1. The Concept of Virtual Circuit (VC)
A virtual circuit is a type of connection-oriented communication method, similar to circuit switching, but instead of using a physical path, it creates a logical path through the network. This logical path, known as a virtual circuit, allows data to be transmitted in an organized and efficient manner.
2. How Data is Transmitted Using Virtual Circuits
(1) Virtual Circuit Setup: Before any data is sent, the sender initiates a call setup by sending a control message containing address information. Each intermediate node along the path checks its current state and assigns a logical channel (LC). These LCs are then linked together to form a complete virtual circuit.
(2) Data Transmission: Once the virtual circuit is established, all data packets from the source travel along this pre-defined path. Since the route is fixed, the order of packet arrival at the destination is preserved, ensuring accurate delivery.
(3) Virtual Circuit Termination: After the transmission is complete, a release message is sent to terminate the virtual circuit. Either the sender or receiver can initiate this process, freeing up network resources for other users.
Due to the time required to establish and tear down the circuit, the virtual circuit approach is not ideal for applications that require frequent short connections, such as interactive communications or real-time services.
3. Permanent vs. Switched Virtual Circuits
Permanent Virtual Circuit (PVC): A fixed, pre-established connection between two nodes. It functions like a dedicated line, allowing instant communication whenever needed without the need for setup.
Switched Virtual Circuit (SVC): A temporary connection created on demand by the user application. It is dynamically set up and released as required, offering more flexibility than PVC.
4. Comparison Between Datagram and Virtual Circuit
Datagram:
- No need to establish or release a connection before transmitting data;
- Each packet carries full addressing information;
- Data may arrive out of order, requiring more complex processing at the receiving end;
- Highly reliable in mesh networks, as alternative paths can be used if a node or link fails;
- Best suited for small data transfers between nodes.
Virtual Circuit:
- Requires a connection to be established and released before data transfer;
- Each data packet contains only minimal address information (such as the logical channel number);
- Data follows a fixed path and arrives in the correct order;
- If a node or link in the virtual circuit fails, the entire connection is disrupted;
- More suitable for large-scale or continuous data transfers between nodes.
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