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Principal Sponsors:
 | Digital Subscriber Line (DSL) Testing |
2. Methodologies for xDSL Loop Prequalification
There are two methods of prequalification testing: single-ended and double-ended testing. Single-ended testing requires test equipment only at the CO or digital loop carrier (DLC) system. Double-ended testing requires test equipment at both ends of the copper loop. In the single-ended case, a copper pair can be tested from the CO or DLC location without involving on-site technicians, and a judgment can be made on the ability of the line to support xDSL transmission (see Figure 1). The single-ended tests enable preservice loop testing from a centralized location and provides information on necessary loop repairs (e.g., remove load coils). Single-ended testing can provide a 90 to 95 percent confidence that a copper loop will support xDSL service. The single-ended methodology eliminates the need to dispatch a technician and is more compatible with automated testing, which will be required to support the projected volume of xDSL services.
One approach to the single-ended xDSL prequalification solution involves current loop testing, OSS, and plant records. The OSS will measure loop length and rely on plant records for both current and previous cable routings to gauge if bridge taps and load coils are present. When ISDN services were first offered, however, some LECs found this method ineffective for loop prequalification. Using current testing techniques and plant records alone is not sufficient to prequalify a copper loop for xDSL services.
In the double-ended testing case, a technician is dispatched to the customer's location to install a modem or test equipment at the customer location that communicates with the reference modem in the CO and a device connected to the customer's network interface device (NID) (see Figure 2). If the service is inoperable, a work order is issued to clean up the pair. Using a reference modem, however, may not provide sufficient information as to why the loop will not support xDSL transmission and may not support a customer service date because the plant may need work prior to initiating service. Because of the extra expense associated with dispatching a technician, the LECs are unable to cost-justify double-ending testing and meet the competitive pricing of cable modems.
Ideally, the LECs will require two techniques when using the single-ended approaches for xDSL prequalification. These prequalification tests should be executed either in a batch mode or on demand. The batch mode will be used to profile an office or area to determine which subscribers can be offered DSL service and to estimate the amount of work required to condition the routes to support DSL if so desired. The on-demand capability is used to determine if a particular customer's line can support xDSL transmission for data or voice/data services. The on-demand tests should not be run while the customer is on the line because of the level of voiceband frequencies that will be used to detect load coils. On-demand tests are also typically run after the necessary loop conditioning has been completed both to close-out the work order and enable a rerun before the service commit date to verify that the service will be available as promised. The predominant POTS–testing OSSs are capable of providing either batch or on-demand testing.
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