Optimization of a DSL-compatible POTS line card

A digital subscriber line (DSL) compatible plain old telephone service (POTS) line card that interface a telecommunications switching system to a subscriber over a two-wire subscriber line, wherein the subscriber line card detects whether a DSL line card is connected to the subscriber line. A digital signal processor on the POTS line card is configured to process voice-band signals with a first set of parameters if a DSL line card is connected to the subscriber line and to process voice-band signals with a second set of parameters if the DSL line card is not connected to the subscriber line. Detecting the presence of a DSL line card may be performed by sending a voice band tone on the subscriber line and the return loss measured. Additionally, a tone above voice band may be sent on the subscriber line and a return loss measured. The first set of parameters adjust frequency response of the subscriber line, return loss of the subscriber line and/or trans-hybrid loss of the POTS line card to compensate for the presence or absence of a DSL card connected to the subsource line.

FIELD OF THE INVENTION

This invention relates to telephone switching systems and to high-speed data communications, and, more specifically, to maintaining response standards in a line card for analog telephone service whether or not the subscriber line is also used for high-speed data service.

BACKGROUND OF THE INVENTION

Many business and home consumers are requesting faster data access than can be provided by voice-band modems over two-wire subscriber lines. The central office switch, because of the voice band CODEC's, limits the data bandwidth to approximately 56 kbps. In response, both incumbent local exchange carriers (LEC's) and alternative carries are offering digital subscriber line (DSL) service. Basically, DSL uses wide frequency band above the voice frequency to carry more data. The same tip-ring pair that supports voice service can thus also support DSL service without rewiring the extant local telephone system.

When DSL service is added over voice service, several filters and other components are added to the subscriber loop in order to isolate the DSL service from the voice service at both the customer premises and at the central office to avoid one interfering with the other. When these components are added, voice band service is affected negatively. Telcordia (formerly BellCore) specified voice-band characteristics in the LSSGR (LATA Switching System Generic Requirements), now Telcordia GR 507 (local switching) and GR57 (DLC). Currently, when a DSL line card is connected to a POTS line card, the POTS line card does not meet these specifications.

Special line cards have been proposed to meet the requirements while still providing DSL services, but this requires that the POTS line card be changed when DSL service is added. Therefore, a challenge in the art is to develop a POTS line card that can meet LSSGR requirements both when it is and is not connected in circuit with a DSL line card.

SUMMARY OF THE INVENTION

A technical advance is achieved in the art in a digital subscriber line (DSL) compatible plain old telephone service (POTS) line card that interface a telecommunications switching system to a subscriber over a two-wire subscriber line. The POTS line card detects whether a DSL line card is connected to the subscriber line. A digital signal processor on the POTS line card is configured to process voice-band signals with a first set of parameters if a DSL line card is connected to the subscriber line. Further, the digital signal processor on the POTS line card is configured to process voice-band signals with a second set of parameters if the DSL line card is not connected to the subscriber line. Detecting the presence of a DSL line card may be performed by sending one or more tones within the voice band frequency on the subscriber line and measuring return loss. Additionally, one or more tones above voice band frequency (advantageously in the DSL frequency band) may be sent on the subscriber line and a return loss measured.

The first set of parameters adjusts a frequency response of the POTS line card, a return loss of the subscriber line card and/or a trans-hybrid loss of the POTS line card to meet LSSGR specifications when a DSL line card connected to the subscriber line. The second set of parameters adjusts a frequency response of the POTS line card, a return loss of the subscriber line card and/or a trans-hybrid loss of the POTS line card to meet LSSGR specifications when a DSL line card is not connected to the subscriber line.

DETAILED DESCRIPTION

FIG. 1is a block diagram of a DSL-compatible POTS line card10connected to a customer premises telephone12via a tip-ring pair14according to one aspect of this invention. The DSL-compatible POTS line card10comprises four main components, as opposed to the three main components of a current or prior art POTS line card. The exemplary embodiment of this invention is described in terms of the DSL-compatible POTS line card as described in U.S. patent application Ser. No. 09/617,446, assigned to the assignee of this application and incorporated herein by reference. According to this exemplary embodiment, DSL-compatible POTS line card10comprises a compensating digital signal processor (“DSP”)16, a CODEC or digital/analog, analog/digital converter18and a battery feed20. In addition, POTS line card10includes a low-pass filter22. Low-pass filter22provides attenuation of the high frequency DSL signal but more importantly lowers the amplitude of any DSL signal from 18 volts peak to peak to between three and five volts peak to peak. In this manner, the signal and voltage is attenuated to that acceptable to a POTS line card, according to prior specifications, if and when a DSL line card is connected to the tip-ring pair14.

According to this exemplary embodiment, the compensating DSP16provides an interface to Public Switched Telephone Network (PSTN)23. Compensating DSP16provides four further functions to its known, prior art functions. DSP16also provides a second order low pass filter30to further attenuate voltage and filter out any unwanted DSL signal or noise to ensure that the signal is appropriate for POTS. The compensating DSP16further includes gain compensator32to restore any signal in the voice band (0 to 4 kHz) that may have been attenuated by either of the two low pass filters22and30. This compensation restores some signal that is lost from the nature of filtering, as is known in the art.

