Even though there can be several 12 Mb/s full-/low-speed buses, there are only at most 127 USB devices attached to any single host controller. 4 Logical Bus Topology While devices physically attach to the USB in a tiered, star topology, the host communicates with each logical device as if it were directly connected to the root port. This creates the logical view illustrated in Figure 5-7 that corresponds to the physical topology shown in Figure 5-5. Hubs are logical devices also but are not shown in Figure 5-7 to simplify the picture.

One message pipe, the Default Control Pipe, always exists once a device is powered, in order to provide access to the device’s configuration, status, and control information. The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents buffers from underrun or overrun situations by using a NAK handshake to throttle the data rate. When NAKed, a transaction is retried when bus time is available. The flow control mechanism permits the construction of flexible schedules that accommodate concurrent servicing of a heterogeneous mix of stream pipes.

The software architecture allows for future extension of the USB by providing support for multiple USB Host Controllers. PERFORMANCE APPLICATIONS LOW-SPEED Keyboard, Mouse Stylus Game Peripherals Virtual Reality Peripherals • Interactive Devices • 10 – 100 kb/s FULL-SPEED • Phone, Audio, Compressed Video • 500 kb/s – 10 Mb/s HIGH-SPEED • Video, Storage • 25 – 400 Mb/s POTS Broadband Audio Microphone Video Storage Imaging Broadband ATTRIBUTES Lowest Cost Ease-of-Use Dynamic Attach-Detach Multiple Peripherals Lower Cost Ease-of-Use Dynamic Attach-Detach Multiple Peripherals Guaranteed Bandwidth Guaranteed Latency Low Cost Ease-of-Use Dynamic Attach-Detach Multiple Peripherals Guaranteed Bandwidth Guaranteed Latency High Bandwidth Figure 3-1.