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APPARATUS FOR TRANSMITTING
INFORMATION VIA TELEPHONE LINES
This invention relates to apparatus for transmitting 5 information via telephone subscriber lines.
It is known, for example from Bell, III et al. U.S. Pat. No. 3,937,889 issued Feb. 9, 1976, to transmit above voice-band data, simultaneously with voice-band telephony signals, via telephone subscriber lines. With ap- 10 propriate separation of the data and telephony signals by filtering, interference therebetween is avoided.
Such an arrangement is not suitable for use with loaded subscriber lines, because the presence of loading coils on such lines gives the lines a low-pass filter char- 15 acteristic with a cut-off frequency just above the voiceband. Data can be transmitted within the voice-band via loaded subscriber lines when telephony signals are not present, but the data transmission must be interrupted whenever telephony signals appear. 20
However, it is desirable to be able to transmit information simultaneously with telephony signals via loaded subscriber lines, especially when the information may contain alarm signals relating to the security of the subscriber's premises. In addition, it is desirable to main- 25 tain a continuous check of the integrity of the subscriber line via which such information is transmitted, in order to avoid a security system using the subscriber line being defeated by a simulated off-hook condition of the subscriber's telephone. 30
Accordingly, this invention seeks to provide apparatus which facilitates the transmission of information via a telephone subscriber line, and which is particularly but not exclusively applicable to loaded subscriber lines.
According to one aspect this invention provides ap- 35 paratus comprising a telephone subscriber line connected at a first end to a telephone and at a second end to a central office, means for transmitting information via the line when the telephone is on-hook, means for applying a pilot tone to the line at the first end when the 40 telephone is off-hook, means coupled to the line at the first end for detecting the presence of ringing signals on the line, means for selecting one of two frequencies of the pilot tone applied to the line when the telephone goes off-hook in dsependence upon the detection of the 45 presence of ringing signals, and means for monitoring the pilot tone at the second end of the line.
Thus in accordance with embodiments of the invention, when the telephone is on-hook information, such as data, control signals, meter readings, and alarm sig- 50 nals, can be transmitted in both directions on the telephone subscriber line. For example the information is transmitted and received via modems, which are conveniently voice-band modems when the line is a loaded line. When the telephone is off-hook a pilot tone having 55 a selected one of two frequencies is transmitted from the subscriber end of the line and is monitored at the central office end of the line. This pilot tone monitoring checks the integrity of the line and enables urgent alarm signals to be communicated to the central office end of the line. 60 The use of two different pilot tone frequencies, respectively for originating and answering subscribers, enables distinct monitoring of each pilot tone from two similarly-equipped subscribers who are interconnected during a telephone call, without necessitating the use of 65 blocking filters in the telephone central office.
Each of the pilot tone frequencies is preferably less than 180 Hz, so that it is below the voice-band, and
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preferably greater than 60 Hz. At such frequencies the pilot tone can be transmitted on the line at a low level so that it is not audible in normal use of the line, and can be detected and distinguished from signals, induced on the line from power lines, at frequencies of 60 Hz and its harmonics.
Advantageously the means for monitoring the pilot tone and the modem coupled to the line at the second end are provided commonly for a plurality of telephone subscriber lines, the apparatus comprising switching means for selectively connecting the means for monitoring the pilot tone and the modem to each of said lines.
According to another aspect this invention provides apparatus comprising: a telephone subscriber line connected at a first end to a telephone and at a second end to a central office; first and second hook state detection means coupled to the line at respectively the first and second ends for determining whether the telephone is on-hook or off-hook; means, responsive to the hook state detection means determining that the telephone is on-hook, for transmitting information via the line; means, responsive to the first hook state detection means determining that the telephone is off-hook, for applying a pilot tone to the line at the first end; first ringing signal detection means coupled to the line at the first end for detecting the presence of ringing signals on the line; control means for selecting one of two frequencies of the pilot tone, applied to the line when the telephone goes off-hook, in dependence upon the detection of the presence of ringing signals; and means, responsive to the second hook state detection means determining that the telephone is off-hook, for monitoring the pilot tone at the second end of the line.
