METHOD AND SYSTEM FOR REDUCING CAPACITIVE IMBALANCE IN TELECOMMUNICATIONS TRANSMISSION SYSTEMS BACKGROUND OF THE INVENTION The telephone system infrastructure that has been in place for many years. and specifically the local loop portion of the telephone plant that is used to connect subscribers'telephones to the telephone central office. can be used to provide high speed data services in addition to traditional analog voice services. The telephone local loop typically comprises a portion of twisted wire pair cable that extends from a central office, a remote terminal, a remote access interface, a curbside terminal, or some other cable interface point. to a residence, an office building, an office complex, or some other subscriber location. When such a configuration is used for data transmission, data signals travel to subscriber locations over a twisted wire pair transmission path.

A twisted wire pair typically consists of two twisted wires, referred to in the industry as the tip wire and the ring wire. Twisted wire pair is used as a transmission medium because it supports differential transmission, a mode of communication in which information is transmitted as the difference between the currents in the tip and ring wires.

The advantage of differential transmission is that noise or interference that couples to both wires of the twisted wire pair is not received as noise at the receiving end of the system. so long as the noise is of equal magnitude on both wires. This phenomenon is known as common mode rejection.

The ability of a system to receive differential information-bearing signals and to reject common mode noise signals is typically characterized by a common mode rejection ratio. A system with a high common mode rejection ratio is able to reject noise

signals that appear on both wires. For example, radio frequency or other interfering signals will not have a significant detrimental effect on communication if the interfering signals couple equally to both wires, and if the receiving system has a high common mode rejection ratio.

Certain conditions can frequently lower a system's common mode rejection ratio by allowing noise and interference to be converted from a common mode signal to a differential mode signal. A differential mode signal is one that has different magnitudes on the tip and ring wires. Common mode to differential mode conversion can arise from physical imbalances between the wires in the twisted wire pair, imbalances in components attached to the wires, or other imbalances that give the tip and ring wires different impedances with respect to a common reference such as ground. One such imbalance is longitudinal capacitive imbalance, which is the difference in the capacitance between the tip wire and ground, on the one hand, and the capacitance between the ring wire and ground, on the other hand.

When imbalances such as longitudinal capacitive imbalance become excessive, they can cause significant distortions in signals transmitted over twisted wire pairs. thereby decreasing the ability of systems comprised of twisted wire pair transmission paths to receive signals transmitted at high data rates reliably. In Digital Subscriber Line (DSL) systems, common mode to differential mode conversion can severely limit the achievable data rates, which may limit the types of services that can be provided over twisted wire pairs. As an example, it may not be possible to provide video services over a particular twisted wire pair transmission path because the noise that appears on the line.

due to common mode to differential mode conversion. limits the data rate to one that is too low to support the transport of video signals.

In a noisy environment, one technique that can be used to reduce signal transmission errors is to use a lower modulation density format (i. e.. less information as measured in bits/ (second*Hz)). However, use of a lower modulation density format necessitates an increase in the occupied bandwidth, if the data rate is to remain the same.

But when common mode to differential mode conversion occurs, the use of additional bandwidth does not always provide a satisfactory solution. Because common mode to differential mode conversion may be frequency-dependent, higher frequency signals might be subject to severe noise. Consequently, the use of higher frequencies may be prohibited.

For the foregoing reasons, there is a need for methods and systems that can reduce or eliminate the effects of longitudinal capacitive imbalance, and thereby optimize the transmission bandwidth utilized in twisted wire pair subscriber line environments that are subject to common mode to differential mode conversion.

SUMMARY OF THE INVENTION An object of the invention is to provide methods and systems for avoiding or reducing the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path. and for minimizing the conversion of common mode noise signals to differential mode noise signals in transmission systems comprised of a twisted wire pair transmission path.

