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A USB CDC device is responsible for device management, call management if
needed, and data transmission. Device management includes controlling and
configuring the device and notifying the host of events. Call management
includes establishing and terminating telephone calls or other connections.
Some devices don??™t need call management. Data transmission is the sending
and receiving of application data such as phone conversations, files, or other
data sent over a modem or network.
CDC supports five basic models for communicating. Each model encompasses
one or more subclasses.
??? The POTS (Plain Old Telephone Service) model is for devices that communicate
via ordinary phone lines and generic COM-port devices. Ethernet
devices that comply with Microsoft??™s USB Remote Network Driver Interface
Specification (NDIS) also use the POTS model.
??? The ISDN model is for communications via phone lines with ISDN interfaces.
??? The networking model is for communicating via Ethernet or ATM (asynchronous
transfer mode) networks.
??? The wireless mobile communications (WMC) model includes cell phones
that support voice and data communications.
Using Generic USB Controllers
337
Table 16-1: The CDC communication class interface supports a variety of subclasses. In
an interface descriptor, the bInterfaceSubClass field contains the subclass code.
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