Classification of Switching system
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    Signalling Tones



    Signalling Tones plays an important role in Telephone Swithing system as well as in mobile switching system.A number of signalling functions are involve in establishing,maintaining and releasing a telephone automatic swithing systems,the verbal signalling of the operator is replaced by a series of distincive tones.
    The five subscriber related signalling functions are performed by the operator-

    1. Respond to the calling subscriber to obtain the identification of the called party.

    2. Inform the calling subscriber that the call is being establised.

    3.Ring the bell of called party.

    4.Inform the calling subscriber,if the called party is busy.

    5. Inform the calling subscriber,if the called party line is unobtainable for some reason.

    presently attempts have been made to standardise the tones for various signals,manny variations are in vouge in different part of the world and even in different parts of the same country.Variation are mainly due to different capabilities and technologies of the switching system used.


    Dial tone:

    This tne indicates that the exchange is ready to acceptdialled digits from the subscriber.

    Most often the dial tone is sent out by the exchange even before the hanset is brought near the ear.some times,however, a few seconds may elapse before the dial tone is heard. This happens particularly in comman controle exchanges which use shared resources for user interfaces.

    The dial tone is a 33 hz or 50 Hz or 400 Hz contineous tone as shown in figure .the 400 Hz signal is usually modulated with 25 Hz or 50 Hz.


    Ringing Tone :

    Usually the exchange controle equipment sends out the ringing current to the telephone set of called party. This ringing current has the familiar double ring pattern.Simultaneously,the controle euipment sends out a ringing tone to the calling subscriber,which has a pattern similar to that of the ringing current.


    The two rings in the double ring pattern by a gap of 2 s. The ring brust has a duration of .4s. The frequency of ringing tone is 133Hz or 400 Hz,sometime modulated with 25Hz or 33Hz. It may be noticed that the ringing curent and ringign tone are two independent quantities.


    Busy Tone:

    It is a bursty 400Hz signal with silence period in between. The brust and silence durations have the same value of .75s or .375s. A busy tone is sent to the calling subscriber whenever the switching equipment or junction line is not available to put through the call or the called subscriber line is engaged.


    Number Unobtainable Tone:

    It has a contineous 400Hz signal.This tone is send to calling subscriber due to variety of reasons such as called party line is out of order or disconnected and error in dialling leading to the selecion of a spare line. In some exchanges the number unobtainable tones is 400 Hz intermittent with 2.5s on period and .5s off period.


    The routing tone:

    The routing tone or call in progress tone is a 400 Hz or 800 Hz intermittent pattern.In electromechanical system,it is usually 800 Hz with 50 percent duty ratio and .5s on/off period. In analog electronic exchange it is a 400 Hz pattern with .5s on period and 2.5s off period.In digital exchanges,it has.01s on/off period at 400 Hz. When a subscriber call is routed through a number of different type of exchanges,one hears different call in progress tones as the call progresses through different exchanges.

    Info: It is not unusual that a subscriber in a new area where frequency or timings of tones are different from those in his own area,confuse signalling tones. in order to overcome this problem,recorded voice that announce message like "Number engaged" or "Busy" are used in some modern exchanges.






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    Submitted By : Suyash Srivastava

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