In contrast to the prior art, the present invention teaches that common mode to differential mode conversion can be avoided, and transmission errors can be minimized.

if a signal is transmitted using a lower portion of the frequency spectrum-less than about 7 MHz. and preferably less than about 5 MHz-even if that requires the modulation density format or baud rate of the signal to be increased. For example, the present invention encompasses transmitting signals carrying data at rates approaching and exceeding 25 Mb/s using high density transmission modulation formats such as 1 6-QAM (Quadrature Amplitude Modulation), constrained to a spectrum not exceeding about 5 MHz. as opposed to using lower density transmission modulation formats such as QPSK (Quadrature Phase Shift Keying) in a spectral range exceeding 5 MHz.

The present invention comprises a method for avoiding the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path. including the step of selecting a portion of frequency spectrum with an upper limit of about 5 MHz, or alternatively about 7 MHz, for the transmission of signals over the twisted wire pair transmission path.

A further aspect of the invention includes a method for achieving a target bit rate while avoiding the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path, comprising the step of selecting an increased modulation density format or baud rate for transmitting signals. in preference to selecting a higher portion of frequency spectrum for transmitting the signals.

In addition, the present invention comprises a method for avoiding or reducing the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path that employs lightning protectors, including the step of using either bias-independent lightning protectors, or lightning protectors with minimal bias-dependent capacitance. The invention also encompasses

twisted wire pair transmission systems that are comprised of bias-independent lightning protectors or lightning protectors with minimal bias-dependent capacitance, including systems in which such lightning protectors are comprised of gas tube protectors or networks of solid state and gas tube protectors.

The present invention further comprises a method for avoiding or reducing the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path that also employs a cable connecting a demarcation point and a telecommunications device inside a subscriber location, including the step of avoiding shielding the cable (i. e., using an unshielded cable), or, if the cable is shielded. avoiding grounding the cable shield. The invention also encompasses transmission systems that are comprised of a cable connecting a demarcation point and a telecommunications device inside a subscriber location, wherein the cable is comprised of tip and ring wires, and wherein the cable is either unshielded, or the cable shield is not grounded.

In addition, the present invention comprises a method for avoiding or reducing the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of a twisted wire pair transmission path that includes a cross-connection device. comprising the steps of cutting the tip and ring wires inside the cross-connection device to approximately equal lengths, twisting the tip and ring wires into a twisted wire pair inside the cross-connection device, or both twisting the tip and ring wires and cutting them to approximately equal lengths within the cross-connection device. The invention also encompasses transmission systems that are comprised of a cross-connection device. wherein the cross-connection device is comprised of a tip wire and a ring wire, and

wherein the tip and ring wires within the cross-connection device are either cut to approximately equal lengths, twisted into a twisted wire pair, or both.

Additional objects and advantages of the present invention are set forth in part in the description which follows, or may be learned by practice of the invention. The objects and advantages of the present invention may also be realized and attained by means of the instrumentalities and combinations particularly set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated into and constitute part of the specification, illustrate preferred embodiments of the present invention as well as transmission systems in which the methods of the invention may be used. Together with the description, the drawings serve to explain the principles of the invention.

FIG. 1 depicts a telecommunications transmission system comprising a central office, a transmission path, and a subscriber location, in which the methods of the present invention may be used, and into which the systems of the present invention may be incorporated.

FIG. 2 is a flow chart depicting a preferred embodiment of a method of the present invention for maintaining a target bit rate while avoiding the effects of excessive longitudinal capacitive imbalance.

FIG. 3 depicts an embodiment of a telecommunications transmission system of the present invention, comprising at least one lightning protector.

FIG. 4 depicts an embodiment of a telecommunications transmission system of the present invention, comprising a demarcation point. a cable, and a telecommunications device.

FIG. 5 depicts an embodiment of a telecommunications transmission system of the present invention, comprising a cross-connection device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The drawings additionallv illustrate telecommunications transmission systems in which the methods of the present invention may be practiced. Like reference numerals in the drawings indicate like elements throughout the several figures. In describing the preferred embodiments of the invention, specific terminology is used for the sake of clarity.

However, the invention is not intended to be limited to the specific terms so selected. and it is to be understood that each specific term includes all equivalents that operate in a similar or substantially similar manner to accomplish a similar or substantially similar result.

