Difference Between Virtual Circuits And Datagram Networks

Virtual Circuits

Virtual circuit (VC) is a means of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data.

Before the data transfer begins, the source and destination identify a suitable path for the virtual circuit. All intermediate nodes between the two points put an entry of the routing in their routing table for the call. Additional parameters such as the maximum packet size are also exchanged between the source and the destination during call setup. The virtual circuit is cleared after the data transfer is completed.

Types of Virtual Circuits

• Permanent virtual circuits (PVCs): the switches are set up manually by the communication management station and offer performance comparable to dedicated lines.
• Switched virtual circuits (SVCs): the switches are set up automatically when a communication is established.

What You Need To Know About Virtual Circuits

• Virtual circuits are networks that provide only a connection service at the network layer.
• There is a reservation of resources like buffers, CPU, bandwidth, etc for the time in which the newly setup VC is going to be used by a data transfer session.
• Virtual circuits are implemented at the edge of the network in the end systems.
• Packets always reach their intended destination in the order in which they were sent. 
• First packet goes and reserves resources for the subsequent packets which as a result follow the same path for the whole connection time.
• The virtual circuit provides a reliable communication function to ensure that each group arrives correctly and the group remains in the original order in which they were sent.
• Virtual circuits are costly to implement because each time a new connection has to be set up with reservation of resources and extra information handling at routers.
• In virtual circuits, destination site address (global header) is only used for the first packet of the connection.  Other packets uses a short virtual circuit number and generally do not require destination site address (global header).
• All virtual circuits passing through the faulty router cannot work.
• It is used by the ATM (Asynchronous Transfer Mode) Network, which is used for telephone calls.

Datagram Networks

A datagram is a basic transfer unit associated with a packet-switched network whereby each packet is routed independently through the network. A datagram is primarily used for wireless communication and is self-contained with source and destination addresses written in the header.

The intermediate nodes examine the header of a packet and select an appropriate link to another node which is nearer to the destination. In this transfer unit, the packets do not follow a pre-established route and the intermediate nodes do not require prior knowledge of the routes that will be used.

The individual packets which form a data stream may follow different paths between the source and the destination. As a result, the packets may arrive at the destination out of order in which they were sent. When this occurs, the packets will have to be reassembled to form the original message.

Because each packet is switched independently, there is no need for connection set up and no need to dedicate bandwidth in the form of a circuit. Datagram packet switches use a variety of techniques to forward traffic; they are differentiated by how long it takes the packet to pass through the switch and their ability to filter out corrupted packets.

Types of datagram packet switches

• Cut through: does not filter errors, it switches packet at the highest throughput, offering the least forwarding delay.
• Store and forward: Protects data until the entire packet is received and checked for errors.
• Fragment Free: filters out most error packets but doesn’t necessarily prevent the propagation of errors throughout the network.

What You Need To Know About Datagram Networks

• Datagram networks are computer networks that provide only a connectionless service at the network layer.
• There is no need for reservation of resources as there is no dedicated path for a connection session.
• Datagram networks are implemented in the routers in the network core as well as in the end systems.
• Packets reach their intended destination in a different order in which they were sent.
• All packets are free to go to any path intermediate router which is decided on the go by dynamically changing routing tables on routers.
• Datagram networks are not reliable because the connectionless property makes data packets reach the destination in any order, different from the order in which they were sent.
• Datagram networks are always cost effective and easy to implement there is no problem of reserving resources each time an application has to communicate.
• In data gram networks, every packet is free to choose any path and therefore each packet must be associated with full address of the destination site (header).
• Faulty routers may lose packets and some routes may change.
• It is generally used in the IP network which is used for data service like internet.

Between Virtual Circuits And Datagram Networks In Tabular Form