PC Hardware in a Nutshell, 3rd Edition

USB Data Transfer Modes

USB uses a set of unidirectional and bidirectional pipes to transfer user data and control information between the host and USB devices. Each device may support multiple pipes for different purposes, and data transferred in one pipe is independent from data transferred in other pipes. For example, a USB printer might have one pipe that it uses to receive page data from the host, and a second pipe that it uses to transfer status information to the host. USB defines the following data flow types:

Isochronous Data Transfers

Isochronous Data Transfers are used for periodic, continuous communication between the host and a device, typically time-critical data such as audio or video streams. Isochronous Data Transfers are enabled by reserving the required amount of bandwidth for the isochronous device, which the USB host controller makes unavailable to other devices whether the isochronous device happens to be using that bandwidth at any given time. Isochronous Data Transfers have the highest priority for bandwidth. If all available bandwidth is reserved for Isochronous Data Transfers, no other device can use the USB.

Interrupt Data Transfers

Interrupt Data Transfers are used for small, limited-latency transfers when timely, reliable delivery of data is required—for example, to receive coordinate changes from a mouse or status changes from a modem. Interrupt Data Transfers have lower priority for available bandwidth than do Isochronous Data Transfers.

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