Asynchronous-transfer mode (ATM) is a core telecommunications
technology that evolved from analog voice service, X.25 and
frame relay. ATM is the evolutionary technology that is best
able to fulfill the quality-of-service (QoS) requirements for
today's isochronous applications. It is likely that IP will
eventually be able to make similar claims, but there are still
issues to be resolved before streaming broadcast video can be
easily carried across public IP networks.  Figure 1. This
comparison between TDM and PSM shows how ATM uses PSM to service
high-priority traffic first. Image courtesy SBC-TRI.  Voice
networks started as analog transmissions over copper wires using
tube-type line amplifiers and mechanical crossbar switches. Such
early voice networks had several problems. They were error-
prone, expensive to lease and relatively unreliable for
transmitting data between facilities. But, at the time, it was
the best technology available. With the advent of solid-state
electronics and integrated circuits, this