This International Standard specifies the 25-pole connector and the assignment of contact numbers at the interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) or parallel automatic calling equipment (ACE). It is applicable to voice band modems, public data network (PDN) facilities, telegraph signal converters, and automatic calling equipment where CCITT1) Recommendations V.24 and V.28 are applicable. In the case of the PDN attachment through the X.20 interface, the functions of the interchange circuits are in accordance with CCITT Recommendation X.24. In the case of the V.20 type outstation interface, the electrical characteristics are in accordance with CCITT Recommendation V.31 or V.31 bis. International Standard ISO/IEC 2110 additionally provides the dimensions of the connector housing, as well as the recommended means of providing a locking device (latching block) and connector shielding.