range extenders with gain that automatically switch their gain setting to provide the 3- or 6-dB net gain as required. This automatic switching removes the need to maintain and administer transmission zones. From this standpoint, MLRD is a single range-extended zone. 5.4.6.3 Concentration with Range Extension and Gain (CREG). The CREG plan is designed for use with finer-gauge copper pair cable. It can accomplish this by providing VF amplifier gain behind a stage of switching concentration. It employs H88 loading beyond 1500 S2. Any two gauges in the combination of 22, 24, or 26 gauge may be employed. Gain and range extension applies only to loops beyond the 1500-S demarcation. 5.4.6.4 Digital Loop Carrier (DLC). Of the three long route design techniques, digital loop carrier is the most attractive, especially for facilities greater than 28,000 ft. Many of these DLC systems are based on T1 digital techniques (described in Chapter 6) with specially designed terminals. One such T1 system can serve up to 40 subscriber loops over a single repeated line consisting of two wire pairs, one for transmission in one direction TRANSMISSION ASPECTS OF VOICE TELEPHONY and one for transmission in the other direction. Such a system is typically used on long routes when relatively high subscriber density is forecast for the planning period and when such feeder routes would require expansion. Another advantage of DLC is that it can provide improved transmission loss distribu- tions. One such system displays a 1000-Hz transmission loss of 2 dB between a serving switch and a remote terminal regardless of the length of the digital section. The low insertion loss of the digital portion of such systems allows up to 6 dB to be apportioned to the analog subscriber loop distribution plant. 5.5 DESIGN OF LOCAL AREA WIRE-PAIR TRUNKS (JUNCTIONS) 5.5.1 Introduction Exchanges in a common local area are often connected in a full-mesh topology (see Section 1.3.7 for a definition of full mesh). Historically, depending on distance and certain other