Virtual – circuit and datagram networks are categories of packet switching network. In virtual circuits, a virtual path is established between the source and the destination systems through which the data packets are transferred from the source to the destination. In the other hand, in datagram networks, each data packet called datagram contain adequate header information so that they can be individually routed by all intermediate network switching devices to the destination.
|Issue||Virtual – Circuit Networks||Datagram Networks|
|Connectivity||Virtual – circuits are connection oriented networks.||Datagram networks are connectionless.|
|Path||In these networks, the path between the source and the destination nodes that is followed by first data packet gets allocated. All other data packets transmitted between them will use the same path.||Since the datagrams are treated as independent units, no dedicated path is fixed for data transfer. Each datagram is routed by the intermediate routers using dynamically changing routing tables. So two successive packets from the source may follow completely separate routes to reach destination.|
|Resource Allocation||All the resources like buffers, processors and bandwidth get reserved before the transmission. The same resources are consumed by all the data packets and are released only after the transmission ends.||No prior resources allocation is done for the individual packets before the transmission commences. When a packet arrives at a router, the resources are allocated on demand on a First-Come First-Serve (FCFS) basis.|
|Communication Phases||There are three phases of transmission: set up, data transfer and teardown.||No such communication phases exists.|
|Header||All the packets belonging to a message contain the same header information since they belong to the same virtual circuit.||The header information is different for the independent datagram packets, even if they belong to the same message.|
|Addressing||The addressing and route is determined at the setup phase. So, each packet contains only the VC number.||Each datagram packet contains the full source and destination addresses.|
|Cost||Virtual circuits are more expensive in terms of installation and maintenance,||Datagram networks are much cheaper to install and maintain.|
|Reliability and Complexity||They provide greater reliability and less complexity owing to fixed paths and fixed resources.||They require more complex logic. Also, they are less reliable than virtual circuits due to error-prone dynamic resource allocation techniques.|
|Application Areas||Virtual Circuits are implemented in networks using Asynchronous Transfer Mode (ATM) communications, as in making telephone calls.||Datagram communication is generally guided by User Datagram Protocol (UDP). They are applied in IP network.|
Virtual – Circuit Network − In the following diagram, we can see that a virtual circuit is created, as denoted by the dotted lines, and all the packets from the sender to the receiver are being routed along this virtual circuit.
Datagram Network: The following diagram shows datagram packets being send by host H1 to host H2. The four datagram packets labelled as A, B, C and D, all belonging to same message are being routed separately via separate routes.