Any appreciable listener echo on telephone lines is perceived as undesirable, and considerable effort is expended to hold echo to acceptable levels. To provide a basis for appropriately controlling listener echo, a series of four subjective tests was conducted at Bell Laboratories to obtain subjective evaluations of the effects of listener echo on telephone transmission quality. The subjective tests included conditions in which the listener echo‐path loss was flat or frequency‐shaped by selective filtering. The test results showed that subjective opinions for conditions with the same singing margin (minimum value of echo‐path loss) were highly dependent on the frequency shaping. A weighted echo‐path loss (wepl) is defined to provide a weighting on the frequency‐shaped test conditions so that subjectively equivalent test conditions have approximately the same levels of wepl. The results of these subjective tests are used to formulate a model of subjective opinion for use in network planning studies. The resulting listener echo opinion model is incorporated into an existing transmission rating model which encompasses loss, noise, and talker echo. Objectives are proposed for listener echo based on wepl.

No abstract

AT&T Information Systems Architecture defines a framework for compatibility among AT&T products as well as with key products of other vendors. A key direction of Information Systems Architecture is the use of standard, open interfaces and protocols. Standards now being defined for the Integrated Services Digital Network (ISDN) play an important role in the architecture. Support of ISDN will provide customers with uniform access to integrated voice and data capabilities which are provided through digital transmission and common channel signaling. Support of ISDN within Information Systems Architecture promises to provide customers with compatibility and high functionality. The AT&T Information Systems PBX product family already supports many capabilities that provide ISDN functionality. These capabilities make incorporation of ISDN in Information Systems Architecture and full support of ISDN a direct step in product evolution.

ISDN is being rapidly deployed in the United States. Each of the Regwnal Bell Operating Companies as well as several independent telephone companies currently offer ISDN commercial service through use of the basic rate interface. AT&T has been leading the way for intererchange carrier implementation of ISDN by providing primary-rate-interface-based services via direct connections to the AT&T service node. The major interexchange carriers have ako deployed Signaling System No. 7 throughout their networks. Equipment for ISDN implementation i s predominantly supplied by AT&T and Northern Telecom; however, European and Japanese vendors are ako active in the United States market.. The local exchange carrier 'sprimary motivation for centralofice based ISDN has been to provide business sewices which are competitive with digital PBX's. There are several impediments to residential ISDN which make it unlikely that it wit1 be common in the United States for several years. One of the most common configurations for deploying central ofice based ISDN is the use of remote vehicles. Remoting strategies have reduced cost through pair gain and, in some installotionst the ability to use T (rather than relatively erpensive U) line carakandeliminate NT1 's. One of the Iorgest drawbacks for ISDN Cenaer today is that it mists in ishnds where end-to-end ISDN functionality i s only available to subscribers on the same switch. Increasing deployment of Signaling System No. 7 by the local erchange carriers and interconnection with intererchange carrier SS7 networks should remedy this situation. Interexchange carriers are deploying the ISDNprimary rate interface to provide more attroctiw service oferings for customers who connect directly to their service node. Customer demand for the ATdiTprimary rate interface has been very strong. The single most sought after function on the primary rate has been delivery of calling party number which is usedpredominantly in telemarketing applicatwns. Other functions such as more eficient trunk usage, the use of switched HO and H l l channels and message associated user to user information are also drawing increasing attention. PBX vendors are placing initial emphasis on the primary rate interface in order to gain access to the nationwide ISDN capabilities offered by AT&T There has been less emphasis on the basic rate because PBX's already support sophisticated voice telephone functionality via proprietary intetfaces. These pmprietary solutions are generally lower cost than basic rate and support a wide range of supplementary services. However, these pmprietary interfaces do not currently support either passive bus or D-channel data finctionality. There is a very limited availobility of ISDN trunk arrangements, all of which are based on the primary rate, by which PBX's can access the h a 1 uchnnge cawier ISDN networks.