3.4). Any traffic system is characterized by two main functions: resources and resource control. This is a programmed control, not to be confused with management. In ISDN, the concepts of user plane with the user traffic, and control plane were introduced and Understanding Changing Telecommunications - Building a Successful Telecom Business Management system Traffic system Customer network Customer management management IT & system management Network operation warehousing Service provisioning Transmission Internet ATM/FR PLMN/PSTN/ISDN Figure 3.3 The management and the traffic systems Telecom business Traffic system = Network M = Management system Figure 3.4 The triangle defined. In PSTN the planes are not described separately, and consequently there are no standardized interfaces between them. In the evolving horizontal network, the distinction between these planes is pronounced. An example is the split into a control layer and a connectivity layer. Packet-switched networks (or packet-mode networks) are sometimes regarded as having no control plane. Especially in connectionless networks, the control layer is difficult to identify, since for example routing is done for every single packet. (Connectionless = 'Any packet can theoretically take its own path to the receiving end'.) However, there Networks and Technologies is always a kind of control plane. In packet-switched networks it just partly deals with other issues than the tasks of the control plane of circuit-switched networks. For IP networks this is a heritage from the original US defence-based requirements on robustness, with quite a lot of intelligence located at the terminal devices, and a less intelligent but more autonomous network part in comparison with the circuit-mode networks. Among IP control functions are traffic supervision (ICMP, Internet control message protocol), routing (BGP, border gateway protocol, for routing to/from external domains; OSPF, open shortest path first, for internal routing; and IGMP, Internet group management protocol for 