PSTN (Public Switched Telephone Network)
The public switched telephone network (PSTN) is the aggregate of the world's
circuit-switched telephone networks that are operated by national, regional, or local
telephony operators, providing infrastructure and services for public telecommunication.
The PSTN consists of telephone lines, fiber optic cables, microwave transmission links,
cellular networks, communications satellites, and undersea telephone cables, all
interconnected by switching centers, thus allowing most telephones to communicate with
each other. Originally a network of fixed-line analog telephone systems, the PSTN is
now almost entirely digital in its core network and includes mobile and other networks,
as well as fixed telephones.
The technical operation of the PSTN adheres to the standards created by the ITU-T.
These standards allow different networks in different countries to interconnect
seamlessly.
PSTN - short for public switched telephone network,
also knows as the plain old telephone system(POTS) is
basically the inter-connected telephone system over
which telephone calls are made via copper wires.
PSTN is based on the principles of circuit switching
Therefore when a call is made a particular dedicated
circuit activates which eventually deactivates when the call
ends
Telephone calls transmits as analogue signals across
copper wires
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 Digital Interoffice
Switch Trunks
CO Class 5
Analog Loop Switch Digital Loop
Residence PBX or KTS
CPE 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
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 – 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
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).
Signaling
Signaling is the controlling of communications
Basically anything but voice transmission is signaling
Ex : call setup, call termination, billing , caller ID etc…
There are types of signaling
Channel associated signaling (CAS) - signaling
information is transmitted within the same voice channels
Also know as in-band signaling
Ex : Dual tone multi frequency signaling (DTMF)
Common channel signaling (CCS) - signaling
information is transmitted via a separate channel
Also know as out-band signaling
Ex : signaling system #7 (SS7)
A Cell Call Placed To A Land-Based Phone
Travels Through The PSTN
CO Class 1
Switch
Cellular User
CO Class 2 Digital Interoffice
Switch Trunks
Radio CO Class 5 Analog Loop
Controller
Switch
Residence
CPE
MTSO
Switching systems
Switching systems, basically are what determines the
routing pathway of a call
Switches are contained in local exchanges and central
offices
Call setup process
Call setup process
Example : Suppose the calling subscriber dialed
“5834975”
At first the exchange(294) which the calling subscriber is
directly connected to, examines the dialed digits “583-4975”
Secondly it acts upon the first three digits and access its
look up table to rout the call to the “583” exchange
Then the “583” exchange acts upon the information
It identifies the dialed number and connects the correct
subscriber loop which matches the “4975” number
When the call is answered conversation begins
