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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