3.7.3 Bus Enumeration

Bus enumeration is the activity that identifies and assigns unique addresses to devices attached to a logical bus. Because Wireless USB allows devices to attach to or detach from the logical bus at any time, bus enumeration is an on-going activity for the USB System Software. Additionally, bus enumeration for Wireless USB also includes the detection and processing of removals.

3.8Data Flow Types

Wireless USB supports the same data transfer types and pipe types as wired USB. Basic usage and characteristics of the transfer types and pipe types are the same as wired USB. Because of the higher error rate characteristic of wireless communications, Wireless USB protocol defines different mechanisms for performing isochronous data transfers. These mechanisms include handshakes on data delivery as well as device specific amounts of buffering to allow devices some measure of control on the overall reliability of the isochronous pipe.

Bandwidth allocation for Wireless USB is very similar to wired USB.

Details of how the basic transfer types are implemented in Wireless USB can be found in Chapter 4.

3.9Wireless USB Devices

Just like wired USB, Wireless USB devices are divided into device classes such as human interface, printer, imaging, or mass storage device. Wireless USB devices are required to carry information for selfidentification and generic configuration. They are also required at all times to display behavior consistent with defined USB device states.

Notably, hubs are NOT a supported Wireless USB device class. Because Wireless USB hosts can support the architectural limit of 127 devices, there is no need for hubs. However, a new device class called Wire Adapter is defined. This device class describes a standard way for a device of one USB type (wired or wireless) to connect devices of the other type. A USB 2.0 connected Wire Adapter (known as a Host Wire Adapter) acts as the host for a Wireless USB system and provides a way to upgrade an existing PC to have Wireless USB capability. A Wireless USB Wire Adapter (known as a Device Wire Adapter) acts as a host for a wired USB system and allows wired USB devices to be connected wirelessly to a host PC. Figure 3-3 shows an example PC system including both a Host Wire Adapter and a Device Wire Adapter.

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USB2.0

USB2.0

Host Wire Adapter: DWA

Device Wire Adapter: DWA

Figure 3-3. Wire Adapters

3.9.1 Device Characterizations

Like wired USB, all Wireless USB devices are accessed by a USB address that is assigned when the device is attached and enumerated. Each Wireless USB device additionally supports one or more pipes through which the host may communicate with the device. All Wireless USB devices must support a specially designated pipe at endpoint zero to which the USB device’s USB control pipe will be attached. All Wireless USB devices support a common access mechanism for accessing information through this control pipe.

Associated with the control pipe at endpoint zero is the information required to completely describe the Wireless USB device. Standard descriptors for Wireless USB devices have been augmented (beyond those required for USB 2.0) to include the necessary information to support wireless communication. Detailed information about these descriptors can be found in Chapter 7.

3.9.2 Devices and MAC Layer

Wireless USB devices must also be well-behaved MAC Layer devices, see reference [3]. Wireless USB defines three categories of devices providing different degrees of awareness of MAC Layer mechanisms. The three categories are:

•Self beaconing device: These devices are fully aware of MAC Layer protocol and do all related beaconing.

•Directed beaconing device: These devices are unaware of MAC Layer protocol and rely on the host for direction to properly beacon and detect neighbor devices.

•Non-beaconing device: These devices have reduced transmit power and receiver sensitivity so that they don’t interfere with, and are not interfered with, by neighbor devices that the host cannot detect.

More information on the behaviors of these devices can be found in Section 4.3.7 of the Data Flow chapter.

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