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I'm reading book about networking now and can't understand one thing. As I've read Manchester encoding uses transition of voltage to interpret incoming signal. Question: When I'm sending data via Ethernet using Manchester encoding how receiver understands whether first byte is 0 or 1.

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I think you are speaking of bits, not bytes.

Before sending a frame, the sender put a preamble of 64 bits (8 bytes):

Byte 1   Byte 2   Byte 3   Byte 4   Byte 5   Byte 6   Byte 7   Byte 8  
10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011

The first 7 bytes are the same: 10101010. The last byte called the Start of Frame byte is slightly different: 10101011.

The preamble is not officially counted as part of the Ethernet frame. The frame begins immediately after the Start of Frame, without a gap.

As the preamble is a fixed and known pattern, it is used by the receiver to "lock-in" the clock and from then it can easily understand the incoming frame.

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The encoding is fixed in the standard,see this desciption

The frame's preamble is known, so that helps to determine what bit this is, or a collision is taking place.

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