sed. While these technologies have been widely used, for secure and reliable inter- connections, physical overlays are expensive because they provide a dedicated set of resources to each customer. Packet network technologies allow resources to be Part 3 Network Technologies shared and SO offer a more economical solution. Frame Relay (which itself was developed from the earlier 64 kbit/s X.25 network standard) typically operates over 2-Mbit/s lines, but it can be made to operate at higher speeds if needed. ATM (which uses short packets called cells) operates at line speeds of up to 155 or 622 Mbit/s and is structured to support realtime traffic. The operation of IP networks was described in Volume 1. Here the transport of IP data across a core network using MPLS switching is described in Chapter 13. IP/MPLS has proved to be successful and is now widely adopted in the center of core networks. However, it is less easy to scale to large numbers of meshed nodes. An alternative solution for metropolitan areas based on Ethernet principles is being developed through the IEEE and is known as "Carrier-Grade Ethernet." Ethernet itself does not scale to carrier network applications nor does it have suit- able management facilities. However, the modified version currently under devel- opment through the IEEE will have a scalable architecture and full operations, administration, and management facilities; it will have a speed target of up to 100 Gbit/s. This is described in Chapter 14. It remains to be seen to what extent Carrier-Grade Ethernet can displace a firmly embedded IP/MPLS infrastructure. 10 Synchronous Optical Networks and Synchronous Digital Hierarchy Mark Matthews Nortel Ltd. Goff Hill GTel Consultancy Ltd. Introduction The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are standards that were designed for multiplexing together many low-rate digital traffic chan- nels (in particular, 64-kbit/s voice channels) into higher-rate channels SO that the low-rate channels can be more efficient