PCIe Gen 3 Questions and Answers

One of the main factors in the wide adoption of the PCIe architecture is its sensitivity

to high-volume manufacturing materials and tolerances such as FR4 boards, low-cost clock sources, connectors and so on. In providing full compatibility, the same topologies and channel reach as in PCIe 2.0 will be supported for both client and server configurations. Another important requirement is the manufacturability of products using the most widely available silicon process technology. For the PCIe 3.0 architecture, the PCI-SIG believes a 65nm process or better will be required to optimize on silicon area and power.

2) What is the bit rate for PCIe 3.0 and how does it compare to prior generations of PCIe?

The bit rate for PCIe 3.0 is 8GT/s. This bit rate represents the most optimum tradeoff between manufacturability, cost, power and compatibility. The PCI-SIG analysis covered multiple topologies and configurations, including servers. All of these studies confirmed the feasibility of 8GT/s signaling with low-cost enablers and with minimal increases in power and silicon die size.

3) How does the PCIe 3.0 8GT/s "double" the PCIe 2.0 5GT/s bit rate?

The PCIe 2.0 bit rate is specified at 5GT/s, but with the 20 percent performance overhead of the 8b/10b encoding scheme, the delivered bandwidth is actually 4Gb/s. PCIe 3.0 removes the requirement for 8b/10b encoding and uses a more efficient 128b/130b encoding scheme instead. By removing this overhead, the interconnect bandwidth can be doubled to 8Gb/s with the implementation of the PCIe 3.0 specification. This bandwidth is the same as an interconnect running at 10GT/s with the 8b/10b encoding overhead. In this way, the PCIe 3.0 specifications deliver the same effective bandwidth, but without the prohibitive penalties associated with 10GT/s signaling, such as PHY design complexity and increased silicon die size and power.

4) Does this mean that PCIe is finished at 8GT/s? What comes next?

The PCI-SIG will study the requirements of its members and of the industry for the next generation of the PCIe architecture following the successful release of the PCIe 3.0 specifications. Higher signaling rates depend on a number of factors. The PCI-SIG is committed to delivering the most robust and high-performance I/O interconnect specifications, while at the same time maintaining an uncompromised focus on low cost, low power, high volume manufacturability and compatibility, by taking advantage of breakthroughs in signaling technologies and silicon process capabilities.

5) Will PCIe 3.0 specs only deliver a signaling rate increase?

The PCIe 3.0 specifications will comprise the Base and the Card Electromechanical (CEM) specifications. There may be updates to other form factor specifications as the need arises. Within the Base specification, which defines a chip-to-chip interface, updates will be made to the electrical section to comprehend 8GT/s signaling. As the technology definition progresses through the PCI-SIG specification development process, additional ECN and errata will be incorporated with each review cycle. For example, the current PCIe protocol extensions that address interconnect latency and other platform resource usage considerations will be rolled into the PCIe 3.0 specification revisions as they become available. The final PCIe 3.0 specification will consolidate all ECN and errata published since the release of the PCIe 2.1 specification, as well as interim errata.

PCIe Gen 3 Questions and Answers

By: Jim Renehan

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