This document discusses PSTN (public switched telephone network) internet connectivity. It describes the hierarchy of the PSTN network, with regional, sectional, primary, toll, and end offices. Subscribers are connected via local loops to end offices. The document explains how a computer can connect to the internet through PSTN using a modem and telephone line. It also discusses how VOIP (voice over internet protocol) allows phone calls to be routed over the internet. In summary, the document outlines the PSTN network structure and how traditional telephone lines can provide dial-up internet access.

1. Sub topic : PSTN Internet Connectivity
Teaching aids : Diagrams
* 1

2. Recap
In the previous topics, you have learnt about
• Internet service provides (ISP)
• Internal architecture of ISP
• High level architecture of ISP
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3. Objectives
On completion of this topic, you would be able to
know about
• PSTN internet connectivity (i.e. public switched
telephone network)
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4. PSTN
• PSTN is an example for circuit switched telephone network.
The switching centers are organized into five classes.
• Regional offices (class1)
• Sectional offices (class2)
• Primary offices(class3)
• Toll offices(class4)
• End offices(class5)
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5. PSTN Hierarchy
Regional office (class 1)
Sectional office (class 2)
Primary office (class3)
Fig .1 Toll office (class 4)
End office (class5)
Local loops
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6. PSTN
• Subscriber telephones are connected through local
loops, to end offices
• A small town may have only one end office
• Large cities will have several end offices
• Many end offices are connected to one Toll office
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7. PSTN
• Several Toll offices are connected to a sectional office,
which normally serves more than one state
• Several sectional offices are connected to one Regional
office
• All the regional offices are connected using mesh
technology
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8. PSTN
• Subscribers connected to the same central office can
communicate with each other by means of central office
switch
• The central offices themselves are connected together
by trunk lines, so that any subscriber can contact any
other subscriber within a local calling area
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9. PSTN
• When over load occurs, it is impossible for a subscriber
to place a call. This situation is called as call blocking
• When there are no available trunks between two central
offices, a connection can some time made through a
tandem office
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10. PSTN – Internet Connectivity
computer
Computer
Connectivity by using
PSTN
Telephone
Modem Telco Telco Modem
network
Fig (2)
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11. PSTN – Internet Connectivity
• If we want to connect the internet only occasionally , dial-
up connection with PSTN is used
• To connect the internet , the computer must establish a
connection to the ISP (Internet service Provider)
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12. PSTN Internet Connectivity
Ordinary
telephone
Gateway
Gateway Local
PSTN
Internet or
Local intranet
PSTN
Fig.3 Ordinary
telephone
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13. PSTN – Internet Connectivity
• Voice over Internet protocol also called VOIP
• Incoming phone calls can be automatically routed to VOIP
phone, regardless of where you are connected to the
network
• While at journey, where ever we get connected to the
internet we can receive incoming calls
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14. PSTN – Internet Connectivity
• Call center agents using VOIP phones can work from
any where with a sufficiently fast and stable Internet
connection
• Many VOIP packages include PSTN features that most
Telcos normally charge extra
• VOIP allows users to travel anywhere in the world and
still make and receive phone calls
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15. Summary
In this class, you have learnt about
• PSTN hierarchy
• PSTN internet connectivity
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16. Quiz
1) In ______ the transmission media is shared between
all the computers on a network
a) LAN
b) MAN
c) WAN
d) X.25
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17. Frequently Asked Questions
1) Define PSTN?
2) Explain PSTN internet connectivity
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