munications network into three simplified layers in order to clarify their structure and the tech- nologies that are used to implement their required functions. 2.7 Local-Access Network The local-access network provides the connection between the customer's telephone and the local exchange. Ordinary telephone and ISDN subscribers use two wires, a pair, as a subscriber loop, but for business customers a higher capacity optical fiber or microwave radio link may be required. Many differ- ent technologies are used in a local-access network to connect subscribers to the public telecommunications network. Figure 2.14 illustrates the structure of the local-access network and shows the most important technologies in use. Most subscriber connections use twisted pairs of copper wires. Sub- scriber cables contain many pairs that are shielded with common aluminum foil and plastic shield. In urban areas cables are dug into the ground and may be very large, having hundreds of pairs. Distribution points that are installed outdoor or indoor cabinets are needed to divide large cables into smaller Overhead Wireless Distribution distribution local loop point Optical cable Underground copper Office cables Concentrator Microwave n X 1.5 or 2 Mbps, or line of sight Local copper or optical cable distribution Factory exchange Figure 2.14 An example of a local-access network. Introduction to Telecommunications Network Engineering ones and distribute subscriber pairs to houses as shown in Figure 2.14. In suburban or country areas, overhead cables are often a more economical solu- tion than underground cables. An optical connection is used when a high transmission capacity (more than 2 Mbps) or very good transmission quality is required. A microwave radio relay is often a more economical solution than optical fiber when there is a need to increase data capacity beyond the capacity of an existing cable network. Installation of optical or copper cables takes more time because per- missions from landowners and city authorities are required. I