Telecommunications Research Laboratories

Develcon Electronics Ltd.

Canadian Patents & Development Ltd.

Telecommunications Research Laboratory

Critical Telecom Corp.

Danby Systems Ltd.

David E Dodds

Ludo A Bertsch

Gregory Jacob Erker

Jan Conradi

Robert James Davies

Andrzej M Sendyk

Oliver Cruder

Michael J Sieben

Bernardo Celaya

Mark Labbe

Donald B Wohlberg

Terence Monteith

A technique for estimating distances and the nature of irregularities and faults on a telephone subscriber line is disclosed. Reflected test signals are measured as a function of frequency. The measurements are compensated for variable propagation velocity and variable line attenuation and then weighted before transformation with the Fourier transform. The amplitude and phase of peaks in the spectral domain identify the distances to the irregularities and the magnitude and phase angle of the reflection coefficients.

Wideband frequency domain reflectometry to determine the nature and location of subscriber line faults

A plurality of metallic telephone lines carrying both baseband POTS (plain old telephone service) and DSL (digital subscriber line) or ADSL (asymmetric digital subscriber line) signals originate at customer premises and pass through a common field cabinet enroute to a telephone central office. Baseband POTS signals flow in an undisturbed manner through the field cabinet and maintain the normal, highly reliable, communication between the subscriber premises and the central office telephone switch. At the field cabinet, DSL/ADSL signals are removed from the metallic telephone line and are then communicated between the field cabinet and the central office using fiber optic broadband transmission. Fiber optic transmission increases the allowed distance between the ADSL transceiver located at the customer location and the respective ADSL transceiver located in the telephone central office. In the optical fiber, DSL and ADSL signals are retained in their 2B1Q, DMT, QAM or CAP analog format and are simply frequency translated in the process of frequency division multiplex (FDM) transmission. The arrangement allows the provision of ADSL service to customers beyond the distance limit normally imposed by a completely metallic telephone line. It is understood that this arrangement is equally applicable to a variety of DSL (digital subscriber line) signals.

Extended distribution of ADSL signals

An optical transmission system has an input light guide and N multipath modulators operated as frequency shifters connected in cascade to receive light from the input light guide and output light along N output light guides where N is 1 or more, wherein the light carried by each ith output light guide, where i&equals;1, . . . , N, is shifted in frequency from light carried by the input light guide by i&ohgr;,, and where each set of multipath modulators contributes a frequency translation of &ohgr;,and where the light from each output light guide is modulated and combined with all other modulated output light to form a multiwavelength transmission system. A receiver equipped with similar frequency shifters is configured to generate a number of optical reference signals of frequencies &ohgr;,&plus;i&ohgr;,, each reference sign to be added as a separate portion of the received signal so as to perform self-homodyne detection of the information signal associated with each respective modulated frequency.

Optical frequency shifter and transmission system

Bandwidth reduction methods and apparatus that offset the effects of dispersion on a signal transmitted through an optical fiber. The apparatus and methods employ a generator of modulated optical signals in cascade with a modulator. One of the generator and the modulator may be an optical intensity or amplitude modulator and the other a phase (or frequency) modulator. With the application of specific signals, the cascaded generator and modulator produce an optical signal with reduced energy in one half of the transmission bandwidth. One important benefit in the method is the fact that by exploiting the less obvious spectral characteristics of single sideband signals, a simple modulator design is achieved. All of the designs exploit a method of generating analytic signals via hybrid modulation.

Hybrid single sideband optical modulator

Apparatus for generation of broadband vestigial sideband (VSB) signals primarily for transmission over optical fibers in which the carrier frequency is in the infrared portion of the electromagnetic spectrum. The modulating signal is normally a broadband digital signal. The apparatus includes an optical fiber, an optical signal source coupled to a modulator with first and second modulation signals modulating the carrier optical signal, plus a method of adding carrier optical signal to the modulator output optical signal and coupling to an output optical fiber. The modulation signals are selected such that an output optical signal transmitted along the optical fiber from the modulator has a vestigial single sideband. Part of the carrier optical signal is transmitted with the output optical signal through the use of a bypass loop or through an appropriate combination of the first and second modulation signals.

