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Non-Intrusive xDSL Circuit Identification & Testing
By J. Hardin, Pleiades Telecommunications Consulting; November, 2014
Telco Network Evolution -
The Telecommunications industry has taken advantage of tremendous technological advances over the years,
enabling the development of communications networks having lower cost, longer reach, higher speed and
improved quality.
Beginning in the 1970’s, Telephone Companies worldwide adopted digital T1 / E1 standards for high-speed
digitized voice and data transmission over paired copper trunk facilities, which increased both speed and reliability
over previous analog technologies. This technological advance, while more cost-effective in bits/second per-unitcost
than previous methods, came with an understood tradeoff – more circuit vulnerability to external influences,
such as Telco technicians connecting test sets in the routine course of their daily jobs. Hence, Telcos provided
metallic access protection for these engineered “Special Service Circuits” by requiring non-conductive, color-coded
“caps” to be placed over T1 / E1 pair access terminals. Theoretically, this measure would both identify the circuits
as “Specials”, and also prevent inadvertent metallic test access by technicians; but in practice, these specials have
often been left un-capped, thus exposed to unintended interruptions. In addition, “live” circuit testing / monitoring
Instruments now required higher input impedances and balanced test cords to access T1 / E1 circuits without
causing excessive bit-errors. While analog networks had the advantage of “graceful degradation” of quality due to
external influences, the new digital technologies basically tended to offer an “all or nothing” proposition, with the
service being either totally up, or totally down.
In the 1990’s Telcos began replacing these distance-limited T1 / E1 trunk facilities with the more robust, greaterreach
“HDSLx” (High-frequency Digital Subscriber Loop) technologies of HDSL, HDSL2, and HDSL4. Despite offering
considerable price and reach advantages over T1 / E1 technology, HDSLx has proven to be more vulnerable to
external metallic access & test, often being interrupted by just connecting test set cords or bulk-access Front Tap
Shoe (FTS) cables. These circuits are still regarded by Telcos as “Specials”, and as such, are supposed to be
protected from inadvertent access by insulating caps….but often are not.
Beginning in the early 2000’s “ADSLx” (Asymmetric Digital Subscriber Loop) technologies ADSL, ADSL2, and ADSL2+
were incrementally deployed by Telcos to provide high-frequency data transmission to individual subscribers over
traditional POTS (“Plain Old Telephone Service”) cables. As such, they were not considered to be “Specials” by
Telcos, and were not capped at metallic access points. As a result, these services are more vulnerable to external
metallic access, with ADSL2+ being the most sensitive due to its higher bandwidth.
While ADSL2+ was the first technology to offer true “Triple Play” service (Internet Data, Television & Telephone) to
the subscriber over copper pairs, its capabilities were limited, and “VDSLx” technology (Very-high-frequency Digital
Subscriber Loop, an extension of ADSLx) was soon developed to provide the greater bandwidth needed for
simultaneous higher-speed Internet sessions and multiple HDTV channel delivery. VDSLx circuits, while also not
considered as “Specials” and therefore also not capped, have unfortunately proven to be even more susceptible to
external metallic connections. VDSLx circuits are supposed to be readily identifiable in cross-connect cabinets by
distinctive jumper wire coloring, but often they are not, using the same color wiring as POTS circuits. Although
Telephone Company records databases should provide accurate circuit assignment data for these services, they are
often not updated after repair & change activities; and thus cannot be relied upon to any extent.
Future improvements to VDSLx service, most recently with the developmental work on “G.fast” technology,
promise gigabit service to the subscriber. However, G.fast extends the required bandwidth even further than
VDSLx, and is thus more vulnerable to interference caused by “bridged taps”, discussed in the following section.
