Line terminal for determining operational status of a U interface

A line terminal for determining operational status of a U interface connecting the line terminal to a digital service unit, the line terminal comprising:

• a time presetting module configured for presetting a plurality of time cycles for the line terminal, wherein the time cycles increase progressively;
• a current detecting module configured for detecting a loop current flowing via the U interface;
• a cycle selecting module configured for selecting a first time cycle from the plurality of time cycles upon the condition that the loop current is detected by the current detecting module;
• a timer configured for time tracking in a first time cycle selected from the plurality of time cycles by the cycle selecting module, wherein the current detecting module is further configured for determining if the loop current is continuously detected flowing via the U interface before the timer times out in the first time cycle;
• a synchronization determining module configured for determining if the line terminal has been synchronized with the digital service unit upon the condition that the loop current has been continuously detected flowing via the U interface and before the timer times out in the first time cycle;
• a timing determining module configured for determining if all time cycles from the plurality of time cycles have been selected by the cycle selecting module upon the condition that the line terminal has not been synchronized with the digital service unit before the timer times out in the first time cycle, and further configured for selecting a second time cycle from a remainder of the plurality of time cycles if not all the time cycles from the plurality of time cycles have been selected by the cycle selecting module, wherein the operational status of U interface is determined to be working normally upon the condition that the line terminal has been synchronized with the digital service unit before the timer times out or to be working abnormally upon the condition that all the time cycles have been selected from the plurality of time cycles and the line terminal has not been synchronized with the digital service unit;
• and
• at least one processor to execute the time presetting module, the current detecting module, the cycle selecting module, the timer, the synchronization determining module, and the timing determining module.