PDF of this tutorialThe history of subscriber and loop carriers is based on the initial deployment of loop electronics, which was driven primarily by transmission needs (i.e., trying to obtain better-quality transmission over longer distances). Hence, deployment was not very economically influenced, nor was there much emphasis on pair-gain capabilities. There were a small number of lines and relatively few subscribers. The line subscribers, typically, were at extended distances from the central office, with no way to receive high-quality service. Some of the early subscriber carriers were actually just analog devices intended to extend service and deliver improved voice quality.
Analog Carrier Systems
Almost every aspect of the analog carrier system was based on 1960s technology—the advent of electronics. The main driver was greatly improved voice quality over long distances. There were some modest pair-gain capabilities: up to four to one. But that was not the key element of the system. Furthermore, there were downsides to the old analog-carrier subsystems: maintenance was difficult, and quality was inconsistent. Basically, once the analog carrier was working, it was fine, but it was not easy to get it working correctly.
The analog carrier architecture was fairly simple. It consisted of an analog box deployed on the subscriber side. There was little need for many special services, and so there was minimal demand for the architecture to support them. On the network side, plain old telephone service (POTS) was analog, and this is what made the network difficult to maintain, set up, and make operational. But the system served its purpose in delivering high-level service to small numbers of subscribers when they were at distances greater than the central office could serve.
Figure 1 depicts a typical example of the old carrier system.
Figure 1. Analog Carrier System
The analog box where the multiplexing took place was set in the central office. Signal was carried over the existing copper cable, and there were wall-mounted devices positioned close to the customer's service drop. There were usually two or three—and sometimes as many as eight—subscribers. But the system was difficult to get working, and it certainly would not be acceptable for the coming future.
Digital Loop Technology
The early 1970s brought digital loop carrier technology. This new type of carrier was based on digital-technology electronics, and it aimed for the performance benefits of digital transmission. It offered the voice-quality upgrade that subscribers were looking for. In essence, the new system was economically driven. There were significant pair-gain capabilities, usually 12 to 1. It was more feature-rich than the old analog carrier system and capable of deploying some of the newer services such as digital data service (DDS) and some of the analog specials. But more importantly, it was more reliable, easier to install, and easier to maintain.
The architecture of the new system was not very different from the analog system—it is just based on digital technology. On the subscriber side, there was, again, analog POTS along with DDS and a TR57–type suite of electronics. The network side typically was digital, which allowed for the pair-gain capabilities.
Figure 2 shows a universal DLC configuration.
Figure 2. Universal DLC Configuration
There is a central office terminal or a local exchange terminal near the subscriber. A remote subscriber terminal could usually pick up 1 to 96 lines. A good example of this is the old 96-type carrier system, in which T1 carrier electronics between the terminals provided the pair-gain benefit. In addition, it was very economical to deploy.
