Diagnosing DSL Cable Pre-Qualification Troubles
By Ralph Marsh
Cable Location Consultant/Trainer
Austin, Texas
The last article in this series discussed the changing responsibilities of field technicians who are being required to handle pre-qualification of copper pairs for services such as internet connections, corporate networks, video distribution, and other sophisticated data communications applications. The required level of performance is determined, in part, by the category of broadband service to be provided by a pair.
While pair qualification for DSL service is important, this is only half of the outside plant broadband challenge. In today's telco environment, the value of individual pairs has increased with service demands. Since every pair is important to the bottom line, the traditional cut-to-clear approach to copper maintenance is not longer tenable, and operating companies can no longer simply abandon faulty pairs. Today the objective is to recover and qualify every pair for broadband performance levels in order to maximize the investment of the installed plant.
If a pair tests bad for data service, the cable repair technician will need to understand why, and be able to locate and repair the problem. In this article we will review common problems that cause copper pairs to fail data transmission qualification testing, and review trouble diagnosis and fault location leading to repair.
Recovering Pairs for Broadband
Broadband pre-qualification problems are generally caused by one of the following copper pair issues:
- Load Coils
- Bridge Taps
- Opens
- Resistance
- Noise/Cross-talk
Load Coils - Perhaps the most common broadband pre-qualification failure problems are related to line loading caused by unknown load coils and bridge taps. While a load coil optimizes POTS service by attenuating signals above 4Khz, xDSL signals are much higher in frequency, and thus are effectively blocked by these devices. Load coils on a cable pair can be detected using the Time Domain Reflectometer (TDR) test set function.
In the TDR mode, the test set generates a pulsed signal on the pair being tested, with a fast rise time that makes it possible to precisely measure the signal's return time. The pulse is reflected from the far end, returning to the input point with a measurable delay that represents pair length. The displayed TDR return signal reflects other important pair properties as well, including impedance, line losses, and the existence and location of splices and connectors.
Some units, such as the 3M™ Dynatel™ 965 DSP Subscriber Loop Analyzer, have a load coil count feature that can measure and confirm the presence of up to four loads on a pair, and determine the distance to the first coil. Any and all such coils must be located and removed to allow a pair to handle data level signals.
Bridge Taps - The TDR function will also identify and locate bridge taps, which add to the effective subscriber-to-C.O. distance. A pair carrying DSL service can tolerate a bridge tap if the aggregate pair length is within the absolute limit. For example, a 15,000 foot line length with a 3,000 foot bridge tap may be workable for a DSL mode that is limited to 18,000 feet - providing the tap is near the middle of the section rather than at either end, where it can cause signal reflection problems. Troublesome taps should be removed at the splice points.
Resistance Faults - A common line resistance problem involves earth return faults that result from some compromise in the cable shield, such as nicks or contact with metal conductors. In time, water can invade a damaged cable, and the smallest void in conductor insulation may eventually result in electrolysis that further complicates a faulty shield. This causes noise and signal degradation that will compromise broadband transmission long before it affects POTS signals.
Resistance measurements are used to resolve those pair deficiencies that are related to splice points, connectors and sheath or insulation quality. The pairs in question are strapped at the far end, and then measurements are taken with a resistance fault locator. Three megohms is typically the minimum insulation resistance that can be tolerated for broadband circuits. Pair resistance can be conveniently measured using a device such as the 3M™ Dynatel™ 965 DSP Subscriber Loop Analyzer.
The ability of a pair to support broadband service is dependent on the performance of all the connector modules, splices, and termination blocks in the loop. Each connector must be certified to deliver Category 5 service. Additionally, cables must be properly shielded, bonded and grounded. Insufficient bonding is a common signal integrity problem in a DSL network, resulting in noise because of electromagnetic interference conducted by the shield. Such problem can be avoided by checking to see that every cable segment is properly bonded at both ends and at each splice.
Balance - Cable pairs normally exhibit proper balance at installation, and if a balance problem is indicated, something has changed. There are two primary categories of balance problems: clean mechanical faults, and corrosion (partial) faults. Clean faults (opens, splits, shorts, crosses) are usually man-made, resulting from incorrect splicing, insulation damage, or a similar error. They often affect one wire in a pair to a greater degree than the other, and this makes such faults relatively easy to find and repair.
Corrosion-related balance problems can result in marginal performance that may be sufficient for POTS, but can have time-varying effects with DSL that are based on battery and loop current. This is particularly true for ADSL because this broadband mode does not incorporate a steady-state current, which serves to control corrosion. Voice signals sharing the line with ADSL will pull in loop current, and this can affect oxide layers and eventually degrade ADSL transmission. In contrast, the HDSL transmission mode uses a steady-state current, which can make marginal splices more stable.
The first step is to determine the nature of the fault - whether it is open or resistive. If a partial open is indicated, the operator moves on to the RFL or TDR functions to measure the distance to the fault, depending on the resistance level. Best practice is to shoot every resistance fault. If the -50V supply is removed, corrosion and balance problems will appear to be eliminated, but the line will fail as soon as power is restored. In the early stages of corrosion only the RFL function will result in a successful locate.
The 3M Dynatel test set mentioned earlier has an autotest feature that cycles through the most useful test modes and provides a useful pair status summary. When used in conjunction with a remote Far End Device (FED) to generate test signals, full-function devices such as this are particularly useful in analyzing, locating and repairing faults that affect broadband qualification. The slave FED is controlled by the test set, and places tones and terminations on the test line so that sequential measurements of insertion loss, slope, loop resistance and resistive balance can be made automatically.
Noise - In a zero noise environment, xDSL transmission operates with a relatively lower signal and at relatively greater distances. However, as noise increases, a stronger signal is required to overcome the noise, limiting transmission distance. Noise on a pair may be related to imbalance, or to cross-talk generated by data activity on another pair in the same binder group. This is a limitation of twisted pair transmission, and can generally only be resolved by moving one of the services to a different binder group. A test set's Spectrum Analyzer function is particularly useful in determining the exact nature of a noise problem so that the repair technician can zero in on both the problem and its location.
Experience shows that 30 percent or more of all cable trouble occurs at pedestals or terminals. If closure protection is not complete, splices will eventually degrade because of temperature cycling, moisture, or chemicals in groundwater that enters through a sheath fault. Careful housekeeping will help ensure that cable sections remain in broadband-ready condition. Such troubles affect broadband capacity well before POTS capacity is degraded.
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