PUBLIC CALL OFFICE GENERAL INFORMATION

Telephone Exchange :

There are three types of Telephone Exchanges. These are Strowger, Crossbar and Electronic. Strowger and Crossbar Exchanges  are mechanical  exchanges. These exchanges are obsolete and  all  new  exchanges being installed now are electronic  exchanges.  At present,  in India, there are mainly two types of electronic exchanges. These are Alcatel E-10 B and Siemens.

Telephone Operation :

The telephone line has two wires known as "ring" and "tip" wires. When the phone is on - hook ( i.e. the handset is placed on the cradle), there is approximately 48V between these two wires. In practice, this voltage may vary between 44 volts and 54 volts. In this condition the telephone instrument does not draw any current from the telephone line. When the telephone is off - hook (i.e. the handset is lifted from the cradle ), the telephone draws approximately 40mA current ( it may vary between 15 to 60 mA ) and the voltage across the ring & tip wires is reduced to about 10V ( it may vary from 8 to 20 volts ). This is shown in the figure given below :

Dialing :

There are two types of dialing:

1. Pulse dialing
2. DTMF or Tone dialing

Pulse Dialing :

This is the oldest method of dialing a number. Telephones with rotary dials and some push button phones also employ this method of dialing numbers. In pulse dialing, a switch in the telephone instrument breaks and makes repeatedly making the line voltage go high and low a number of times depending upon the digit being dialed. If number "1" is dialed, the switch breaks and makes once only. This makes line voltage go high (48 volts) for a short duration (67 msec) and then low (10V) again. If another number, say "5" is dialed, there will be 5 high pulses of 67 msec high duration & 33 msec low duration on the line. These pulses are generated at the rate of 10 pulses per second. There is a gap of minimum 500msec (Typically 850 msec) between two digits.

DTMF (Dual Tone Multi Frequency) or Tone Dialing :

This is a new and faster method of number dialing. In Tone dialing, each digit corresponds to a set of two frequencies i.e. when a key is pressed the phone instrument produces a particular distinct tone which is recognized in the telephone exchange. Most new models of telephones have a slide switch which has "P" (for pulse dialing) on one side and "T" (for tone dialing) on the other side. Slide the switch to "T" to dial a number in Tone dialing mode. On some other models of phones, tone dialing mode is enabled by pressing the " * " button on the phone.

If the number is being dialed in tone dialing mode, the exchange will recognize the number i.e. the dial tone will be cut only if the P&T line supports tone dialing facility.

Reversal :
After dialing the number, ring goes to the called party. When the called  party lifts phone, the polarity of the  voltage  on  the caller's  telephone  line reverses i.e.  the  positive  terminal becomes negative and vice versa. This is known as line reversal. This is as shown in the diagram given below :

This phenomenon of the line reversal was there on all telephone lines connected to Strowger and Crossbar exchanges. However in electronic exchanges, normally reversal is not given on P&T line. In Alcatel E-10B exchange, normally subscribers are connected on XEJ16 cards. This card supports 16 subscribers but does not give line reversal. It also does not generate 16Khz metering pulses. There is another card named XEJ8 card. This card gives line reversal and also generates 16Khz metering pulses. The lines to PCOs are given from this XEJ8 card. However, even with XEJ8 card, the facility of line reversal and 16Khz metering pulses has to be enabled on the computer in the telephone exchange. On Siemens exchange, there is no provision of line reversal. Only 16Khz metering pulses are generated on PCO lines.

Call Charges :

The  telephone calls are charged in terms of units.  At  present, one  unit  rate is Rs. 1.20 + 0.1224  service  tax  (Total  Rs. 1.3224). Local calls has a pulse rate of 180 sec.  (i.e. after every 3 min. (180sec) charges are increased by one unit.). For example, if the call duration is up to 3 minutes, the charges will be  one unit. If the call duration is 3 minutes and 1 second to 6  minutes, the charges will be of 2 units and so on.

For STD calls, pulse rates depends on the distance of the  called party.  

STD PULSE  AND TIMINGS  :

A.    Intra Circle Calls : Calls made with in same circle.

Unit call rates has been taken at the rate of Rs. 1.20

B.    Inter Circle Calls  : Calls made between two different circles.

Unit call rates has been taken at the rate of Rs. 1.20

ISD Tariff  (With effective from 21-05-2007) :

BSNL reduces the ISD charges for Rest of World with a 6 seconds pulse

Comparative of  existing and revised rates is as under :-

*Based on unit call rate for Rs. 1.20, for other unit calls rates, the amount will be charged accordingly.

The revised rates are applicable with effect from 21st May, 2005 (00.00 hrs on the midnight of 20th and 21st May, 2005) and shall apply to all calls originated from fixed, Cellular and WLL(M) Phones of BSNL and also be applicable to trunk calls and PCO's.

Note : The revised pulses above shall apply to trunk calls as well.

There  is  a mathematical relation between the number  of  units, total call duration and pulse rate.

                         Total call duration is seconds
No. of Units = -----------------------------------------------  
                                       Pulse rate

Suppose  a 600 sec. call is made to Bombay and the pulse rate  of Bombay is 15 sec. for this call:

Units = 600 / 15     =    40

Note  that if on division the result is not a whole number,  then the number will be rounded up to its nearest higher integer number. For  example  if  time duration is 77 seconds and the pulse rate is 15 seconds, then:

Units = (77/15) 
            = 5.1
            = 6

Also  note that the first unit is counted as soon as  the  called party  picks up the phone. The second unit will be counted  after time equal to the pulse rate. For example if the pulse rate is 30 seconds, units are as per the table given below:

Time duration Units

... and so on.

Metering :

16KHz Metering : To ensure accurate metering, DOT gives 16 KHz metering pulses  to PCO's  from the telephone exchange. Such instruments  are called "Call  Charge  Indicators". These pulses are given  on  telephone line every time the subscriber meter in the exchange is incremented.  Telephone  call charge indicator uses  these  pulses  to increment  the chargeable units. The  pulse  rate  automatically changes  in  the  evening and night hours and  on  holidays.  All models  of PCO Monitor manufactured by PEP have 16  KHz  metering facility.

At the telephone exchange, the amplitude of the 16KHz signal is 2.4Volts rms (2400 millivolts ). However, with distance this amplitude decreases as per the table given below :

Since the sensitivity of the PCO monitor is 50mV, the maximum distance up to which 16Khz metering will work satisfactorily is 6KM from the exchange.

Service Charges :

Current  Govt. Policy allows PCO owners to charge a  maximum  of Rs.  2.00 in urban areas and Rs. 1.00 in rural areas on STD/ISD calls and on group  calls (calls starting with 95)  as services charges.