ttery feed circuit illustrated in Figure 5.3. Battery voltages have been standardized at -48 V dc. It is a negative voltage to minimize cathodic reaction. This is a form of corrosion that can be a thermal noise source. 5.4.2 Subscriber Loop Length Limits It is desirable from an economic standpoint to permit subscriber loop lengths to be as long as possible. Thus the subscriber serving area could become very large. This, in turn, would reduce the number of serving switches required per unit area affording greater centraliza- tion, less land to buy, fewer buildings, simpler maintenance, and SO forth. Unfortunately, there are other tradeoffs forcing the urban/suburban telecommunication system designer to smaller serving areas and more switches. The subscriber loop plant, sometimes called outside plant, is the largest single invest- ment that a telecommunication company has. Physically, we can extend a subscriber loop very long distances: 5, 10, 20, 50, or even 100 miles. Such loops require expen- sive conditioning, which we will delve into later in this chapter. It is incumbent on a telecommunication company to optimize costs to have the fewest possible specially conditioned loops. The two basic criteria, which limit loop length, must be considered when designing a subscriber loop: 1. Attenuation (loss) limits 2. Resistance limits Attenuation (loss) must be limited to keep within loudness rating requirements. Loudness rating was discussed in Section 3.2. If a subscriber loop has too much loss, the telephone user signal level suffers and she/he cannot hear the signal well enough; the user may consider the connection unsatisfactory. In North America the maximum loss objective 8 dB for a subscriber loop. In some other countries, that value is 7 dB. Remember that it takes two subscriber loops to make a connection: the subscriber loop of the calling subscriber and the loop of the called subscriber. The attenuation is referenced to 1000 Hz in North America and 800 Hz elsewhere in the world. In other words, when we measure