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Digital Subscriber Line (DSL) Testing

3. xDSL Testing Needs

The following is a list of the testing elements required to support single-ended xDSL loop prequalification. The testing should be able to prequalify the loop for the common xDSL variants of IDSL, MDSL, SDSL, HDSL, CAP ADSL, and DMT ADSL. The testing needs for DSL–lite are expected to be the same as DMT ADSL.

• load coil detection—Load coils block the transmission of any xDSL on a loop by constricting the bandwidth. Prequalification testing must include the capability to determine if load coils are present in a circuit.
• measurement of line length—All xDSLs are sensitive to the line length between the CO or DLC and the subscriber's premises. xDSL throughput is inversely proportional to loop length. Prequalification testing must be able to determine the total length of the line accurately.
• detection of bridge taps—The length of bridge taps impacts the performance of each xDSL differently. Prequalification testing should determine the total length of all bridge taps. Some of the xDSL technologies available today claim not to be affected by the presence of bridge taps.
• wide band noise measurement—Noise or the presence of other transmissions in the same binder group can adversely affect some xDSLs. There are three categories of noise: 200-kHz noise, which impairs the performance of all 2B1Q DSL transmission (ISDN, MDSL, SDSL, HDSL); noise up to 1.1 MHz, which degrades the performance of ADSL services; and the presence of T1, which can significantly impair some ADSLs.
• metallic tests and longitudinal balance—Prequalification testing should include metallic fault and longitudinal balance testing. Metallic fault tests measure the standard tip-to-ring (T-R), tip-to-ground (T-G), and ring-to-ground (R-G) parameters including DC voltage and resistance along with AC voltage, resistance, and capacitance. The longitudinal balance of the loop is important for efficient operation of xDSL services. Most existing OSSs support these type of tests.

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TABLE OF CONTENTS: Definition and Overview 1 Need for xDSL Loop Prequalification 2 Methodologies for xDSL Loop Prequalification 3 xDSL Testing Needs 4 Discussion of Load Coils 5 Measurement of Line Length 6 Bridge Tap Detection 7 Crosstalk, Metallic Fault, and Longitudinal Balance Testing 8 Gaining Access to the Copper Loop for Test 9 References Self-Test Correct Answers Glossary Comment on This Tutorial