AT&T 3B2/300 Technical Reference Manual page 426

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Ethernet Data Packet Format
The format of an Ethernet Data Packet is as follows:
PREAMBLE
PREAMBLE DESTINATION SOURCE TYPE DATA CRC
FIELD
ADDRESS ADDRESS
SIZE 64 BITS 48 BITS 48 BITS 16 BITS 46 TO 1500 BYTES 32 BITS
The Preamble is a 64-bit field of alternating l's and O's, ending with two
consecutive l's. This field synchronizes the receiving circuits to the incoming data
packet.
DESTINATION ADDRESS
The 48-bit Destination Address field is the nodal address to which the data is being
transmitted.
SOURCE ADDRESS
TYPE
DATA
CRC
The 48-bit Source Address field is the nodal address from which the data is being
transmitted.
The 16-bit Type field is used for a high-level data protocol.
The Data field is the data to be sent over the network and is from 46 to 1500 bytes
in length.
The 32-bit Cyclic Redundancy Check (CRC) field is calculated on all of the other
fields. This field is also called the Frame Check Sequence field.
FUNCTIONAL DESCRIPTION 3-173