March 7, 2001
Issues in Pair Qualification for Broadband Services
By Ralph Marsh
Cable Location Consultant/Trainer
Austin, Texas
This writer has presented articles on a range of cable locating, testing and repair topics over the past several years, focusing primarily on POTS issues. Today the twisted-pair network is evolving into a broadband network to handle POTS as well as high-speed data and other communication signals, and the explosive growth in demand for broadband services across the country has added an entirely new dimension to cable location and maintenance work. Network management requires new local loop tools, skills and strategies.
In this article we will review issues that are becoming increasingly important to field technicians as their responsibilities are expanded to include broadband services for internet connections, corporate networks, video distribution, and other applications.
There are nearly one billion copper pairs in service today across the U.S., designed and installed to meet the needs of narrow-band analog voice communication. The Telecommunications Act of 1996, and the introduction of DSL technologies over the past several years has created the potential for vastly expanded communication capabilities. This change began with fax service and slow speed data modems, and has grown to include ISDN and a variety of even faster digital subscriber line (DSL) transmission schemes. Virtually every operating company in the U.S. is gearing up to deliver commercial DSL services, and most report far more growth in the demand for broadband services than for POTS.
Most telcos have at least 10% of their installed base tied up in faulty pairs that have been idled by trouble that has not been isolated and resolved because it has not been necessary or cost-effective to do so. The new demand for broadband service makes it critical that these resources be recovered, and passable voice grade circuits be upgraded to broadband performance levels.
It has been estimated that the typical operating company can meet new service demands using the existing copper plant at a savings of approximately 66% of the cost to convert to fiber optics. This savings takes into account labor, materials, new test equipment, and the new electronics that would be required. With the proper test devices, training and planning, copper networks can be upgraded incrementally as service demands require.
The decades-old twisted-pair network incorporates bridge taps to facilitate subscriber installation, and load coils to optimize voice-grade service - both of which present problems for broadband signals. In some cases the installed plant is at best only marginally adequate for POTS, and ineffective for digital services. The over-riding outside plant challenge today is to qualify pairs for digital service, and to diagnose and upgrade existing resources to meet the new telco demands.
Broadband Alternatives
The array of new digital services and technical characteristics is complex, and each type of service has its own performance requirements and copper pair quality standards. Following is a brief review of the xDSL alternatives that are most commonly encountered. Table 1 compares their primary characteristics.
ISDN is the oldest digital service, running at a maximum speed of 128 Kbps over a dedicated pair, using end-to-end digital connectivity, and incompatible with simultaneous POTS. ISDN signals are carried by bearer channels that occupy a bandwidth of up to 64 Kbps. Customers must be within 18,000 feet of a central office, and require special terminal adapters to communicate with the switch and other ISDN devices.
IDSL is replacing ISDN as a robust 144 Kbps service that is easily extended using repeaters for communication over distances up to 36,000 feet. It has data transmission capacity comparable to ISDN, but IDSL is not a switched service. ISDN and IDSL are generally reserved for use where distance from the central office is a challenge, and a dedicated pair is available. IDSL prospects include both business and residential subscribers.
ADSL (asymmetric digital subscriber line) is a high volume, consumer residential digital service that operates over a single pair and supports simultaneous POTS service. It has a typical loop length limitation of 18,000 feet from the central office. ADSL provides speeds up to 8 Mbps to the user (downstream) and up to 1 Mbps upstream, depending on line length and condition, and the rate purchased by the user. Many ADSL subscribers are able to receive service at 640 Kbps or less. For the average technician, ADSL will be the most common broadband technology encountered because of its growing use by residential customers.
SDSL is an un-regulated, symmetric DSL technology that was created prior to industry standards, and tends to be used only in localized areas. SDSL requires a dedicated line and does not support simultaneous voice service.
T1 is a dedicated 1.544 Mbps service that requires repeaters at 6,000-foot intervals. It works only on a clean, balanced line, and generally requires high maintenance. T1 signals present cross-talk problems, and commonly interfere with most DSL services in the same binder group. This data communication format has been around for a number of years, and is often used by banks and other businesses.
HDSL (data only) was developed primarily as a replacement for T1, and works more dependably than T1 on unconditioned loops. This transmission format carries up 12,000 feet from the central office, with the option of using a repeater to extend the loop. It offers only half the data rate of T1, but HDSL transmission capacity can be doubled with the addition of a second pair.
G.SHDSL is a new data specification that provides double the capacity of HDSL on a single pair, using a unique modulation technique for data rates up to 2 Mbps. This service type was designed from the outset to be spectrally friendly, but it is not yet widely deployed.
VDSL is a new POTS-compatible technology with very high-speed capacity (up to 52 Mbps) on 24 gauge twisted pair lines, but this transmission mode is limited to relatively short distances. The VDSL digital format was designed primarily for switched digital video application, and presents special line qualification demands.
Test Specifications
Each xDSL format has its own set of network performance requirements. Table 1 lists the performance specifications of each of the listed DSL technologies.
Table 1 - xDSL performance specifications.
Qualifying Pairs for xDSL
DSL signals are sophisticated, and data transmission requires specialized electronics at both ends of the line. Fortunately, there are subscriber loop analyzers and associated far end devices available that make it possible to dependably qualify and maintain lines for DSL without requiring a detailed understanding of broadband technology and without additional equipment.
For example, the 3MTM DynatelTM 965DSP Subscriber Loop Analyzer, a hand-held, multi-function test set, can be used to evaluate pairs for POTS and digital service, including 56 and 64 Kbit DDS, ISDN, HDSL, T1, E1 and ADSL. The necessary broadband tests can be conducted manually with this unit, or, with a companion tone device connected at the central office frame, it will sequence automatically through a full series of broadband qualification tests and display the results. The slave 3M Far End Device is controlled by the test set, placing tones and terminations on the tested line so that insertion loss, slope, loop resistance and resistive balance measurements can be made sequentially.
In the manual mode, an HDSL line qualification process typically begins with an insulation resistance test; the measured value must be greater than 3 megohms. The user next checks for the presence of load coils using the test set's load coil count feature. Multiple tone sweeps can sequentially detect up to four loads on a pair. Alternately, the TDR mode can be used for go/no go load coil detection. This method is only able to show the first coil. No coils are permissible on a qualified digital line.
The TDR function is used to determine the presence of any bridge taps, which may also disqualify a pair for digital service if they exceed 2000 feet or are close to either end of the line. Longitudinal balance is tested, and the measured value for the 3M unit must exceed 60 dB.
The loop resistance for a broadband pair must be less than 1300 ohms, and the resistive balance of the pair should not be greater than three ohms. Line loss at 196 Khz is checked, and should not exceed -45 dB. Measured capacitance length must be 12,000 feet or less.
Summary
Broadband services are supported by proven systems for both ends of the line, and standardized specifications. A common truism is that if the media (the copper plant) is good, xDSL service will work. However, if pair performance is marginal or intermittent, broadband service will not be satisfactory. Therefore, close is not good enough for broadband qualification. Current on the line, changes in splices, water in the cable or a splice case, or degraded terminations that would not threaten POTS can cause a digital service to crash.
Pair qualification for DSL service is important, but it is only half of the broadband services challenge for field technicians. If a pair tests bad, it is important to understand why, and to be able to locate and repair the problem so that the network can be optimized for today's growing communication requirements. The next article in this series will review the copper cable problems that cause pairs to fail data transmission qualification testing, and review trouble diagnosis, location and repair.
