Communications Device Class

This chapter describes the Communications Device Class (CDC) class and the associated CDC subclass supported by μC/USB-Device. μC/USB-Device currently supports the Abstract Control Model (ACM) subclass, which is especially used for serial emulation.

The CDC and the associated subclass implementation complies with the following specifications:

• Universal Serial Bus, Class Definitions for Communications Devices, Revision 1.2, November 3 2010.
• Universal Serial Bus, Communications, Subclass for PSTN Devices, Revision 1.2, February 9 2007.

CDC includes various telecommunication and networking devices. Telecommunication devices encompass analog modems, analog and digital telephones, ISDN terminal adapters, etc. Networking devices contain, for example, ADSL and cable modems, Ethernet adapters and hubs. CDC defines a framework to encapsulate existing communication services standards, such as V.250 (for modems over telephone network) and Ethernet (for local area network devices), using a USB link. A communication device is in charge of device management, call management when needed and data transmission. CDC defines seven major groups of devices. Each group belongs to a model of communication which may include several subclasses. Each group of devices has its own specification document besides the CDC base class. The seven groups are:

• Public Switched Telephone Network (PSTN), devices including voiceband modems, telephones and serial emulation devices.
• Integrated Services Digital Network (ISDN) devices, including terminal adaptors and telephones.
• Ethernet Control Model (ECM) devices, including devices supporting the IEEE 802 family (for instance cable and ADSL modems, WiFi adaptors).
• Asynchronous Transfer Mode (ATM) devices, including ADSL modems and other devices connected to ATM networks (workstations, routers, LAN switches).
• Wireless Mobile Communications (WMC) devices, including multi-function communications handset devices used to manage voice and data communications.
• Ethernet Emulation Model (EEM) devices which exchange Ethernet-framed data.
• Network Control Model (NCM) devices, including high-speed network devices (High Speed Packet Access modems, Line Terminal Equipment)