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According to OSI model, does the physical layer add any header to the date it receives from layer 2? Or is it simply sending the data frames it gets from layer2 as shown in the figure below? Are the Protocol Data Unit (PDU) and the Service Data Unit (SDU) for PHY layer equal?

OSI ISO model

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  • Short answer is "No", because L1 it is just sequence of signals. Radio, fiber, copper, whatever, with different type of modulation - but in the end of the story it is just sequence of bits. Of course, some hardware has some servise signals (w/o any IP inside, for example), but these signals are more belongs to L2, than to L1.
    – Konstantin Goncharenko
    Commented Oct 26, 2017 at 9:44
  • Remember that the OSI model is just a theoretical model, and noting in the real world actually uses it. For example, no OS has separate layers 5 to 7; they are lumped together as the application layer.
    – Ron Maupin ♦
    Commented Oct 26, 2017 at 12:45
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Feb 19, 2018 at 19:55

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That depends on the physical layer - there's a multitude out there - and your point of view.

For example, Ethernet adds a preamble/syncword combination to a layer 2 frame, providing bit and word synchronization. Some might call that a "header". The L1 PDU is the Ethernet packet, the SDU the frame.

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    Also, not a header, but encoding can add bits on the wire.
    – Ron Maupin ♦
    Commented Oct 26, 2017 at 12:43

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