TDC 461: Basic
Telecommunications
DePaul University
9 April, 2001
LoriLee M. Sadler
Agenda
• Telephone Network
• Telephone Numbering
Format
• Signaling on Subscriber
Loop
• Analog vs. Digital
Information
• Central Office Switch
Technologies
• How is a Voice Call
Made?
• Carrier Services
Public Switched Telephone
Network:
IXC
POP
IX
C
Tr
un
Ameritech
Tandem Office
k
Ameritech
Central Office
Direct
IXC Trunk
IXC
POP
k
T run
m
e
Ta nd
Ameritech
Central Office
Inter-Office Trunks
PB
X
u
Tr
nk
s
PBX
Centrex Lines
Subscriber Lines
POP = Point of Presence
PBX = Private Branch Exchange
The PSTN:
• Topology of Connections
• Network=>
– Demarcation point (Demarc)=>also called
station drop.
Network Configuration
• Star configuration from the Central Office
(CO) to the demarc
• Star or ring configuration from the Central
Office to other COs or higher-level offices
• Wire Center or Frame
Switch
• Physical vs. logical connections
• Facilities
The Subscriber (Local) Loop
Outside Plant
• Demarcation Point (Demarc)
• Feeders:
• Lateral distribution:
• Branch feeders:
Subscriber Loop Signaling
• Carrier Sense=>
Central
Office
Battery
-48 v.
The Subscriber Loop Central
Office
Battery
-48 v.
• The Subscriber Line or Subscriber Loop is
made up of 2 copper wires twisted together
(wires called “tip” and “ring”)
The Subscriber Loop
Idle Line
• Central Office provides battery power (48
volts DC) to subscriber loop.
• When telephone is ON-HOOK, it keeps
loop open (wires are not connected) and
there is no current flow.
Central
Office
Battery
-48 v.
The Subscriber Loop
Service Request
• Off-Hook signals request
• Request fulfilled by CO
Central
Office
Battery
-48 v.
North American Numbering
Plan (NANP)
• Determines format for dialed numbers.
• Administered by BellCore
• Format before 1995:
Prefix + 0/1 + N(0/1)X - NNX - XXXX
(N digits are 2-9, X digits are 0-9)
• Format as of January, 1995:
Prefix + 0/1 + NXX - NXX - XXXX
(N digits are 2-9, X digits are 0-9)
Address Signaling
Rotary Pulse Dial
– After pulling dial for digit ‘N’,
telephone opens and closes
subscriber loop ‘N’ times as dial
returns to its original position.
697 Hz
770 Hz
852 Hz
ABC
DEF
1
2
3
GHI
JKL
MNO
4
5
6
PRS
TUV
WXY
7
8
9
Oper
941 Hz
*
0
#
1209 Hz 1336 Hz 1447 Hz
• Dual Tone Multifrequency
(DTMF) or Touch Tone
– One frequency assigned to each
row and each column.
– Pressing any key sends a 2frequency signal tone.
Business Station Set Features
• Access to multiple lines:
–
–
–
–
–
Multiple Directory Numbers
Multiple Access to single Directory Number (MADN)
Call Pickup privileges to other lines
Call status indicator lamps and display
Intercom dialing
• Speakerphone
• Voice Mail Access
– Message Waiting lamp
Business Station Set Features
• Calling Features:
–
–
–
–
–
–
–
Hold
Drop
Conference
Transfer
Speed Dial
Call Forward
Call Waiting
Trunks
Ameritech
Central Office
Ameritech
Central Office
Inter-Office Trunks
Repeaters
•
•
•
Repeaters
Analog Trunks
Ameritech
Central Office
Ameritech
Central Office
Inter-Office Trunks
Repeaters
• Trunk types:
– 2-wire analog trunk
– 4-wire analog trunk
Repeaters
Digital Trunks
Ameritech
Central Office
Ameritech
Central Office
Inter-Office Trunks
Repeaters
• 1962 (T-1 carrier)
Repeaters
Digital Trunks
Ameritech
Central Office
Ameritech
Central Office
Inter-Office Trunks
Repeaters
Repeaters
• Advantages of Using Digital Trunks:
Voice Digitization
1 Voice Channel = 64,000 bits/second
Sample
8000 samples/sec
Quantize
8 bits/sample
100111011
Call over Digital Trunk
Ameritech
Central Office
Ameritech
Central Office
Inter-Office Trunks
Regenerators
Regenerators
• Codec (Coder/decoder) circuits in COs
do analog-digital conversion.
