What is a Virtual Circuit?

Virtual circuit (English: Virtual circuit, abbreviated as VC), also known as virtual circuit , virtual connection or virtual channel , is one of the transmission modes of exchanged data on a packet-switched computer network. It is a connection-oriented or circuit-switched data transmission method. A connection is created between two end systems to exchange data. Before using a virtual circuit, you must first create a connection between two nodes or software applications. After the connection is created, data flow can be exchanged between the two nodes. The concept comes from circuit switching, and its operation is like creating a dedicated physical layer line connection between two endpoints, so it is also called a virtual connection or virtual channel.

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A virtual circuit, sometimes called a logic circuit, is one between two or more points, similar to a fixed physical path, but is actually one of many arrangable physical paths. A permanent virtual circuit (PVC) is a virtual circuit that guarantees connection between two or more points, and does not need to be reserved in advance or submitted to a special physical path. This allows multiple companies to share a common circuit. This method is used for network / relay, which can provide an effective resource set to a telephone company's customers at a relatively low price, which is more cost-effective than customers renting their own circuits. A switched virtual circuit (SVC) is similar to a permanent virtual circuit in that it allows users to regulate the network of virtual circuits.

In the connection establishment phase, a "connection state" needs to be established on each switch between the source host and the destination host. The connection state consists of the "VC" table records in each switch that the connection passes through, and the VC table on a switch One of the records includes:

1. Virtual Circuit Identifier (VCI), which uniquely identifies the connection on this switch, and sends it in the packet header that belongs to this link.

2. The input port of the packet that this VC reaches the switch.

3. The output port of the packet leaving the switch from this VC.

4. A possibly different VCI for output packets.

When establishing a new connection, a VCI value must be allocated on each link through which the connection passes, and ensure that the selected VCI value on a link is not used by a link that already exists on the link. There are two ways to establish the connection state. One is the connection state configured by the network administrator. Such a virtual circuit is a permanent virtual circuit (PVC), which can be regarded as a long-lived or manageable VC. PVC can also be deleted; the other type is that the host sends a message to the network to establish a connection. The virtual circuit thus established is called a switched virtual circuit (SVC), which can be dynamically created and deleted by the host.

1. The user communication must establish a connection, no longer need to find a path during data transmission, and the delay is relatively small compared to the datagram method.

2. Usually the packets follow the same path, so they reach the receiver in order.

3. High resource utilization.

4. The packet header does not include the destination address but VCI, and the overhead of the relative datagram method is small.

5. If there is a switch or link failure on a connection, the connection will be destroyed. A new connection must be established, and the original connection must be cancelled at the same time to release the storage space of the virtual circuit table in the switch.

6. There is a certain delay before the first packet is sent.