Public Switched Telephone Network PSTN Explained

A student looks up public switched telephone network ppt and downloads a black rotary phone 2009 slide. When you dial +91 it still picks up the network in 2026 but the middle is fibre and IP and radio. What is a public switched telephone network is not “the landline” It is generally believed that a number can find another number across the operators.

Quick Answer Box

PSTN is the interconnected world public telephone network: numbering, switching, interconnect. Access in India is via licensed TSPs. Your mobile connects through a point of interconnect and the cellular network. Country codes and gateways are used for international calls . WhatsApp to WhatsApp is not equal to a business SIP trunk blasting Indian mobiles.

What Is a Public Switched Telephone Network and How Does It Work?

What is a public switched telephone network and how does it work?

The working definition of Wikipedia is still valid: PSTN is the sum of telephone networks operated by national and local operators, built from lines, fibre, microwave, cellular, satellite and undersea cables, interconnected by switches so that any subscriber can call any other. It was analogue to start with. The core is now largely digital. Cellular and satellite are part of the reachable public network, not another planet.

Three properties remain if you strip the copper:

Public. Anyone who pays for a service can join. It is not a company PBX.
Switched. The network builds a path for this call, then tears it down. It does not hard-wire every phone to every other phone.
Numbered. ITU-T E.164: country code plus national digits, at most 15 digits. That is why +91 40 … means Hyderabad to a switch in Frankfurt.

How a classic call works

You lift the handset or press Call. The digits are received by the local exchange or mobile switch .
Signalling (primarily SS7 in the past, with ISUP messages between exchanges) asks the next hop to reserve a voice channel and reports ringing, answer or busy. The signalling runs on a plane parallel to the voice path.
Voice on the old digital PSTN is often G.711 at 8 kHz, 8 bit, in a 64 kbit/s timeslot (DS0). The slot is yours whether you speak or stay silent. That’s what circuit-switched means.
The far end chimes. The answer is on the path. Hang-up lets it go.

Hierarchy in plain language

Local loop: last metres or kilometres to a home or a cell site.
Local exchange / MSC: where your line or IMSI lives.
Tandem and long-distance switches: between cities and circles.
International gateway: out of the country.

Modern cores may carry the same call as IP packets after a media gateway. To the user it is still “a phone call to a phone number.” To regulation it still hits licensed interconnect.

Public Switched Telephone Network PDF and PPT

There is no single official public switched telephone network pdf or public switched telephone network ppt that replaces standards.

Use these as the spine of a deck:

ITU-T E.164 numbering.
SS7 / ISUP call-setup overview.
TRAI Telecommunication Interconnection Regulations (including PSTN-to-PSTN point-of-interconnect rules).
Your operator’s tariff and ISD page.

A campus PPT that stops at “analog copper” will fail a 2026 viva. Add one slide: core digital, access mixed, identity still the number.

Which Companies Provide Public Switched Telephone Network Services in India?

Which companies provide public switched telephone network services in India?

Anyone with the right authorisation to offer public voice and to interconnect.

Fixed / landline access (the picture people still call PSTN)

The 2026 TRAI fixed-line share snapshots still have the big names like Jio, Airtel, Tata Teleservices, BSNL and MTNL, plus smaller licensees. BSNL and MTNL are still the historic PSU copper and fibre plant. Modern Fixed Voice Broadband from Jio and Airtel. Tata is a huge fixed base. Vi's fixed footprint is small compared to its movable book.

Mobile (PLMN interconnected with PSTN)

BSNL, Airtel, Jio, Vi, A mobile to landline call is two licensed networks meeting at a point of interconnect (POI). Where operators cannot agree on where those POIs should be, TRAI has identified the general locations of them within a service area. That paperwork is the PSTN in legalese.

National and international long distance

The same large groups plus specialised NLD/ILD licensees carry calls between circles and out of India.