FIG. I illustrates a telecommunications transmission system in which the methods of the present invention may be used, and into which the systems of the present invention may be incorporated. The system supports the transmission and reception of signals. The transmitted signals may comprise a telephone signal, a data signal. an Internet signal, a video signal, an audio signal, a facsimile signal, or any other type of signal, or combinations thereof.

As depicted in FIG. 1. the transmission system includes a central office 1'0. a transmission path 130. and a subscriber location 140. The transmission system depicted in FIG. 1 is connected by a network transmission path 115 to a communications network 110. The network transmission path 115 may be comprised of fiber optic or coaxial cable. copper wire or twisted wire pair, radio spectrum, or some other medium (or combination thereof) capable of transporting signals, as known to persons of skill in the art. The communications network 110 may be comprised of a public switched telephone network. a packet-switched network, a public network, a private network, a data network, or any other type of network (or combination thereof) used in whole or in part to provide telecommunications services, as known to persons of skill in the art. In another configuration not depicted in FIG. 1, the transmission system is not connected by a network transmission path to a communications network.

The central office 120 depicted in FIG. 1 may be comprised of a switch or other components for routing signals, as known to persons of skill in the art. In the transmission system depicted in FIG. 1, the central office 120 is connected to a transmission path 130 by means known to persons of skill in the art. The transmission path 130. which may be used to transport one or more signals, is comprised of a twisted wire pair or other differential transmission medium.

In the configuration depicted in FIG. 1, the transmission path 130 is comprised of a tip wire 131 and a ring wire 132. The tip wire 131 and the ring wire 132 are used to carry or transport signals between the central office 120 and a subscriber location 140. The tip wire 131 and the ring wire 132 may be comprised of copper wire or

other mediums used for carrying or transporting signals, as known to persons of skill in the art.

The transmission system depicted in FIG. I can support differential transmission using tip wire 131 and ring wire 132 through the use of currents, which. when generated at one end of the twisted wire pair, result in the transmission of signals across transmission path 130. As an example, a differential signal transmitted from central office 120 results in currents on tip wire 131 and ring wire 132, which cause a wave to propagate on transmission path 130. and which can be received at subscriber location 140. A wave containing different information can be simultaneously transmitted from subscriber location 140 and received at central office 120.

The system depicted in FIG. 1 may comprise means for transmitting in digital formats, as known to persons of skill in the art. The system may, for example, encompass means for transmitting signals in QAM format, including 4-QAM, 8-QAM. 16- QAM. 64-QAM, 128-QAM. or 256-QAM, with an associated bit rate, as known to persons of skill in the art. The QAM system may transmit a plurality of signal streams, wherein two or more of the streams have different modulation densities and bit rates. In another configuration. the QAM system can transmit one or more signal streams, using only one modulation density and bit rate. The bit rate of the QAM transmission may remain constant, or it may vary.

The system depicted in FIG. 1 may also comprise means for transmitting signals in DSL format, as known to persons of skill in the art. Signals may be transmitted in Asymmetric Digital Subscriber Line (ADSL) format, Very High Speed Digital Subscriber Line (VDSL) format, or in some other DSL format, as known to persons of skill

in the art. A DSL system may transmit a plurality of signal streams, wherein at least two streams have different DSL formats and associated bit rates. Alternatively, a DSL system may transmit one or more signal streams using only one DSL format with its associated bit rate. The bit rate of the DSL transmission may remain constant, or it may vary.

In other configurations, the system depicted in FIG. 1 may comprise a QPSK or a Vestigial Sideband (VSB) system, comprising means for transmitting signals in QPSK or VSB format, as known to persons of skill in the art. In still other configurations. the system depicted in FIG. 1 may use other systems. facilities, and techniques, as known to persons of skill in the art, for transmitting signals between a central office 120 and a subscriber location 140.

As set forth below, the present invention comprises systems and methods for avoiding the effects of excessive longitudinal capacitive imbalance between the tip and ring wires of transmission systems that transmit signals using differential transmission.

With reference to FIG. 1, a preferred embodiment of the present invention is a method of avoiding the effects of excessive longitudinal capacitive imbalance between the tip wire 131 and the ring wire 132, comprising the step of selecting an upper limit. for the portion of the frequency spectrum used to transmit signals over the transmission path 1, 0. of about 5 MHz. In an alternative preferred embodiment, the upper limit is selected to be about 7 MHz.

