The
Public Switch Telephone Network
Lesson 1 Topics
PSTN Overview
The Role of The PSTN
CPE – An Example, How A
Telephone Works
Thought For The Week
Building on Lessons Learned In TCM 134
Study This Set Of PowerPoint Slides
Complete Assignment 1 And Post All Answers
In The Lesson 1 Drop Box By The Due Date.
Objectives
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.
Section 1.1 – PSTN Overview
What Is The PSTN?
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.
Major Elements Of The PSTN
CO Class 1
Switch
PSTN
CO Class 2
Switch
Analog Loop
Digital Interoffice
Trunks
CO Class 5
Switch
Residence
CPE
Digital Loop
PBX or KTS
Office Bldg
The Four Major Elements Of The PSTN
There Are Four Major Elements Of Today’s Public
Switched Telephone Network (PSTN).
Customer Premises Equipment (CPE)
Access System
Transport
Signaling
Customer Premises Equipment
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 Includes:
Telephones
PBXs
Fax Machines
Key Systems
Wiring & Jacks
CSUs
Multiplexers
DSUs
Cross Connect Systems
Video Codecs
Computers
Etc.
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.
Demarcation Point
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.
Access System
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.
Central Office Termination Equipment
Some Samples Of End Office Termination Equipment.
Access System
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 – Residential Access
Each Customer Circuit Is A Two-Wire Cable From The
Customer Demarc Out To The First Cross Connection Box
Class 5 Switch
A & B – Cross
Connection Pedestal
Or Cross Connection
Box
Access System – Residential Access
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
Class 5 Switch
2-Pair Cable
100-Pair
Cable
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.
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.
Plastic Insulator
Metal Conductor
Access System – Business Access
Trunks
Business W/ PBX
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.
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.
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.
Section 1.2 – The Role Of The PSTN
The Role Of The PSTN
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 - Continued
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?
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!
Several Examples Of The Role Of The PSTN
Cellular – Many Major Cellular Features Depend On
The PSTN – Two Examples Follow
A Cell Call Placed To A Land-Based Phone
Travels Through The PSTN
CO Class 1
Switch
Cellular User
CO Class 2
Switch
Digital Interoffice
Trunks
CO Class 5
Switch
Radio
Controller
MTSO
Analog Loop
Residence
CPE
Cellular Roaming Depends On The PSTN SS7 Network
Roaming
Cellular User
STP
SSP
STP
Internet
SS7/
SSP
PSTN
VLR
Roaming
MTSO
HLR
Home
MTSO
Cellular Roaming Would Not Be
Possible Without SS7 Network In PSTN
Another Example Of The Role Of The
PSTN
Private Line Data Networks Rely On Several
Elements Of The PSTN
PSTN Backbone Circuits Are Provisioned To Create
Corporate Private-Line Data Networks
Site D
Site A
OC3 Private Line
ASN Router
Router
ASN Router
Router
56 Kbps
Private Line
T3
Private Line
T1
Private Line
Site E
56 Kbps
Private Line
ASN Router
Router
56 Kbps
Private Line
ASN Router
Router
Site B
ASN Router
Router
Site C
Internet Access Relies On The PSTN – Another
Example
ISP Access Use PSTN Circuits
Corporate
End-User
CO Class 1
Switch
PSTN
MCDL
CO Class 2
Switch
Digital Interoffice
Trunks
CO Class 5
Switch
Analog Loop
Multiplexed/Channelized
Digital Loop
Residential
End-User
ISP
ASN Router
Router
Section 1.3 – Customer Premises Equipment
Customer Premises Equipment
CPE Includes All Telecommunications-Related Equipment
That Is Located On The Consumer’s Side of the Demarcation
Point.
Customer Premises Equipment
Let’s Study How Telephones, One Type Of
CPE, Works
Example Of Some
Modern Telephones
Direct Current (DC)
- An Electrical Charge
Flows Steadily In One
Direction Over the
Conductor.
Voltage
Do You Remember From TCM 134 How DC Current Flows?
+
-
Time
Voltage Over Time In A DC Circuit
Ohm’s Law: V = I x R
I Flows When
Resistor R
Switch Closed
Resistor R
I
DC
Voltage
DC
Voltage
DC Series Circuits With & W/O A Switch
Ohm’s Law: V = I x R
Do You Remember From TCM 134 How A Capacitor Affects
Current Flow In A Circuit?
Capacitor
Resistor R
Capacitor
Does I Flow?
DC
Voltage
Resistor R
Does I Flow?
AC
Voltage
Does Current Flow In Circuits A Or B?
A View Of A Telephone Set
Functional Diagram – Telephone Set
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.
Switch Hook
T
Receiver
Line Ringer
From
CO
R
Closed
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
What Does The Central Office Do To Allow You
To Dial Your Telephone?
What Should The CO Switch Do First?
What Should The CO Switch Do Next?
T
Line Ringer
From
CO
Switch Hook
Receiver
Closed
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
R
How Does DTMF Dialing Work?
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 Rotary Dialing Work?
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? - Continued
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
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?
Operation Of The Receiver And Transmitter
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?
T
Line Ringer
From
CO
Switch Hook
Receiver
Closed
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
R
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.
Hybrid Coil And The Balancing Network
What Is The Role Of The Hybrid Coil Or Hybrid Network?
What Is The Role Of The Balancing Network?
T
Line Ringer
From
CO
Switch Hook
Receiver
Closed
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
R
How Does The Central Office Ring Your
Telephone?
What Should The CO Switch Do First?
What Should The CO Switch Do Next?
T
Line Ringer
From
CO
Switch Hook
Receiver
Open
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
R
Functional Diagram – Telephone Set: Switch
Hook Closed
T
Line Ringer
From
CO
Current
Switch Hook
Receiver
Closed
1
2
3
4
5
6
7
8
9
*
0
#
Hybrid
Coil
Transmitter
R
Current
When The Switch Hook Is Closed, Current Flows.
Summary Of CPE
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.