Telephonic communications – Transmission line conditioning – Interference suppression
Reexamination Certificate
1998-02-27
2001-05-22
Korzuch, William R. (Department: 2641)
Telephonic communications
Transmission line conditioning
Interference suppression
C379S413020, C375S220000
Reexamination Certificate
active
06236726
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the transmission of high speed digital data. Specifically, the invention provides a system and a method for decreasing the amount of crosstalk occurring within a data transmission system in which digital data is transmitted at high speeds over a telephone network.
The proliferation of home computers and the increase in usage of data networks, such as the Internet, has resulted in a marked increase in the amount of digital data transmitted over telephone networks. Data transmission systems utilizing voice band modems are approaching technological limits in efficiency and speed. Demand for both access to high speed data networks and higher data transmission rates has necessitated the development of new technologies for facilitating data transmission over traditional telephone lines.
One such newly developing family of technology involves digital subscriber lines (xDSL). For example, an Asymmetric Digital Subscriber Line (ADSL) allows for the transmission of both voice and data signals over the same twisted copper pair, at the same time. It allows a computer user access to high speed data networks, such as the Internet, at generally a faster rate than is available using traditional voice band modems. Since data signals are transmitted along with voice signals over traditional telephone lines, the ADSL user need not incur the expense of acquiring separate telephone lines or specialized data lines. Accordingly, demand for this technology has been considerable and its use within today's telephone networks is increasing rapidly.
In xDSL technologies, one end of the telephone line, usually of the twisted pair type, terminates at the location of the user in a subscriber unit. The other end of the telephone line terminates in a digital terminating unit at an access location within the telephone network, such as a central office or a remote access point such as a digital loop carrier. From this point, the user gains access to a digital data network, such as the Internet.
A problem arises when the telephone lines from multiple users transmitting data signals converge along the way to and at a common location such as the central office or digital loop carrier. Traditional telephone lines do not provide perfect separation between signals on different pairs. Some signal may leak from one pair and interfere with the signal on another nearby pair. This type of interference is known as crosstalk.
Two different types of crosstalk are known to those familiar in the art. These are Near-End Crosstalk (NEXT) in which the disturbed unit is located at the same end of the transmission line as the disturbing unit and Far-End Crosstalk (FEXT) in which the disturbed unit is located at the opposite end of the transmission line as the disturbing unit. In the case of xDSL systems as described herein, NEXT is alleviated by using a different frequency band for each direction of transmission and only FEXT remains as a possible reach limiting factor. Whenever mention is made of crosstalk in the present document, it is implied to be Far-End Crosstalk.
Traditional telephone lines provide much better separation and are less susceptible to crosstalk when they carry voice signals as opposed to data signals. While voice signals generally occupy bandwidth in the frequency range of 0 kHz to 4 kHz, data signals, in the case of xDSL technologies, occupy bandwidth on the telephone line at much higher frequencies; usually from 30 kHz and up to approximately the low megahertz range. For some xDSL systems, this range can extend to approximately 25 MHz. The physical characteristics of the traditional telephone lines are such that the higher data signal frequencies tend to leak more easily than the lower voice signal frequencies. Accordingly, the data signals carried over telephone lines are inherently susceptible to interference and crosstalk.
Exacerbating the problem is the disparity in the signal strengths of the data signals received at the central office. In existing xDSL systems, the strength of the data signal received at the central office depends, to a large degree, on the length and quality of the telephone line connecting the central office to the user's subscriber unit. Typically, the longer the distance between a user and the central office, the weaker the data signal received at the central office. Conversely, the data signal received from another user who is closer to the central office is much stronger.
When the telephone lines of two users converge along the way to and at the central office, especially if the lines are in physical proximity to each other, the stronger signal will tend to interfere and cause crosstalk with the weaker signal. The result is a decrease in the signal to noise ratio of the data signal from the more distant user and loss of data. As the number of such users in system increases and as more telephone lines carrying high speed data converge along the way to and at the central office, the potential for crosstalk and corruption of data increases exponentially.
SUMMARY OF THE INVENTION
The present invention solves the above identified problem. Specifically, the invention is a system and a method for use in a data transmission system for decreasing the amount of crosstalk occurring along the way to and near a central location within a telephone network. The invention makes use of intelligence at a plurality of the remote subscriber units to reduce differences in received data signal strengths at the central location, thus reducing the potential for crosstalk and corruption of data.
In the principal embodiment of the invention, a communication system and method are disclosed which reduce crosstalk between a plurality of communication lines within a telephone network. The invention includes: receiving a signal at a subscriber unit from a central location over a communication line; determining the signal strength of the received signal at the subscriber unit; adjusting the transmit signal power using the received signal strength such that a data signal transmitted over the communication line from the subscriber unit at the adjusted transmit signal power would be received at the central location at approximately a desired signal strength; and transmitting a data signal to the central location at that adjusted transmit signal strength. The desired signal strength can be predetermined for a central location such that data signals received at the central location from a plurality of subscriber units are received at approximately the desired signal strength.
Also disclosed is a subscriber unit for use in the present invention including a data signal transceiver and power scaling circuitry which makes use of the method of the present invention.
Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.
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Azad Abul K.
Foley & Lardner
Korzuch William R.
Nortel Networks Limited
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