Wire and Four-Wire Circuits When you release energy into space, it loses power as it travels. Because networks were designed to carry communications over a distance, they need tools to regen- erate attenuated signals-that is, signals that lose power as they travel across the network. These tools are called amplifiers and repeaters. An amplifier boosts an attenuated signal back up to its original power level SO it can continue to make its way across the network. The PSTN traditionally used copper wires. Based on how quickly the signals attenuate over the copper wires, there's a certain distance limi- tation between amplifiers. The distance limitation between amplifiers is relatively short on copper wires-generally about 6,000 feet (1,800 m). As networks were built, these distance considerations were kept in mind. (Repeaters are discussed later in this chapter, in the section "Digital Transmission.") Network builders had to give some thought to another aspect of amplifiers: First-generation amplifiers were unidirectional. They could only amplify a signal moving in one direction, SO to provision a circuit that was going to be crossing a dis- tance, it was necessary to literally provision two circuits-one to amplify the infor- mation in the transmit direction and a second to amplify the information in the receive direction. Therefore, whenever a network was crossing a distance, it needed to use a four-wire circuit. But in building the millions of local loops for subscribers, it was seen as being cost-effective to pull only two wires into every home rather than four. Therefore, the local loops were intentionally engineered to be very short; some 70% to 80% of the local loops worldwide are less than 2 miles (3.2 km) long. Because the local loops are short, they don't need amplifiers, and therefore the sub- scriber access service can be provisioned over a two-wire circuit. However, the local loop is increasingly being digitalized, so as we migrate to an end-to-end digital envi- ronment, everything becomes four-wire. Fig