g voice stream through a process of digitization called Pulse Code Modulation (PCM) convert the analog voice into a digital bitstream that can be transported over the digital carrier. (Codecs are discussed in Chapter 1, "Telecommunications Technol- ogy Fundamentals," and PCM is discussed later in this chapter.) Beyond the channel bank, the multiplexing hierarchy steps through the indi- vidual transmission levels. In the case of T-carrier, the levels are T-1 through T-4; for E-carrier, they are E-1 through E-5; and for J-carrier, they are J-1 through J-5. The Transport Network Infrastructure DLCs DLCs-also called remote terminals, concentrators, or remote concentra- tors-were introduced in the mid-1970s, specifically as a way to economically expand the telco network. They were deployed to improve efficiency and to lower costs. DLCs reduced analog facilities by up to 80%, and they led to building, real estate liquidation, and maintenance efficiencies as well. They also eliminated the need for loading coils, which are used to improve transmission on wire pairs for distances greater than 3.4 miles (5.5 km). DLCs also reduced the number of pairs of copper wires required between the local exchange and the subscriber; they did this by sharing pairs or transmission facilities among many multiplexed conversa- tions. Essentially, the DLC architecture, shown in Figure 4.6, reduces the loop lengths and makes more effective use of high-capacity trunks from a neighborhood into the local exchange. DLCs continue to evolve, and as they do so, they become smaller systems. The original DLCs were built so that an individual system could service around 600 subscribers, but these boxes achieved only about a 50% fill ratio, which meant that half of the capacity was not being used. Now, given the distribution and density of neighborhoods and populations, smaller DLCs are being created. These systems service up to about 96 subscribers, and utilization is at around a 90% level. These smaller DLCs allow for faster service rollout and a sho