ISDN & DSL Line Conditioning

(removal of Load Coils)

Standard voice runs over POTS (Plain Old Telephone System) lines.  Unfortunately, POTS lines cannot carry ISDN or DSL !! 

Line Conditioning and Load Coils

The longer a telephone line is, the more noise it will incur.  For good voice quality on long lines, extraneous noise (and noise is high frequency) is removed by insertion of inductors, or Load Coils, placed every 3000 feet or so, in serial with the lines.  The load coils inhibit high frequencies, which are needed for digital signals.  Therefore, for every ISDN and DSL line, all the load coils must be either removed or bypassed.  This is called line conditioning, and is a major headache, because finding the load coils is not an easy task.

Standard voice phone calls degrade noticeably when the copper portion of a phone line is > 18Kft long. In order to restore call quality, load coils are inserted at specific intervals along the loop. Load coils are a simple low-pass filter - they allow the voice frequency range (300 - 4000Hz) to pass, and they impede the high frequencies (anything > 4000Hz).

Since DSL and ISDN depend on frequencies > 4Khz, neither can pass through a load coil. They require 'unloaded' copper pairs.

Where the Load Coils are for POTS circuits

We need to make sure that the distances between the CO, load coils, and customer are always between 3000 and 6000 feet !!!  So, going from the CO (Central Office) toward the customer:

1. the first load coil is placed 3000 feet from the CO (Central Office)
2. the next load coil is place 6000 from that last load coil
3. repeat inserting load coils at 6000 foot increments, until you get to within 6000 feet or less from the customer end.  At this point, you place the final load coil as follows:

• if exactly 6000 feet from the customer end - insert a load coil in the middle (3000 feet from the customer end and 3000 feet from your last load coil)
• if less that 6000 feet, do NOT insert another load coil.  This does mean that the final load coil may be more than 3000 feet from the customer, but there is no way to insert a load coil in this case, and have both distances stay at 3000 feet or more

Here are a few examples of where you'd expect load coils to exist on a copper loop.  The loop runs from the CO (Central Office) to the customer:

  

Example:  18,000 feet from CO to Customer

  

  CO---------LC------------------LC------------------LC---------cust



            3Kft                9Kft               15Kft        18Kft 



  

Example:  19,000 feet from CO to Customer

  CO---------LC------------------LC------------------LC------------cust



            3Kft                9Kft               15Kft           19Kft 



  

Example:  20,000 feet from CO to Customer

   CO---------LC------------------LC------------------LC---------------cust



            3Kft                9Kft               15Kft              20Kft 



  

Example:  21,000 feet from CO to Customer

   CO---------LC------------------LC------------------LC---------LC---------cust



            3Kft                9Kft               15Kft      18Kft        21Kft





 

Note that there were NO additional load coils added for the 19,000 and 20,000 foot loops.  This is because, if they added a 4th coil, it would be too close to customer (<3KFt)

Example:  a line conditioned for ISDN or DSL

all load coils have been removed:

  CO------------------------------------------------------------cust 

           (unloaded - Line is conditioned for ISDN or DSL)