me relay, ATM, PSTN/ISDN and wireless networks. Figure 12.4 A common packet backbone simplifies the interworking needs enterprise communications video and voice streaming Multi-service packet backbone networks need advanced QoS and high network resilience. Single faults should not impact the quality. Terminology: multi-service does not mean multimedia! In this case service equals a network service, such as PSTN or frame relay. Understanding Changing Telecommunications - Building a Successful Telecom Business The backbone does not really operate at a service media level, rather on a 'pipe level'. The pipe characteristics, priority etc. are set by the edge nodes and the control layer. If the backbone is operated exclusively with best-effort pipes there is no need for a control layer. The mediation between backbone and access is performed in the edge nodes. See Chapter 11. 12.3 CAPACITY DEVELOPMENT For the packet backbone there are two important dimensions to consider: The layered dimension and the distributed dimension. In order to understand the situation, a brief history may be useful. Early Phase After the Digitization of the Transmission. Switched Circuit Mode in Fixed TDM It is a well-known fact that ordinary voice calls for quite some time have been sent in time slots in a transmission system utilizing a particular transmission medium, such as a window in an optical fibre. Normally, the network designer did not care very much about the layering of this traffic within the medium. The transmission system was inflexible with manual cross-connection facilities in digital distribution frames in the nodes. Some traffic was through-connected and other traffic was dropped to the switching function of the node. Much attention was/is required regarding resilience in the case of faults and the hier- archical design of the backbone network at a local, transit and international level. The traffic was/is bundled by the switches into routes between the switching points. Packet Traffic in Leased Time Slots. Switched Packet M