14 Facts About Ethernet frame

In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms.

An Ethernet frame is preceded by a preamble and start frame delimiter, which are both part of the Ethernet packet at the physical layer.

Each Ethernet frame starts with an Ethernet header, which contains destination and source MAC addresses as its first two fields.

The middle section of the frame is payload data including any headers for other protocols carried in the frame.

The Ethernet frame ends with a Ethernet frame check sequence, which is a 32-bit cyclic redundancy check used to detect any in-transit corruption of data.

An Ethernet packet starts with a seven-octet preamble and one-octet start frame delimiter.

For Ethernet variants transmitting serial bits instead of larger symbols, the on-the-wire bit pattern for the preamble together with the SFD portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011; The bits are transmitted in order, from left to right.

Fast Ethernet frame transceiver chips utilize the MII bus, which is a four-bit wide bus, therefore the preamble is represented as 14 instances of 0x5, and the SFD is 0x5 0xD.

Gigabit Ethernet frame transceiver chips use the GMII bus, which is an eight-bit wide interface, so the preamble sequence followed by the SFD would be 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5.

When used as EtherType, the length of the Ethernet frame is determined by the location of the interpacket gap and valid Ethernet frame check sequence.

End of a Ethernet frame is usually indicated by the end-of-data-stream symbol at the physical layer or by loss of the carrier signal; an example is 10BASE-T, where the receiving station detects the end of a transmitted Ethernet frame by loss of the carrier.

Different Ethernet frame types have different formats and MTU values, but can coexist on the same physical medium.

Ethernet II framing, defines the two-octet EtherType field in an Ethernet frame, preceded by destination and source MAC addresses, that identifies an upper layer protocol encapsulated by the frame data.

Ethernet frame II framing is the most common in Ethernet frame local area networks, due to its simplicity and lower overhead.