| Electrical Communication is a free textbook on the basics of communication technology. See the editorial for more information.... |
|
Home  Telephone Toll Service and Systems Telephone Repeaters |
|
 Search the VIAS Library | Index
|
  |
Telephone RepeatersThe attenuation loss of the largest size telephone open-wire line is 0.03 decibel per mile (page 222), and of the largest and heaviest loaded telephone cable about 0.27 decibel per mile (page 253)12. The loss that can be tolerated in a given section of line depends on many factors, such as the grade of service and the amount of crosstalk and noise present (Chapter 14). For open-wire lines the line transmission loss should not exceed about 10 decibels, and for a cable (which is less susceptible to noise) about 20 decibels. This means that for voice-frequency telephony,1 effective transmission over open-wire copper lines of the largest size wire requires amplifiers at most about 300 miles apart. For voice-frequency transmission over the largest and heaviest loaded cables amplifiers must be at most about 60 miles apart. The amplifiers1 used in telephone lines and cables are often small class A power amplifiers (page 291) for the voice-frequency circuits.1 The entire amplifying device is called a telephone repeater, defined1 as "a combination of one or more amplifiers together with their associated equipment for use in a telephone circuit." If the signal magnitude is permitted to fall until it is comparable to the noise and crosstalk (that is, if the signal-to-noise ratio becomes low), then the signal is masked out and cannot be recovered. Also, if the signal is amplified at any point until it is strong compared with the signal strength in adjacent circuits, the strong signal may cause excessive crosstalk to these adjacent circuits.
|
|
| Home Telephone Toll Service and Systems Telephone Repeaters |
|
Last Update: 2011-05-30