Ringing Supply, Tone Supply, and Recorded Announcements in Digital Switching System
5.38.1 Ringing Supply
The digital exchange shall be equipped with duplicated or redundant ringing supply on a main and standby basis. Automatic changeover to standby shall take place on failure of the main unit, and an alarm condition shall be given when changeover takes place.
Ringing Output voltage shall be 90 Vac at 20 ± 2 Hz. The cadence of the ringing cycle shall be 1 second on and 4 seconds off for local calls and 2 seconds on, 1 second off for long distance calls. The first transmission of ringing to the called line shall start write away, once it has been determined that the desired subscriber’s line is free.
This can be achieved either by starting the first pulse of ringing of the cycle when the free line is detected, or by applying a single burst of ringing (of 1 second duration) followed by the random application of the ringing cycle.
The ringing signal shall be removed as soon as the called party goes off hook, regardless of whether the silent or ringing period of signaling applies at that time.
The ringer impedance shall be based on a maximum of four (4) telephones per line.
5.38.2 Tone Generation and Outpulsing
The CO digital switch shall have the capacity to generate 512 individual frequencies, which can be used to produce various distinct waveforms. All tones shall be digitally generated. The frequency and cadence of each tone shall be maintained within 0.5% of nominal value respectively. Provision for adjusting tone levels shall be included.
a) Dial Tone
Dial tone shall be transmitted (a) in response to a request for service from a calling subscriber, or (b) as a result of initiation of a custom service. It shall consist of a continuous signal at a frequency of 450 Hz modulated by a 50 Hz sine wave voltage with adjustable output, 80 to 100% modulation. The tone level shall be -12 dBm/600 ohms.
b) Ring-Back Tone
Rink-back tone shall be returned to the calling subscriber as an indication that the exchange has connected the call to the desired subscriber and that the line is free. It can also be returned on certain calls to operators and special services. It shall consist of a frequency of 450 Hz modulated at 50 Hz in the following sequence; 1 second on, 4 seconds off for local calls and 2 seconds on, 1 second off for long distance calls. The tone level shall be -12 dBm/600 ohms.
c) Busy Tone
Busy tone shall be returned to the calling subscriber if the desired subscriber’s line is busy or all the lines belonging to a series of collective numbers are busy. It shall consist of a frequency of 450 Hz in the following sequence, 0.50 second on, 0.50 second off. Busy tone shall be returned from the subscriber’s line circuit so that the transmission matrix is released when the busy condition is detected. Each tone level shall be -12 dBm/600 ohms.
d) Congestion Tone (Reorder Tone)
If a calling subscriber does not reach the desired subscriber due to congestion of equipment or unavailability of a trunk circuit group, the existing exchange shall return a busy tone to the calling subscriber. However, the digital system shall have the potential to provide a congestion signal having a different cadence from the busy signal, (to indicate to the subscriber calling, that there is congestion at the exchange or trunk group). Both congestion and busy tones shall employ the same frequency of 450 Hz. Each tone level shall be -12 dBm/600 ohms.
e) Pay-Station Tone
In case that might be required, the pay-station tone shall be an audible indication to the operator that the call is coming from a coin box. This signal shall consist of a burst of 950 Hz for 0.25 second, which is applied before the transmission path to the calling subscriber is completed. Tone level shall be -12 dBm/600 ohms.
f) Information Tone
Information tone shall be applied to the recorder announcements in order to indicate to the incoming international callers that, if they cannot understand the language of the recording, they should call their national assistance operator.
It consists of:
950 ± 50 Hz for 0.33 second followed by
1400 Hz for 0.33 second followed by
1800 Hz for 0.33 second followed by
silence for 1.00 second.
