In general, a communication network is a transport system for message traffic between people and systems. A communications network provides the end-to-end connection between the end subscribers or the subscriber devices so that they can communicate with each other.
In terms of spatial extent, communication networks are divided into local area networks( LAN), access networks ( AN), metropolitan area networks( MAN), wide area networks(WAN) and global area networks( GAN). In their physical structure, communication networks can work with radio waves, as well as with electrical signals on wired transmission media, via light signals in optical fibers, or via infrared beams in the case of fixed connections.
In terms of switching technology, communication networks can be circuit-switched, packet-switched or cell-switched. Furthermore, communication networks can be differentiated according to the type of transmission paths, interfaces, subscriber devices and transmission methods, and last but not least, the services supported.
The development of communication networks over the years shows a wide variety of network structures and switching technologies. Depending on the conception, these networks were and are company-oriented like the corporate networks( CN), service-oriented like the value-added networks( VAN) or protocol-oriented like the IP networks. The current state of development relies on end-to-end quality-of-service networks, which in most cases operate as transport networks on Layer 2, such as Frame Relay, ATM, Leased Lines, or as Virtual Private Networks( VPN) on Layer 2, such as Permanent Virtual Circuit ( PVC) over Frame Relay( FR) or Virtual Circuit( VC) over ATM. These concepts rely on the overlying mechanisms, such as IPsec, to establish VPNs on Layer 3.