• Analog Central Office Switch:
– Analog signal is switched internally.
• Digital Central Office Switch:
– Digital signal is switched internally.
Channel Banks
Analog
Switch
T-1
Channel Analog
Bank Switch
T-1
Channel
Bank
T-1 Trunk
Repeater
...
...
Repeater
24 Analog
Voice Channels
• T-1 Channel Bank converts between 24 analog
voice channels and a single T-1 trunk.
– Digitizes 24 voices (codec function)
– Combines 24 DS0 signals into a DS1 signal
(multiplexing function)
Central Office Switch
Technologies
Ameritech
Central Office
Wire Center Components
Central Office / Wire Center
C.O. Switch
Se rv ice
Circuits
Common
Control
Data
Store
Switch M atrix
Line Cards
Trunk Cards
Switch Side
Main Distribution Frame (MDF)
Outside
Plant
Cable Side (Protected)
Cable Vault
• Central Office Switches
• Main Distribution
Frame (MDF)
• Cable Vault
Wire Center Components
Central Office / Wire Center
C.O. Sw itch
Se rv ice
Circuits
Common
Control
Data
Store
Switch Matrix
Line Cards
Trunk Cards
Switch Side
Main Distribution Frame (MDF)
Outside
Plant
Cable Side (Protected)
Cable Vault
• Central Office Switch
Components
– Common Control
– Data Store
– Service Circuits
Wire Center Components
Central Office / Wire Center
C.O. Sw itch
Se rv ice
Circuits
Common
Control
Data
Store
Switch Matrix
Line Cards
Trunk Cards
Switch Side
Main Distribution Frame (MDF)
Outside
Plant
Cable Side (Protected)
Cable Vault
• Central Office Switch
Components
– Switch Matrix
– Line Cards
– Trunk Cards
Wire Center Components
Central Office / Wire Center
–
–
C.O. Sw itch
Se rv ice
Circuits
Common
Control
Data
Store
Switch Matrix
Line Cards
Trunk Cards
Switch Side
Main Distribution Frame (MDF)
Outside
Plant
Cable Side (Protected)
Cable Vault
• Distribution Frame
• Cable Vault
• Outside Plant
–
–
How is a Voice Call
Made?
A Voice Call
Central Office
Service Common
Circuits Control
Data
Store
Switch Matrix
Line
Cards
Trunk
Cards
MDF
• Originating phone goes offhook.
A Voice Call
• Customer dials number.
Central Office
Service Common
Circuits Control
Data
Store
Switch Matrix
Line
Cards
Trunk
Cards
MDF
A Voice Call
Central Office
Service Common
Circuits Control
Data
Store

Switch Matrix
Line
Cards
Trunk
Cards
MDF
Phones go on-hook.

