 NETWORKS (ISDN) 12.4.1 Background and Objectives The original concept of ISDN dates back to the early 1970s. Its design, in the context of the period, was built around the copper distribution plant (subscriber loop and local trunk plant). The designers saw and understood that by the early 1980s there would be a digital network in place controlled by CCITT Signaling System No. 7 (Chapter 13). It was revolutionary for its time by bringing 64-kbps digital channels right into the home and office. With the ISDN design, the 64-kbps digital channel handles: Voice telephony (digital) Digital data, both packet switched and circuit switched Telex/teletext Facsimile (e.g., CCITT Group 4) Conference television (56, 64, or 128 kbps) The goal of ISDN is to provide an integrated facility to incorporate each of the services listed above on a common 64-kbps channel. In North America, one gets the distinct feeling that the certain technologies [e.g., ATM (Chapter 18), frame relay (Section 12.5) and gigbit enterprise networks (Chapter 11), and EHF wireless local loops)] have leap-frogged ISDN. 12.4.2 The Future of ISDN ISDN as an entity has become obsolete in many parts of the world. It has specific channel configurations such as B and H channels, with fixed bit rates. Customers are demanding much higher data rates. ISDN is inflexible and cannot provide these rates such as 45 Mbps and higher. Frame relay has greater flexibility and can provide such data rates. A customer may also choose to use DS3 or E3 or even higher specific data rates where the only interface considered is intra-network. This may well be a proprietary network scheme. ENTERPRISE NETWORKS II: WIDE AREA NETWORKS ISDN does leave two of its subsidiary protocols, LAPB and LAPD. LAPD and its variants are used with frame relay. 12.5 SPEEDING UP THE NETWORK: FRAME RELAY 12.5.1 Rationale and Background It would seem from the terminology and the popular press that somehow we were making bits travel faster down the pipe. By some means we'd broken the velocity barrier by dr