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Overview

A CDC EEM device is composed of the following endpoints:

  • A pair of Bulk IN and OUT endpoints.

describes the usage of the different endpoints:

CDC EEM Messages

CDC EEM defines a header that is prepended to each message sent to / received from the host. The CDC EEM header has a size of two bytes. A USB transfer on the Bulk endpoints can contain multiple EEM messages. An EEM message can also span on multiple USB transfers. 

describes the content of a CDC EEM message.

describes the content of a CDC EEM header.

  • bmType represents the message type, either a regular Ethernet frame or a CDC EEM specific command.
describes the content of a CDC EEM data message header.
  • bmCRC indicates if the CRC was calculated on the Ethernet frame. If not, CRC is set to 0xDEADBEEF.
describes the content of a CDC EEM command message header. Note that the EEM commands provide USB local link management. This management is opaque to the network stack.
  • bmEEMCmd represents the command code to execute. 
  • bmEEMCmdParam contains command data. Fomat depends on the bmEEMCmd.

For more information on CDC EEM messages format, see "Universal Serial Bus Communications Class Subclass Specification for Ethernet Emulation Model Devices" revision 1.0. February 2, 2005, section 5.1.

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