Basic Telephone
Subscriber
Line
Telephone
T
dial switch
cradle switch
CO
(Local Exchange)
tip (+)
off-hook
on-hook
mouth
ear
R
DTMF
C
ringer
SLIC
ring (-)
telephony
Slide 1
Subscriber Line Interface
Subscriber Line Interface Circuit
ring switch
Telephone
T
tip (+)
Subscriber
Line
ring (-)
hybrid
current
detector
crossconnect
switch
control channel
ring
generator
(100Vrms 25 Hz)
~
-48 VDC
Processor
Call States
idle
dialing
calling
ringing
called
on hook
dialing in progress
after dialing
incoming call
call in progress
telephony
Slide 2
PSTN
Review
The PSTN circa 1900
pair of copper wires
“local loop”
manual routing at local exchange office (CO)
• Analog voltage travels over copper wire end-to-end
• Voice signal arrives at destination severely attenuated and distorted
• Routing performed manually at exchanges office(s)
• Routing is expensive and lengthy operation
• Route is maintained for duration of call
telephony
Slide 3
PSTN
Review
Multiplexing
1900: 25% of telephony revenues went to copper mines



standard was 18 gauge, long distance even heavier
two wires per loop to combat cross-talk
needed method to place multiple conversations on a single trunk
1918: “Carrier system” (FDM)



5 conversations on single trunk
later extended to 12 (group)
still later supergroups, master groups, supermaster groups
1963: T-carrier system (TDM)



channels
f
timeslots
T1 = 24 conversations per trunk
later T3 = 28 T1s
still later SDH rates with 1000s of conversations per trunk
t
telephony
Slide 4
Cross-connect switch
Analog Crossbar switch
1
2
3
4
5
6
Digital Cross-connect (DXC)
7
1
2
3
4
5
6
7
8
1
2
3
4
5
t
3
t
processor
2
1
5
4
Complexity increases rapidly with size
telephony
Slide 5
Basic TDM CO
DXC
Subscriber
Lines
SLIC
Hybrid
4w
2w
SLIC
Hybrid
PCM
MUX
TDM
port
Tone
&
Ann
TDM
port
E1/T1
E1/T1
TDM
port
TDM
port
DTMF
decode
Processor
telephony
Slide 6
PSTN
Review
PSTN Topology
local loop
Local
Exchange
Local
Exchange
Long distance
network
subscriber line
Local
Exchange
trunk
circuit
Many local telephone exchanges had sprung up
Bell Telephone acquired them
and interconnected them for long distance
telephony
Slide 7
PSTN
Review
Old US PSTN
Regional centers
Class 1
Class 2
Class 3
Class 4
Class 3
Class 4
Sectional centers
Class 2
Class 4
Class 3
Class 4
Primary centers
Toll (tandem) offices
circuits,trunks
Class 5
Class 5
local loop
Class 5
subscriber lines
Class 5
Class 5
Central (end) offices
last mile
Class 5 switch is the sole interface to the subscriber lines
telephony
Slide 8
Numbering Plans
An E.164 International Number has the format :
Country-Code Area-Code Exchange-Code Line-Number
MAXIMUM 15 DIGITS
EX:
972
2
588
9159
Country-Codes
1
2
3
4
5
6
7
8
9
:
:
:
:
:
:
:
:
:
N America (US, CA, Caribbean) 1 digit
Africa 2 or 3 digits
20
Europe 2 or 3 digits
31
Europe 2 or 3 digits
44
S America 2 or 3 digits
54
Australia & S Pacific 2 or 3 digits 61
Russia 1 digit
China & N Pacific 2 or 3 digits
86
Middle East 2 or 3 digits
90
Egypt 27 South Africa 235 Chad
Netherlands 354 Iceland
UK 49 Germany 420 Czech Republic
Argentina 595 Paraguay
Australia 675 Papua
PR China
855 Cambodia
Turkey
972 Israel
telephony
Slide 9
PSTN
Review
Signaling
PSTN with automatic switching requires signaling
The present PSTN has thousands of features
and all require signaling support
Examples:
On-hook / off-hook
Pulse / Tone dialing
Receiver off-hook
Call waiting
Caller number identification
Call forwarding
Hook-flash
Fax transmission detect
Inter-CO messaging
Echo cancellation
Voice mail
Conference calls
Coin-drop
Billing
telephony
Slide 10
PSTN
Review
Signaling Methods
Signaling can be performed in many ways

Analog voltage signaling loop-start, ground-start, E&M

In-band signaling DTMF, MFR1, MFR2

Channel associated signaling (CAS) AB bits, ABCD bits

Common channel signaling (CCS) SS7, QSIG
– Trunk Associated CCS
– Separate signaling network CCS
telephony
Slide 11
Subscriber - Exchange
Signaling*




On/off hook DC current flow
Dial-tone 350+440 continuous
Pulse Dialing
DTMF
L1 697
H1 1209
L2 770
H2 1336
L3 852
H3 1477
L4 941
H4 1633
off-hook
>100ms
(60:40)
>700ms
make
break
H1
L1 1
L2 4
L3 7
L4 *
H2
2
5
8
0
H3
3
6
9
#

