This version of ITU-T Rec. V.250 integrates Amendment 1 (07/2001), Amendment 2 (03/2002) and Amendment 3 (07/2003) with the 05/1999 version of the Recommendation. This Recommendation defines commands and responses for use by a DTE to control a V-series DCE using serial data interchange over an asynchronous interface. It contains four elements: - codifies existing practice in common GSTN-DCE that use the ATtention (AT) command set, - defines a format for orderly extension of the AT command set, - provides a set for standardized extensions for common functions to identify the DCE, to control the DTE-DCE interface, and to control DCE-DCE protocols (signal conversion, error control and data compression), - provides a mapping for these commands into V.25 bis frame format for use with DCEs employing synchronous serial interfaces.