1. Field of the Invention
The present invention relates to a ringing signal sending apparatus, and more particularly to an apparatus for sending a ringing signal to inform the called terminal of the presence of an incoming call addressed to it.
2. Description of the Related Art
Telephone exchanges employ subscriber interface units to provide various control functions over local loop connections, such as handling of incoming and outgoing calls to/from customer premises equipment, or telephone. Sending a ringing signal (or call signal) is one of those subscriber interface functions. The subscriber interface unit notifies a remote telephone set that there is an incoming call addressed to it, by sending a predetermined pattern of ringing and silent intervals.
FIG. 20 shows the duty cycle and waveform of a typical ringing signal. The ringing signal is an alternate current (AC) signal with a frequency of 20 to 25 Hz superimposed on a certain direct current (DC) bias voltage (e.g., −48 volts). The very first cycle of ringing and silence is referred to as the “initial ringing cycle”; similar cycles that follow are called the “cadence ringing cycles.” The illustrated signal has a duty cycle of 1.2-second ringing and 2.8-second silence for the initial ringing cycle, and 1.2-second ringing and 3.0-second silence for the cadence ringing cycles. The subscriber interface unit supplies the called telephone set with a ringing signal with such a cyclic pattern.
The silent period in the initial ringing cycle is often used to provide various service functions, during which a data transfer path is established between the called telephone set and the local exchange. For example, the customer can see the caller's phone number displayed on his/her telephone set when an incoming call is signified. This service, known as the “caller number display” or “calling line identification presentation” (CLIP), exploits the silent period in the initial ringing cycle to transfer the originating phone number information.
FIG. 21 shows subscriber interface units, where the function of sending a ringing signal is illustrated in a simplified form. Here, a plurality of subscriber interface units 101-1 to 101-n are installed in a telephone exchange 100. The subscriber interface unit 1011 comprises a relay switch 111, a feed voltage source 112, and a ringing voltage source 113. The illustrated subscriber interface unit 101-1 supports only one channel for simplicity. In the actual implementations, however, a single subscriber interface unit serves multiple telephone channels (e.g., 32 channels).
The terminal “a” of the relay switch 111 is connected to the feed voltage source 112, which produces a feed voltage of −48 volts. The terminal “b” is connected to the ringing voltage source 113. The terminal “c” is connected to a telephone set 20 through the ring wire (also called “B wire”) of its local loop. The terminal “e” is grounded. The relay switch 111 is actuated by a driving command supplied to its terminal “d.” The tip wire (also called “A wire”) extending from the telephone set 20 is grounded at the subscriber interface unit 101-1.
Suppose here that the telephone exchange 100 is to send a ringing signal to the telephone set 20. This process is initiated by giving a driving command to the relay switch 111. The armature contact (sw) of the relay switch 111 then moves to the “b” side. This creates a circuit that connects the ring wire with the ringing voltage source 113, thus sending out a ringing signal to the telephone set 20.
Referring to FIG. 22, a conventional subscriber interface unit 101, which is configured as above, is electrically connected to a telephone set 20 via tip and ring wires. When modeling the telephone set 20 and subscriber line for transient analysis, one should consider their inductance and stray capacitance. The lower half of FIG. 22 shows such parasitic components. In this model, the telephone set 20 is represented as an inductor L21 and a capacitor C21 connected in series between the tip and ring wires. The tip wire itself is modeled by series inductors L22 and L23 and a capacitor C22 representing its stray capacitance to the ground. Likewise, the ring wire is modeled by series inductors L24 and L25 and a capacitor C23 representing its stray capacitance to the ground. Further, there is a parallel capacitor C24 between the tip and ring wires.
The above-described parasitic components may cause noise interference, particularly when the subscriber interface unit 101 is sending a ringing signal. At every transitional point between a ringing period and a silent period, it is likely that the parasitic inductors produce counter electromotive forces, and the parasitic capacitors discharge their electric energy, thus causing impulse noises. Since the subscriber interface unit 101 simultaneously handles many telephone channels, such impulse noises developed on a channel may cause interfere with its adjacent channels via closely arranged wiring patterns on the print circuit board. Frequent impulses on a channel would be heard by the user of an adjacent channel as unpleasant crosstalk noises.
As previously mentioned, a single telephone exchange 100 is designed to accommodate many subscriber interface units 101. This means that impulse noises developed on a certain unit could reach the adjacent units through the wiring on the backplane, causing adverse effects on their operation.
As described earlier in FIG. 20, the telephone exchange 100 establishes a path to a remote telephone set 20 to transport data during the silent period in the initial ringing cycle. Conventionally, however, the hardware of the subscriber interface unit 101 is configured to maintain the established path in the cadence ringing cycles that follow. In other words, an unnecessary path is activated during such silent periods that have no data to send. This brings about a practical problem. To activate a data transfer path is to reduce the impedance of the loop. This low loop impedance helps the development of larger impulse noises, making it difficult to suppress them.