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

How to support a reliable communication in transport layer ?

At the Transport layer, each particular set of pieces flowing between a source application and a destination application is known as a conversation.To identify each segment of data, the Transport layer adds to the piece a header containing binary data. This header contains fields of bits. It is the values in these fields that enable different Transport layer protocols to perform different functions.

Reliability means ensuring that each piece of data that the source sends arrives at the destination. At the Transport layer the three basic operations of reliability are:
- tracking transmitted data.
- acknowledging received data.
- retransmitting any unacknowledged data.

This requires the processes of Transport layer of the source to keep track of all the data pieces of each conversation and the retransmit any of data that did were not acknowledged by the destination. The Transport layer of the receiving host must also track the data as it is received and acknowledge the receipt of the data. These reliability processes place additional overhead on the network resources due to the acknowledgement, tracking, and retransmission. To support these reliability operations, more control data is exchanged between the sending and receiving hosts. This control information is contained in the Layer 4 header.

Determining the Need for Reliability
Applications, such as databases, web pages, and e-mail, require that all of the sent data arrive at the destination in its original condition, in order for the data to be useful. Any missing data could cause a corrupt communication that is either incomplete or unreadable. Therefore, these applications are designed to use a Transport layer protocol that implements reliability.

The Transport Layer - Layer 4 of OSI model

The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.

- This layer converts the data received from the upper layers into segments and prepares them for transport.
- The Transport layer is responsible for end-to-end (source-to-destination) delivery of entire messages.
- It allows data to be transferred reliably and uses sequencing to make sure that the order of packets is maintained.
- It also provides services such as error checking and flow control.
- In case IP, lost packets arriving out of order must be reordered.
- The size and complexity of a transport protocol depends on the type of service it can get from the network layer.
- The transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer.
- Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.
- TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. It keeps a track of the packet delivery order and the packets that must be resent.
- UDP provides a low overhead transmission service, but with less error checking.
- The Transport layer protocols are either connectionless or connection-oriented.
- Connection-oriented means that a connection (a virtual link) must be established before any actual data can be exchanged. e.g. TCP.
- In Connectionless, the sender does not establish a connection before it sends data, it just sends it without guaranteeing delivery. e.g. UDP.

Data Segmentation

Data segmentation is the process by which the transport layer uniquely handles all data passed to and from different upper-level applications. For example, if a user is browsing the web and checking email at the same time, each program would be passing data and waiting for a reply on a unique port number. The Transport layer ensures that data is passed to the correct application.