# Recommendation X.21 (11/1988): Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for synchronous operation on public data networks

> superseded recommendation/international standard from the International Telecommunication Union

**Wikidata**: [Q26928834](https://www.wikidata.org/wiki/Q26928834)  
**Source**: https://4ort.xyz/entity/recommendation-x-21-11-1988-interface-between-data-terminal-equipment-dte-and-data-circuit-terminating-equipment-dce-for-synchronous-operation-on-public-data-networks