PDF of this tutorialThe 1980s brought the advent of the next-generation digital loop carrier (NGDLC), which is now common. The NGDLC was based primarily on very large-scale integration (VLSI) technology, so it took advantage of many of the electronics enhancements that computers were driving. That technology began to be integrated into the local loop and resulted in very cost-effective electronics. It was used primarily as a service-delivery tool. ISDN was the buzzword then—many of the NGDLC architectures were actually designed to be able to deploy ISDN to every single customer. Although that need did not materialize, it was still a good system for delivering high bit-rate services such as T1, ISDN specials, and DDS.
As far as transport on the network side, most of these systems were designed for fiber. So, the key element as far as deployment of this system was concerned was making sure that fiber ran between the central office and the remote subscriber’s terminal. Unfortunately, that was not always the case. Nonetheless, if fiber was in place between the two locations, this was a very good system and the intent was to serve large numbers of customers.
Early NGDLC was optimized for high concentrations of subscribers, up to 2000 lines. As with the previous carrier system, this system was economically driven. The key was to connect high concentrations of subscribers over fiber very economically. The system aimed at advanced services distributed over wide areas, which was the important downfall of this type of device. Another important feature enhancement of this device was software-based remote provisioning. It became the de facto standard that as one wanted to increase service capacity or change services, everything had to be software downloadable. Finally, early NGDLC had a time-slot interchanger. This is a key aspect of the cost-effectiveness of the technology.
This architecture builds on earlier ones. On the subscriber side, it carried a host of analog services: analog POTS and private line; DS–0 switch 56; DS–1/E1; ISDN basic-rate and primary-rate interface; analog specials; DDS, and subrate data. All of those services were important factors in deploying this type of system. On the network side, the system was typically fiber. Eventually, some of the systems migrated to copper because it became apparent that fiber was not always available in those types of network pathologies. So, the primary interface on the network side was generally OC–1 or OC–3. Later there was some nxT1/E1 and digital-transmission capability over copper.
Figure 3 shows a typical network architecture of an early NGDLC configuration.
Figure 3. Early NGDLC Configuration
There was a central office terminal which was typically a large device—it contained a whole shelf worth of common control. The fact that there had to be an investment in the shelf of common control in order to get the first subscriber up and running made the system less cost-effective than was ideal at low line sizes. It was, however, very cost-effective at high numbers of subscribers. A remote terminal would be deployed close to a heavily populated area and the plan was to serve about 2,000 subscribers in order to obtain the maximum benefit.