In order to avoid unduly loading the line, which could adversely affect normal use of the line, preferably each hook state detection means comprises means, having a high input impedance, coupled to the line for monitoring the d.c. voltage across the line. Desirably the second hook state detection means is responsive to dial pulses on the line to inhibit the means for monitoring the pilot tone during the dial pulses.
The apparatus preferably includes second ringing signal detection means, coupled to the line at the second end, each of the first and second ringing signal detection means being responsive to the presence of ringing signals on the line to inhibit the means for transmitting information via the line during ringing signals. Inhibiting the transmitting means during ringing signals avoids the risk of desired information being mutilated by the ringing signals.
In order to check the line, the control means is preferably responsive to the second hook state detection means and to the monitoring means for causing the transmitting means to transmit interrogating information via the line in response to a change in or loss of the monitored pilot signal while the second hook state detection means determines that the telephone is off-hook. Preferably the control means includes means for temporarily interrupting the connection of the line at the second end to the central office during transmission of the interrogating information.
For detecting a predetermined condition, such as an alarm condition, at the subscriber end of the line, preferably the control means is responsive to the monitoring means detecting a shifted frequency of the monitored pilot tone to cause transmission of the interrogating information, and the apparatus includes control means for causing the shifted frequency pilot tone to be ap4
plied to the line at the first end in response to a predetermined condition occurring while the first hook state detection means determines that the telephone is offhook.
The transmitting means conveniently comprises first 5 and second modems, which are preferably voice-band modems if the line is a loaded telephone subscriber line, coupled to the line at respectively the first and second ends.
The control means are preferably responsive to the 10 second hook state detection means determining that the telephone is off-hook for decoupling the second modem from the second end of the line and responsive to the first hook detection means determining that the telephone is off-hook for including a series impedance in 15 the coupling between the first modem and the first end of the line. This reduces loading of the line during its normal use for telephony.
Advantageously the control means, the second hook state detection means, the means for monitoring the pilot tone, and the second modem are provided commonly for a plurality of telephone subscriber lines, the apparatus comprising switching means for selectively connecting the second hook state detection means, the ^ means for monitoring the pilot tone, and the second modem to the second end of each of said lines. Conveniently, the switching means comprises a first switch for selectively connecting the second hook state detection means to the second end of each of said lines and a second switch for simultaneously connecting at least one of the means for monitoring the pilot tone and the second modem to the second end of the respective one of said lines.
In this case conveniently the means for temporarily 35 interrupting the connection of the respective line during transmission of the interrogating information comprises relay means in respect of each line, the relay means being responsive to a d.c. signal produced by the control means during transmission of the interrogating in- 4q formation to connect an impedance across the respective line at its second end, thereby to maintain any telephone connection established for the line via the central office, and to open the line between said impedance and the coupling of the line to the first and second switches, 45 the d.c. signal produced by the control means being applied to the respective relay means via the second switch.
The invention also extends to apparatus for connection to a telephone subscriber line comprising: a high 50 impedance buffer having an input for connection to the line; a hook state detector, having an input coupled to an output of the buffer, for determining whether a telephone connected to the line is on-hook or off-hook; means for transmitting and receiving information via 55 the line when the hook state detector determines that the telephone is on-hook; means for generating a pilot tone when the hook state detector determines that the telephone is off-hook; coupling means for coupling the pilot tone generating means and the information trans- 60 mitting and receiving means to the line; a ringing signal signal detector, having an input coupled to the output of the buffer, for detecting the presence of ringing signals on the line; and control means for selecting one of two frequencies of the pilot tone, to be generated by the 65 pilot tone generating means when the telephone goes off-hook, in dependence upon the detection of the presence of ringing signals by the ringing signal detector.
The invention will be further understood from the following description with reference to the drawings, in which:
FIG. 1 is a generally block diagram illustrating apparatus in accordance with an embodiment of the invention;
FIGS. 2 and 3 show circuit diagrams of parts of the apparatus of FIG. 1;
FIG. 4 shows a block diagram of a pilot tone detector of the apparatus of FIG. 1;
FIG. 5 illustrates a switching arrangement of the apparatus for a plurality of lines; and
FIG. 6 illustrates an alternative arrangement for a plurality of lines.