Signals may be transmitted over transmission path 130 in the system depicted in FIG. 1 at a target bit rate, using a selected modulation density format and a target portion of the frequency spectrum. A preferred embodiment of the present invention is a method for achieving or maintaining the target bit rate. while avoiding the effects of

excessive longitudinal capacitive imbalance between the tip wire 131 and the ring wire 132. by increasing the modulation density format of the transmitted signals in preference to selecting a higher portion of the frequency spectrum to transmit the signals.

Signals may also be transmitted over transmission path 130 in the system depicted in FIG. 1 at a target bit rate, using a selected baud rate and a target portion of the frequencv spectrum. A preferred embodiment of the invention is a method for achieving or maintaining the target bit rate, while avoiding the effects of excessive longitudinal capacitive imbalance between the tip wire 131 and the ring wire 132, by increasing the baud rate of the transmitted signals in preference to selecting a higher portion of the frequency spectrum to transmit the signals.

The flowchart in FIG. 2 depicts a preferred embodiment of the method of the present invention for achieving or maintaining a target bit rate while avoiding the effects of excessive longitudinal capacitive imbalance. In the context of a transmission system that is to transmit signals at a predetermined target bit rate. step 201 comprises the identification of potential combinations of modulation density and baud rate that can achieve the target bit rate. For example, if the target bit rate is 25. 92 Mb/s. potential combinations of modulation density and baud rate that are capable of achieving that target bit rate might be identified as either the combination of 32 QAM and 5. 184 Mbaud. or 16 QAM and 6.48 Mbaud.

In step 202 of FIG. 2, one or more of the candidate combinations of modulation density and baud rate are selected, based on the ability of the combinations to operate at frequencies below about 5 MHz. In an alternative embodiment, candidate combinations are selected based on the ability to operate at frequencies below about 7

MHz. The selection of candidate combinations that can operate at frequencies below either 5 or 7 MHz is based on the discovery that longitudinal capacitive imbalance increases materially above those frequencies, and that. as a consequence, signal transmission typically degrades significantly above those frequencies. Thus, in the example provided above, in which the target bit rate is 25.92 MB/s. and the candidate combinations of modulation density and baud rate are either 32 QAM and 5.184 Mbaud. or 16 QAM and 6.48 Mbaud. the former combination-32 QAM and 5.184 Mbaud-is selected in step 202. because of its ability to operate at a frequency of below about 5 MHz.

In step 203 of FIG. 2, a final combination of modulation density and baud rate is selected from the combinations that were identified in step 202. This selection may be made based on a number of factors, such as the desire to minimize overlap with other transmission frequencies being utilized either on the same or on adjacent transmission paths. In a preferred embodiment, the selection in step 203 is made by choosing the combination with the lowest upper frequency limit, in preference to combinations with lower modulation densities but higher upper frequency limits.

In an alternative embodiment (not depicted), step 202 may be omitted. so that no combination of modulation density and baud rate is eliminated from consideration based on a predetermined upper frequency limit.

FIG. 3 depicts an embodiment of a telecommunications transmission system of the present invention, comprising a Network Interface Device (NID) 135 at subscriber location 140. In a preferred embodiment, NID 135 provides for high-voltage protection and serves as the interface between the transmission path 130 and the telecommunications system wiring or cabling inside subscriber location 140. The interface between the

network side 136 and the subscriber side 137 of NID 135. known in the industry as the demarcation point 138, marks the division between a telecommunications network. such as a local exchange carrier network, and the wiring or cabling inside subscriber location 140.

Demarcation point 138 is typically located at a point in or near subscriber location 140. as known to persons of skill in the art.

With reference to the preferred embodiment depicted in FIG. 3. NID 135 comprises at least one lightning protector 139. An embodiment of the method of avoiding the effects of excessive longitudinal capacitive imbalance between the tip wire 131 and the ring wire 132 comprises the step of using one or more bias-independent lightning protectors 139. In an alternative embodiment, one or more lightning protectors 139 with minimal bias-dependent capacitance are used. In a preferred embodiment. the bias- dependent capacitance of at least one lightning protector 139 is less than or equal to +1/2 picofarad.

