The Measure Loop Delay/ERL application provides the capability to measure and display loop delay and echo return loss (ERL) on one or more timeslots. Multiple instances of this application may be opened simultaneously, allowing measurements to be taken on multiple timeslot ranges and measurement strategies.
Modes of operation. This application provides two basic modes of operation:
Theory of Operation:
- Non-Intrusive: Two GL boards are used, one of which monitors the original signal while the other monitors the returned or looped-back signal. The board monitoring the original signal operates in outward loopback mode, so that the signal is forwarded by the board hardware while being monitored by this application.
- Intrusive: One or two GL boards are used. A signal is injected into one or more timeslots and the returned signal is monitored. The injected signal may be sourced either from a file containing A-Law or µ-Law encoded signal values, or may be a white noise (Gaussian) signal generated internally from within the application. The drop/insert feature of the GL boards may be used to pass through the traffic in channels not being tested by the application.
The application computes the delay by taking snapshots of the data stream on the receive and send paths. Snapshots are taken at approximately 2-second intervals, or as soon thereafter as permitted by the speed of the host machine. The send path snapshot is larger than the receive path snapshot by an amount corresponding to the maximum delay. It then computes the statistical correlation between the receive path and send path data streams at each successive point in the send path stream. The offset giving the highest correlation is taken as the delay in the returned signal. The return loss (ERL) is then calculated as the ratio of the average power in the corresponding segments of the receive path and send path streams. The procedure is shown in block diagram form below.
The measurement procedure is tempered by several caveats:
- There must be a minimum average power in the original signal. The threshold is -40 dBm over a 32 ms segment. If this threshold is not met by at least one portion of the original signal within a snapshot, the application judges that there is insufficient signal strength to perform a meaningful correlation. In this case, the current delay and ERL values are left unchanged.
- "Only when Off Hook" has been checked, but the channel is not off hook. In this case, the application displays "XX" for the delay and "X.X" for the ERL of the affected channel.
- Infinite return loss has been detected. This situation is somewhat similar to the first caveat above, but it is now the send path signal that has insufficient power to perform a meaningful calculation. This condition is declared if the computed return loss is greater than 60 dB, or if the signal power in the receive path is below -65 dB. In this case, the application displays "0" for the delay and "-inf" for the ERL of the affected channel.
- A "double-talk" condition has been detected. Double-talk is declared when the computed ERL is less than 6 dB. The application judges that the returned signal has been corrupted by injection injected into the double-talk