Network Topology Supervision with Distributed Tags (17-Aug-2009)

Network Topology Supervision with Distributed Tags

Idea: Nils Asmussen

Traditionally, subscriber loops are used to carry voice telephone-calls on an analog basis via copper cable. Today the same loops also carry digital data by using DSL (Digital Subscriber Line) technology (see ITU (International Telecommunication Union) recommendations G.99x). Although glass fibers are better suited to transport data the existing copper network will not be replaced soon because replacing would be rather expensive.

The quality of voice and data services is influenced by the characteristics of the subscriber loop. Therefore, a network carrier needs recent information about the attenuation of the loop and about disturbing signals (e.g. crosstalk). The carrier can run measurements to collect this information. But without certain previous knowledge about the network topology or without sophisticated or costly test procedures the measurement results may be misleading. Finding the location of a loop fault is also a difficult measurement task. Therefore, the following questions need to be answered:

a) Are the correct devices connected to the loop?

b) Is the wiring correct? Does the loop lead to the right subscriber? If it doesn't, where is the wrong connection?

Usually, the loop is tested with an automated analyzer connected to the end of the Central Office of the loop. Certain DSL modems allow additional remote controlled measurements from the end of the subscriber such as DELT (Dual Ended Line Test) which is a test method involving DSL modems on both sides. If the results are not sufficient to localize a line fault, technicians have to be sent out to check the line. Then fault isolation becomes really expensive.

For identifying devices at the subscriber's end (CPEs: Customer-Premises Equipment) the electrical characteristics of these devices are measured. A telephone set in on-hook state has a ringer capacity that can be measured. These characteristics differ depending on the type of telephone, especially if it has an electronic ringer. A DSL modem (when used without a splitter) can be detected by an analyzer if it has a wetting current sink. A splitter or a DSL modem can be detected if it has an additional signature circuit consisting of components like diodes, resistors and capacitors. Interpretation of measurement results is rather difficult if several devices with various signatures are simultaneously connected to the loop. Besides, all these measurement results are degraded by the line attenuation.

Mostly, there is no chance to find a wrong connection at a distribution frame by only measuring from one end of the loop.

Detection of bridged taps may be possible with SELT (Single Ended Line Test) which is a test method involving DSL-modem in the Central Office. This test requires ADSL2 or VDSL equipment. Unfortunately, the accurac...