SUBSCRIBER LOOP DESIGN
Work the following examples based on a maximum loss of 8 dB. Here we are to
calculate the maximum loop length for that 8-dB loss. Use simple division with the
values in column 2 of Table 5.2. The answers, of course, will be in kilofeet.
/0.615 = 13.0 kft
/0.51 = 15.68 kft
/0.41 = 19.51 kft
/0.32 = 25.0 kft
/0.21 = 38.1 kft
/0.14 = 57.14 kft
Copper is costly. Thus, many telecommunication companies employ gauges with diam-
eters no greater than 22 gauge in the local trunk plant and 26 gauge in the subscriber
Extending the Subscriber Loop
In many situations, subscribers will reside outside of the maximum subscriber loop lengths
described above. There are five generally accepted methods that can be used to extend
these maximums. They are:
1. Increase conductor diameter (covered above).
2. Use amplifiers and/or range extenders.
3. Employ inductive loading.
4. Use digital subscriber line (DSL) techniques (covered in Chapter 6).
5. Employ remote concentrators or switches (see Section 4.3).
Amplifiers in the subscriber loop extend the transmission range. Perhaps better said, they
compensate for loop loss. Commonly such amplifiers are set for about 7-dB gain. Care
must be used to assure that dc signaling is not lost.
. Inductive loading of a subscriber loop (or metallic VF
trunk) tends to reduce the transmission loss at the expense of amplitudefrequency
response beyond 30003400 Hz, depending on the loading technique employed. Loading
a particular subscriber loop (or metallic pair trunk) consists of inserting series inductances
(loading coils) into the loop at fixed distance intervals. Adding load coils tends to:
Decrease the velocity of propagation
Increase the impedance
Loaded cables are coded according to the spacing of the load coils. The standard code
for the spacing of load coils is shown in Table 5.3. Loaded cables typically are designated
A range extender increases the battery voltage to either
-84 or -96 V dc. In some texts the term loop extender
is used rather than range extender.
Velocity of propagation is the speed (velocity) that an electrical signal travels down a particular transmis-