Inter-Office Calls
Central Office
Central Office
Service Common
Circuits Control
Service Common
Circuits Control
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Data
Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
Inter-Office Trunk
Network Structure
IXC
POP
C
IX
un
Tr
Ameritech
T andem Office
k
Ameritech
Central Office
Tand
unk
m Tr
Direct
IXC Trunk
IXC
POP
e
Ameritech
Central Office
Inter-Office T runks
PB
ru n
XT
ks
PBX
Centrex Lines
Subscriber Lines
POP = Point of Presence
PBX = Private Branch Exchange
Residential Services
• Direct Distance Dialing or DDD service
• Party Line Service
Business Services
• Line vs. Trunk Service
• Private vs. Switched Service
Private Line Services
• Private Line Service provides a dedicated
circuit between two locations
Private Line Services
Central Office
Central Office
Service
Circuits
Com m on
Contr ol
Service
Circuits
Data
Store
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on
Contr ol
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
Inter-Office Trunk
• Private Line / Leased Line / Dedicated Line
Private Line Services
• Foreign Exchange (FX) Line
• Off Premises Extension (OPX)
Private Line Services
Central Office
Central Office
Service
Circuits
Service
Circuits
Com m on Data
Contr ol Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on Data
Contr ol Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
Inter-Office Trunk
• Foreign Exchange (FX) Line
Private Line Services
Central Office
Central Office
Service
Circuits
Service
Circuits
Com m on Data
Contr ol Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on Data
Contr ol Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
Inter-Office Trunk
• Off Premises Extension (OPX)
PBX
FX Line Example
• LhasaLabs Corporation in Chicago has many
customers in New York
– DDD Chgo-NY costs 15 cents per minute
– FX line Chgo-NY costs $800 per month
– LhasaLabs makes about 25 calls to NY each day, each
call lasting about 10 minutes
– SHOULD LhasaLabs GET AN FX LINE?
Private Line Services
• Tie Line
• Dedicated Access Line to IXC POP
Private Line Services
Central Office
Central Office
Service
Circuits
Service
Circuits
Com m on Data
Contr ol Store
Trunk
Cards
Trunk
Cards
• Tie Line
Line
Cards
MDF
MDF
PBX
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on
Contr ol
Inter-Office Trunk
PBX
Private Line Services
IXC Switch
LEC Switch
Service
Circuits
Service
Circuits
Data
Store
Common
Control
Switch Matrix
Switch Matrix
Line
Cards
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
PBX
Data
Store
Common
Control
IXC Trunk
• Dedicated access to IXC POP
Switched Line Services
• Direct Distance Dialing (DDD)
• Wide Area Telecommunications Service
(WATS)
• IN-WATS (800, 888 services)
WATS, IN-WATS
• WATS=>Wide-area telecommunications
services; Refers to outbound calls only, not
inbound.
• old style with banded service rates
• Full-time WATS=>
• Measured WATS=>
WATS Example
• Lhasa Labs is located in Chicago and many
customers in New York. Lhasa Labs wants to offer
a toll free number to its New York customers.
Lhasa Labs anticipates using 200 hours per month.
– $37.50 per line per month; additional usage at $.15 a
minute
–
– Compare to DDD?
– Compared to a T1?
Switched Line Services
• 900 Services
–
• International WATS
–
• Remote Call Forwarding
–
–
Switched Line Services
• Directory Services
–
• Wide Area Telecommunications Service
(WATS)
–
• IN-WATS (800, 888 services)
–
RCF Example
• LhasaLabs Corporation in Chicago has many
customers in New York. LhasaLabs wants to offer
a local New York number.
–
–
–
–
DDD Chgo-NY costs 15 cents per minute
RCF service costs $50 per month
FX line Chgo-NY costs $800 per month
Customers in NY make about 10 calls to LhasaLabs
each day, each call lasting about 10 minutes
– SHOULD LhasaLabs GET AN FX LINE?
Residential and/or Business
Features
•
•
•
•
•
•
•
•
Call Forwarding
Call Waiting
Three-Way Calling
Call Transfer
Calling Card
Speed Calling
Automatic CallBack
etc. etc.
Network Structure
IXC
POP
C
IX
un
Tr
Ameritech
T andem Office
k
Ameritech
Central Office
Tand
unk
m Tr
Direct
IXC Trunk
IXC
POP
e
Ameritech
Central Office
Inter-Office T runks
PB
ru n
XT
ks
PBX
Centrex Lines
Subscriber Lines
POP = Point of Presence
PBX = Private Branch Exchange
Business Services
• Line vs. Trunk Service
– Is access to network directly through telephone
sets or through customer switch (Private
Branch Exchange or PBX)?
• Private vs. Switched Service
– Does network just provide connection (private)
or dial tone (switched)?
Line vs. Trunk Addressing
• Each line has a telephone number
associated with it.
–
–
• A trunk does not have any telephone
number associated with it.
–