Conclusion

There are two basic types of switching – circuit switching and packet switching. A physical communication channel is established for the exclusive use of a call in a circuit-switched network. In contrast, physical communication channels are shared in a packet-switched network. Packet switching is described as store-and-forward because a switch will store packets and then forward them when resources become available.

Networks can provide connection-oriented and connectionless services. In a connection-oriented service, a connection must be established before user data is transferred, whereas this is not necessary for a connectionless service. The connections need not be physical channels, and generally exist to organise the flow of packets rather than to reserve resources.

The OSI reference model is an approach to the separation of the communication functions of systems into seven layers: application, presentation, session, transport, network, data link and physical. The concept of layering is common with most protocols, although the seven layers advocated by OSI are not always followed. The vertical flow of data is represented by primitives being issued and received across layer boundaries. The four basic primitive types are: request, indication, response, and confirm. The flow of data between peer layers in different systems is called horizontal communication and is governed by protocols, which enable a layer to provide services to the layer above. One or more protocols is available to each layer. A protocol defines the structure and the meaning of protocol data units and therefore how the systems communicate. A protocol data unit in a higher layer is encapsulated in a protocol data unit in a lower layer.

Each layer, except the lowest, works at a level of abstraction based on the services provided by lower layers. For instance, higher layers can ignore the problems that arise with specific physical media, such as optical fibre and twisted pair, because the physical layer deals with problems associated with the types of physical medium.

The Internet is based on the TCP/IP protocol suite, which generally has a four-layer model: application, transport, internetwork, and host-to-network. The hypertext transfer protocol (HTTP) is an example of an application layer protocol, but there are several others. The principal transport layer protocol is the transmission control protocol (TCP), which provides a connection-oriented service. A three-way handshake provides a reliable means of establishing a connection. Sequence numbers allow any missing data to be identified. The main internetwork layer protocol is the internet protocol (IP), which provides a connectionless service. IPv4 addresses identify all hosts on an internetwork and have 32 bits. The number of bits allocated to host and network addresses is determined either by the uppermost bits if the class form of addressing is used, or by masks if the classless form of addressing is used. The forwarding of IP datagrams is accomplished by forwarding tables, which are set up by the routing process of an internetwork. Entries in a forwarding table denote either a direct path or an indirect path for a network address. In the former case the router is able to forward the datagram over a direct connection. In the latter case the final destination is on a network to which the router does not have a direct connection.

The asynchronous transfer mode (ATM) protocol is based on 53-byte cells. The ATM layer provides a connection-oriented service and several service categories are available: constant bit rate, real-time variable bit rate, non-realtime variable bit rate, unspecified bit rate, available bit rate and guaranteed frame rate. A virtual connection is identified by the combination of a virtual path identifier and a virtual channel identifier.

One approach to providing IP over an ATM network is label switching. Edge routers of an ATM network convert between forwarding equivalence classes (i.e. significant parts of IP network addresses) and labels. The labels are transferred in the virtual path identifier and/or virtual channel identifier fields in ATM headers.