Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – With blocking of normal usage
Reexamination Certificate
1999-07-07
2001-05-22
Kuntz, Curtis (Department: 2643)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
With blocking of normal usage
C379S027010, C379S028000, C379S030000
Reexamination Certificate
active
06236714
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to Digital Subscriber Line (DSL) technology for communicating data over telephone subscriber loops, and more particularly, to reducing voiceband distortion in a splitterless DSL system.
BACKGROUND OF THE INVENTION
DSL technologies provide subscribers with high-speed access to computer networks over existing twisted-pair copper telephone wires. Competing DSL technologies include, for example, High-bit-rate DSL (HDSL), Rate Adaptive DSL (RADSL), and Asymmetric DSL (ADSL). Each of these DSL technologies offers different speeds, ranges, and operating characteristics.
One of the more promising DSL technologies is ADSL. A typical ADSL circuit connects an ADSL modem on each end of a twisted-pair of copper telephone lines. The ADSL modems communicate over three information channels: a high speed downstream channel, a medium speed upstream channel, and a Plain Old Telephone Service (POTS) channel.
In conventional ADSL systems, the POTS channel is split off from the other channels by a splitter installed at the customer's premise. The installation is typically done by technical personnel, and may include the re-wiring of existing telephone lines. This kind of installation can incur significant costs and inconvenience for the customer.
For new DSL systems intended to be deployed in large volumes to the general public, it is desirable to provide for easy installation of the ADSL modem at the customer's premise. One way to simplify installation is to remove the splitter entirely from the system. The ADSL modem can then be plugged directly into an existing telephone jack. This splitterless configuration enables the ADSL modem to share the same wiring as the customer's telephone sets without the need for installing a splitter or rewiring the customer's premise.
The deployment of ADSL systems in a splitterless environment prompted the development of splitterless ADSL modems. One example of a splitterless ADSL modem is defined in International Telecommunication Union Standardization Sector (“ITU-T”) recommendation G.992.2 (October 1998), which is incorporated by reference herein in its entirety. With splitterless ADSL modems, the installation of the splitter and/or rewiring is no longer necessary. Without the splitter, high frequency ADSL signals are transmitted directly to the customer's telephone sets. The ADSL signals include downstream signals transmitted from the Central Office (CO) modem and upstream signals transmitted from the Customer Premises Equipment (CPE) modem.
High frequency ADSL signals have a higher transmit power than POTS signals and may be electronically “clipped” by internal telephone set circuitry if such power exceeds a predetermined power threshold designed to protect the user's hearing. This is especially problematic for upstream signals since the CPE modem is typically located close to the telephone set. Thus, the upstream signal level at the telephone set is basically equal to the transmitted signal level, while the downstream signals are attenuated by the telephone lines. This “clipping” effect can introduce nonlinear distortion within the POTS frequency band, which can interfere with POTS services (e.g., telephony, voiceband modems, and facsimile machines).
A fast retrain procedure for estimating the proper level of transmit power for ADSL modems in a splitterless environment is described in ITU-T recommendation G.992.2. During this fast retrain procedure, a single R-line-probe signal is transmitted to the telephone sets from the CPE modem to estimate the proper transmit power level. The characteristics (i.e., shape and level) of this R-line-probe signal is vendor-discretional, and thus not defined in G.992.2.
Unfortunately, estimating the proper transmit power is difficult because splitterless environments can include several different types of telephone sets, each connected to the same telephone wiring as the CPE modem. These telephones sets typically have different electrical characteristics, thereby making it difficult for the CPE modem to set the proper transmit power level without prior knowledge of the types of telephone sets being used. Depending on the types of telephone sets used, if the transmit power level is reduced too much, the reduction of the signal-to-noise ratio (SNR) of the ADSL signals may cause significant data rate reduction, or even become inadequate for data communication. On the other hand, if the transmit power is reduced too little, distortion may still be audible to the user.
Accordingly, there is a need for a system and method of controlling the transmit power level of a DSL modem operating in a splitterless environment without regard to the number and type of telephones sets being used. The system and method should minimize audible distortion in the voiceband while providing a signal-to-noise-ratio (SNR) adequate for DSL data communication. Such a system and method should operate on the fly without prior knowledge of the electrical characteristics of the telephone sets being used.
SUMMARY OF THE INVENTION
The present invention determines the proper transmit power level in a splitterless DSL environment having one or more telephone sets by transmitting a probe signal at different power levels to the telephone set and measuring the distortion signal reflected from the telephone set for each power level. The proper transmit power level is determined by detecting the “corner point” of the reflected distortion signal.
A method of reducing distortion in the voiceband of a splitterless DSL comprises the steps of: transmitting a probe signal to at least one telephone set, the probe signal having a first power level; receiving a distortion signal reflected from the telephone set; computing a first measurement of the distortion signal; adjusting the first power level to a second power level; computing a second measurement of the distortion signal; and detecting a comer point in the received distortion signal from the first and second measurements. A third power level associated with the detected comer point is saved for later use in data transmission.
A system for reducing distortion in the voiceband of a splitterless DSL includes a probe signal generator, a distortion level detector, and a comer point detector. The probe signal generator is coupled to a telephone line for transmitting a probe signal to at least one telephone set at a plurality of power levels. The distortion level detector is coupled to the telephone line for receiving and measuring a distortion signal reflected from the telephone set for each one of the plurality of power levels. The corner point detector is coupled to the probe signal generator and the distortion level detector for detecting a corner point in the received distortion signal from the measurements.
In a preferred embodiment, the probe signal is divided into a plurality of sections, wherein each section has a plurality of symbols that are transmitted at a predetermined power level. The power level can also be determined by the corner point detector during retrain to provide coarse adjustments in the power level. The corner point detector computes a ratio having a numerator equal to the difference between the first and second measurements and a denominator equal to the difference between the first and second power levels, and compares the ratio value against a predetermined value.
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Splitterless DMT System Design and Measurements; Aware, Inc.; ITU; Temporary Doc. RB-019-E; Redbank NJ, Oct. 27-29, 1997.
Splitterless G. Lite Interoperability With ANSI T1.413 and/or G.DMT; Aware, Inc.; ITU; Temporary Doc. RB-022-E; Redbank NJ, Oct. 27-29, 1997.
Long Guozhu
Zheng Qingyi
Centillium Communications Inc.
Fenwick & West LLP
Kuntz Curtis
Tieu Binh K.
LandOfFree
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