The document provides an overview of topics to be covered in Lesson 1, including the Public Switch Telephone Network (PSTN), its role, and an example of how customer premises equipment (CPE), specifically telephones, work. It will discuss the PSTN's original design for voice services and how it has evolved to support modern data and video. It will also explain the major elements of the PSTN - CPE, access systems, transport networks, and signaling - using telephones as an example CPE to illustrate how one piece of equipment interfaces with the PSTN. The lesson objectives are to understand the PSTN and learn how telephones connect to and utilize the network.

1. Lesson 1 Topics
PSTN Overview
The Role of The PSTN
CPE – An Example, How A
Telephone Works
Lesson 1 Topics
PSTN Overview
The Role of The PSTN
CPE – An Example, How A
Telephone Works
Public Switch Telephone NetworkPublic Switch Telephone Network
Thought For The Week
Building on Lessons Learned In TCM 134
TheThe

2. Study This Set Of PowerPoint Slides
Complete Assignment 1 And Post All Answers
In The Lesson 1 Drop Box By The Due Date.

3. In This Lesson, You Will Learn How The Public
Switch Telephone Network (PSTN) Was
Originally Designed To Carry Voice Services.
Next, We Will Learn That The Original Design
Of The PSTN Has Evolved To Carry Most Of
Today’s Advanced Voice, Data And Video
Services.
You Will Study A Brief Overview Of The PSTN –
A Building Block For Our Course.
Finally, You Will Learn How One Piece Of CPE
Works In Conjunction With The PSTN.
Objectives

4. Section 1.1 – PSTN Overview

5. The Public Switch Telephone Network (PSTN)
Is A Mesh Network Of Lines, Trunks, Switches,
Etc. That Connects Customer Provided
Equipment (CPE) Together To Allow The CPE To
Communicate Together.
The PSTN Was Originally Designed To Optimize
The Transmission Of Voice Services.
What Is The PSTN?

6. Major Elements Of The PSTN
CO Class 5
Switch
CO Class 2
Switch
Residence
CPE
CO Class 1
Switch
PBX or KTS
Digital LoopAnalog Loop
Digital Interoffice
Trunks
Office Bldg
PSTN

7. There Are Four Major Elements Of Today’s Public
Switched Telephone Network (PSTN).
Customer Premises Equipment (CPE)
Access System
Transport
Signaling
The Four Major Elements Of The PSTN

8. Prior To 1984 And The Breakup Of The AT&T Monopoly,
All Parts Of The Public Switched Telephone Network Was
Owned And Operated By The Telephone Company.
After 1984, The Ownership And Maintenance Of All
Telecommunications Equipment And Wiring On The
Customer’s Site Became The Responsibility Of The Customer.
Customer Premises Equipment

9. Customer Premises Equipment Includes:Customer Premises Equipment Includes:
Telephones
Fax Machines
Wiring & Jacks
Multiplexers
Cross Connect Systems
Computers
PBXs
Key Systems
CSUs
DSUs
Video Codecs
Etc.

10. Customer Premises Equipment - Continued:Customer Premises Equipment - Continued:
Each Of These Pieces Of CPE Are Different.
They Each Work Differently Electronically, And Many
Have Different Ways In Which They Connect To The
PSTN.
Later In This Lesson We Will Study In More Detail, As
One CPE Example, How Telephones Work Electronically
And How They Connect To The PSTN.
Later In This Course, We Will Study How Several Other
CPEs Work Electronically And How They Connect To
The PSTN.

11. The FCC Defined A Point, The Demarcation Point, In The
Telecommunications Network That Separates CPE From
Telephone Company-Provided Equipment And Wiring.
The Demarcation Point Is Often Referred To As The Demarc.
Demarcation Point

12. The Access Component Of The PSTN Provides Customers
All Of The Wiring And Equipment For The Customer’s
CPE To Be Connected To The Central Core Of The Public
Switched Telephone Network.
The Access System Includes Both Lines And Trunks And The
Termination Equipment In The End Office.
Access Lines And Trunks Along With The Telephone Poles,
Conduits, Connector Boxes, Etc. Are Referred To As The
Local Loop.
Access System

13. Some Samples Of End Office Termination Equipment.
Central Office Termination Equipment

14. Most Access Circuits Run From The Carrier’s Central Office
And A Residence Are Analog Circuits (Ref. Slide 6).
Most Access Circuits Run Between The Carrier’s Central
Central Office And Businesses With Private Branch
Exchanges Are Digital Circuits (Ref. Slide 6).
Since Most Circuits That Connect Central Offices To
Customer Premises Are Connected To Residences, Then, Most
Circuits In The Local Loop Are Analog.
Access System

15. Access System – Residential Access
Class 5 Switch
Each Customer Circuit Is A Two-Wire Cable From The
Customer Demarc Out To The First Cross Connection Box
A & B – Cross
Connection Pedestal
Or Cross Connection
Box

