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.6

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.

5.4.4.1 Inductive Loading. Inductive loading of a subscriber loop (or metallic VF trunk) tends to reduce the transmission loss at the expense of amplitude-frequency response beyond 3000-3400 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 propagation7

• 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

6A 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.

7Velocity of propagation is the speed (velocity) that an electrical signal travels down a particular transmission medium.

Table 5.3 Code for Load Coil Spacing

Code

Spacing

Spacing

Letter

(ft)

(m)

A

700

213.5

B

3000

915

C

929

283.3

D

4500

1372.5

E

5575

1700.4

F

2787

850

H

6000

1830

X

680

207.4

Y

2130

649.6

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 Q 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.

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