Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1997-12-19
2002-01-29
Chan, Wing F. (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S387020, C379S167050, C379S093050
Reexamination Certificate
active
06343126
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of telephony. More particularly, the invention relates to an adaptive interface for interfacing a two-wire or a four-wire analog telephone instrument to a digital, analog or hybrid telephone switching system.
BACKGROUND OF THE INVENTION
A two-wire analog telephone set of the type commonly found in the homes of telephone service subscribers typically includes a base unit connected to a central office of a telephone service provider via a bi-directional, two-wire, telephone line and also includes a handset connected to the telephone base unit via a four-wire handset cable. The handset cable has four wires because, for two-way voice communication, the handset includes both a microphone and a speaker, each of which requires a pair of wires. Typically, the telephone base supplies audio signals to the speaker and a DC biasing voltage to the microphone, while the telephone base receives audio signals from the microphone. A two-wire to four-wire converter included in the telephone base unit converts the two central office signals into the four handset signals. In addition, the telephone set includes a ring detector for detecting an AC ring signal provided by the central office and a hook-switch for signalling the central office for answering or placing calls. When the handset is removed from its cradle, the hook switch controls draw of DC current from the central office by the telephone set which is detected by the central office.
A conventional modem transmits digital data over a two-wire telephone line by modulating an analog carrier signal according to the digital data. Typically, the digital data is generated by a computer or facsimile machine connected to the modem. The carrier signal is a tone within the frequency range of telephone transmission line. Upon reception by a second modem at the other end of the transmission line, the digital data is reconstructed by demodulating the received signal.
Business organizations often utilize a telephone switching system for providing telephone service to telephone users within the organization. The telephone switching system can have an all-digital interface with its corresponding compatible telephone sets, such as in a digital private branch exchange (PBX). Alternately, the telephone switching system can have an all-analog interface, such as is provided by an analog line card in a PBX or by a central office. In addition, the telephone switching system can provide a combined digital and analog interface with its corresponding compatible telephone sets, such as in a hybrid PBX or a key telephone system (KTS). For the purposes of this document, the term “PBX” is utilized to encompass equipment similar to those the above-listed types of telephone switching equipment.
Telephone sets that are compatible with a particular PBX utilized by a business organization are located on the desks of the users. Each PBX-compatible telephone set is connected to the PBX via a corresponding extension line, while the PBX is connected to a telephone service provider via one or more outside lines. The PBX typically includes capability for appropriately connecting incoming calls to the user telephone sets and for connecting outgoing calls from the user telephone sets to an outside line. In this way, fewer than one outside line per telephone set is needed, thus, reducing the cost of the telephone service. In addition, the PBX typically provides a variety of features to the users of the PBX, such as connecting calls among the users and providing voicemail services.
To implement all of the functions of the PBX, certain control and overhead communications must take place between each user's telephone set and the PBX. These communications typically include digital status, initialization and command signals in addition to the two-way voice signals necessary to carry on a telephone conversation. For example, the PBX must know whether a telephone set is connected to a particular extension line in order to know whether or not to route calls to that extension. As another example, the PBX must interact with the user telephone sets in order for the users to receive incoming calls, initiate outgoing calls, terminate telephone calls and to access voicemail and other features of the PBX.
In general, communication protocols utilized for control and overhead communications differ among the various manufacturers of PBX's. In addition, in an all-digital PBX, the voice signals are communicated between the telephone sets and the PBX as digital samples. Thus, analog voice signals are digitally sampled and encoded according to various different schemes (e.g. &mgr;-law or A-law) before they are communicated. Upon reception, the digital samples are decoded and converted back into analog voice signals. In a hybrid system, voice signals are communicated as analog signals, while control and overhead communications are digital signals. Therefore, a two-wire analog telephone instrument, such as a modem, fax modem, facsimile machine or teleconferencing device, cannot generally interface directly with a PBX. Nor can a four-wire analog telephone instrument, such as a headset, handset or modem, generally interface directly with a PBX.
This creates a problem for users of a PBX who wish to use universally available analog telephone instruments, such as modems, fax modems, facsimile machines, teleconferencing devices, headsets or handsets, in addition to their PBX-compatible telephone sets. This problem has intensified by the recent increase in demand for access to the world wide web, which is typically accessed through use of a modem connected to a personal computer. A proposal has been to provide a dedicated outside line for each such analog telephone instrument. This solution is not entirely satisfactory, however, because it negates the savings which result from the PBX limiting the number of required outside lines. Another solution has been to provide an analog line card in the PBX and a separate line connecting the two-wire analog telephone instrument to the PBX. This solution can be costly due to the need to install separate extension lines to connect each of the PBX-compatible telephone set and the analog telephone instrument to the PBX.
Another solution has been to provide a device which interfaces a modem with a telephone set through the handset port of the telephone set. For example, U.S. Pat. No. 4,907,267 discloses a modem interface device for use with a telephone set having a base unit and a handset. The telephone set can be a two-wire telephone set or a telephone set designed for use with a PBX. To use the modem interface device, the handset is unplugged from the handset jack of the base and plugged into a handset jack in one end of the device. Extending from the device is a four-wire cable which is connected to the handset jack of the base. The device also includes a modular jack for accepting a two-wire cable which connects the device to a two-wire telephone instrument, such as a modem. A series of switches are manually positioned to select between voice and data communications and to configure the interface device to match the signalling characteristics of the particular telephone set being used.
The manually operable switch arrangement described in U.S. Pat. No. 4,907,267 is improved upon in two products manufactured by Unlimited Systems Corp. of San Diego, Calif. A first of these products, the “KONEXX Office Konnector,” connects to the base of a telephone set and to the handset to provide an interface for a two-wire telephone, facsimile machine or modem. The device detects when the two-wire telephone, facsimile machine or modem is placed off-hook for switching between voice and data communications. A second of these products, the “KONEXX Konference,” is similarly connected between the base and handset, but provides an interface for a teleconferencing device. For each of these devices, a manually operable switch is positioned in one of four positions for adjusting the device to the signalling characteristics of th
Chan Wing F.
Haverstock & Owens LLP
Hello Direct, Inc.
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