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PCI Express (PCIe) and USB are two of the most common serial bus interface standards. USB is a simple, low-cost replacement for all legacy versions of serial and parallel ports, incorporating hot swap capability and external device interfacing. PCIe was designed to replace the general-purpose PCI expansion bus, as well as PCI-X and AGP. A PCIe host interfaces to one device per slot, whereas a USB host controller interfaces to up to 127 devices.

PCIe is structured around point-to-point links called lanes. This is unlike the bus-based system of PCI. In PCIe 1.1 (most common at present), the data rate is 250 MB/s in each direction for each lane (PCIe 2.0 doubles this data rate). PCIe slots come in a variety of physically different sizes referred to by the maximum lane count they support (x1, x2, x4, x8, x16, x32), with the product giving rise to a (theoretical) transfer rate. In practice, 8b/10b encoding in the physical layer limits the transfer rate.

A USB system consists of one or more host controllers, downstream USB ports, and multiple peripheral devices. Communication is based on unidirectional endpoints (also called pipes), with up to 32 endpoints per USB device. No USB can transfer any data on to the bus without explicit request from the host controller. USB supports three data rates: Low-speed (1.1, 2.0) of 1.5 Mbit/s, Full-speed (1.1, 2.0) of 12 Mbit/s, and Hi-Speed (2.0) of 480 Mbit/s.


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