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Personal Blog

This post is written for a particular family member who wanted to know what this means.

By the year 2025, BT aims to abolish the good old POTS or Plain Old Telephone Service, also known as the Public Switched Telephone Network or PSTN.

Wait? What?! Does that mean no more landline calls?!

No. Let me explain how the system works presently. We have a hybrid system where our internet is sent on approximately three-quarters of the bandwidth of the line that we use. We use Copper To The Cabinet* (CTTC), Fibre To The Cabinet (FTTC) or Fibre To The Premises (FTTP) to deliver an internet connection. Broadband as it is called, refers to the fact that three-quarters or more of the cable are given to the internet whilst the remaining quarter is given to the phone line or PSTN.

When broadband first arrived, to cope with the increased internet speed and the ability to use the phone and internet at the same time the phone signal was compressed. This meant it went from taking a full 100% of the bandwidth to just about 0.5% (0kHz to 4kHz). The rest of it went to the internet connection where 87.5% of the line was given to download (138kHz to 1104kHz) and the remaining 10% to upload (25kHz to 138kHz). Finally, to ensure that the analogue PSTN does not receive interference, there is 2% of the bandwidth reserved between it and the upload. Remember, the way that these signals are transmitted is as frequencies in pulses. This also explains why upload tends to be a lot less than download. Below is how this is all separated out:

As you are no doubt aware, removing the copper cabling that is in use at present and replacing it with fibre makes the bandwidth increase so faster connections are available.

The abolishment of the PSTN from the signal would have an increase on speeds because that existing 25kHz would allow the internet to use that instead.

So what would happen to phone lines?

The purpose of this article was to inform someone in my family of what we have recently chosen to do in our own home - abolish the PSTN from it. Yeah that's right, as of this week we've got no PSTN telephones in the house and we now use a PBX powered by Asterisk (that I setup back in November for my own line) and a bunch of SIP phones. Businesses have done this for years, but the flexibility of these phones is what makes them great.

When BT gets rid of the POTS and PSTN, we will all use phones running on VoIP or Voice over IP technology, basically internet phones. But that doesn't mean you need to buy any new phones, BT or whoever is in control of the phone line in your home will need to provide a compatible option to connect these phones to the new VoIP network.

I'm a bit of an expert now on SIP, VoIP and the PSTN so if you've got any questions just fire away!

*officially, Copper To The Cabinet is not a thing, it's just what I've called it here!

Posted by J B Balfour in Tech talk
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