Telephonic communications – Line equalization or impedance matching
Reexamination Certificate
2000-02-28
2001-11-06
Chan, Wing F. (Department: 2643)
Telephonic communications
Line equalization or impedance matching
C379S394000, C379S093050, C370S494000
Reexamination Certificate
active
06314181
ABSTRACT:
TECHNICAL FIELD
The present invention relates to terminating bridged taps on a subscriber loop used to carry digital subscriber line signals.
BACKGROUND ART
A subscriber loop connects a telephone company central office to a demarcation point on customer premises. The subscriber loop is typically a pair of wires such as a twisted pair. Traditionally, a subscriber loop is used to carry plain old telephone service (POTS) signals between central office equipment and customer equipment.
Bridged taps are sometimes introduced into the subscriber loop. Bridged taps are typically wire splices on a cable pair. Bridged taps simplify outside plant design and administration by making a wire pair physically available for termination at different customer locations. Although only one customer typically uses the cable pair at any time, the bridged tap provides flexibility as to which particular customer is assigned to any particular cable pair. This flexibility simplifies network design that results from frequent customer moves, requests for second lines, and new constructions. Often, a bridged tap is left unterminated once installed in the subscriber loop.
Recently, services based on digital subscriber lines (DSLs) have been provided to customers over subscriber loops. DSL signals operate at a higher bandwidth than POTS signals, permitting increased access to voice, data, and video services. Also, DSL signals may be modulated to a frequency range outside the range covered by POTS signals.
Bridged taps in subscriber loops may severely attenuate DSL signals. This is because the unterminated bridged tap appears as a low impedance element over certain frequency ranges depending, in part, on the geometry of the bridged tap. If DSL signals are operating within these frequency ranges, the signals may be severely attenuated. Previous efforts to terminate bridged taps reduce the effects of notches or dips in the spectrum due to the bridged tap but result in an overall attenuation of the DSL spectrum.
What is needed is proper termination of bridged taps to allow adequate transmission of DSL signals without significant signal loss. The termination should cancel the effects of the bridged tap. Bridged tap cancellation should be adaptable to different tap geometries and subscriber loop variations as well as be economical to produce.
DISCLOSURE OF INVENTION
It is a primary object of the present invention to provide termination of bridged taps in communication lines.
Another object of the present invention is to improve DSL reception on a subscriber loop.
Still another object of the present invention is to provide bridged tap cancellation without significant loss of signal strength.
Yet another object of the present invention is to provide bridged tap cancellation without adversely affecting POTS transmission.
A further object of the present invention is to provide bridged tap cancellation that is economical to produce.
In carrying out the above objects and other objects and features of the present invention, a method is provided for improving reception on a subscriber loop used to carry digital subscriber line (DSL) signals. The subscriber loop contains a bridged tap creating at least one local minimum at a notch frequency in the frequency response of the subscriber loop within the frequency range of the DSL signals. Coefficients are determined for a terminating filter. The terminating filter has at least one filter element for each local minimum created by the bridged tap in the DSL signal range. Each filter element provides substantially complete reflection of DSL signals at the corresponding notch frequency. The terminating filter is constructed based on the determined coefficients. The bridged tap is then terminated with the constructed terminating filter.
In various embodiments of the present invention, determining coefficients comprises selecting band pass filter coefficients such that the resulting band pass filter has a bandwidth substantially the same as the corresponding local minimum, selecting low pass filter coefficients such that the resulting low pass filter has a bandwidth including at least one local minimum, and selecting high pass filter coefficients such that the resulting high pass filter has a bandwidth including at least one local minimum. The terminating filter also includes a terminating load impedance.
A termination for a subscriber loop bridged tap is also provided. The termination includes a band pass filter connected to the bridged tap for each local minimum. The band pass filter passes frequencies including a notch frequency at which the corresponding local minimum occurs. A load impedance is connected to the band pass filter.
A subscriber loop system is also provided. The subscriber loop system includes a central office having service provider POTS communication equipment. Service provider DSL communication equipment is also provided. Customer premises, including customer POTS communication equipment and customer DSL communication equipment, are joined through subscriber loops connecting customer POTS communication equipment with corresponding provider POTS communication equipment and connecting customer DSL equipment with corresponding provider DSL equipment. At least one subscriber loop has a bridged tap that would otherwise cause the subscriber loop to have a frequency response with at least one local minimum within the frequency range of DSL signals. A bridged tap canceller connected to the bridged tap cancels the effects of the local minima. The bridged tap canceller has a terminating filter with a filter element for each local minimum created by the bridged tap. Each filter element provides substantially complete reflection of DSL signals at a notch frequency corresponding to the local minimum.
In an embodiment, each filter element minimizes loading within the POTS band. This may be accomplished by using a cutoff frequency above the frequency range of the POTS signals in each filter element.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with accompanying drawings.
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Brooks & Kushman P.C.
Chan Wing F.
Qwest Communications International Inc.
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