Pci Express Specification -

This layered approach means that higher-level protocols (like NVMe for SSDs or CXL for coherent memory) can run seamlessly over the PCIe transport layer.

To appreciate PCIe, one must understand the problem it solved. Its predecessors, including the original PCI and PCI-X, used a . Multiple devices shared a single, wide bus (32 or 64 bits) and communicated over a common clock signal. While conceptually simple, this approach faced severe physical limitations. As clock speeds increased, signals on parallel lines began to interfere with each other (a phenomenon known as crosstalk), and skew—where signals on different lines arrive at slightly different times—became impossible to manage. The parallel bus had hit a "speed wall." pci express specification