One version of splitterless ADSL service requires microfilters to separate ADSL signals from POTS signals. The data rate that a subscriber can buy may be limited by the characteristics of the subscriber's local loop. Therefore, some customers may not be able to subscribe to the service they want because their local lines won't support their preferred data rates. For the telco to know how each local loop performs, it must test each loop. Here's another place where ADSL testing has moved out of the lab.
Technicians use portable equipment to test the local loops for throughput and noise.
The local-loop measurements also help set the maximum and minimum rates of service that the ADSL modem can use. If noise appears at certain frequencies during the tests on a local loop, the technician can find the source of the noise and crosstalk in the bundle of wires that contains the subscriber's loop. For example, noise at 772 kHz indicates that noise is coming from a T1 line in the same bundle. That frequency is the Nyquist frequency of the T1's 1.544 MHz. Right now, ADSL service that U S West sells requires subscribers to use the ATU-R equipment that the company provides.