TRANSMISSION ASPECTS OF VOICE TELEPHONY
Code for Load Coil Spacing
19H44, 24B88, and so forth. The first number indicates the wire gauge, the letter is taken
from Table 5.3 and is indicative of the spacing, and the third number is the inductance
of the load coil in millihenries (mH). For example, 19H66 cable has been widely used
in Europe for long-distance operation. Thus this cable has 19-gauge wire pairs with load
coils inserted at 1830-m (6000 ft) intervals with coils of 66-mH inductance. The most
commonly used spacings are B, D, and H.
Table 5.4 will be useful in calculating the attenuation (loss) of loaded loops for a given
length. For example, in 19H88, the last entry in the table, the attenuation per kilometer
is 0.26 dB (0.42 dB per statute mile). Thus for our 8-dB loop loss limit, we have 8/02.6,
limiting the loop to 30.77 km (19.23 mi).
When determining signaling limits in loop design, add about 15
per load coil as
a series resistor. In other words, the resistance values of the series load coils must be
included in the total loop resistance.
``Cookbook'' Design Methods for Subscriber Loops
Resistance Design Concept
. Resistance design (RD) dates back to the 1960s
and has since been revised. It was basic North American practice. Our inclusion of resis-
tance design helps understand the "cookbook" design concept. At the time of its inception,
nearly all local serving area switches could handle loops up to 1300
resistance. In vir-
tually every case, if the RD rules were followed, the attenuation limit of 8 dB would be
complied with. The maximum resistance limit defines a perimeter around a local switch
which is called the resistance design boundary. For subscribers outside of this boundary
served by the switch, long route design (LRD) rules were imposed. LRD is briefly covered
in Section 184.108.40.206.
The following additional terms dealing with RD are based on Ref. 5.
1. Resistance design limit is the maximum value of loop resistance to which the RD
method is applicable. The value was set at 1300
primarily to control transmission
loss. In the revised resistance design (RRD) plan, this value is increased to 1500
2. Switch supervisory limit is the conductor loop resistance beyond which the operation
of the switch supervisory equipment (loop signaling equipment) is uncertain.
3. Switch design limit. With RD procedures, this limit was set at 1300
(in RRD it
is increased to 1500
4. The design loop is the subscriber loop under study for a given distribution area to
which the switch design limit is applied to determine conductor sizes (i.e., gauges
or diameters). It is normally the longest loop in the cable of interest.