In further embodiments of the present invention, at least one lightning protector 139 is comprised of a gas tube protector, a solid state protector, or a network of gas tube and solid state protectors. In yet other preferred embodiments, at least one lightning protector 139 is comprised of other materials capable of absorbing the effects of lightning, as known to persons of skill in the art.

FIG. 4 depicts an embodiment of a telecommunications transmission system of the present invention, comprising a telecommunications device 146 inside subscriber location 140, and a cable 144 connecting NID 135 and the telecommunications device 146.

In the transmission system depicted in FIG. 4. the transmission path 130 is connected to cable 144 in the subscriber location 140 at the demarcation point 138. In an alternative

configuration (not depicted), the transmission path 130 may be connected to cable l 41 at a place other than the demarcation point 138. The telecommunications device 146 shown in FIG. 4 may comprise a telephone, a computer, a video device, an audio device, a facsimile machine, a VCR. a set-top box, or another device for receiving signals, as known to persons of skill in the art.

Cable 144, as depicted in FIG. 4, is comprised of materials known to persons of skill in the art. In one embodiment, cable 144 comprises tip and ring wires.

Alternatively, cable 144 may comprise a coaxial cable, or another form of cable that uses differential transmission. Cable 144 may also comprise a shield, and may or may not be grounded.

It has been discovered that shielding causes a difference in the 1 between the tip wire and ground, on the one hand, and the capacitance between the ring wire and ground, on the other hand. Accordingly, the present invention teaches the use of an unshielded cable 144 in the subscriber location 140 in order to avoid the effects of excessive longitudinal capacitive imbalance between the tip and ring wires in cable 144.

The present invention also teaches a method comprised of the step of avoiding shielding the cable 144.

Furthermore, it has also been discovered that when a shielded cable is used. grounding the cable shielding causes a difference in the capacitance between the tip wire and ground, on the one hand. and the capacitance between the ring wire and ground, on the other hand. Hence, the present invention teaches that when cable 144 is shielded. the cable shielding is not to be grounded. in order to avoid excessive longitudinal capacitive

imbalance between the tip and ring wires in cable 144. The present invention also teaches a method comprised of the step of avoiding grounding the cable shield of cable 144.

The effects of excessive capacitive imbalance may also be reduced or avoided by ensuring that the tip and ring wires are cut to the same length and/or twisted around each other. FIG. 5 depicts an embodiment of a telecommunications transmission system of the present invention, comprising a cross-connection device 160. As known to persons of skill in the art. the cross-connection device 160 provides a means for configuring a telecommunications network and distributing signals from a central office 120, which serves multiple subscribers, to a single subscriber location 140. The cross- connection device 160 also provides an efficient means for changing facility configurations and subscriber services. As depicted in FIG. 5. a cross-connection device 160 may be external to subscriber location 140. Alternatively, a cross-connection device may be located inside a subscriber location (not depicted).

As depicted in FIG. 5, the cross-connection device 160 comprises a tip wire 131 and a ring wire 132. In an embodiment of the present invention, the tip wire 131 and the ring wire 132 within the cross-connection device 160 are cut so that they are approximately of equal length. In a preferred embodiment, the difference in length between tip wire 131 and ring wire 132 is less than 1/2 inch. In another preferred embodiment, tip wire 131 and ring wire 132 are twisted around each other so as to create a twisted wire pair within cross-connection device 160. In yet another preferred embodiment, tip wire 131 and ring wire 132 and both twisted around each other and cut so that they are of approximately equal length. The present invention also teaches the methods comprising the steps of cutting tip wire 131 and ring wire 132 within cross-

connection device 160 to approximately the same length, twisting tip wire 131 and ring wire 132 into a twisted wire pair, or both.

The invention is intended to be protected broadly within the spirit and scope of the appended claims. Although the systems and methods of the present invention have been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made that fall within the scope of the invention. It is intended that such changes and modifications shall be covered by the systems and methods of the appended claims and their equivalents.