The CCNA Basic: Ethernet frame

We have learned that the encapsulation data defined by the network access layer is called Ethernet frames. Ethernet frames begin with a header that contains the source and destination MAC addresses and other data. The middle part of the frame is the actual data. The frame ends with a field called the frame check sequence (FCS).

The Ethernet frame structure is defined in the IEEE 802.3 standard. This is the graphical representation of the Ethernet frame and the description of each field in the frame:

Preamble – informs the receiving system that a frame is starting and enables synchronization.

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 in the Ethernet trailer. It allows the receiver to find out if an error has occurred in the frame. Note that Ethernet-only detects data corruption in transmission-no attempt to recover lost frames. Other higher-level protocols, such as TCP, perform error recovery.

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