Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2000-06-05
2003-03-25
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S534000, C324S535000, C379S022040, C379S027030
Reexamination Certificate
active
06538451
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to determining the make-up of subscriber loops in the public switched network and more specifically to methods and systems that determine the make-up of subscriber loops via single ended measurements.
BACKGROUND
The mainstay of the telephone company local network is the local subscriber loop. The great majority of residential customers, and many business customers, are served by metallic twisted pair cables connected from a local switch in the central office to the subscriber's telephones. When customers request service, request a change in service, or drop service, these facilities must be appropriately connected or arranged in the field, referred to as the “outside plant,” and telephone companies have specially trained craft dedicated full time to this task. Obviously a company needs to have an understanding of its subscriber loops including where they are connected and the location of the flexibility points such as junction boxes, etc. These records historically were kept on paper, called “plats,” and more recently are manually entered into a computer database. However, even when entered into a database there are still problems associated with keeping the records accurate and up-to-date.
Having accurate records of the loop plant is critically important to many aspects of a telephone company's business. In addition to the need for accurate records to provide traditional voice services, there will be a need for even more accurate and more detailed records in order to deploy a whole new class of “xDSL” based services, including those based on integrated services digital network (ISDN), high-rate digital subscriber line (HDSL), asymmetrical digital subscriber lines (ADSL) and very high rate digital subscriber lines (VDSL) technology. These technologies are engineered to operate over a class of subscriber loops, such as nonloaded loops (18 kft), or Carrier Serving Area (CSA) loops (9 to 12 kft). In fact, the need to be able to “qualify” a loop for provision of one of these technologies is becoming critical, as the technologies emerge and deployment begins. The ability to easily and accurately qualify loops will allow telephone companies to offer a whole range of new services; problems and high expenses associated with qualifying loops can potentially inhibit deployment and/or lower or forego associated new revenues. The unscreened multipair cables in the existing subscriber loop network constitute the main access connection of telephone users to the telephone network. Recently, the demand for new services such as data, image and video has increased tremendously, and telephone companies have planned to deliver broadband ISDN services via fiber optic local loops. However, the deployment of fiber optic cables in the access plant will require at least twenty years, so that, in the meantime, it is extremely important to fully exploit the existing copper cable plant.
Although there are many different digital subscriber line services, for example, ISDN basic access, HDSL, ADSL, VDSL, and Synchronous DSL (SDSL), these services are not always available to every customer since copper lines seem to present more problems than expected. In fact, the cable length and the presence of load coils and bridged taps may deeply affect the performance of DSL services. Unfortunately, loop records are unreliable and often don't match the actual loop configuration, so that the existing databases cannot be fully exploited.
Loop prequalification is an important issue not only because it can help an economic deployment of DSL services, but also because it can help telephone companies in updating and correcting their loop-plant records. From this point of view, the feasibility of accurate loop make-up identification would have a much higher economic value than simple DSL qualification.
One way to obtain accurate loop records is to manually examine the existing records and update them if they are missing or inaccurate. This technique is expensive and time consuming. Furthermore, new technologies such as xDSL require additional information that was previously not kept for voice services, so there is the potential that new information needs to be added to all existing loop records. Test set manufacturers offer measurement devices that can greatly facilitate this process, but typically they require a remote craft dispatch.
Another way to obtain accurate loop records is by performing a loop pre-qualification test. There are essentially two ways of carrying out a loop pre-qualification test: double ended or single ended measurements. Double-ended measurements allow us to easily estimate the impulse response of a loop by using properly designed training sequences. Double-ended testing, however, requires equipment at both ends of the loop. Specifically, in addition to equipment at the Central Office (CO) or near end of the loop, double ended testing involves either the presence of a test device at the far end of the loop (Smart Jack or MTU) or dispatching a technician to the subscriber's location (SL) to install a modem that communicates with the reference modem in the CO. An exemplary double ended system and method that extrapolate voice band information to determine DSL service capability for a subscriber loop are described in Lechleider et al. U.S. Pat. No. 6,091,713, issued Jul. 18, 2000, entitled “Method and System for Estimating the Ability of a Subscriber Loop to Support Broadband Services” (which is assigned to the assignee of the present invention).
In contrast, single ended tests are less expensive and time consuming than double-ended tests. Furthermore, single-ended testing requires test-equipment only at the CO. In fact, no technician dispatching is required and the CO can perform all the tests in a batch mode, exploiting the metallic access with full-splitting capability on the customer's line. An example of such a single ended test system is the “MLT” (Mechanized Loop Testing) product that is included as part of the widely deployed automated loop testing system originally developed by the Bell System. The MLT system utilizes a metallic test bus and full-splitting metallic access relays on line card electronics. By this means, a given subscriber loop can be taken out of service and routed, metallically, to a centralized test head, where single-ended measurements can be made on the customer's loop. The test head runs through a battery of tests aimed at maintaining and diagnosing the customer's narrowband (4kHz) voice service, e.g., looking for valid termination signatures via application of DC and AC voltages. This system is highly mechanized, highly efficient, and almost universally deployed. In addition, the MLT system is linked to a Line Monitoring Operating System (LMOS) thereby providing a means to access and update loop records which are useful in responding to customer service requests or complaints. However, because this system exclusively focuses on narrowband voice services, the system misses important loop make-up features that will be deleterious to supporting broadband services via DSL technologies.
Another well known single-ended measurement technique relies on the observation of echoes that are produced by medium discontinuities to fully characterize the link. Specifically, these single ended measurements typically rely on time domain reflectometry (TDR). TDR measurements are analogous to radar measurements in terms of the physical principles at work. TDR test systems transmit pulses of energy down the metallic cable being investigated and, once these pulses encounter a discontinuity on the cable, a portion of the transmitted energy is reflected or echoed back to a receiver on the test system. The elapsed time of arrival of the echo pulse determines its location, while the shape and polarity of the echo pulse(s) provide a signature identifying the type of discontinuity that caused the reflection or echo. Basically, if the reflecting discontinuity causes an increase in impedance, the echo pul
Galli Stefano
Waring David L.
Deb Anjan K.
Giordano Joseph
Le N.
Telcordia Technologies Inc.
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