Ring AC with cadence
Ring-back 440+480 with 2:4 cadence
Busy 480+620 with 1/2:1/2 cadence
Trunk busy 480+620 but 0.2:0.3 cadence
Receiver off-hook 1400+2060+2450+2600 with ).1:0.1 cadence

Invalid (nonworking, unobtainable) number




H4
A
B
C
D
announcement, SIT-tone
* US cadences - national differences
telephony
Slide 12
Subscriber - Subscriber
Subscriber - Exchange
Signaling


Hook-flash on-hook for 0.1 - 1 sec
Echo suppressor disabler 2100 continuous
– FAX CED 2100 2.6 - 4 sec
– Modem ANS 2100 with phase reversals every 450 ms for 3.3 s


– ANSAM 2100 with phase reversals and 15 Hz AM modulation
CNG
– FAX 1100 with 0.5:3 cadence
– Modem 1300 with 0.5:2 cadence
Caller Line Identification (CLI/CND)
– 1200 bps FSK (V.23) data signal 1300 = 1 2100 = 0
– Between 1st and 2nd rings
RING
>300 ms
CLI
> 475 ms
RING
telephony
Slide 13
Supervision procedures

FXO/FXS
Foreign Exchange Office / Foreign Exchange Subscriber
FXS is like exchange - provides voltage, ringing, dialtone
FXO is like a phone - requires voltage, detect ring, etc.
When connecting PBX to CO, PBX is FXO, CO is FXS

Analog loop start

Digital loop start

Analog ground start

Digital ground start

E&M (wink)

telephony
Slide 14
Interexchange Signaling

CAS - R2

CAS - R1

CCS - SS7
telephony
Slide 15
PSTN
Review
Optimized Telephony Routing
Circuit switching (route is maintained for duration of call)
Route “set-up” is an expensive operation, just as it was for manual switching
Today, complex least cost routing algorithms are used
Call duration consists of set-up, voice and tear-down phases
telephony
Slide 16
PSTN
Review
The PSTN circa 1960
trunks
circuits
local loop
subscriber line
automatic routing through universal telephone network
• Analog voltages used throughout, but extensive Frequency Division Multiplexing
• Voice signal arrives at destination after amplification and filtering to 4 KHz
• Automatic routing
• Universal dial-tone
• Voltage and tone signaling
• Circuit switching (route is maintained for duration of call)
telephony
Slide 17
PSTN
Review
The Digitalization of the PSTN
Shannon (Bell Labs) proved
Digital
Communications
Analog
is better than Communications
and the PSTN became digital
Better means

More efficient use of resources (e.g. more channels on trunks)

Higher voice quality (less noise, less distortion)

Added features
telephony
Slide 18
PSTN
Review
Timing
In addition to voice, the digital PSTN transports timing

This timing information is essential because of
– the universal use of TDM
– the requirement of accurate playback (especially for fax/modem)

Receiving switches can recover the clock of the transmitting switch

Every telephony network has an accurate clock called “stratum 1”
Clocks synchronized to it are called “stratum 2”
Clocks synchronized to them are called “stratum 3”
and so on
telephony
Slide 19
PSTN
Review
The Present PSTN
core
backbone
subscriber line
PSTN Network
• Analog voltages and copper wire used only in “last mile”,
but core designed to mimic original situation
• Voice signal filtered to 4 KHz at input to digital network
• Time Division Multiplexing of digital signals in the network
• Extensive use of fiber optic and wireless physical links
• T1/E1, PDH and SONET/SDH “synchronous” protocols
• Signaling can be channel/trunk associated or via separate network (SS7)
• Automatic routing
• Circuit switching (route is maintained for duration of call)
• Complex routing optimization algorithms (LP, Karmarkar, etc)
telephony
Slide 20
PSTN
Review
Nonvoice services
The PSTN can even be used to transport non-voice signals
such as
FAX
or
VoP course
DATA
PSTN

These services disguise themselves as voice by using a modem

Proper timing is essential

Special signaling is required
– turn off LEC
– turn off call waiting
– service recognition
– capabilities negotiation
– mutual identification
– end of page/document
– modem recognition
– modem training
– data compression
telephony
Slide 21
PSTN
Review
Digital Loop Carrier
Pushes the digital PSTN closer to customer
AT&T SLC-40, SLC-96, Nortel DMS P-phone,
TR-08 Mode 1 pair-gain:
Replace 96 pairs with 5 T1s
Access Network
CLASS
5
“pair-gain”
(one spare for “span protection”)
UTP/coax/fiber Street
FTTB/FTTC cabinet
CPE
96 – 10 = 86
TR-08 Mode 2 pair-gain:
Replace 96 pairs with 2 T1s
pedestal
(without “span protection”)
UTP
96 – 4 = 92
TR-08 multiplex 96 lines on:
Mode 1: 4 T1s
Mode 2: 2 T1s (2:1 concentration)
GR303/V5.1/V5.2 multiplex up to 2048 lines
telephony
Slide 22