This paper investigates the relationship between open and closed models for networks of queues. In open models, jobs enter the network from outside, receive service at one or more service centers, and then depart. In closed models, jobs neither enter nor leave the network; instead, a fixed number of jobs circulate within the network. Open models are analytically more tractable, but closed models often seem more realistic. Hence, this paper investigates ways to use open models to approximate closed models. One approach is to use open models with specified expected equilibrium populations. This fixed-population-mean method is especially effective for approximately solving large closed models, where “large” may mean many nodes or many jobs. The success of these approximations is partly explained by limit theorems: Under appropriate conditions, the fixed-population-mean method is asymptotically correct. In some cases the open-model methods also yield bounds for the performance measures in the closed models.
There are two ways to profile the behavior of a program: timing and counting. Timing is traditional in UNIX™ operating systems. This paper describes an easy implementation of count profiling, and gives several examples and applications. It has been implemented on the Motorola 68000, VAX™, and AT&T 3B20 computers.
This is the second issue of the Technical Journal devoted exclusively to papers on the family of computer operating systems bearing the UNIX trademark of AT&T Bell Laboratories. The UNIX operating system was created in 1969 by K. Thompson and D. M. Ritchie. Its growth since then, in both the commercial world and the research community, has been truly remarkable.
Like all digital transmission systems, digital subscriber loop carrier seems destined to play an ever-increasing role in modern communication networks. This trend derives its strength from basic and pervasive factors: (1) the great ease with which digital transmission can support new services, particularly those related to any form of data transmission; (2) the seemingly inexhaustible potential of digital integrated circuit technology to realize ever-decreasing cost, size, and power-per-circuit function; (3) the great promise digital transmission holds for synergy with digital switching. This paper provides an overview to a series of papers that describe in detail AT&T Technologies eminently successful entry in this market, the SLC® 96 Subscriber Loop Carrier System. Emphasis is placed on the background leading to the development and the forces that shaped the system's architecture.
