Carriers have two routes to ADSL testing

Subscribers are signing up for asymmetric digital subscriber line services in rapidly rising numbers worldwide. But for service providers to effectively provision all those new customers cost-effectively, they must have useful information about the lines.

Data gathered from line testing has to be collected end-to-end from the service provider's central office (CO) to customer premises equipment. Such intelligence will allow service providers to diagnose potential challenges before new DSL services are deployed, spot problems once service begins and devise solutions that in many cases will prevent the need for technicians to be sent into the field.

Such intelligence can be gathered through dual-ended line testing (DELT) or loop diagnostics mode. Standards bodies have adopted this testing measure under the ITU-T ADSL2 or G.992.3 numbering scheme. Single-ended line testing (SELT) is also under investigation by the ITU-T member companies as the standard called G.selt. SELT and DELT can be used together to yield the best results for the service provider. However, economics may force the providers to adopt one technology over the other.

In general, SELT is more useful for preinstallation loop qualification because the remote end need not be connected. It can be used for loop diagnostics when the loop impairment is so severe that DELT can't be used because of a load coil or short. But DELT is more useful when both ends are connected because it provides better loop diagnostics and more detailed information.

Understanding SELT

The primary force driving SELT is cost: Eliminating costly truck trips to the client's premises will enable service providers to roll out more DSL subscriptions and earn higher profits. SELT is used to identify the loop the CO will use to fulfill a new subscriber order before a modem is sent to the customer. SELT can be performed while the line is either open-ended or connected to a telephone.

SELT technology can be used to estimate and ascertain particular characteristics of the subscriber line. For loop topology, SELT can be used to determine loop length, the location of bridge taps, the length of bridge taps and the gauge of loop segments. For loop conditions, SELT can also help service providers determine where shorts are occurring in the line and the location of load coils. This gives the providers a clear idea about line noise and other interference characteristics.

Before SELT was developed, technicians tested DSL lines in a double-ended fashion using equipment at the CO and the customer's premises. SELT can perform the same probing tasks more economically because it occurs only on the CO side using ADSL hardware and chip sets. However, chip sets that can handle SELT require hybrid circuitry on the front end of the CO modem, a modification that can involve costs that may hinder competitiveness since it requires installation on every port.

An alternative approach is to deploy a dedicated SELT line card per each ADSL shelf or rack residing in the CO. Again, economic viability comes into question, because this method requires rerouting the ADSL line under test through a SELT line card in the main distribution frame and then back onto the subscriber line to the DSL port on the DSL access multiplexer. The technical feasibility of this method is also in question.

Time domain reflectometry is perhaps the oldest method used for SELT in telephone lines. A rectangular pulse is transmitted across the channel; the reflected pulse is then examined for time characteristics. By examining the pulses sent back to the CO, technicians can determine where a short in the line occurs or the presence of a load coil.

Frequency domain reflectometry is another method that extracts loop information from the spectrum of the channel. It is a sweeping sinusoid tone with varying frequency that is transmitted and measured for loop response at each frequency. The information sent back can be used to determine loop length in addition to the number of shorts and load coils. In short, the technique provides unique information about the loop but it cannot provide time-sensitive information such as location of these elements.

Deciphering DELT

The use of DELT provides three crucial benefits. The first is improving stability and connection rates for the current customer base by identifying loop anomalies. The second is identifying sources of transient noise and interference as causes for rate loss, connection drops and failures. The third is developing an up-to-date and evolving database of the copper plant and loop-specific data rate capacities. This last feature allows service providers to increase customer rates with prior knowledge of the limits of the loop.

DELT, as defined by the ADSL2 (G.992.3) ITU-T standard, falls under loop diagnostics mode. This built-in function enables the measurement of line conditions at both ends without dispatching maintenance technicians to attach test equipment to the end of the line. The information helps to isolate the location and the sources of impairments caused by crosstalk, radio-frequency interference and bridge taps. The standards board refers to the data collection as "DELT physical-layer technology" and the data-processing portion as "Loop Identification."

Both sides now

Loop diagnostics can be performed when the two sides of the ADSL connection (CO and customer premises equipment) convey all relevant "raw" channel information for processing. This information includes scalars such as loop attenuation, signal attenuation, signal-to-noise ratio margin, attainable net data rate and actual aggregate transmit power.

DELT can also be used to determine subcarrier information, such as channel characteristics, quiet line noise and signal-to-noise ratio. This information is vital in providing the best ADSL service. It is important for service providers to not only gather data but also to process and evaluate the info to improve on and resolve problems within their networks.

To generate tangible results from the information collected by applying the physical-layer technology of DELT to the line, a set of algorithms is devised to provide information pertaining to the subscriber line's loop topology. The information includes loop length, location of bridge taps, length of bridge taps, gauge of loop segments, and line noise and interference characteristics.

With the use of these specific algorithms, a service provider can determine important information about the network. When the loop is known to have one or no bridge taps, it is possible to fully identify the loop with great accuracy. If the loop is known to have two bridge taps, it is possible to find a reasonable estimate of the loop topology.

Afshin Shaybani is product-marketing engineer in networking and Jalil Kamali is senior staff software engineer at Centillium Communications Inc. (Fremont, Calif.).M

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