Extraction of two wire Loop topology using Hybrid Single Ended Loop Testing

Abstract

Performance estimation of a digital subscriber line (DSL) is critical for an operator to commit on the quality of service (QoS) requirements. The performance of the subscriber line depends on the line topology and the noise power spectral density (PSD).  Single ended loop testing (SELT) is the preferred and economical method for identifying the loop topology. In this paper SELT based on a combination of Correlation Time Domain Reflectometry (CTDR) and Frequency Domain Reflectometry (FDR) is proposed to identify the topology of a two wire line. The proposed CTDR and FDR use the existing modem for probing the line. In CTDR, complementary codes are used to probe the line and the reflections are correlated with the probe signal. This correlated signal is then analyzed to predict the loop. The prediction accuracy of the CTDR is limited but the main advantage is, it does not need any prior knowledge of the loop.  FDR uses the CTDR prediction as an initial estimate and employs optimization algorithm to accurately predict the loop. So the combined method has the advantage of predicting the loop accurately without any prior knowledge of the loop. As the SELT measurements are done online, the effect of cross talk and AWGN is considered. The developed algorithm is tested for standard ANSI loops and the results shows good prediction capability of this algorithm.