 The methods used to transport telephone communications have evolved over time from the use of uninsulated open-wire pairs strung on poles to fiber optic systems capable of transporting over 100,000 simultaneous calls. The early open-wire systems gave way to the use of insulated twisted pairs, often bundled together in cables. It is common to- day for the cable drop to a residence to consist of six twisted pairs in a cable, and ca- bles containing as may as 2700 pairs are in use. Today there are many methods in use for transporting information through the net- work. Some of the more common methods will be described in this section. We prefer to use the term information because today's public switched telephone network (PSTN) transports not only voice but also various forms of data. It even carries portions of the Internet itself. First, we will introduce several general concepts that will aid in the un- derstanding of later sections of this chapter. Traditional telephony has as its roots the concept of a circuit. A circuit can be thought of as a dedicated pair of wires or a channel in a digital facility such as the DS-1 (de- scribed in Chap. 11). Traditional telephony creates connections between telephones connecting circuits end to end until the entire transmission path is complete from caller to called party. This approach is commonly referred to as circuit switching. The term circuit applies most precisely to a two-way point-to-point link. However, the term is also used to refer to a series of circuits connected through switches connecting two end points. Data communication uses a statistical approach instead. Communications between many devices on a data network may pass over the same transmission path, each iden- tified uniquely by address information imbedded in the communication. This sharing of a transmission path is referred to as statistical multiplexing. Many transmission technologies can transport far more bandwidth than is either re- quired, desired, or affordable for a particular data communications