There is a balance network impedance compensator34in the compensating DSP16to bring the balanced network into conformity with standard non-loaded balance network. Further, a termination impedance compensator36provides proper termination impedance in PSTN23.

According to the exemplary embodiment of this invention, a processor40controls the functionality of the entire board. Processor is connected to memory42, which contains a plurality of operational programs for the compensating DSP16, as will be described further, below. Of course, processor40is shown for the sake of clarity; the functionality of processor40may be incorporated in any component on POTS line card10. Advantageously, the functionality of processor40is incorporated into DSP16.

Memory42stores a first set of parameters that sets the frequency response, the return loss and the trans-hybird loss of the POTS line card for POTS with DSL service present. Memory42stores a second set of parameters that sets the frequency response, the return loss and the trans-hybird loss of the POTS line card for POTS without DSL service present.

To complete the context in which this invention is described, XDSL line card50may be connected to tip-ring pair (local loop)14by a second tip-ring pair52(shown in phanton). XDSL line card50connects a digital device, such as PC56via XDSL Modem54to digital network58. When XDSL is present on local loop14, Filter60blocks DSL signals from interfering with POTS telephone12service, as is known in the art.

When a DSL-compatible POTS line card10according to this invention is initialized, it determines whether a DSL line card such as50is connected to local loop14. According to one embodiment, as shown inFIG. 2, processing starts at circle200and moves to action box202. In action box202, the DSP16generates a tone in the voice band and plays it on the subscriber line, tip-ring pair14. The tone may be, for example, 2.4 kHz, 3.0 kHz or 3.4 kHz, depending on the application. Other frequencies may be used without departing from the scope of this invention.

Processing then continues to action box204, where reflected energy of the tone is measured. As is known in the art, local loop14and telephone12reflects a percentage of the energy delivered on tip-ring pair14, with a determinable amount of attenuation. If a DSL card is present, then the attenuation is greater than it would be if only a POTS telephone is connected to the line. Therefore, in decision diamond206, a determination is made whether the reflected energy is less than a threshold, then a DSL card is present. In this case, processing moves to action box208where the DSL present coefficients are loaded into compensating DSP16.

If, in decision diamond206, the reflected energy is above the threshold, then a DSL card is not present and the POTS only coefficients are loaded into compensating DSP16. Processing ends in circle212.

Turning now toFIG. 3, another embodiment of making the determination as to whether a DSL card is present is shown. Processing starts in circle300and moves to action box302, where a tone is generated in the frequency band that is used by DSL. The reflected energy is measured in action box304, and a determination is made in decision diamond306whether the reflected energy is below a threshold. If the reflected energy is below the threshold, then a DSL card is detected, and the DSL present coefficients are loaded into compensating DSP16in action box308. If the reflected energy is above the threshold, then a DSL card is not present and the POTS only coefficients are loaded into compensating DSP16in action box310. Processing ends in circle312.

Turning now toFIG. 4, a further embodiment of making the determination as to whether a DSL card is present is shown. Processing starts in circle400and moves to action box402, where energy in the DSL frequency band is measured. A determination is made in decision diamond404whether the energy is above a threshold. If the reflected energy is above the threshold, then a DSL card is detected, and the DSL present coefficients are loaded into compensating DSP16in action box406. If the reflected energy is below the threshold, then a DSL card is not present and the POTS only coefficients are loaded into compensating DSP16in action box408. Processing ends in circle410.

Turning now toFIG. 5, yet another embodiment of making the determination as to whether a DSL card is present is shown. Processing starts in circle500and moves to action box502, where a check is made on tip-ring pair14for a DSL pilot tone. A determination is made in decision diamond504whether the DSL pilot tone is present. If the DSL pilot tone is present, then a DSL card is detected, and the DSL present coefficients are loaded into compensating DSP16in action box506. If the pilot tone is not detected, then a DSL card is not present and the POTS only coefficients are loaded into compensating DSP16in action box508. Processing ends in circle510.

Turning now toFIG. 6, still yet another embodiment of making the determination as to whether a DSL card is shown. Processing starts in circle600and moves to action box602where a plurality of tones are generated at various frequencies. These tones may all be in the voice frequency band, the DSL Frequency band, or, advantageously, frequencies from several points in the local loop range. Processing continues to action box604where a reflected energy is measured. A determination is made in decision diamond606whether the reflected energy indicates the presence of a DSL card connected to the local loop. In this scenario, one or more profiles may be developed for reflected energy of various frequency patterns. Such patterns are functions of the characteristics of the local loops and are determinable in the laboratory without undue experimentation.

If a reflected energy/frequency pattern is detected, then processing moves to action box608, where the DSL-Present coefficients are loaded into DSP16. If a reflected energy/frequency pattern is not present in decision diamond606, then the POTS-only coefficients are loaded into DSP16in action box610. Processing ends in circle612.

It is to be understood that the above-described embodiments are merely illustrative of the principles of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention. For example, a component on the DSL-ready POTS line card may query a database on the switch to determine if a DSL line card is provisioned on the line, and load compensating DSP16accordingly. It is, therefore, intended that such variations be included within the scope of the appended claims.