What are Virtual Circuits?
– Virtual circuits or VC (in short form) are components of computer networks and telecommunications.
– A connection oriented communication service related to the virtual circuits is known as the virtual channel and is provided via the packet mode communication.
– Once a virtual circuit or connection has been set up between two application processes or more commonly nodes, it is then possible to deliver a byte stream (bit stream) between these two.
– In order to avoid the higher level protocols to deal with the data division in to various units such as frames, packets or segments, virtual circuit protocol.
– To some extent there is a resemblance between the virtual circuit communication and circuit switching because of both being connection oriented.
– This implies that in both the cases, there is delivery of the data is done in desired order and while the connection is still being established, a signaling overhead is needed.
– One major difference between circuit switching and virtual circuit is that the former provides latency and constant bit rate while in the latter these factors vary due to the following reasons:
- Varying length of the packet queue in the nodes of network.
- Variations in the bit rate produced by the application.
- Variation in the load from the several users that share the resources on a network via statistical multiplexing and so on.
– There are a number of virtual circuit protocols but all of these do no guarantee reliable communication services via data re-transmission due to factors such as the ARG (automatic repeat request) and error detection.
– Datagram is an alternate network configuration for this.
Types of Virtual Circuits
There are two types of virtual circuits as discussed below:
1. Layer 4 virtual circuits:
– Protocols such as the TCP which are connection oriented layer 4 (transport layer) protocols depend up on the connection less packet switching network layer protocols.
– Here, the packets have to be routed to different paths and thus delivery does not take place in an orderly way.
– There are possibilities of virtual circuit because of the inclusion of the segment numbering in TCP.
– This will help in the prevention of the out of order delivery.
2. Layer 2/3 virtual circuits:
– Virtual circuit protocols of the data link and the network layers are based up on packet switching that is connection oriented.
– This implies the delivery of the data always takes place through the same network path.
– This one offers more advantages than the previous one.
What is Circuit Switching?
– Circuit switching is just another means for the implementation of the telecommunications network where a circuit or a communications channel is established by two network nodes.
– This connection guarantees for the full bandwidth and the connection remains throughout the communication session.
– The functioning of the circuit is like if there is a physical electrical connection.
– In packet switching the data is divided in to packets that are then transmitted independently.
– Here, the packets from various sessions share the same network links which leads to the loss of QoS (quality of service) provided by the circuit switching.
– Circuit switching provides a constant bit delay during the connection which is not the case with the packet switching.
– In packet switching there might be very indefinite packet transfer delays.
– In circuit switching, if currently no communication is taking place, the circuit remains as such in protection from the users.
– Virtual circuit switching is a technology based up on packet switching that is used for the emulation of the circuit switching.
– The connection is thus established before the transfer of the packets and the delivery of these packets takes place in order.
– For connecting the voice circuits, circuit switching is commonly used.
– The main advantage is that there is a continuous transfer of the packets thus maximizing the use of the bandwidth available for the communication.
– Disadvantage is that it is inefficient when compared to virtual circuit switching.