AN 690: PCI Express* Avalon® -MM DMA Reference Design

ID 683824
Date 5/08/2017

1.7. Understanding PCI Express Throughput

The throughput in a PCI Express system depends on the following factors:

  • Protocol overhead
  • Payload size
  • Completion latency
  • Flow control update latency
  • Devices forming the link

Protocol Overhead

Protocol overhead includes the following three components:

  • 128b/130b Encoding and Decoding—Gen3 links use 128b/130b encoding. This encoding adds two synchronization (sync) bits to each 128-bit data transfer. Consequently, the encoding and decoding overhead is very small at 1.56%. The effective data rate of a Gen3 x8 link is about 8 gigabytes per second (GBps).
  • Data Link Layer Packets (DLLPs) and Physical Layer Packets (PLPs)—An active link also transmits DLLPs and PLPs. The PLPs consist of SKP ordered sets which are 16-24 bytes. The DLLPs are two dwords. The DLLPs implement flow control and the ACK/NAK protocol.
  • TLP Packet Overhead—The overhead associated with a single TLP ranges from 5-7 dwords if the optional ECRC is not included. The overhead includes the following fields:
    • The Start and End Framing Symbols
    • The Sequence ID
    • A 3- or 4-dword TLP header
    • The Link Cyclic Redundancy Check (LCRC)
    • 0-1024 dwords of data payload
Figure 4. TLP Packet Format