame elements:

• The frame format consists of six fields (as shown in Figure 2.13) such as Preamble and Start Frame Delimiter (SFD), Destination MAC address, Source MAC address, EtherType, Data, and Frame Check Sequence (FCS). The Preamble and the SFD together (a total of 8 bytes) form the point of data transmission in the Ethernet frame concept. Destination MAC, a 6-byte field, can be a unicast, multicast, or broadcast address. Like Destination MAC, Source MAC is a 6-byte field that defines the full address of the sender of the frame. The EtherType field (2 bytes) indicates the protocol that is used to encapsulate the data in the frame, and it is often used to indicate the size of the frame. The data field (from 46 bytes to 1500 bytes) is actually the data (also known as the payload) that came from the upper layers of the OSI reference model. FCS is a 4-bytes field, which represents the mechanism for error checking in the Ethernet frame.
• The frame size is defined from the Destination MAC address field to the FCS field. Thus, the minimum size is 64 bytes, and the maximum size is 1,518 bytes. As you can note, Preamble does not count the size of the frame. The Ethernet is very accurate in terms of frame size. This is observed in frame transmission. If the frame size is smaller than the minimum size, or greater than the maximum frame size, then the receiving device drops the frame. Thus, frames smaller than 64 bytes are called runt frames and are the result of collisions, whereas frames larger than 1,500 bytes are called jumbo frames (or giant ) and are used on LAN networks to support Gigabit Ethernet speeds: