11.2 LOCAL LOOP

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11.2 LOCAL LOOP

The local loop, which is a dedicated connection between the terminal equipment (the telephone) and the switch, is the costliest element in the telephone network. Generally, from the switch, a cable is laid up to a distribution box (also called a distribution point) from which individual cable pairs are taken to the individual telephone instruments.

To reduce the cable laying work, particularly to provide telephone connections to dense areas such as high-rise residential complexes, digital loop carrier (DLC) systems are being introduced. The DLC system is shown in Figure 11.4. The telephone cables are distributed from the DLC system, and the DLC is connected to the digital switch using a single high-bandwidth cable.

Figure 11.4: Digital loop carrier (DLC) in PSTN.

To reduce the installation and maintenance effort (finding out where the cable fault is), wireless local loops are now being introduced. Wireless local loops (WLLs) using CDMA technology are becoming widespread. The advantages of WLL are (a) low maintenance costs because no digging is required; (b) low maintenance costs because equipment will be only at two ends (either the switch or the distribution point and the terminal equipment; (c) fast installation; and (d) possibility of limited mobility.

Most of the present local loops using copper can support very limited data rates. To access the Internet using the telephone network, the speed is generally limited to about 56kbps. Nowdays, user demands are increasing for voice and video services that cannot be supported by the present local loops. Hence, fiber will be the best choice so that very high bandwidth can be available to subscribers, supporting services such as video conferencing, graphics, etc. In the future, optical fiber would be the choice for the local loop. Experiments are going on to develop plastic optical fibers that can take slight bends and support high data rates. Plastic optical fiber would be the ideal choice for fiber to the home.

Because the data rate supported by twisted pair copper cable is limited, optical fiber will be used as the preferred medium for local loop in the future to provide high-bandwidth services. For remote/rural areas, wireless local loop is the preferred choice because of fast installation and low maintenance costs.