Enterprise voice

Cloud PBX and CPaaS companies don’t turn into “the PSTN.” They buy termination from licensed TSPs or they are breaking the law. In India, domestic VoIP dial-out to a mobile or landline is still perceived as restricted. International IP-to-IP and a lot of OTT app calls are a different bucket. First, design the architecture. Buy a pretty dashboard.

How to Connect to a Public Switched Telephone Network From a Mobile Device

How to connect to a public switched telephone network from a mobile device?

You already do, every time you dial a normal number from the Phone app.

Path:

Handset radio attaches to a 2G/3G/4G/5G cell (or VoWiFi).
The mobile core (MSC or IMS for VoLTE/VoNR) authenticates the SIM and reads the digits.
If the B-party is on another mobile network or a landline, the call leaves through an interconnect toward that operator’s switch.
The far phone rings on PSTN or PLMN numbering.

You do not configure a “PSTN setting.” You need:

A licensed SIM in a coverage area.
The number written in E.164 or national format.
For Wi-Fi calling, the operator’s VoWiFi profile.

Dialling +91 from a mobile overseas uses international roaming and foreign interconnects, not some magic PSTN app. Calling an Indian landline from Jio is mobile to PSTN interconnect and charged as a mobile call.

The intra-circle roaming on select rural towers funded by DoT can put a Jio, Airtel or BSNL 4G user on to another operator’s radio for coverage. The number and the home operator are yours. The radio-hop moves. This is still the public network.

Grey “softphone to Indian mobile” apps that bypass licensed termination are not a supported how-to.

Can I Use a Public Switched Telephone Network for International Calls?

Can I use a public switched telephone network for international calls?

Yes. That is what country codes and international gateways are for.

From an Indian PSTN or mobile line:

Prefix the destination with the international access code (00) plus country code, or use + on a mobile.
The originating operator hands the call to an ILD route.
The far country’s operator terminates it on their PSTN or mobile network.

Quality and price are determined by the route. ISD direct operator is the consumer compliant path. Calling cards and some VoIP apps can reduce the international leg. They are not a substitute for a business CLI that Indian networks will accept.

India-specific business caution:

Indian PSTN numbering is not dialled by OTT app-to-app (WhatsApp call to WhatsApp).
The sensitive case is the unlicensed VoIP to Indian PSTN outbound.
Commercial outbound that needs to show a legitimate CLI increasingly needs to use TRAI aligned series (including 160-series for many service calls), not a random foreign DID.

So: yes, PSTN can call the world. No, “international VoIP” is not a free pass to look like a local Indian mobile on the receiving handset.

Technical & Performance Data Matrix

Piece	What it is	India face	User action
Number	E.164 public identity	+91 + national number	Dial as usual
Access	Local loop or radio	Copper, FTTH, cell	Landline or SIM
Switch	Sets up and tears down the call	Local exchange, MSC, IMS	None
Signalling	SS7 / SIP after a gateway	Operator core	None
Voice channel	Historically 64 kb/s reserved	Now often IP after MGW	Talk
Interconnect	POI between licensees	TRAI POI rules	Call another network
Mobile to PSTN	PLMN then POI	Jio/Airtel/Vi/BSNL to landline	Phone app
International	ILD gateway	00 + country code	ISD tariff
CPaaS / cloud PBX	Application on top	Must buy licensed termination	Do not grey-route

The lecture slide is the matrix. The rows are PSTN for number , switch and interconnect . The last row is your contact-centre vendor.

If a PDF is still pulling just copper pairs, add the mobile and IP rows or the doc is history not a network.

Advice vs Strategic Thinking Matrix

Question	Generic advice	Strategic thinking
What PSTN is	Old landline	Public numbering + switching + interconnect
How it works	Copper analog forever	Digital core, mixed access, reserved or gated path
India providers	Only BSNL	BSNL/MTNL plus Airtel, Jio, Tata, mobiles at POIs
Mobile connect	Install a PSTN app	SIM + Phone app + interconnect
International	Use any VoIP	ISD on-net, or licensed route; watch CLI and Indian law
PDF / PPT	First Google deck	E.164 + TRAI interconnect + current tariff