Optical modulation system

Networking computers via shared use of voice telephone lines

An impedance matched branch connection for local area networks (LANs). A stub or branch connection increases the number of computers that can connect to a LAN. A star connection is formed when several stub lines are connected at the same point. Stub connections introduce an impedance discontinuity in the line and this discontinuity causes signal reflections which interfere with normal data transmission. The impedance discontinuity is avoided by providing a negative impedance device at the star point which makes the impedance at each transmission line appear as though the stub lines were not added.

LAN star connection using negative impedance for matching

A dataset apparatus is powered by voltages extracted from electrical signals passing between the data terminal apparatus and the dataset by circuits comprising a capacitor and diode combination. The transmitter of the dataset includes capacitance isolation of the signal from the transmission lines by applying an oscillated voltage and an inverted oscillated voltage dependent upon the signal to capacitors in a first circuit to charge the capacitors through a diode whereby the oscillating voltage causes the capacitors to discharge through a second path formed by further diodes to charge a pair of capacitors coupled across the transmission lines. A second circuit of similar impedance to the first charges the capacitors to the opposite polarity. The arrangement can handle high common mode voltages on the transmission lines. The receiver of the dataset includes a doubly balanced resistance bridge network connecting the transmission lines to a differential amplifier, the circuit being balanced such that signals from the transmitter are not applied to the amplifier and also common mode signals are not applied to the amplifer.

Dataset powered by control and data signals from data terminal

An interface circuit for providing power and signal voltages to a telephone line under the control of a microcomputer within the telephone exchange comprises four charge pumps, each comprising a pair of driver/amplifiers which charge a pair of capacitors through a first diode with two further diodes discharging the capacitors to charge a capacitor connected across the telephone line. The four charge pumps are arranged symmetrically to provide positive or negative potential at the TIP terminal and positive or negative potential at the RING terminal. Modulation of the voltages to transmit a desired signal is obtained by modulating the power voltage supplied to the amplifier/drivers. The current drawn by the telephone line is measured by sensing the current drawn from the voltage source by the amplifier/drivers and this is used to extract the received signal from a telephone line and also to extract a measurement of peak current drawn from the TIP and RING leads separately. The amplifier/drivers are driven by continuous square wave signal.

Telephone line interface circuit

A dataset apparatus is powered by voltages extracted from electrical signals passing between the data terminal apparatus and the dataset by circuits comprising a capacitor and diode combination. The transmitter of the dataset includes capacitance isolation of the signal from the transmission lines by applying an oscillated voltage and an inverted oscillated voltage dependent upon the signal to capacitors in a first circuit to charge the capacitors through a diode whereby the oscillating voltage causes the capacitors to discharge through a second path formed by further diodes to charge a pair of capacitors coupled across the transmission lines. A second circuit of similar impedance to the first charges the capacitors to the opposite polarity. The arrangement can handle high common mode voltages on the transmission lines. The receiver of the dataset includes a doubly balanced resistance bridge network connecting the transmission lines to a differential amplifier, the circuit being balanced such that signals from the transmitter are not applied to the amplifier and also common mode signals are not applied to the amplifier.

Coupling an electrical signal to transmission lines

Balanced circuit for connecting receiver and transmitter to transmission

An adaptive delta modulation system wherein the input analog signal is periodically sampled and a binary bit is generated for each period, the logic level of the binary bit being dependent on whether the sampled signal is greater or smaller than the approximate signal of the previous sample. Decoding apparatus converts the stream of binary bits to approximate the analog signal by periodically charging or discharging a capacitor integrator by predetermined variable steps. The charging or discharging of the integrator during each period is determined by the logic level of the binary bits, whereas the increase or decrease in step size for successive periods is determined by successive similar signal binary bits or successive dissimilar bits, respectively. The increase or decrease in step size is achieved by storing a binary step size number S in a register and adding or subtracting a fraction of the number to or from itself during each period, producing a new step size number for each period. In addition, each added fraction may be increased by a fixed least significant number to enhance step size recovery between a transmitter and a receiver. The charge on the capacitor integrator is controlled by a pulse having a width that is directly related to the value of the S number. The pulse controls the conduction time of constant current sources which are connected to the integrator to produce the analog output signal.

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Saskatoon, Canada

David E Dodds