Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
2000-01-07
2004-06-01
Eng, George (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S093050, C379S106090
Reexamination Certificate
active
06744883
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the arts of data communications and telephony, and more particularly, to a filter system (and associated methodology) for connection between a source of large-amplitude, high-frequency noise and at least one broadband communications channel which is frequency-division multiplexed into a communications medium of a communications system.
BACKGROUND OF THE INVENTION
With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data performance over the two-wire, copper twisted-pair, transmission lines that connect the telephone central offices (COs) to the customer premises (CPs). The telephone system architecture connects customer premises equipment (CPE) to CO switches over transmission lines known as “local loops,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. These loops are part of the cable plant that provides the physical means by which transmission services are implemented. To allow connectivity between COs, the CO switches are interconnected to each other over high-capacity, multiplexed transmission facilities known as trunk lines. Unlike the CO-to-CO trunk connections, which have generally been digital for some time, digital service provision to the customer premises is a more recent development. These digital CO-to-CO trunk connections are part of a telephone network architecture known as integrated services digital network (ISDN).
Historically, the PSTN (public switched telephone network) evolved with analog, subscriber loops connected to a telephone network with circuit-switched capabilities which were designed to carry voice communications. The evolution of the telephone network from a system just designed to carry analog, voice communications to a system which could carry digital voice and data led to the development of the ISDN architecture. One of the goals of the ISDN architecture is to provide customers (or subscribers) with integrated access (over a single loop) to the services of a digital network.
Because of the costs of replacing or supplementing the cable plant to the customer premises, technologies are needed that utilize existing subscriber loops to provide digital connectivity to the customer. Furthermore, to provide customers with additional flexibility and enhanced services, multiplexing techniques are needed to fill a physical subscriber loop with multiple logical channels. In contrast to the ubiquitous use of frequency-division multiplexing (FDM) and/or time-division multiplexing (TDM) on CO-to-CO trunk connections, the standard analog POTS (plain old telephone service) capability provided over subscriber loops generally has not been multiplexed with any other channels. To solve the problems of digital connectivity and multiple channels of communication, subscriber loops capable of carrying digital channels have been developed and are known as digital subscriber lines (DSLs). Logical channels within a subscriber line which carry digital signals are known as DSL channels while logical channels within a subscriber line which carry POTS signals are known as POTS channels.
Frequency-division multiplexing modulates (or frequency shifts) signals to different frequency channels before placing them on the communications medium. This modulation of signals to higher frequency ranges creates a broadband signal which has been frequency shifted to a different frequency band than the base band frequencies (or baseband) of the signal. A communications channel associated with a broadband signal is known as a broadband channel, while a communications channel associated with a baseband signal is known as a baseband channel. The equipment that MOdulates and DEModulates signals onto a communications medium is called a MODEM. Broadband is an adjective often used to describe equipment, networks, and systems that use FDM to modulate signals, while narrowband is an adjective often used to describe equipment, networks, and systems that do not modulate signals out of their base frequency bandwidth range. Digital signals that utilize square waveforms theoretically use an infinite frequency spectrum which limits the practicality of using FDM to multiplex other signals onto the same communications medium with a square waveform signal. In contrast to FDM, the bandwidth of a communications medium is broken up into time-slots in TDM.
One of the initial DSL solutions is the service of narrowband ISDN which has several drawbacks. For example, although the narrowband ISDN BRI (Basic Rate Interface) service communicates three logical channels over a subscriber loop, it does not allow direct compatibility with POTS because the narrowband ISDN channels occupy the baseband frequencies (0-4 KHz) that are used by POTS to carry a voice frequency (VF) channel. Because narrowband ISDN uses the frequency baseband of POTS, voice service over a narrowband ISDN DSL requires the use of actively powered terminal adapters (TAs) to provide POTS functionality by utilizing one of the logical channels in the narrowband ISDN DSL.
As a result of some of the drawbacks from narrowband ISDN, and as a result of the customer demand for increased bandwidth, newer DSL (digital subscriber line) technologies have been developed. Some of these newer DSL technologies use frequency-division multiplexing and/or time-division multiplexing to provide a higher-bandwidth digital service over a local subscriber loop without significantly interfering with standard POTS functionality. This backward compatibility with standard POTS functionality offers easier and lower cost customer migration to the newer DSL technologies without the previously mentioned drawbacks of narrowband ISDN. The newer DSL technologies accomplish this functionality by modulating their broadband, digital signal above (at higher frequencies than) the 0 KHz to 4 KHz baseband of standard, analog POTS signals. This FDM capability to provide multiple channels on a single subscriber line will work with both baseband and broadband analog signals as well as with baseband and broadband digital signals. However, the common existence of older, legacy, analog POTS loops and equipment coupled with the demand for newer, digital service has caused the development of new DSL technologies that allow a new, digital channel to be frequency-division multiplexed above an old, analog POTS channel. Several variants of new DSL technology exist, e.g., ADSL, SDSL, RADSL, VADSL, etc., with such variants generally referred to collectively as xDSL. Generally, when multiple channels are communicated across an xDSL connection, a POTS splitter is utilized to decouple the frequency-division multiplexed channels. A POTS splitter usually is situated at the CO as well as at the customer premises.
Although the transmission of both digital broadband and analog POTS baseband signals over a subscriber loop between a CO telephone switch and a customer premises offers many potential advantages for customers, several practical problems must be solved in implementing DSL solutions. One significant problem results from the high-frequency transient noise signals generated by the legacy, analog POTS equipment connected to the subscriber loop. These transient signals from the POTS equipment are in frequency ranges above the POTS baseband and have components which interfere with the one or more broadband channels. Many functions of the legacy POTS equipment generate these high-frequency noise components.
Historically, the POTS subscriber loop was designed with the functions needed to communicate both baseband, voice-conversation signals and call-state/line-state signaling information (known generally as subscriber loop signaling or POTS loop signaling as opposed to CO-to-CO trunk signaling). Although POTS is a service deployed by the telephone company, it also can be viewed as defining an interface standard that specifies how to communicate both baseband, analog voice conversations and subscriber loop signaling information. Because call-state/line-
Bingel Thomas J.
Bremer Gordon
Hazen Ramon B.
Eng George
Paradyne Corporation
Thomas Kayden Horstemeyer & Risley
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