What is "Twisted pair with loading" cable ? please explain and if possible please share a picture. Thanks,

Note: Below image is taken from: Data And Computer Communications - 8e - WilliamStallings

Point-to-Point Transmission Characteristics of Guided Media


A twisted pair cable with loading is a cable that has added inductance.

It is a cable used for voice lines. A long span of twisted pair wires acquires capacitance and that's the case with voice lines.

Capacitance is harmful to voice because it reduces voltage variations and in cosequence it tends to flat out the signal by blocking higher frequencies.

Coils are added to counteract capacitance on the twisted pair lines by adding inductance.

Load coils are placed at intervals along the wire. Load coils improve the amplitude of the voice frequency range (300 - 4000Hz) but are detrimental in frequencies above 4000Hz.

  • Thanks, I have searched the internet after reading your comment and found this useful for future readers; earlier i was looking into wrong direction. :: link May 19 '16 at 11:20
  • If I understand correctly,. loading coils have been removed and/or made unnecessary in many areas (of the US, at least) because it is not possible to provide DSL through a loading coil, so during the DSL boom, last mile infrastructure in the US was upgraded in many areas to remove/bypass loading coils. The number I remember for the distance between loading coils on a POTs line was 10,000 feet, which is why DSL required one to be less than 10,000 feet from the nearest central office. May 19 '16 at 15:38
  • @ToddWilcox That's true. To provide DSL the telco have to remove or replace the old coils with ones which are transparent to DSL.
    – jcbermu
    May 20 '16 at 8:34

Ideally for good transmission line behaviour you want the characterstic impedance of the line matched to the impedance of the devices. If they don't match then you get reflections which when translated into the frequency domain result in a non-flat frequency response.

Unfortunately telephone equipment is designed for an impedance of 600 or 900 ohms (depending on country) while twisted pair cable has an impedance of about 100 ohms. From some googling it seems the reason for this discrepancy is historical (bare wires on seperate insulators had a much higher characteristic impedance). On long phone lines this leads to poor frequency response.

The charactersitic impedance of a line is determined by the square root of the ratio of distributed inductance to distributed capacitance. We can fake an incresed characterstic impedance by periodically inserting inductors into the line. For a balanced line the inductors have to be installed in pairs to maintain balance.

These inductors were known as loading coils. This was done to improve the performance of long voice lines and I expect such lines are what the "twisted pair with loading" entry in your table refers to.

Unfortunately such coils only work correctly over a limited frequency range. For good transmission line behaviour the distance between coils needs to be small compared to the wavelength and the coils themselves must also have good inductor behaviour in the desired frequency range. Introdcing coils also reduce the effective propagation speed of the line. As a result loading coils have fallen out of favor in modern telephone networks.

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