Southwestern Bell Telephone Co says it has started testing for ISDN primary access interface. The primary access feature enables call processing capabilities between a Siemens EWSD public telephone exchange and customer premises equipment carrying 23 B channels at 64Kbps each and one D channel used for signalling information. Southwestern Bell started applications testing for basic rate ISDN capability last year and, as reported then, currently has four customers for its product: Tenneco Inc and Shell Oil Co in Houston, 3M in Austin and AT&T Network Systems in St Louis. The first ISDN lines will be cut over to Tenneco on June 1.
