The network remembered each conversation
A virtual circuit gave packet-switched data a prepared route through the network. Before sending data, the endpoints established a connection, and each switch along the way stored a small entry linking that connection to the next step. When packets arrived, a switch could consult its entry and forward them without working out a route from scratch each time.
The route was logical, not a dedicated wire. Many conversations could share the same links, and packets still traveled separately. What made the service feel more like a continuous connection was the network’s retained state: the switches remembered which packets belonged to which established path.
The tradeoff: simpler forwarding, more to remember
That memory could make forwarding predictable and support orderly delivery. But it also created setup work before data flowed, and every switch had to keep connection information while the circuit remained active. If a route or switch failed, the stored entries might no longer describe a usable path; the connection could need to be re-established.
In a datagram-style network, by contrast, packets carry destination information and are routed individually, without requiring every switch to maintain a record for each conversation. That approach reduces per-connection bookkeeping in the network, though packets from one message may take different paths.
Virtual circuits were therefore a compromise: packets kept the efficiency of sharing network links, while a setup phase gave the network a persistent, remembered path for each connection. The “appointment book” mattered because it shifted some responsibility from each packet to the switches—and made their stored memory part of the connection itself.
