The preamble of an Ethernet packet consists of a 56-bit (seven-byte) pattern of alternating 1 and 0 bits, allowing devices on the network to easily synchronize their receiver clocks, which is followed by the SFD to mark a new incoming frame.
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; since octets are transmitted least-significant bit first, the corresponding hexadecimal representation is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5.
The SFD is the eight-bit (one-byte) value that marks the end of the preamble, which is the first field of an Ethernet packet, and indicates the beginning of the Ethernet frame. The SFD is designed to break the bit pattern of the preamble and signal the start of the actual frame. The SFD is immediately followed by the destination MAC address.
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