1️⃣ Packet Creation in Transaction Layer

• Input: Device core presents:

  • Command type (read, write, completion, message)
  • Address
  • Attributes (priority, ordering, etc.)

• Transaction Layer does:

  • Builds a Transaction Layer Packet (TLP).
  • Stores it in a Virtual Channel buffer (essentially a queue).
  • Waits until flow control says there’s space on the link.

2️⃣ Data Link Layer Adds Reliability

• Adds:
  • Sequence number
  • LCRC (Link CRC) for error checking
• Keeps a copy of the packet so it can resend if the receiver NAKs (negative acknowledge).
• Passes the packet down to the Physical Layer once ready.

3️⃣ Physical Layer Encodes & Transmits

• Encodes packet (8b/10b or 128b/130b).
• Scrambles data to ensure good signal integrity.
• Serializes and sends bits across all available lanes using differential signaling.
• Link Training (done earlier) guarantees both sides agree on speed and width.

4️⃣ Reception – The Reverse Process

When the other side receives:

1. Physical Layer

   • Recovers clock using CDR (Clock and Data Recovery).
   • Deserializes bitstream → parallel data.
   • Decodes 8b/10b or 128b/130b.
   • Passes up to DLL.

2. Data Link Layer

   • Checks LCRC, sequence numbers.
   • If good → sends Ack back.
   • If bad → requests retransmission (Nak).
   • Forwards to TL if valid.

3. Transaction Layer

   • Buffers packet, extracts:

     • Command
     • Address
     • Attributes

   • Passes to device core (or switch routing logic).

   • If this is a switch → creates a new TLP for the outgoing port.

🧠 Big Picture: Layer-to-Layer Communication

• Upper layers talk in terms of packets.

  • TL → organizes data into TLPs.
  • DLL → wraps them with reliability info (DLLPs).
  • PL → just moves bits reliably, doesn’t “understand” the content.

• Each layer only talks to its peer on the other side.

  • TL on one side ↔ TL on the other.
  • DLL ↔ DLL.
  • PL ↔ PL.

💡 Why This Matters

• Ensures modularity: Each link segment has its own error detection/retransmit, so errors are handled locally (no need to resend across the entire chain).
• Makes switches work: Since each port terminates a link, it must fully decode/encode packets before forwarding.
• Allows concurrent traffic: Different links can operate independently, each with its own flow control