nstallation of cables is also very expensive when they must be sunk into the ground. One technology for implementation of ordinary subscriber loops for fixed telephone service is known as wireless local loop (WLL). WLL uses radio waves and does not require installation of subscriber cables; it is a quick and low-cost way to connect a new subscriber to the public network. With the help of this technology, new operators can provide services in an area where another old operator owns the cables. WLL is also used for replacement of old fixed overhead subscriber telephone lines in rural areas. When cable network capacity for subscriber connections needs to be increased, it may be more economical to install concentrators, remote sub- scriber units, or subscriber multiplexers SO as to utilize existing cables more efficiently. We use one of these terms to describe the switching capability of the remote unit. Concentrators may be capable to independently switch local calls among the subscribers connected to them. A remote subscriber unit is basically the subscriber interface part of the exchange that is moved away close to the subscribers. Subscriber multiplexers may only connect each subscriber to a time slot (channel) in the PCM frame. The detailed functionality of these systems depends on the manufacturer, but we can say that only those sub- scribers who have picked up their handsets reserve a channel to the local exchange. Digital transmission between an exchange and a concentrator fur- ther improves cable utilization SO that two cable pairs serve tens of subscribers. We have explained the access alternatives shown in Figure 2.14 mainly from a fixed telephone service point of view, but they can also be used to pro- vide access to the Internet. Technologies used for Internet access are explained in Chapter 6. 2.7.1 Local Exchange Local or subscriber loops connect subscribers to local exchanges, which are the lowest-level exchanges in the switching hierarchy. These are the main tasks of the digital local exchange: The T