USB System Architecture

Audio Class-Specific Requests

The audio class specification defines class-specific requests that control various audio properties. In general, the properties can be divided into the following two groups:

•Audio Control Properties — These audio class-specific requests control the audio functions such as volume and tone. These properties are controlled via the audio control blocks defined by the audio class specification. The audio control block contains parameters that can be manipulated by software via the class-specific requests.

•Endpoint Properties — These properties control various aspects of the audio data transfers such as sampling frequency. These properties are manipulated by changing the characteristics of the isochronous audio data endpoint.

The properties that all standard USB audio devices should support are termed general properties. However, the audio class specification permits other vendor implementations to define additional properties. A class-specific request called the Get/Set System Exclusive properties is provided as a mechanism for controlling vendor-defined properties. Refer to the audio class specification for details regarding the definition and use of the audio class-specific requests.

Communications Device Class

Any USB device that connects to a telephone line falls within the definition of a communications class device. At the time of this writing, two subclasses had been defined:

•Telephony Interface

•Vendor-Specific

However, numerous protocols are defined within the telephony subclass as shown in Table 21-2. These protocols specify the control protocol used to control the communications function.

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Chapter 21: Device Classes

Table 21-2: Telephony Protocol Types and Codes Used by Telephony Devices

Communications Device Interfaces

USB communications class devices have interfaces that vary depending on their characteristics. The endpoints that would be used in typical implementations include:

•Control Endpoint Zero — Used to send information that is not time sensitive and that requires relatively little bus bandwidth. This ensures that the control pipe does not become saturated with some of the extremely long command sequences required by some control protocols.

•Interrupt Endpoint — Used to report device-generated events (i.e., on/off hook and user key presses) and communications network events such as incoming call notification. May also be used to determine interface availability and related data formats.

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