For packet-mode terminals connected to public data networks, the X.25 standard…

Concept: A communication standard may define character framing, an access method, an equipment interface, or a physical transmission rate. The scope is identified from the standard’s formal subject and the layer of communication it governs.

Application: X.25 defines packet-mode operation at the boundary between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). DTE is the subscriber-side terminal or host, while DCE is the network-side equipment that provides access to the packet-switched network.

• Technique for start-stop data: this is asynchronous character framing with start and stop bits.

• Technique for dial access: this is the procedure for establishing a temporary switched telephone connection.

• DTE/DCE interface: this is the boundary through which subscriber equipment communicates with network-terminating equipment.

• Data bit rate: this is a physical-link speed measured in bits per second.

Cross-check: X.21 also specifies a DTE/DCE interface, but specifically at the physical layer for synchronous operation. That does not conflict with X.25: X.25 is the packet-mode DTE/DCE interface recommendation and may use a physical-layer specification underneath it.

Therefore, the standard specifies a DTE/DCE interface.