16. Access System – Residential Access
Class 5 Switch
Even Though Customer Circuits Are Two-Wire Cable,
Carriers Will Install Either 2-Pair, Or 4-Pair Cable.
Customer Circuits Are Combined With Other Circuits At The
Cross Connection Box
2-Pair Cable
100-Pair
Cable

17. Access System – Residential Access
Class 5 Switch
Each Customer Circuit Is A Two-Wire Cable From The
Customer Demarc Out To The First Cross Connection Box.
From The Cross Connection Box To The Central Office The
Customer Two-Wire Circuit Is Carried Inside Of A Much
Thicker Cable That Carries 100 Pairs Of Cable Or More.
Each Customer’s Two-Wire Circuits Remain Separate From
Other Customer’s Circuits Which Are Carried On Separate
Sets Of Wires.

18. Plastic Insulator
Metal Conductor
Sample 4-Pair Cable
The Photo Below Shows A 4-Pair Copper Wire Cable
Similar To The Type Of Cable That Might Be Run To A
Residence.
Each Pair Of Cable Supports A Separate Circuit.
Because Of The Cost Of Installing Outside Cable, Carriers
Usually Install More Than 1 Pair Of Wires.

19. Access System – Business Access
Class 5 Switch
Access For Small Business Customer Circuits Are Similar To
Residential Access Circuits.
Business Trunks From PBXs Are Digital, Usually T1s Or T3s.
If The Trunks Are T1s, 24 Separate End-User Applications
Are Carried On One Set Of Wires At A Time.
We’ll Study More About Data Rates Of Trunks In The
Lessons In Module 3 Of Our Text.
Trunks
Business W/ PBX

20. Transport Component Of The PSTN
Transport Means Transmission – Taking Voice, Data, And
Video Signals From One Subscriber’s Access Line And
Delivering Them To Another Customer’s Access Line.
The End-Office, Or Class 5 Central Office Is The Point Where
All Customer Access Lines Connect.
The Trunks Within The Transport Core Include A Wide
Variety Of Transmission Speeds (e.g. T1, T3, OC-1, OC-12)
And Transmission Media (e.g. Copper And Optical Fiber).
We’ll Study More About The Transport System Later In This
Course.

21. Signaling Component Of The PSTN
Signaling Means The Controlling Of Calling Including:
Setting Up A Path For A Conversation Through The
Transport Core.
Maintaining And Terminating The Conversation Path.
Collecting Billing Information.
Handling Other Supervisory Functions.
We’ll Study More About The Signaling System Later In
This Course.

22. Section 1.2 – The Role Of The PSTN

23. Many People View Voice Services, And The PSTN
Used To Deliver Those Services, As “Old, Ho- Hum”
Topics.
Those People Reason That The Most Lucrative,
Future Growth Areas In Telecommunications Will
Be In Data Services As Opposed To Voice Services.
Many Of Those People Believe That
Telecommunications Students Should Not Study
Topics Related To Voice Communications Services
And The PSTN.
The Role Of The PSTN

24. Those Same People Believe That Data
Communications Will Underpin The New
Emerging Technologies And Will Be The “Wave
Of The Future”.
Why Then Study Plain Old Telephony?
The Role Of The PSTN - Continued

25. Throughout This Course We Will Learn How POTS
And The PSTN Play Vital Roles In Providing Many
Services Such As:
Voice-over-IP (VoIP)
Third Generation
Cellular
Internet Applications
E-Mail
E-Commerce
WANs & MANs
Etc., Etc., Etc.
Today’s
Emerging
Technologies
Do These
Services Seem
Like POTS!

26. Cellular – Many Major Cellular Features Depend On
The PSTN – Two Examples Follow
Several Examples Of The Role Of The PSTN

27. A Cell Call Placed To A Land-Based Phone
Travels Through The PSTN
CO Class 5
Switch
CO Class 2
Switch
CO Class 1
Switch
Analog Loop
Digital Interoffice
Trunks
Radio
Controller
MTSO
Cellular User
Residence
CPE

28. InternetSS7/
PSTN
Cellular Roaming Depends On The PSTN SS7 Network
Roaming
MTSO
Home
MTSO
VLR HLR
SSP
SSP
STP STP
Roaming
Cellular User
Cellular Roaming Would Not Be
Possible Without SS7 Network In PSTN

29. Private Line Data Networks Rely On Several
Elements Of The PSTN
Another Example Of The Role Of The
PSTN

30. Site A
Site B
56 Kbps
Private Line
Site E
56 Kbps
Private Line
T3
Private Line
Site D
OC3 Private Line
Site C
56 Kbps
Private LineT1
Private Line
PSTN Backbone Circuits Are Provisioned To Create
Corporate Private-Line Data Networks
Router
Router
Router
Router
Router

