INDAX is a local area home information system developed by Cox Cable Communications, Inc. for the delivery of videotex services to residential Customers. The large bandwidth provided by this medium allows integration of one-way (teletext) with two-way (videotext) transmissions into a cohesive delivery vehicle. As a data delivery mechanism, INDAX is well suited to many different types of servies. Among the earliest service offerings are generalized information retrieval and transaction services such as home banking and shopping. Implementation and traffic characteristics of these services are described in the following sections. In addition, INDAX supports interactive entertainment options such as opinion polling in conjunction with the standard entertainment programming. Status monitoring and control services such as security monitoring and energy management can also be delivered via INDAX. While the traffic model described is oriented towards a residential service, the modular system architecture readily supports differing traffic statistics by the addition of communications and processing modules.

This algorithm consists of two logically distinct parts: (1) a package of six F O R T R A N subroutines to facilitate the use of a special form of computer floating-point arithmetic that we call extended-range arithmetic; and (2) a FOR-T R A N subroutine that computes values of normalized Legendre polynomials according to an algorithm that generates (for some inputs) floating-point numbers that are outside the range of any computer.Our desire to produce a robust F O R T R A N subroutine to compute these polynomials stimulated the development of the extended-range software package.This package may prove to be useful for many other computations.Normalized Legendre polynomials are defined by the formula ~) (1 -x~) ~/2 d P.(x),where ~ and ~ are nonnegative integers, x is a real variable lying in the closed interval [ -1 , 1], and P,(x) is the ordinary Legendre polynomial.These functions satisfy a three-term recurrence relation in ~ that is useful in c o m p u t i n g / ~( x ) for fixed ~ and x, and sequences #1, ~1 + 1 , . . ., #2 of values of #.For stability reasons, the recurrence is applied in the backward direction, starting at # = r + 1 and # -v and proceeding through ~ -1, p -2 . . . . .#1. T h e starting value of P~(x) is determined from a first-order recurrence relation, and P;+l(x) = 0 for all r, x.When x is close to =i:l and r is moderately large, this method fails because of Recewed

Nahonal Bureau of StandardsAn algorithm is presented for the computation of normalrzed Legendre polynomials.In order to permit wide ranges of argument, degree, and order of these functions, an "extended-range" arithmetic is introduced whereby a separate storage location is allocated to the exponent of a floatmg-pomt number.Since this device may have other applications, separate subroutines are developed for addition of extended-range numbers and also for conversion to and from ordinary floating-point form.