It is argued that the high bit rate digital subscriber line (HDSL), permitting low-cost 1.5 Mb/s copper access, will ease the transition to fiber access by accelerating the use of higher speed services. Copper will dominate over fiber customer access for at least the next ten years. During this period, the success of high-speed switched services will depend on the connectivity provided by both fiber and copper access. HDSL will initially be used to serve private-line DS 1, ISDN primary rate access, and digital loop carrier feeders. Later, the HDSL will be applied to switched services such as metropolitan area networks (MANs) and circuit switched DS1s.>

This paper discusses a powerful priority mechanism that can be used on a physical communication medium supporting multiple, independent, logical data links. This scheme allows a high priority message to immediately interrupt a lower priority message which is in the process of being transmitted, and yet will not require the retransmission of the first part of the lower priority message; hence, the term graceful preemption. Graceful preemption may be extended to furnish multiple priority levels similar to existing processor interrupt structures which implement multiple levels. This protocol may serve a single processor which handles several different processes, or several independent stations that share a common bus. Hence, priority levels can be enforced without the need for a centralized bus “master” or “arbitrator” unit. As an example, the application of graceful preemption to the Integrated Services Digital Network (ISDN) link access procedure, LAPD, is discussed.