Unique VDSLx Problem: “Bridged Taps” -
Telcos deploying VDSLx service have spent, and continue to spend, much time and energy in pre-qualifying POTS distribution pairs prior to service cutover. One of the biggest obstacles to successful VDSLx operation has proven to be the presence of “bridged taps”, or lateral (parallel) wire-pair connections, which in the past have been widely used in POTS distribution cables to provide flexibility to support future subscriber connections. Potential VDSLx pairs should have all bridged taps removed prior to deployment of the service. This is due to their interference with the VDSLx modem’s advanced echo-cancellation algorithms, resulting in either the inability to synchronize (“train”) with the far-end modem, or the inability to attain the desired throughput speed. Longer bridged taps (>20 feet) have a more adverse effect on ADSLx circuits due to their lower frequency range. Conversely, shorter bridged taps (< 20 feet) cause more interference on VDSLx circuits due to their higher frequency range. Bridged taps as short as a few feet long can have a disastrous effect on the ability of a VDSLx modem to attain or maintain synchronization, especially when placed near the DSLAM modem.
Connecting a legacy test set’s 5-foot test cord to a live VDSLx circuit will have exactly the same effect as a 5-foot long bridged tap on the circuit. The most vulnerable location for a short VDSLx bridged tap is near the DSLAM, which is usually co-located with a cross-connect cabinet, and is thus one of the most common locations of technician activity. Circuit re-training can take from several seconds (T1 / E1 circuits) to two minutes (xDSL circuits). Often the circuit may not be able to re-train at all until the test set is disconnected.
To further complicate the problem of inadvertent, intrusive circuit access, technicians will most likely not even know they have taken a vulnerable circuit down during their routine installation and testing activities. For example, if a technician using a “Data-Safe” Butt-set gets a “Data” indication after accessing a VDSLx circuit, there is no indication if the circuit is still synchronized. Even if the disturbed circuit is immediately re-accessed and tested, the Butt-set will still show a “Data” indication due to the VDSLx Modem’s “training” tones being sent, with the technician having no indication that the circuit was ever taken down.
Service Disruption Ramifications –
For individual, non-business subscribers, the impact of losing internet access or television service for up to two minutes at a time or experiencing dropped phone calls is problematical, but not catastrophic. That being said, a two-minute TV interruption during a critical event in the Olympic Games, World Cup or Super Bowl could be maddening enough to cause the subscriber to consider switching to alternative TV services such as Cable or Satellite. However, continuing TV, Internet, or phone service interruptions, potentially caused by extra-normal technician activity at the VDSLx serving cabinet, will most certainly erode subscriber confidence in the serving Telco, eventually leading to service terminations and lost Telco revenue.
For business subscribers served by VDSLx, the problem is far more severe. Loss of internet service data, especially during critical monetary transactions, could have an enormous impact on businesses. “Quality of Service” agreements are routinely provided by Telcos to business customers, which specify the maximum allowable frequency and duration of service downtime; and are directly affected by the service intrusions described herein.
It has been estimated in the past that T1 / E1 service downtime for certain types of business transactions could cost up to thousands of dollars per minute lost, at a data bitrate of 192 / 256 kilobits per second. Consider the lost revenue repercussions at the much higher VDSL2 data bitrate of 50 to 100 Megabits per second.
The Quality of Service provided by the Telco directly affects the “Quality of Experience” by the subscriber, and in today’s competitive marketplace for information delivery systems, this commitment should be uppermost in the minds of Telco management.
Existing Product Solutions -
Up until now, Telephone Companies have had no viable options to safely identify live xDSL services and perform subsequent testing without disturbing these circuits. Existing options have thus far proven to be inadequate:
1. “Data-Safe” Butt-Sets - Some vendors have integrated POTS Splitter/Filter circuitry into their product “front-ends” to raise the high-frequency input impedance of the instrument. However, this does not address the problem caused by the capacitance of the test cord itself. By the time the Butt-set indicates that there is “Data” on the circuit, it is already too late to prevent potential xDSL re-training.
2. “High-Impedance” Test Cords – One solution to avoid circuit intrusion is to employ a test cord with high-impedance characteristics built-in to the cord itself, to reduce its effective capacitance. Although protecting the circuit during attachment, little testing can be performed with any accuracy. The special cord must be attached to safely access unknown circuit types, and then swapped with the standard cord for non-xDSL circuit testing….a cumbersome procedure, coupled with the lengthy test times required by existing instruments.
