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How IVR System is connected to PSTN or Mobile Network

Any Voice Application finally needs to get connected to telephone network or PSTN as we call them, so that it reaches real people. Now a days, it seems, PSTN is being replaced by Mobile Network as mobile phone users outnumber fixed-phone users. My own experience of looking at caller data of IVR systems installed in various industries, shows that at least 80% callers any any IVR System is from Mobile Phones.

IVR System Architecture

IVR System Architecture

Here are ways how an IVR System gets connected to real world telephone network irrespective of their wire-line or wireless nature.

1. Analog Lines
This is the Last mile solution to end connectivity to end user equipment. So, here instead of connecting this line to a Telephone Instrument, we connect those lines to Analog CTI Boards which in turn is installed inside a PC or a Server through a RJ1 connector. It uses the old protocol of connecting telephone which are basic as known/heard by anyone for signalling. It uses Ringing to indicate incoming call from telephone Exchange or switch to voice application, polarity reversal or battery reversal or even 16KHz tone for connection establishment connection. It has to detect all telephony related conditions by detecting audio pattern available in the channel, for example busy tone, engage tone, FAX Tone, DTMF etc which are exactly as we hear in telephone speaker.

This has now become almost obsolete and not many people use them anymore. Specially after ISDN PRI lines were become more and more affordable as well as available. Progressing to digital lines from analog lines seems a natural development.

Even Mobile Connections can be connected to this kind of arrangements as there are many FXO-GSM converter equipment which converts mobile number to analog lines a sin POTS!

2. E1/T1 PRI Line
Modern digital connection which is now most widely used. Same two pair cable is terminated on user-premise and connected to CTI Boards or even stand-aloe equipments like Donjin Keygoe Series using RJ45 connectors, the same used in LAN Connections.
Its completely digital system, at least the signalling protocol part. Here are few signaling protocols used :-
This is legacy protocol, no longer in use. It is slower to connect than ss7 or ISDN PRI.


This is most widely used signalling protocol. It gives 30 voice channels per E1 lines.

3. SS7
SS7 is most advanced signalling which give complete information of a call. One should use SS7 signalling is using more than 10 PRI lines.

Apart from these, presently, PRI lines are being delivered using SIP protocol too. This will eliminate requirements of CTI Boards and may bring down cost of IVR Systems.

This is not delivered at customer premise, but it assumes that customer is already connected to internet cloud and has enough bandwidth already to make voice calls using those infrastructure! How convenient! VOIP is growing fast and I think Asterisk is also fueling its growth.

So, these are all how IVR System is connected to real world telephone network!

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{ 2 comments… add one }
  • S S Patel September 3, 2014, 10:37 am

    Sir, please clarify the ISDN technic in detail .What is the difference between 2G and 3G SIM.

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