31. Internet Access Relies On The PSTN – Another
Example

32. ISP Access Use PSTN Circuits
CO Class 5
Switch
CO Class 2
Switch
CO Class 1
Switch
Multiplexed/Channelized
Digital Loop
Analog Loop
Digital Interoffice
Trunks
Residential
End-User
ISP
MCDL
Corporate
End-User
PSTN
Router

33. Section 1.3 – Customer Premises Equipment

34. CPE Includes All Telecommunications-Related Equipment
That Is Located On The Consumer’s Side of the Demarcation
Point.
Customer Premises Equipment
Customer Premises EquipmentCustomer Premises Equipment

35. Let’s Study How Telephones, One Type OfLet’s Study How Telephones, One Type Of
CPE, WorksCPE, Works
Example Of Some
Modern Telephones

36. Direct Current (DC)
- An Electrical Charge
Flows Steadily In One
Direction Over the
Conductor.
Voltage Over Time In A DC Circuit
Time
Voltage
+
-
Do You Remember From TCM 134 How DC Current Flows?

37. DC
Voltage
Resistor R
I
DC
Voltage
Resistor R
I Flows When
Switch Closed
DC Series Circuits With & W/O A Switch
Ohm’s Law: V = I x R

38. DC
Voltage
Resistor R
Do You Remember From TCM 134 How A Capacitor Affects
Current Flow In A Circuit?
Does I Flow?
Does Current Flow In Circuits A Or B?
Capacitor
AC
Voltage
Resistor RCapacitor
Does I Flow?
Ohm’s Law: V = I x R

39. A View Of A Telephone Set

40. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
Examine The Diagram Below To See If You Can Apply Any
Of The Lessons Learned From The Previous Two Slides.
Under What Conditions Will Current Flow (1) Through The
Ringer, (2) Through The Keypad, and (3) Into The Hybrid
Coil.
Closed
Functional Diagram – Telephone Set

41. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
What Should The CO Switch Do First?
What Should The CO Switch Do Next?
Closed
What Does The Central Office Do To Allow You
To Dial Your Telephone?

42. A Touch Tone Phone Operates By Transmitting A
Combination Of Two Frequencies Each Time A Button Is
Pressed.
Pressing Each Button Sends Out A Different Combination
Of Frequencies.
How Does DTMF Dialing Work?

43. Rotary Dialing - A User Chooses A Number And Turns A
Wheel From That Number To The Finger Stop Then
Releases The Wheel.
Finger Stop
How Does Rotary Dialing Work?

44. The Phone’s Internal Electronics Repeatedly
Breaks The Electric Current To Signal The
Number Dialed.
The Interrupts In The Current Causes Pulses Of
Specific Lengths,
These Pulses Of Interrupted Current Flow Are
Used In Pulse Dialing Rather Than Frequencies
To Represent Each Digit.
How Does Rotary Dialing Work? - Continued

45. To Dial The Number “2” The Phone’s
Electronics Breaks The Current 2 Times.
To Dial The Number “9” The Phone’s
Electronics Breaks The Current 9 Times.
Question: How Is The Number “0” Dialed?
How Does Rotary Dialing Work? - Continued

46. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
How Is Current Transitioned From The Two-Wire CO Circuit To
The Four Wires Required To Operate The Receiver And
Transmitter?
What Is An Impedance Mismatch & How Does It Apply Here?
Closed
Operation Of The Receiver And Transmitter

47. Carbon Filled Receiver/Transmitter
Carbon Particles Used To Create Current.
Transmitter - The Input Voice Alternately Compresses
The Carbon Particles Which Create A Matching
Oscillating Current.
Receiver – The AC Current
From The Telephone Wires
Moves A Membrane Which In
Turn Alternately Compresses
The Carbon Particles Which
Produces Sound.

48. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
What Is The Role Of The Hybrid Coil Or Hybrid Network?
What Is The Role Of The Balancing Network?
Closed
Hybrid Coil And The Balancing Network

49. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
What Should The CO Switch Do First?
What Should The CO Switch Do Next?
Open
How Does The Central Office Ring Your
Telephone?

50. RingerLine
From
CO
R
T
Switch Hook
Hybrid
Coil
Balancing
Network
Receiver
Transmitter
1 2 3
4 5 6
7 8 9
* 0 #
When The Switch Hook Is Closed, Current Flows.
Closed
Current
Current
Functional Diagram – Telephone Set: Switch
Hook Closed

51. The Electronics Within Each CPE Listed Earlier Works
Differently.
All CPE Are Similar, However, Since They All Eventually
Create Electrical Signals Which Are Transmitted Over
Telecommunications Wiring And Those Signals Are
Received By A Compatible Piece Of CPE.
Later In This Course, We Will Have The Opportunity To
Study How Additional CPE Works.
Summary Of CPE