The X.25 standard specifies a

An X.25 network specifies the interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). It operates in packet mode and connects to public data networks using dedicated circuits. X.25 networks use connection-mode network services.

X.25 is a packet-switched network protocol developed by ITU-T for reliable communication over public data networks. It provides a connection-oriented service, meaning a logical connection (virtual circuit) is established before data transfer begins. X.25 mainly works on the Physical, Data Link, and Network layers of the OSI model.

The most important feature of X.25 is that it defines the interface between DTE (Data Terminal Equipment) and DCE (Data Circuit-Terminating Equipment).

• DTE: End-user devices that generate or receive data, such as computers, terminals, or routers.

• DCE: Network devices that connect DTE to the communication network, such as modems or packet switches.

The DTE sends data to the DCE, and the DCE forwards it through the packet-switched network. X.25 also provides error control, flow control, and packet acknowledgement to ensure reliable communication.

Therefore, the correct answer is:

✅ DTE/DCE interface