Telephonic communications – Supervisory or control line signaling – Using line or loop condition detection
Reexamination Certificate
1999-08-02
2002-02-26
Chan, Wing F. (Department: 2643)
Telephonic communications
Supervisory or control line signaling
Using line or loop condition detection
C379S382000, C379S398000, C379S413010
Reexamination Certificate
active
06351528
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to an apparatus for use with a subscriber line interface circuit in a telephone system. More particularly, the present invention relates to a method and apparatus for detecting an AC electrical characteristic of the subscriber line in the presence of said time vowing ringing signal and providing a ring trip indication in response to the AC electrical characteristic reaching a predetermined threshold.
BACKGROUND OF THE INVENTION
A telephone system generally includes one or more subscriber telephone devices, each subscriber telephone device being coupled with a telephone central office by a subscriber line. The subscriber telephone devices may be telephones or other telephone equipment. The telephone central office handles switching of telephone signals between subscriber telephone devices. Industry standards govern the electrical signal levels used for switching, coupling and signaling within the telephone system.
When a telephone call is placed to a particular subscriber telephone device, the telephone central office must send signals to the telephone device to indicate the incoming call. The telephone central office sends ringing signals which cause the subscriber telephone device to generate a ringing indication. The ringing indication may be a ringing bell, an electronic tone or some other audible or visible indication of ringing. The telephone central office applies the ringing signals directly to the subscriber telephone line.
Each subscriber telephone device is coupled to the telephone central office by a subscriber line. The subscriber line generally includes two conductors, one labeled “tip” and the other labeled “ring.” Tip conductors and ring conductors carry both AC (time varying) and DC signals. The subscriber telephone device plus, the associated tip and ring conductors is commonly known as a loop or subscriber loop.
The subscriber telephone device can be modeled electrically as a resistor, an inductor and a capacitor in series and selectively coupled by a switch to the tip and ring conductors. The switch is known as the hook switch. When the telephone is on hook, and not in use, the hook switch couples the resistor-inductor-capacitor combination between the tip and ring conductors. When the telephone is off hook, or in use, the switch couples the tip and ring conductors through a resistor only. By detecting the D.C. impedance between the tip and ring conductors, the telephone central office can determine whether the subscriber telephone device is off hook (in use) or on hook (not in use).
When providing ringing signals to an intended receiving subscriber telephone device in response to a call originated by another (call originating) subscriber telephone device, the telephone central office must be able to determine whether the intended receiving subscriber telephone device is on hook or off hook for several reasons. First, if the intended receiving subscriber telephone device is off hook, the telephone central off ice must not send ringing signals, but must rather send a busy tone to the call originating subscriber telephone device. Secondly, if a user answers the phone by taking the receiving subscriber telephone device off hook in response to the ringing signal, the telephone central office must detect the change from on hook to off hook so that the receiving telephone does not ring loudly in the user's ear, causing the user discomfort. Further, in response to the receiving telephone going off hook, the telephone central office completes the connection between the call originating telephone device and the receiving subscriber telephone device. The process of providing ringing signals and detecting the on hook or off hook state of the receiving subscriber telephone device by the telephone central office is known as ring trip detection.
There are numerous industry standards governing ring trip detection. The telephone central office, or other equipment providing the ringing signals to the subscriber lines, must detect that the receiving subscriber telephone device has gone off hook and terminate the ringing signals within a predetermined time period, such as 200 msec. Further, the telephone central office or other device which provides ringing signals to the subscriber line must be able to provide a ringing indication to any telephone coupled to the subscriber line. The telephone central office or other device providing the ringing signals should provide no false ring trip detections and should not miss any ring trip detections. Still further, the device which provides the ringing signals to the subscriber line must work with any length phone line, including both short (low impedance) or long (high impedance) subscriber lines.
Short subscriber lines couple the telephone central office to subscriber telephone devices which are physically near the central office, perhaps just a few blocks away. Long subscriber lines couple the telephone central office to subscriber telephone devices which are physically distant from the central office, perhaps miles away. Long subscriber lines have a greater impedance, measured from the central office, than short subscriber lines. Long subscriber lines are also more susceptible to noise due to coupling from adjacent noise sources such as other subscriber lines and power lines, than are short subscriber lines.
One industry standard requires the ringing signal to be applied as an AC voltage with a DC offset. The AC voltage, measured at the telephone central office, is preferably a 90 volt rms, 20 Hz, AC signal in order to provide at least 40 volts vms at the receiving telephone device. The DC offset is preferably 48 volts, measured at the central office. The ringing signal is generally applied as an unbalanced or single-ended ringing signal. That is, the AC signal is applied to either the tip conductor or the ring conductor with either polarity of DC offset (referred to as ring-plus, ring minus, tip-plus and tip-minus ringing). The conductor to which the AC and DC ringing signals are not applied is grounded in an unbalanced ringing design. The goal when applying ringing signals is to place the 90 volt rms AC signal across the tip and ring conductors.
When the subscriber telephone device is on hook, no DC path exists in the subscriber telephone device to permit DC current to flow in response to the applied DC offset voltage. However, with the telephone on hook, alternating current may flow in response to the applied AC ringing voltage to cause the subscriber telephone device to generate a ringing indication. When the subscriber telephone device is taken off hook, a DC path is established to couple the tip and ring conductors and allow DC current to flow in the loop. The central office detects the flow of the DC current in the loop to determine that the subscriber telephone device has been taken off hook and interrupts the ringing signal. Thus, prior art telephone apparatus have detected ring trip by detecting the DC impedance between the tip and ring conductors, typically by measuring DC current flow in response to a known applied DC voltage.
Application of the 90 volt rms signal and the 48 volt DC offset to the subscriber line means that the ringing generator must be able to handle potential differences substantially equal to 250 volts. It has heretofore been uneconomical to fabricate a ringing generator using a silicon integrated circuit.
Silicon integrated circuits which can handle 250 volts are expensive. Individual circuit elements, such as transistors, resistors and capacitors, designed to handle such large voltages must be physically large in order to sink and source the large currents associated with such large voltages. This large size requires substantial “real estate” on the surface of an integrated circuit which reduces the scale of integration of such devices as well as reducing manufacturing yield. Both the large size and the reduced yield increase manufacturing cost.
Moreover, circuit elements which can handle up to 250 volts must be fabricat
Chan Wing F.
Williams Morgan & Amerson
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