This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminating equipment (DCE) employing serial binary data operation via the 100-series interchange circuits or data operation over equivalent logical circuits. This standard specifies extensions to the protocol elements, procedures, and behaviors described in ITU-T V.250. These extensions are based on the "+" command prefix and adhere to the extended command syntax conventions established by ITU-T V.250. This standard generalizes the protocol elements, procedures, and behaviors described in ITU-T V.250, making them applicable to DCE which operate over arbitrary data networks. This standard defines the commands that the DTE may issue to: (1) Interrogate the capabilities of the DCE, including determination of what data network types and associated network-specific AT command set extensions are supported by the DCE; (2) Select among supported data network types for subsequent DCE processing of automatic calling and automatic call answering functions; and, (3) Select among supported DTE-DCE interface protocols. This standard defines the responses the DCE shall issue to those commands. Where possible, these commands and responses are taken from existing TIA standards. This standard establishes required mappings between manufacturer-specific data network services and the DCE protocol elements, procedures, and behaviors referenced in ITU-T V.250 (e.g. ring, dial, answer, busy, on-line data state). This standard establishes conventions for interworking of network-specific AT command sets in DCE which support multiple data network types. This standard establishes a minimal DCE Common AT Command Set consisting of: (1) Those commands defined in ITU-T V.250, and (2) Required extensions using the +W command prefix in accordance with ITU-T V.250. This standard establishes conventions for uniform extension of its own scope and content through Annexes. Annex D defines optional extensions to the protocol elements, procedures and behaviors defined in the original standard, based on +WS and +WC command prefixes reserved for standardized extensions. The parameters in Annex D only affect operation of the DCE in Transparent Character Stream mode and in Reliable Transparent Character Stream mode, as selected by +WS45=0 or by +WS45=1.