emembered that the spacing of this first coil is most critical to achieve acceptable return loss and must be placed as close to the recommended location as physically and economically possible. 5.4 SUBSCRIBER LOOP DESIGN 5.4.5.2 Long Route Design (LRD). The long route design procedure uses several zones corresponding to the resistance of the loop in excess of 1300 S2. Of course, each subscriber loop must be able to carry out the supervisory signaling function and meet the 8-dB maximum loop attenuation rule (North America). LRD provides for a specific combination of fixed-gain devices (VF repeaters/amplifiers) to meet the supervision and loss criteria. On most long loops a range extender with gain is employed at the switch. A range extender (loop extender) boosts the standard -48 V by an additional 36-48 V, and an amplifier provides a gain of 3-6 dB. Inductive loading is H88. Any combination of cable gauges may be used between 19 and 26 gauge. 5.4.6 Present North American Loop Design Rules There are three subscriber loop design methods in this category: RRD (revised resistance design), MLRD (modified long route design), and CREG (concentration range extender with gain). 5.4.6.1 Revised Resistance Design (RRD). RRD covers subscriber loop as long as 24 kft. Loop length is broken down into two ranges: From 0 to 18,000 ft the maximum loop resistance is 1300 S2, and from 18,000 to 24,000 ft the maximum loop resistance is 1500 S2. H88 loading is used on loops longer than 18,000 ft. Two gauge combinations may be employed selected from the following three wire gauges: 22, 24, 26 gauge. 5.4.6.2 Modified Long Route Design (MLRD). Loop resistances up to 1500 S are served by RRD procedures. The range beyond 1500 So is served by MLRD, CREG (see below), or DLC (digital loop carrier; see Chapter 6). Under MLRD, loop resistances from 1500 to 2000 So are placed in the RZ18 category and require 3-dB gain. The loop resistance range from 2000 to 2800 So is designated RZ28, and loops in this range require 6-dB gain. New switches have 