Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-12-18
2003-05-13
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C375S225000, C370S487000, C379S093010
Reexamination Certificate
active
06563864
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to high rate digital transmissions over existing telephone lines including local subscriber loops and more particularly to a method and system of connecting telephones and high rate modems directly to the same telephone line without a splitter.
BACKGROUND OF THE INVENTION
The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the public switch telephone network (PSTN). An example includes the emerging modem communication standards which have pushed the limit of transmission speeds close to 56 Kbps. Another example includes the emerging variety of the DSL communications protocols including asymmetric digital subscriber line (ADSL), symmetric digital subscriber line (SDSL), high bit rate digital subscriber line (HDSL), and very high rate digital subscriber line (VDSL). Each DSL variant represents a different transmission speed over a different distance of copper pair wiring. Currently, industry is producing equipment capable of implementing one or more of the DSL protocols. Modems, routers, line cards, and digital loop carrier systems are all examples of such equipment.
In principle, a DSL modem and a plain old telephone system (POTS) or other voice band device can operate simultaneously over the same wire line pair since they use different frequency bands. The connection, however, of a POTS to the same wire line pair as a DSL modem can suffer from several problems that result from the change in input impedance of the POTS as it is added to the line. In essence, a POTS is not designed to handle frequencies outside the voice band of 300-3.4 kHz. In addition, a POTS often contains nonlinear components that may create harmonic interferences. Examples of such nonlinear components include Zener diodes, transistors, varistors, triacs, and other devices used for overvoltage protection, sidetone generation and overvolume protection. For specific details on the circuitry of the POTS, reference is made to Whitman D. Reeve, “Subscriber Loop Signaling and Transmission Handbook: Analog,” IEEE Press, 1992, included herein by reference.
In a splitterless configuration, when the POTS goes off-hook during a DSL transmission, these components can distort the DSL signal causing intermodulation and producing nonlinear echos of the upstream DSL signal. Harmonics of these echos lie in the downstream DSL band, and thus severely corrupt the downstream signal. To overcome these problems, a POTS splitter may be used to isolate the DSL high frequency signals from the voice band frequencies.
In principle, a splitter is a combination of low-pass and high-pass filters. The low-pass filter is used to isolate the voice band signals and route them to/from the telephone and the central office. The high-pass filter is used to isolate the passband DSL signal and route it to and from the DSL modem and the central office. The splitter can be part of the DSL modem, in which case the telephone(s) has to be connected to the modem. Alternatively, the splitter can be installed at the line drop to the home, which may require professional installation and re-wiring of the telephone lines going to the DSL modem inside the customer premises.
Besides the installation issues which the customer may be faced with, the use of a splitter in a DSL modem adds to the overall complexity and cost of the user's communication equipment. The filters, transformers, resistors and other discrete components needed to implement a particular splitter design can be bulky and expensive. In addition, the application programs and the control logic necessary to operate the splitter and cause it to activate voice band or DSL filtering functions also increase overall complexity of the system. Utilizing separate connections for the voice band and DSL functions would allow the subscriber to utilize separate devices, although such a configuration restricts the way a customer can connect the equipment and adds the costs of a separate line. It would be advantageous to utilize a DSL modem in the same manner in which voice band modems are currently operated.
SUMMARY OF THE INVENTION
The present invention is a solution for connecting a POTS and DSL modem directly to the same line without a splitter, thus enabling the customer to plug the DSL modem and a normal telephone in any phone jack in the home similar to the way voice band modems are installed.
According to one embodiment, the invention discloses a communications device that accommodates changes in transmission line characteristics over a wire line pair that connects a customers plain old telephone system (POTS) to the central office or other centralized call exchange facility. The device includes a transceiver with an analog interface coupled to the wire line pair and arranged to communicate with the central office. An on/off-hook detector is coupled to the analog interface and arranged to detect when the POTS seizes the line corresponding to the off-hook state of the POTS. The device includes an impedance analyzer function coupled to the on/off-hook detector and the analog interface with an output so that the difference of line impedance between the on-hook and off-hook conditions of the POTS can be determined. The output of the impedance analyzer function is fed to a control logic function that is configured to adjust the transmission parameters of the transceiver in response to changes in the transmission characteristics of the line pair.
The communications device is able to compute and store transmission line profiles corresponding to the on-hook and off-hook states of the POTS. Since the impedance of the customer premise equipment changes as the POTS goes from on-hook to off-hook, the transmission line profiles contain transmission line parameters which the communications device can use to avoid the effects of nonlinearities produced as the POTS transitions between states. The transmission line parameters can include the transmission power levels, echo cancellation coefficients, and equalizer coefficients used by the transceiver during a DSL connection.
In another embodiment, disclosed is a digital subscriber line (DSL) modem capable of operating with multiple transmission line profiles depending on the current line characteristics of a wire line pair coupling a customer to the central office. The DSL modem includes an interface to the Wire line pair and a signal converter with an output terminal coupled to the interface. An on/off-hook detector drives an impedance analyzer function that is able to measure and produce a figure of the impedance presented to the wire line pair by the combination of the DSL modem and the voice band equipment. The impedance analyzer function is coupled to control logic which, in turn, is configured to determine the transmission line characteristics of the wire line pair as the voice band device transitions between states.
In one embodiment, the DSL modem includes a memory space, such as Random Access Memory (RAM), EEPROM, Flash memory, magnetic hard disk, or other suitable memory configuration. The memory space is accessible by the control logic block and stores transmission line profiles computed by the impedance analyzer function and the control logic block. The profiles can contain transmission line parameters corresponding to the on-hook and off-hook conditions of the voice band device coupled to the wire line pair. Examples of such transmission line parameters include the upstream power, echo cancellation and equalizer coefficients used by the DSL modem transceiver during on-hook and off-hook operation of the voice band device. In one embodiment the DSL modem is able to dynamically switch from one transmission line profile to another as it detects transitions of the voice band equipment between states.
Also disclosed is a method of simultaneously operating a digital subscriber line modem and a voice band device, such as a telephone or voice band modem, over the same connection. The method incl
Ibrahim Yaser
Payne Ralph E.
Polley Michael O.
Brady III W. James
Hernandez Pedro P.
Pham Chi
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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