The signal cycle shall be applied before and after each announcement message. Tone level shall be -12 dBm/600 ohms.
g) Offering Tone
Offering tone shall be used to indicate to a subscriber that an operator will be connected into the conversation. It shall consist of three pulses of 950 Hz, 0.15 second on, 0.1 second off, followed by 0.15 second on, 0.1 second off, followed by 0.15 second on immediately before the transmission path to the operator is completed. The tone level shall be -16 dBm/600 ohms.
h) Intrusion Tone
Intrusion tone shall provide an indication to the parties of a call that an operator has connected to that call. It shall consist of a 0.5 second burst of 450 Hz, 0.5 second before and after the transmission path to the operator is completed. The tone level shall be -16 dBm/600 ohms.
i) Receiver Off Hook Tone (Howler)
Receiver Off Hook (ROH) shall be applied telephone lines which are timed-out because of the receiver being off the hook without a call being in effect. It shall consist of 1400 Hz + 1800 Hz for 0.2 second on, 0.3 second off in recurring cycle at a maximum level of +3 dBm/600 ohms.
j) Charge Interval Indicating Tone – (NOT NEEDED FOR A LOCAL SWITCH)
This signal shall be sent 12 seconds before the end of a charging interval. It shall consist of a burst tone of 950 ± 50 Hz which last for 0.25 second. The tone level shall be -16 dBm/600 ohms.
k) Vacant Code or Number Unobtainable Tone
This signal shall be applied to calling lines (without effecting a charge for this call). It shall consist of 450 Hz applied for 2.75 seconds on with 1.25 seconds off in a recurring cycle. The interval allows for the release of echo suppressors on long distance four-wire transmission circuits. The tone level shall be -12 dBm/600 ohms.
l) Call-Waiting Tone
A call-waiting tone is used only in conjunction with call-waiting service that allows a busy line to answer an incoming call by flashing the hook switch.
An audible ring is applied to the calling line and the call-waiting tone is applied to the called line.
Call-waiting tone shall be:
1. Tone A – 440 Hz + 480 Hz, one pulse of 1.0 second duration is sent to the wanted subscriber “A” if it is already engaged on a call.
2. Tone B – 350 Hz + 440 Hz, 0.25 second on, 0.25 second off, 0.25 second on, 3.25 seconds off is sent and repeated periodically to both talking parties “A” and “B” when a connection to one of them is required by another subscriber “C.”
3) Holding Tone – 0.5 second tone A, 0.5 second break, 0.5 second tone B, 2.5 seconds off, repeated periodically, is sent to the second party “B” of the original call if it continues off hook after the wanted subscriber “A” has changed over to speak to “C.”
m) Confirmation Tone
A confirmation tone is used to acknowledge receipt by exchange of information necessary for special services, including abbreviated dialing, call forwarding, do-not-disturb, etc. A confirmation tone shall be a tone of 350 Hz + 440 Hz, 0.1 second on, 0.1 second off, 0.3 second on.
n) Special Dial Tone
A special dial tone is returned to a calling party as a reminder that a certain class of service feature is being activated. The tone shall consist of three bursts of 350 Hz + 440 Hz, 0.1 second on, 0.1 second off, each, then steady on.
5.38.3 Recorded Announcements
The CO digital exchange shall include announcement equipment to store digital recorded messages. The announcement equipment shall be connected directly to the digital switching network in the exchange. The announcements shall be accessible to both originating calls from local lines and terminating calls from lines located in other exchanges, also, it shall be possible to give announcements to called subscribers for special services, such as wake up service, etc. The exchange shall be capable of sending the same message to a number of subscribers or trunks, while the number of simultaneous connections shall not be limited.
The output level of any recorded message shall be, on average, -13 dBmO measured at the Main Distribution Frame (MDF) where the two-wire line is terminated. The output level shall not be affected by the number of subscribers or trunks to which the same message is being sent simultaneously. The CO digital exchange shall release the connection of the announcement equipment when the calling subscriber clears, and when the recorded message has been repeated two times. Upon release, the exchange shall send a busy tone to the calling subscriber or trunk. Announcement equipment translation and routing for at least sixteen (16) different announcements shall be available in each CO digital exchange.