1. Basic Telephone Subscriber Line CO (Local Exchange) Telephone
dial switch T
cradle switch
tip (+)
off-hook
on-hook
mouth
DTMF C
ear
SLIC
ringer R
ring (-)

2. Subscriber Line Interface Subscriber Line Interface Circuit
ring switch
Telephone T
tip (+)
crossconnect
switch
Subscriber Line
current
detector
hybrid
ring (-)
control channel
ring
generator
(100Vrms 25 Hz) ~
Processor
-48 VDC
Call States
idle on hook
dialing dialing in progress
calling after dialing
ringing incoming call
called call in progress

3. The PSTN circa 1900 PSTN Review
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

4. Multiplexing f t 1900: 25% of telephony revenues went to copper mines
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)
T1 = 24 conversations per trunk
later T3 = 28 T1s
still later SDH rates with 1000s of conversations per trunk f
channels t
timeslots

5. Cross-connect switch t t Analog Crossbar switch
Digital Cross-connect (DXC) 1 2 4 5 6 7 8 3 t 1 2 3 4 5
processor t 2 1 5 4 3
Complexity increases rapidly with size

6. Basic TDM CO DXC Subscriber Lines SLIC Hybrid TDM port TDM port Tone &
Ann 2w 4w
PCM
MUX
SLIC
Hybrid
E1/T1
E1/T1
TDM
port
TDM
port
DTMF
decode
Processor

7. PSTN Topology local loop Local Exchange Local Exchange Local Exchange
PSTN Review
PSTN Topology
local loop
Local
Exchange
Local
Exchange
Local
Exchange
Long distance
network
trunk
circuit
subscriber line
Many local telephone exchanges had sprung up
Bell Telephone acquired them
and interconnected them for long distance

8. PSTN Review
Old US PSTN
Class 1
Regional centers
Class 2
Class 2
Sectional centers
Class 3
Class 3
Class 3
Primary centers
Class 4
Class 4
Class 4
Class 4
Toll (tandem) offices
circuits,trunks
Class 5
Class 5
Class 5
Class 5
Class 5
Central (end) offices
local loop
subscriber lines
last mile
Class 5 switch is the sole interface to the subscriber lines

9. Numbering Plans An E.164 International Number has the format :
Country-Code Area-Code Exchange-Code Line-Number
EX:
Country-Codes
1 : N America (US, CA, Caribbean) 1 digit
2 : Africa 2 or 3 digits Egypt 27 South Africa 235 Chad
3 : Europe 2 or 3 digits Netherlands 354 Iceland
4 : Europe 2 or 3 digits UK 49 Germany 420 Czech Republic
5 : S America 2 or 3 digits Argentina 595 Paraguay
6 : Australia & S Pacific 2 or 3 digits 61 Australia 675 Papua
7 : Russia 1 digit
8 : China & N Pacific 2 or 3 digits PR China Cambodia
9 : Middle East 2 or 3 digits Turkey Israel
MAXIMUM 15 DIGITS

10. Signaling PSTN with automatic switching requires signaling
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

11. Signaling Methods Signaling can be performed in many ways
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

12. Subscriber - Exchange Signaling*
On/off hook DC current flow
Dial-tone continuous
Pulse Dialing
DTMF
L L L L4 941
H H H H4 1633
Ring AC with cadence
Ring-back with 2:4 cadence
Busy with 1/2:1/2 cadence
Trunk busy but 0.2:0.3 cadence
Receiver off-hook with ).1:0.1 cadence
Invalid (nonworking, unobtainable) number announcement, SIT-tone
* US cadences - national differences
make
break
>700ms
>100ms
(60:40)
off-hook
H1 H2 H3 H4
L A
L B
L C
L4 * 0 # D

13. Subscriber - Subscriber Subscriber - Exchange Signaling
Hook-flash on-hook for sec
Echo suppressor disabler 2100 continuous
FAX CED 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 = = 0
Between 1st and 2nd rings
RING
CLI
>300 ms
> 475 ms

14. 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)

15. Interexchange Signaling
CAS - R2
CAS - R1
CCS - SS7

16. Optimized Telephony Routing
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

17. The PSTN circa 1960 PSTN Review
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)

18. The Digitalization of the PSTN
PSTN Review
The Digitalization of the PSTN
Shannon (Bell Labs) proved
is better than
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
Digital
Communications
Analog
Communications

19. Timing In addition to voice, the digital PSTN transports timing
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

20. The Present PSTN PSTN Network PSTN Review
core
backbone
PSTN Network
subscriber line
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)

21. PSTN Review
Nonvoice services
The PSTN can even be used to transport non-voice signals
such as FAX or DATA
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
VoP course
PSTN
capabilities negotiation
mutual identification
end of page/document
modem recognition
modem training
data compression

22. Digital Loop Carrier Pushes the digital PSTN closer to customer
PSTN Review
Digital Loop Carrier
Pushes the digital PSTN closer to customer
AT&T SLC-40, SLC-96, Nortel DMS P-phone, “pair-gain”
Access Network
TR-08 Mode 1 pair-gain:
Replace 96 pairs with 5 T1s (one spare for “span protection”)
96 – 10 = 86
TR-08 Mode 2 pair-gain:
Replace 96 pairs with 2 T1s (without “span protection”)
96 – 4 = 92
CLASS 5
UTP/coax/fiber
Street cabinet
CPE
FTTB/FTTC
pedestal
UTP
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