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. Specifically, the coils are placed at 3000' intervals from either end, and at 6000' intervals along the wire. Load coils boost the strength of the voice frequency range (300 - 4000Hz) at the expense of high frequencies (anything > 4000Hz).

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

Here is where you'd expect load coils to exist on a copper loop:

  
  CO------------------------------cust (unloaded)

                                 <18Kft 
  
  CO-------LC---------LC------LC----cust

          3Kft       9Kft    15Kft  18Kft 

  

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

          3Kft       9Kft    15Kft    19Kft 

  

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

          3Kft        9Kft   15Kft      20Kft

Note: NO additional LC's added for 19 and 20ft loops because 4th coil would be too close to customer (<3KFt)

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

        3Kft       9Kft    15Kft  18Kft   21Kft 

LC = load coil