Chapter 16
phones and modems, ISDN terminal adapters, and cell phones. Networking
devices include ADSL modems, cable modems, and Ethernet adapters and
hubs. The USB interface in these devices can carry data that uses applica-
tion-specific protocols such as V.25ter/V.250 for modem control or Ethernet
for a network.$
The CDC specifications are available from the USB-IF (www.usb.org). The
class includes a series of subclasses (Table 16-1). The main CDC specification
defines most of the subclasses. WMC and EEM were defined after the release of
the CDC specification and thus have their own documents. The CDC standard
refers to the V.25ter standard, which evolved from the AT command set for
modems. A more recent edition of the standard is V.250 (www.itu.int). The
OBEX protocol used by some WMC devices is defined in IrDA Object
Exchange (OBEX) Protocol from http://www.irda.org.88
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 commu-
nicate 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 inter-
faces. - The networking model is for communicating via Ethernet or ATM (asyn-
chronous transfer mode) networks. - The wireless mobile communications (WMC) model includes cell phones
that support voice and data communications.