Data transmission modeled after network protocols.

PCI-e is treated as a tree network topology, with the controller (root
complex) at the base and nodes or switches that branch off.

Application layer
  Application performs a system read/write call.

  OS addresses mapped memory port.

  PCI controller acts as the address decoder, latching the target address
    and data.

* PCI-e layers
    Software layer - driver/controller performing a transaction request.
      Remember, actual target addressed with standard PCI protocol,
        bus, device, function.

    Transaction - generates Transaction Layer Packet (TLP)
      Header, Data being transmitted, Error check.
      Handles handshaking between sender and receiver.

Physical - serialization and transmission.
  Serializes packet.

  Encodes bytes in appropriate RLL symbols.

  Creates differential signals on lanes.

  Distributes across available lanes. 

Data link layer - delivery of individual Transaction Layer Packet (TLP)
  Encapsulates TLP, adds sequence ID, and additional error check.

  This layer deals with delivering a packet correctly between nodes on
     PCIe connection.

  Data link layer uses ACK/NAK handshaking to acknowledge success of
    transmission of each packet.

    Transmitting node has a Replay buffer for repeating current packet
      if NAK returned.

  Packet size : 64 bytes (Intel desktop), 128, 256, 512 1024, 2048, 4096
    possible. If controller supports, it will confirm size with client
    device. Settable in BIOS(?).

  Failure to transmit data is a PCIe failure not data transfer failure,
    failure not communicated to applications requesting task.

Transaction layer
  Check out :  http://www.fpga4fun.com/PCI-Express.html
  http://xillybus.com/tutorials/pci-express-tlp-pcie-primer-tutorial-guide-1

  Contents of packet.

    Header : 3 or 4 Double-Words (32-bit)
      Used to indicate type of transaction and size.

    Payload : 0-1023 Double-Words




STP
Sequence
Number
Header
Data
Digest
CRC
End



STP - start of packet, specific bit pattern that ids start. May also
include length of packet.

Sequence number - because PCIe is considered a network bus, it is possible
  for packets to be lost, corrupted, or duplicated. Sequence number is a check.

  Allows for retransmission of packet.

Header - Transaction type (address, data, control, interrupt, etc.),
  Target address, ID, etc.,
  Transfer size,
  Attributes,
  Traffic Class.

Data - data being sent. The data packet itself.

Digest (optional) - End to End CRC check. Not always used.

CRC - CRC check for checking as packet moves from node to node.