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USB Complete fourth- P42:This book focuses on Windows programming for PCs, but other computers and operating systems also have USB support, including Linux and Apple Computer’s Macintosh. Some real-time kernels also support USB. | Chapter 15 UPSTREAM-FACING PORT DOWNSTREAM-FACING PORTS Figure 15-6. A USB 3.0 hub contains a USB 2.0 hub and a hub for SuperSpeed. Adapted from Universal Serial Bus 3.0 Specification. Bus Speeds The speed of a hub s upstream port determines what bus speeds are available to downstream ports. If the upstream port connects at SuperSpeed the hub can communicate with downstream devices at any speed. If the upstream port connects at high speed the hub can communicate downstream at low full and high speeds. If a USB 3.0 hub s upstream port connects at full speed the hub can communicate downstream at low and full speeds. A downstream-facing port that connects internally to an embedded device can support a single speed. At the hub s upstream port traffic to and from downstream SuperSpeed devices uses the SuperSpeed wires and traffic to and from downstream low- full- and high-speed devices uses the USB 2.0 wires. As with USB 2.0 hubs all upstream traffic on the USB 2.0 wires uses high speed unless a USB 1.x hub is upstream from the hub . Components The SuperSpeed portion of a USB 3.0 hub consists of a repeater forwarder and a hub controller. Like the hub repeater in a USB 2.0 hub the repeater for 386 All About Hubs warder re-transmits received packets detects device attachment and removal establishes the connection of a device to the bus detects bus faults such as over-current conditions and manages power to the device. A hub may partially store a Data Packet before beginning to forward it and the hub stores and forwards all other packets. Buffers help to manage the traffic that passes through the hub. Buffers enable storing packet headers for later delivery to a downstream device that must exit a low-power mode before receiving traffic. Buffers also enable receiving asynchronous messages from multiple downstream devices at once and holding received payload data to repeat. To enable retrying after transmitting a Data Packet the buffer retains the packet until receiving a .
