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Monday, July 15, 2013

What is a virtual circuit? What are the advantages of virtual circuit?

The term VC or virtual circuit is synonymous with the term virtual channel or connection in the field of computer networks as well as in the telecommunications. 
- These are the connection oriented communication service.  
- The packet mode communication is the means through which this service is delivered. 
- After the establishment of a virtual circuit or connection between the two application processes or nodes, the stream of bytes or bit stream can be delivered between the two. 
- The higher level protocols are allowed by the virtual circuits in order to avoid the task of unnecessary dealing with the data division.
- This task may involve dividing data in to frames, packets or segments. 
The virtual circuits bear a resemblance to the circuit switching mode under the fact that both of them are connection oriented.
- This means that both of them require correct order of delivery of the data. 
Also, they both require signaling overhead during the establishment phase of the connections.
- The difference between the two is in the terms of the latency and bit rate. 
- It is constant in circuit switching and may vary in virtual circuits. 

This happens because of the following 3 major causes:
1. Varying length of the packet queues in the nodes.
2. Varying bit rate as generated by the application.
3. Varying load generated by the users who share the same resources on the network through statistical multiplexing. 

- A number of virtual circuit protocols are known for providing reliable services for communication but not all. 
- These services are provided by the means of data re-transmission because of the ARQ (automatic repeat request) and error detection. 
- Data-gram represents an alternate configuration for the virtual circuit. 

There are two types of virtual circuits namely:

Layer 4 virtual circuits: Data-link protocols such as TCP which are connection oriented and include segment numbering and thus reordering on the receiver’s side use this kind of virtual circuits. Thus out of order delivery is prevented. 

Layer 2/3 virtual circuits: The virtual circuit protocols of the data link  layer and the network layer are based up on the packet switching that is connection oriented. this implies that the delivery path of the data is always the same. 

Advantages of Virtual Circuits
There are several advantages of this kind of virtual connections:
1. They support the bandwidth reservation while the connection is being established.  This in turns increases the possibilities of QoS (quality of service). 
2. They produce less overhead. This is because of the fact that there is no individual packet routing and exclusion of the complete addressing info from the packet header. Each packet contains only a small VI or virtual channel identifier. The remaining routing info is provided to the network nodes during the establishment of the connection. 
3. Theoretically speaking, the nodes here have high capacity and are faster because their only task is to carry out routing. On the other hand, the network nodes in a connection-less network carry out routing for every packet individually. In switching, it requires only to look up the VCI in the table instead of analyzing the full address. Implementation of the switches is quite easy in the ASIC hardware but the complexity of the routing increases and demands software implementation. But as we know there is a huge market of the IP routers and layer 3 switching is supported by the advanced IP routers. 

Below mentioned are some protocols that provide VC facility:
- TCP or transmission control protocol
- SCTP or stream control transmission protocol
- X.25
- Frame relay
- ATM or asynchronous transfer mode
- MPLS or multi-protocol label switching

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