X.25 is which type of network?

Concept — A packet-switched network is classified by whether its packet layer must set up state before any user data can flow. In a connection-oriented (virtual-circuit) service a logical circuit is first established end to end, every packet of that session then travels the same agreed path and arrives in sequence, and the circuit is cleared when the session ends. In a connectionless (datagram) service there is no setup phase: each packet carries a full destination address and is routed on its own, so packets of one session may take different paths and arrive out of order.

Application to X.25 — X.25 is the ITU-T standard for packet-switched wide-area data networking over public data networks. It is defined over three layers, the physical layer (X.21), the data-link layer (LAPB) and the packet layer (PLP), and it is the Packet Layer Protocol that fixes its transfer model:

1. Call setup: the calling DTE sends a Call Request packet and the remote DTE answers with Call Accepted, which binds a switched virtual circuit (SVC) end to end. A permanent virtual circuit (PVC) is configured in advance and skips this exchange.

2. Data transfer: every data packet carries a 12-bit Logical Channel Number that names the virtual circuit instead of the full destination address, so each intermediate switch forwards it along the stored path. LAPB adds per-hop error and flow control, and packet sequence numbers keep delivery in order.

3. Call clearing: a Clear Request and Clear Confirmation exchange tears the circuit down and frees the logical channel for reuse.

Cross-check — This setup, transfer and clear lifecycle, together with per-circuit state held in every switch along the path, is exactly the definition of a connection-oriented service. A datagram facility did appear in the 1980 X.25 recommendation, but it was never implemented and was withdrawn in 1984, so the standard X.25 service is purely virtual-circuit based. Compare the two transfer models:

Because the packet layer accepts data packets after a virtual circuit has been established, and offers no datagram mode beside it, X.25 is a Connection-Oriented Network.