1. From Data Link to Physical Layer

• TLPs (Transaction Layer Packets) and DLLPs (Data Link Layer Packets) move down into the Physical Layer.
• At this point, the PHY:
  • Adds framing characters (Start + End markers) so the receiver knows exactly where each packet begins and ends.
  • Sends everything into buffers before transmission.

2. Byte Striping Across Lanes

• PCIe Links can have multiple lanes (x1, x4, x8, x16).
• Each lane is a separate serial channel.
• Data is striped (split byte by byte) across all active lanes, then recombined at the receiver.
• Example: With 4 lanes, byte #1 goes on lane 0, byte #2 on lane 1, etc., in parallel.

3. Scrambling

• Before sending, data is scrambled (shuffled with a known pattern).
• Purpose: avoid repetitive bit patterns that cause electromagnetic interference (EMI) on the wires.

4. Encoding (depends on PCIe generation)

• Gen1 / Gen2 (2.5 GT/s, 5 GT/s):

  • Use 8b/10b encoding.
  • Every 8 data bits are expanded into 10 transmission bits.
  • Adds 25% overhead, but ensures:
    • Enough “transitions” for clock recovery
    • DC balance (no buildup of 1s or 0s)
    • Special symbols for framing/management

• Gen3 (8 GT/s):

  • Uses 128b/130b encoding.
  • Much more efficient: only ~1.5% overhead.
  • Still provides transition density and error detection.

5. Serialization

• The encoded bytes are serialized (converted into a continuous bit stream).
• Each lane sends data differentially at its defined rate:
  • Gen1 = 2.5 GT/s
  • Gen2 = 5 GT/s
  • Gen3 = 8 GT/s

6. Reception Path

When receiving:

1. Deserializer converts serial bit stream back into parallel form (symbols/bytes).
2. Elastic buffer absorbs tiny timing differences between transmitter and receiver clocks (clock tolerance compensation).
3. Depending on generation:
   • Gen1/Gen2:
     • 10-bit symbols → pass elastic buffer → 8b/10b decode → 8-bit characters.
   • Gen3:
     • Bytes directly → pass elastic buffer → skip 8b/10b decode.
4. De-scrambling recovers the original byte sequence.
5. Un-striping recombines lanes into the single data stream.
6. The original packet is now fully reconstructed and passed back up to the Data Link Layer.

✅ In short:

• Transmit side: Add framing → stripe → scramble → encode → serialize → send.
• Receive side: Deserialize → buffer → decode (if needed) → de-scramble → un-stripe → deliver.