Referring to FIG. 1, the apparatus shown therein comprises a subscriber's telephone 1 connected via a telephone subscriber line 2 to a telephone central office switch (not shown). The line 2 is assumed to be a loaded line, via which above voice-band signals can not practically be transmitted, but it could alternatively be a nonloaded line.
The remainder of the apparatus shown in FIG. 1 serves for transmitting information, other than telephony signals, via the line 2. This information can be of any type, such as remote control or meter reading information, data signals, etc., but is assumed here to include one or more alarm signals the transmission of which must be possible at substantially all times, whether or not the telephone 1 is on-hook.
The apparatus at the subscriber comprises a microprocessor 3, a voice-band modem 4, a relay and driver circuit 5, a pilot tone generator 6, an a.c. coupling network 7, a high impedance d.c. buffer 8, a hook state detector 9, and a ringing detector 10. The apparatus at the central office comprises a microprocessor 11, a voice-band modem 12, relay and driver circuits 13 and 14, a pilot tone detector 15, an a.c. coupling network 16, a high impedance d.c. buffer 17, a hook state detector 18, and a ringing detector 19.
The information to be transmitted via the line 2 is shown as being coupled via input/output lines 20 and 21 to the microprocessors 3 and 11 respectively, for bidirectional transmission via lines 22 and 23 and via the voice-band modems 4 and 12 and the a.c. coupling networks 7 and 16 when the telephone 1 is on-hook and no ringing signals are present on the line 2. Such bidirectional transmission is effected serially between the modems 4 and 12 in known manner, preferably using minimum shift keying. To this end, at the subscriber the on-hook state of the telephone 1 is detected by the detector 9 whose output on a line 24 is coupled to the generator 6, which is consequently disabled, and to the microprocessor 3, and the absence of ringing is detected by the detector 10 whose output on a line 25 is coupled to the microprocessor 3. In response to the signals on the lines 24 and 25 the microprocessor 3 controls the circuit 5 to close its normally-open contact 5', thereby short-circuiting a resistor 26 and coupling the modem 4 to the line 2 via the network 7.
At the central office, output lines 27 and 28 of the detector 18, and an output line 29 of the detector 19, indicating respectively dial-pulsing on the line 2, the hook state of the telephone 1, and the presence or absence of a ringing signal on the line 2, are coupled to the microprocessor 11. When the telephone 1 is on-hook and no ringing signal is present, the microprocessor 11 controls the circuit 13 to close its normally-open contact 30 to couple the modem 12 to the line 2 via a
capacitor 31 and the network 16. In this state the microprocessor 11 ignores the outputs of the pilot tone detector 15, present on multiple lines 32.
Thus bidirectional transmission is effected via the line 2 when the telephone 1 is on-hook and no ringing signal 5 is present. In the event that a ringing signal occurs on the line 2, this is detected by the detectors 10 and 19, and in response to the consequent signals on the lines 25 and 29 the microprocessors 3 and 11 control the modems 4 and 12 to stop transmission for the duration 10 of the ringing signal. Transmission between the modems 4 and 12 is resumed during the pauses or silent intervals between successive ringing signals while the telephone 1 remains on-hook.
When the telephone 1 goes off-hook, this is detected 15 by the detector 9, and in response to the consequent signal on the line 24 the pilot tone generator 6 is enabled and the microprocessor 3 controls the circuit 5 to open the contact 51, whereby the resistor 26 is inserted in series in the coupling between the line 2 and the modem 20 4. The resistor 26 has a resistance of about 20kfi, which is sufficient to prevent excessive loading of the line 2 by the modem 4 during normal use of the telephone 1. Depending upon whether the subscriber is answering or originating a call, and thus in dependence upon whether 25 or not a ringing signal has been detected by the detector 10, the microprocessor 3 controls the pilot tone generator 6 via one of two lines 33 to generate a tone of 100 Hz or 80 Hz respectively. This tone is coupled via the network 7 to the line 2. 30
Correspondingly, the off-hook state of the telephone 1 is detected at the central office by the detector 18, and in response to the consequent signal on the line 28 the microprocessor 11 controls the circuit 13 to open the contact 30, whereby the modem 12 is decoupled from 35 and does not load the line 2. The microprocessor 11 then monitors the outputs of the pilot tone detector 15 on the lines 32 for detection of the relevant pilot tone. As the presence of dial pulsing on the line 2 may adversely affect the detection of the pilot tone, the detec- 40 tor 18 also supplies dial pulses via the line 27 to the microprocessor 11, which interrupts its monitoring of the pilot tone detector outputs during each dial pulse.
