Telephonic communications – Reception of calling information at substation in wireline... – Blocking caller id transmission
Utility Patent
1999-05-06
2001-01-02
Woo, Stella (Department: 2743)
Telephonic communications
Reception of calling information at substation in wireline...
Blocking caller id transmission
C379S093050, C379S183000, C379S372000, C379S399010
Utility Patent
active
06169792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of data communication equipment (DCE), and in particular to automatic number identification system for data access arrangement (DAA).
2. Background Art
Telephone systems were originally designed for voice communication. As new technologies have emerged, methods have been devised to transmit other types of information over telephone lines. These methods typically require the connections of other equipment besides the voice telephone set to the telephone line. Examples of such equipment include computer modems, facsimile (“fax”) machines, answering machines, voice mail systems, phone patches, and automatic number identification (ANI) or “Caller ID” systems.
An automatic number identification (ANI) system allows a modem or a telephone to identify the caller ID signals without user intervention. Some telephones and modems are equipped with ANI capability to provide users the convenience of ANI system. However, the prior art telephones and modems with ANI capability suffer significant ANI signal attenuation and have limited system design flexibility.
A modem enables two digital electronic systems to communicate over the telephone network. A typical telephone network comprises a single twisted pair of wires (called the “tip” and “ring” lines). Digital equipment systems, on the other hand, require two separate paths of communication to communicate with each other. Also, the signals present within the above equipment often have different electrical characteristics than the signals that may be transmitted on a telephone line. Thus, an interface is required to connect such equipment to a telephone line.
The modem is placed between the digital computer and the analog telephone system, providing the necessary interface between the telephone network and digital systems over the tip and ring lines. The modem accepts a serial stream of bits as input and produces a modulated carrier as output, thus converting the digital signals of the computer to analog signals for transmission on telephone lines, or vice versa.
To meet regional telephone companies' requirements, a modem typically comprises a DAA (Data Access Arrangement), which is used to connect to a dial-up switched telephone line. Traditionally, especially for computer modems, a DAA is used to connect equipment to a telephone line. A DAA typically provides isolation, impedance matching, hybrid circuit and sometimes amplification, filtering and control functions.
When a modem is equipped with an ANI system, it allows to identify the caller ID and register the caller ID in a memory device. If the modem is connected to a computer, then other options are possible to use the caller ID in more sophisticated ways. For example, the computer can receive the caller ID from the modem, display it on a screen, store it in its memory, or write it in a file.
An ANI system is also useful for such state-of-the-art technology as “recall.” When an attempted telephone call goes unanswered, the caller ID is identified by an automatic number identification system on the called telephone and stored in a memory device such as a random access memory (RAM). When a user on the called telephone side wishes to call the last number the user missed, the user can press a special “recall” button or a combination of buttons, such as the “*” key followed by the “9” key, on the user's telephone key panel to initiate an outgoing call to the last number that called. Upon detecting a special key sequence for “recall,” the user's telephone makes a “recall” to the telephone number that last called.
In an ANI system, caller ID signals are sent to a called modem or a telephone when a call is made to the called telephone number. The caller ID signals provide the called telephone or modem with identification of the calling telephone. The called telephone uses the caller ID (ANI) signals to identify the calling telephone or modem. Typical caller ID signals include frequency shift keyed (FSK) modem tones transmitted between rings of the ringing signal.
Prior art ANI systems were implemented by adding extra circuit components to DAA circuits to implement a caller ID signal path. These prior art ANI systems often led to a more complex, expensive design architecture, and resulted in significant caller ID (ANI) signal attenuation up to 18 dB or more.
FIG. 1
shows a block diagram of a prior art DAA circuit for a European-compatible modem with ANI capability. The circuit in
FIG. 1
has a caller ID signal path that comprises switch SW
3
, capacitor C
2
, and resistor R
1
. Capacitor C
2
is used to block DC current from being supplied to transformer T
1
.
In
FIG. 1
, transmitted signal TXA is amplified by amplifier A
1
and provided to transformer T
1
through impedance network Z
1
. The typical value of Z
1
is approximately 300 &OHgr; for U.S. designs and 300 &OHgr;+(374 &OHgr; resistance in parallel with 200 nf capacitance) for international designs. Received signals from the telephone line (represented by the tip and ring lines) are fed to amplifier A
2
and to line RXA. Transformer T
1
isolates the outgoing signal from the received signal. Transformer T
1
has a primary winding and a secondary winding.
The primary winding of transformer T
1
is coupled to capacitor C
1
, which is coupled to the “tip” line through switch SW
1
. Protection circuit PC
1
comprising Zener diode ZD
1
and diode bridge DB
1
(not shown in detail, well-known in the art) is coupled across the primary winding of transformer T
1
via capacitor C
1
. Protection circuit PC
1
is used to protect the modem from voltage spikes and to ensure proper operation of a DAA circuit since the polarity of tip and ring lines are not known in advance. The primary winding of transformer T
1
is also coupled to the “ring” line. The tip and ring lines (local loop) are connected to the telephone central office that provides power, switching, and signaling.
On a local loop represented by the tip and ring lines, an idle (on-hook) state is indicated by an open circuit and no current flow. A connect (off-hook) state is signaled by a closed circuit and continuous current flow. In an off-hook state, the telephone line is said to be “seized.” Referring to
FIG. 1
, during the idle state (on-hook) before the modem has seized the line, an incoming ring signal is detected by the ring detector block (not shown, well known in the art) and switch SW
3
is closed.
The caller ID signals (ANI information) are transmitted from the telephone system central office to the local DAA, and usually come between the first and second incoming rings in the form of a small AC signal. The modem can detect the caller ID signals at the RXA line if an additional AC coupling path is provided.
The additional AC coupling path for the caller ID (ANI) signals is provided in the prior art example of
FIG. 1
along switch SW
3
, capacitor C
2
, and resistor R
1
. The existing path along switch SWl and capacitor C
1
cannot be used as the AC coupling path for the caller ID (ANI) signals since capacitor C
1
by itself would exhibit an impedance below the regulated value of two (2) K&OHgr;. Although a resistor can be used in series with capacitor C
1
to increase the impedance, the addition of such series resistor would also increase the DAA's off-hook impedance value to an unacceptable range.
Referring to
FIG. 1
, switch SW
3
is closed for the duration of an automatic number identification process, and an AC current path is formed by components R
1
and C
2
to allow the caller ID (ANI) signals from the telephone line to reach transformer T
1
and RXA. Switches SW
1
and SW
2
remain open during ANI operations so that no DC current flows between the DAA and the telephone central office, thereby preventing the telephone line seizure. After an automatic number identification process is complete and the caller ID has been detected, switch SW
3
is opened. In another prior art embodiment which is compatible with the U.S. and Canadian regulations but incomp
TDK Systems, Inc.
The Hecker Law Group
Tieu Binh K.
Woo Stella
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