etworking, despite its obvious advantages. A big reason is simply that it is different in concept and management from the packet-switched and routed networks that form the basis current-day LANs and WANs. ATM is different and complex (perhaps the most complex networking scheme ever developed). Hardware-intensive switches require the overlay of a highly sophisticated, software intensive, protocol infrastructure. This infrastructure is required to both allow individual ATM switches to be linked into a network, and for such networks to internetwork with the vast installed base of extant LANs and WANs. Packet-switched Networks How ATM Works Adaptation Physical layer layer layer Data cell + header Voice, text and The adaptation The ATM layer The physical video data needs to layer divides the adds a 5-byte layer prepares the be sent over the data into 48-byte header with cell for the physical network. pieces, called cells. signaling and transport (coaxial, service information. twisted pair, fiber optic) in use. Physical Physical layer layer layer Switch The switch uses physical and ATM layers to guide cells to their destination. The physical layer prepares the cells for a different physical transport, if neccessary. Physical Adaptation layer layer layer 6 The recipient reverses the implementation layers to remove the header and reassemble cells into the proper data format. Figure 12-1. How ATM Works (Laura Long). Video Communications: The Whole Picture Cost and complexity have prevented organizations from implementing ATM in the LAN, but the WAN (which is largely managed by carriers) is different. Carriers have not aggressively sold ATM, and are now purging it altogether as they transition to running IP directly over SONET or directly over optical networks. As virtually all traffic (voice, video, and data) becomes IP, large-scale MPLS networks are displacing both ATM and SONET. CONCLUSION The connection-oriented nature of ATM might have guaranteed it a permanent quality advantage over connectionless networks, were it n