In simple terms a virtual circuit is a dedicated communication line between two end points usually on a packet switched or cell relay network. A common use is to provide a temporary or dedicate link through a router or switch connected network. Any devices along the circuit will be programmed with the specific circuit number so that when packets arrive the switch has the correct information to forward them. This saves the potentially lengthy process of examine the packet header in detail.
Using a predefined path like this can improve performance substantially and also reduces the size of frames and packets specifically by ensuring the header sizes are much smaller. The underlying physical routes of these connections may change in a standard packet switching network however the two end stations will retain a connection and update paths as appropriate. Typically this could happen when the network is experiencing congestion or perhaps some sort of physical problem with a downed line.
There are two main types of virtual circuits which can be described as follows:
PVC: (Permanent Virtual Circuit) – a connection between end points defined in advance. often with a predetermined bandwidth and speed allowance. In commercial public switched carrier networks (like ATM) or frame relay the customers will be allocated the endpoints of the PVC in advance. In internal networks the administrators create the PVCs to direct applications or certain traffic to specific parts of the network. For example a common use would be to retain bandwidth and define a network path for video enabled applications such as video conferencing. Video needs specific quality to operate correctly so it makes sense to define specific routes – although this could also be done to block access to external video applications like Netflix.
SVC: (Switched Virtual Circuit) – an on-demand connection which is normally temporary between two stations. An easy way to visualize an SVC is something like a phone call which is a temporary connection created to transfer voice. Connections on an SVC will only last as long as necessary to complete the transaction, they are then taken down. Many carriers let customers establish these ‘on the fly’ or a carrier may set up a number of defined SVCs which can be used when required. Perhaps these could be useful for establishing internal secure channels such as a VPN or IP Cloaker application.
It’s best remembered that PVCs are most effective when there is a large amount of specific data anticipated between two locations on a regular basis. Using an SVC is much more suitable for temporary or recurring connections for example unscheduled video or voice conferences. Most commercial carriers prefer to set up PVCs because they are easier to manage bandwidth requirements in advance than SVCs. It is very common for PVCs to have monthly costs, rates or bandwidth allowances assigned to them making it easier to allocate costs and budgets against them.