The CO switch routes the call to the destination subscriber, who may be served by the same CO switch, another CO switch in the same local area, or a CO switch reached through a long- distance network. Figure 4.1: The telephone network The portion of the telephone network that connects the subscriber to the closest CO switch is referred to as the local loop. Since all calls enter and exit the network via the local loop, the nature of the local connection directly affects the type of service a user gets from the global telephone network. With a separate pair of wires to serve each subscriber, the local telephone network follows a logical star architecture. Since a Central Office typically serves thousands of subscribers, it would be unwieldy to string wires individually to each home. Instead, the wire pairs are aggregated into groups, the largest of which are feeder cables. At intervals along the feeder portion of the loop, junction boxes are placed. In a junction box, wire pairs from feeder cables are spliced to wire pairs in distribution cables that run into neighborhoods. At each subscriber location, a drop wire pair (or pairs, if the subscriber has more than one line) is spliced into the distribution cable. Since distribution cables are either buried or aerial, they are disruptive and expensive to change. Consequently, a distribution cable usually contains as many wire pairs as a neighborhood might ever need, in advance of actual demand. Implementation of ISDN is hampered by the irregularity of the local loop plant. Referring back to Figure 4.3, it is apparent that loops are of different lengths, depending on the subscriber's distance from the Central Office. ISDN cannot be provided over loops with loading coils or loops longer than 18,000 feet (5.5 km). 4.0 Internet Access This section will outline the contrasts of access via the cable plant with respect to access via the local telephon network. 4.1 Internet Access Via Cable The key...
