Final answer:
In the X.25 protocol, the virtual circuit number is different for two communicating stations because each packet-switching node in the network assigns its own local virtual circuit numbers to manage routing, error recovery, and flow control efficiently within the network.
Step-by-step explanation:
In X.25 protocol suite, the concept of virtual circuits (VCs) provides a way for data to travel across a network. Each virtual circuit is a logical connection between two endpoints, and it is full-duplex, meaning data can be sent in both directions simultaneously. The use of different virtual circuit numbers for two communicating stations even though it is the same full-duplex circuit, is due to the way X.25 infrastructure is built. It operates on a packet-switched network, and these numbers are used to uniquely identify the path or channel of communication from one end to the other within the network.
When the data travels through the network, it passes through multiple switches or nodes. To keep track of the data packets belonging to a specific virtual circuit, each switch might assign a different local virtual circuit number to the same connection. This is because each switch maintains its mapping of virtual circuit numbers, which allows it to route the packets correctly toward their destination. The virtual circuit number used by one station will therefore differ from the number used by the other station because these numbers are assigned locally at each switch along the path from source to destination.
This approach provides flexibility and efficiency in the use of network resources, as the switches can dynamically manage and remap virtual circuit numbers as needed. It also allows for error recovery and flow control to be managed locally at each switch, which is an important consideration in a packet-switched network like X.25.