During the off-hook state of the telephone 1, information transmission between the lines 20 and 21 is gener- 45 ally interrupted, information to be transmitted merely being stored for transmission as decribed above when the telephone 1 goes on-hook. However, if one or more urgent alarm states requiring immediate transmission occur in the information on the lines 20, this is detected 50 by the microprocessor 3 which consequently controls the generator 6, via the other of the two lines 33, to change the frequency of the generated pilot tone to 150 Hz.
In response to detection by the detector 15 of a pilot 55 tone of 150 Hz, or in response to the relevant 80 Hz or 100 Hz pilot tone not being detected by the detector 15 while the telephone 1 is off-hook, the microprocessor 11 determines that the integrity of the line 2 and the occurrence of an urgent alarm from the apparatus connected 60 to this line should be checked. This check is carried out as described below. It is noted that the loss of the 80 Hz or 100 Hz pilot tone, without occurrence of the 150 Hz pilot tone, could be caused by a short-circuit or opencircuit of the line 2. The two different frequencies, 80 65 and 100 Hz, are used to ensure that, if two similarlyequipped subscribers are interconnected via the central office switch for a telephone conversation, loss of the
pilot tone from either of them can be readily detected. If only a single frequency were used for both subscribers, then loss of the pilot tone from one of them could not be detected unless passage of the tone through the central office switch, necessitating complex and expensive filtering, were prevented.
It is also noted here that the pilot tone frequencies and levels are selected so that they are not audible to subscribers during a normal telephone call, and so that the pilot tones can still be detected by the detector 15. To this end the pilot tone frequencies are chosen so that they are below the voice-band, above possible ringing signal frequencies, and distinct from the a.c. mains frequency of 60 Hz and its harmonics. Thus the pilot tone frequencies selected are between 60 Hz and 180 Hz, two of them being between 60 Hz and 120 Hz. The particular frequencies of 80, 100, and 150 Hz selected are not particularly critical, but are chosen for ease of detection.
The check mentioned above is carried out as follows. The microprocessor 11 controls the circuit 14 via a line 34 to close its normally-open contacts 35 and to open its normally-closed contacts 36, whereby a resistor 37 is connected on the central office side of the line 2 to maintain the telephone connection while the line 2 itself is disconnected from the central office. The microprocessor 11 then controls the circuit 13 to couple the modem 12 to the line 2 via the contact 30, and via the modem 12 and the line 2 the apparatus coupled to the line 2 is requested to transmit information including any alarm signal which may be present. If the microprocessor 11 detects no response to this request, it concludes that there is a fault on the line 2 and activates alarms and instigates maintenance accordingly, and then returns the circuits 13 and 14 and their contacts to their previous states.
If there is no fault on the line 2 the above request is received by the modem 4 via the resistor 26 and is detected by the microprocessor 3. In response, the microprocessor 3 controls the circuit 5 to close the contact 51, and controls the modem 4 to transmit the urgent alarm signal which has occurred, and any other information such as any non-urgent signals awaiting transmission, to the line 2 and thence to the modem 12 and microprocessor 11, which activates alarms or instigates maintenance as required. Following such transmission, the microprocessor 3 returns the circuit 5 and its contact, and the microprocessor 11 returns the circuits 13 and 14 and their contacts, to their previous states, whereupon the telephone connection is resumed.
The above-described interruption of the telephone connection can be completed in a fraction of a second using typical data transmission rates, so that it is hardly noticeable by the subscriber. However, as the interruption occurs only when there is a possibility of a fault or an alarm, it may be considered desirable to extend the interruption time, and possibly to transmit under the control of the microprocessor 11 an audible warning to the subscriber during the interruption, to alert the subscriber to this possibility.
Although the above description relates to only a single subscriber line 2, parts of the apparatus at the central office can be provided commonly for a plurality of subscriber lines, the individual lines being polled in turn for individual monitoring of pilot tones or transmission of information. More particularly, as described below with reference to FIG. 5, switches may be inserted at the two points marked * to enable the compo
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