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In L2 Ethernet fields we see [Preamble][SFD][src][dst][tag#][ip/tcp data payload] [FCS] [trailer?]

Does the L1 Physical layer contain [IPG 12octets] field or is that part of L2. What fields sit at L1 [trailer?] ?

2 Answers 2

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In L2 Ethernet fields we see [Preamble][SFD][src][dst][tag#][ip/tcp data payload] [FCS] [trailer?]

The L2 Ethernet frame actually starts after SFD. Preamble, SFD and IPG belong to the physical layer (L1).

Since the L1 PDU is a simple bit (frames belong to L2), there are no headers as on higher layers. You could count the PCS line code (Manchester, 4b5b, 8b10b, 64b66b, ...) though if you wanted.

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The Preamble, SoF Delimiter, and Inter-Packet Gap are all handled by the hardware (layer-1). The Preamble is 56 bits of alternating ones and zeros. The SoF Delimiter is seven bits of alternating one and zeroes, ending in two one bits. Then the symbols of the encoded layer-2 frame bits are sent. The Inter-Packet Gap is really just an unmodulated carrier (silence) for the length of 96 bits, and that can throw off clocking, which is why the Preamble is used at the beginning to synchronize the sender and receiver clocks.

Layer-1 ethernet encodes the layer-2 frame bits and places the resulting symbols "on the wire." How the bits are represented to symbols depends on the ethernet standard used. For example, 10Base-T uses Manchester encoding, but other ethernet standards use various other encoding.

On the wire, the bits you see are not the pre-encoded bits, but they are symbols that prevent too many of the same bit in a row from getting the clocking out of synchronization.

Cisco has a summary document that covers some points, but it is far from a comprehensive list:

Each Ethernet technology uses an encoding method such that the signals exhibit desirable data rate, frequency, signal-to-noise ratio and clocking characteristics.

  • Manchester encoding is used on Legacy Ethernet
  • Fast Ethernet 100BASE-TX data is encoded twice; 4B5B encoding is used first then MLT-3 line encoding is applied.
  • Fast Ethernet 100BASE-FX data is encoded twice; 4B5B encoding is used first then NRZ-I line encoding is applied.
  • Gigabit Ethernet 1000BASE-T data is encoded twice; 8B1Q4 encoding is used first then 4D-PAM5 line encoding is applied.
  • Gigabit Ethernet 1000BASE-X data is encoded twice; 8B10B encoding is used first then NRZ line encoding is applied.
  • 10 Gigabit Ethernet 10GBASE-X data is encoded twice. Sometimes 8B10B encoding is used first, sometimes 64B/66B. The variety of line encoding applied second depends on the fiber used.
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  • Does that mean [IPG] sits at L2 than ? and by symbols you mean not 1 and 0's, but actual electrical signals? Lastly does 10Mb, 100Mb, 1 / 10Gbps use Manchester encoding ?
    – JoelP
    Jan 13, 2023 at 17:40
  • "Does that mean [IPG] sits at L2" No, the preamble, SoF, and IPG are all handled by the hardware. It is not as simple as asking about the ethernet speed, There are various ethernet standards at each speed per medium. Also, each standard can use a different encoding method. For example, 100Base-TX uses 4B5B (sending a five bit symbol for every four bits of the frame) and MLT encoding. This subject is far too large for this site. Simply search on the Internet for ethernet encoding methods.
    – Ron Maupin
    Jan 13, 2023 at 17:52

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