Unit 1

Lesson 1 - Connectionless and Connection-Oriented Networks

An IP network is an example of a packet-switched, connectionless network, as shown on the Packet-Switching Network Diagram. The packet switches in an IP network are really network routers. Many packets can be moving between the same nodes at the same time along different paths. Packets in a packet-switched network are also referred to as datagrams, although the former is technically a subset of the latter.

Packet-Switching Network

Connection-oriented networks establish a physical connection between two nodes. The data entity is passed between the endpoint nodes through intermediate nodes (switches) along a predetermined route. Connection-oriented networks are analogous to the voice telephone system, and such connections are often termed virtual circuits. The virtual circuit establishes a single route for data between the nodes that does not vary for the life of the connection. Therefore, they are connection-oriented. IBM's Systems Network Architecture (SNA) and Asynchronous Transfer Mode (ATM) are examples of circuit-switched networks. This is illustrated on the Circuit-Switched Network Diagram.

Circuit-Switched Network

Circuit-Switched Network

In the early days of data communications, all networks were circuit switched. More recently, emphasis has shifted to packet switching simply because it permits the interconnection of far more nodes into a single network. With packet switching, fewer communication channels are required because they are shared by many users, and interconnection of networks is much easier to accomplish.

Layering of data communications protocols, discussed later in this unit, gives rise to situations where the lower layers of a network are connectionless, but the higher layers establish a [logical] connection--an example being Transmission Control Protocol (TCP). The opposite, where the lower layers establish a connection and the upper layers do not, can also occur.

Characteristics of Packet- and Circuit-Switched Networks

There are benefits and drawbacks to the different types of networks, depending on requirements and application. Some of these are:

Packet-switched networks carry addresses in every packet, making them more flexible with regard to paths.

Circuit-switched networks may have additional setup time if the circuit is not established on a permanent basis. Once the circuit is established, data can move without a lot of decisions at the intermediate nodes.

Packet-switched networks are generally more economical than circuit-switched networks because of their ability to share traffic.

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