3. “Proximity” Detector Tools - “Proximity-access” (vs. metallic-access) electromagnetic-field-sensing products have been available for some time, which can safely detect T1/E1 and DDS circuit activity. However, even if upgraded for detection of the higher-frequency xDSL signal activity, these tools still have major drawbacks:
a. Due to the extremely low level of electromagnetically-coupled signals, it is not practically possible for proximity sensors to effectively distinguish between a true low-level xDSL signal and crosstalk from an adjacent pair; resulting in circuit mis-identification.
b. The proximity sensor’s AC-coupling mechanism effectively prohibits DC Voltage measurements.
c. Standard ANI / VF testing is not possible on xDSL-Over-POTS lines, due to non-metallic access.
Conclusion –
In order to meet the above requirements for an effective tool to allow Telco technicians to continue to do their routine jobs without disrupting service on the burgeoning number of xDSL circuits in the outside plant, a potential viable product must have the following features as a very minimum:
1. Metallic access (vs. proximity), allowing discrimination of very low-level signals, plus DC voltage measurement.
2. Very short test leads (< 6” long) with pair capacitance < 10 picofarads, minimizing connection “Impedance shock” and limiting the “bridged tap” effect to frequencies above the G.fast range (> 100 -200 MHz).
3. High input-impedance front-end circuit (> 100 kilohms), having minimal impact on 100 Ohm T1/E1/xDSL circuits, while still allowing low-level AC and DC voltage measurements.
4. Low-level signal measurement accuracy, allowing discrimination between true xDSL signals and coupled crosstalk from adjacent circuits.
5. xDSL signal discrimination, allowing the different types of xDSL technologies to be accurately identified.
6. Rapid measurement results; allowing quick circuit identification and testing.
7. Cost Effective, comparable to existing “Data-Safe” Butt-sets.
A recent product from Custom Assembly Incorporated, the xDSL Safe-ClipTM, provides all these features plus many more, allowing Telco technicians to be more productive with legacy test sets in the xDSL world; thereby extending the useful life of existing equipment and minimizing future replacement expenses:
1. Multiple, interchangeable Test Input leads -Providing clip-on access for all common terminal block types.
2. Isolated Test Output terminals - Allowing safe connection of legacy test sets and existing cross-connect cabinet test cords for VF testing and Broadband Monitoring operations.
3. Integrated POTS xDSL Splitter circuit - Switchable to Isolated Test Output terminals; allowing safe VF Dialing, ANI (Automatic Number Identification) testing, Voice Frequency measurement, and Circuit ID tone-tracing on xDSL-Over-POTS circuits using legacy test sets.
4. Broadband Monitor capability - Allowing the accessed T1/E1/xDSL signal to be safely monitored by legacy test sets at the Isolated Test Output terminals.
5. Test-through mode - Allowing direct (intrusive) testing of non-xDSL circuits via connected legacy test sets without having to remove and re-connect test leads.
6. User-Friendly Operation – With Intuitive controls and indicators, enabling rapid technician learning curves.
7. Hand-held & Battery-Powered - Allowing easy usage, storage, and simple, off-the-shelf battery replacement.
For more Safe-ClipTM information, go to http://www.customassemblyinc.com/products/category/safe-clip
Jim Hardin is President / Owner of Pleiades Telecommunications Consulting in Fort Worth, Texas, USA. He has more than 30 years experience in the Telecommunications industry, having held positions of Design Engineer, Project Manager, and Engineering Manager for large Telco provider companies such as Reliance Comm-Tec, Communications Technology Corporation, and Fluke Networks. Coming from a background of digital switching and transmission systems design, he supervised the development of a successful line of handheld and bulk-pair access copper test systems, utilizing a unique combination of Time Domain (TDR) and Frequency Domain testing to detect and locate Bridged Taps and cable faults on copper distribution pairs. These products are currently being used by a major U.S. Telephone Company in a nationwide pre-qualification program for subscriber cutover to VDSLx service.