1.4 Virtual Circuit

Some network protocols create virtual circuits in networks. A virtual circuit is conducted through the network and all packets of a particular connection go via the circuit. If the circuit gets interrupted anywhere, then the connection is interrupted, a new circuit is established, and data transmission continues.

Figure 1.16: Virtual circuit

In the figure above, a virtual circuit between nodes A and D is established via nodes B, F, and G. All packets must go through this circuit.

Datagrams can be transmitted via the virtual circuit in two ways:

• The circuit does not guarantee the datagram’s delivery to its destination. (If network congestion occurs, the circuit can even throw the datagram away.) An example is the Frame Relay protocol.
• The virtual circuit can establish a connection and guarantee the data delivery, i.e., the data packets transmitted are numbered and the destination confirms their reception. If any data gets lost, a request to resend the data is made. For example, this mechanism is used in the X.25 protocol.

The advantage of virtual circuits is that they are first established (using signalization) and then the data is inserted only into the established circuit. Each packet does not have to carry the globally unique address of the destination (complete routing information) in its header. It only needs the circuit ID.

The virtual mechanism is not used on the Internet, which was primarily aimed for use by the U.S. Department of Defense, since the destruction of a node in the virtual circuit would result in the transmission being interrupted—a fact that the authors of TCP/IP did not like. For this reason, IP does not use virtual circuits. Each IP datagram carries a destination IP address (complete routing information) and is therefore transported independently. If a node is destroyed, only the IP datagrams currently being transmitted through that particular node are destroyed. The remaining datagrams are routed via different nodes.

Figure 1.17: IP does not use virtual circuits.

As the figure above shows, IP datagrams 1, 2, and 3 start from the node A to node B, but from this point, datagrams 1 and 3 are routed through a different path than datagram 2. The destination (node D) is then reached by each of them via a different path. Generally, IP datagrams may reach their destination in a different order than the order in which they were sent. So our IP datagrams could be received in the following order: 2, 1, and then 3.

In the Internet hierarchy, TCP—a higher-layer protocol that establishes a connection and guarantees the delivery of data—is used above the connectionless IP. If some of the data packets are lost, their retransmission is requested. If the data packets were lost due to the destruction of a node along the way and there is another routing possible within the network, then the transmission is automatically repeated using the other path.

Virtual circuits are divided into the following groups:

• Permanent (Permanent Virtual Circuit (PVC)) , i.e., circuits permanently built by the network administrator.
• Switched (Switched Virtual Circuit (SVC)), i.e., virtual circuits that are created dynamically as the need arises. An SVC is created with the help of signalizing protocols that can be used for communicating between the user and the network itself. The network signalizes to the user various events that can be used for network monitoring and administration. SVC communication consists of two steps: creating the virtual circuit and using it for communication.

PVC corresponds to leased lines and SVC corresponds to the dial-up lines of a phone network.

Note

Protocols using virtual circuits are called Connection-Oriented Network Services (CONS) and protocols transporting their packets without using virtual circuits are called Connection-Less Network Services (CLNS) .