telephony Slide 1

1
Basic Telephone
CO (Local Exchange)
SubscriberLine
Telephone
dial switch
tip ()
T
cradle switch
off-hook
on-hook
mouth
C
ear
SLIC
ringer
ring (-)
R
2
Subscriber Line Interface
Subscriber Line Interface Circuit
ring switch
Telephone
T
tip ()
crossconnect switch
SubscriberLine
hybrid
current detector
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
PSTN Review

• 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

5
Cross-connect switch
Analog Crossbar switch
Digital Cross-connect (DXC)
processor

• Complexity increases rapidly with size

6
Basic TDM CO
DXC
SubscriberLines
Tone Ann

PCM MUX
4w
2w
E1/T1
E1/T1

DTMF decode
Processor
7
PSTN Topology
PSTN Review
Local Exchange
local loop
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
Old US PSTN
PSTN Review
Class 1
Regional centers
Class 2
Class 2
Sectional centers
Class 3
Class 3
Class 3
Primary centers
Toll (tandem) offices
Class 4
Class 4
Class 4
Class 4
circuits,trunks
Central (end) offices
Class 5
Class 5
Class 5
Class 5
Class 5
last mile
subscriber lines
local loop
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 972 2 588
  9159
• Country-Codes
• 1 N America (US, CA, Caribbean) 1 digit
• 2 Africa 2 or 3 digits 20
  Egypt 27 South Africa 235 Chad
• 3 Europe 2 or 3 digits 31
  Netherlands 354 Iceland
• 4 Europe 2 or 3 digits 44 UK
  49 Germany 420 Czech Republic
• 5 S America 2 or 3 digits 54
  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 86 PR
  China 855 Cambodia
• 9 Middle East 2 or 3 digits 90
  Turkey 972 Israel

10
Signaling
PSTN Review

• 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
PSTN Review

• Signaling can be performed in many ways
• Analog voltage signaling loop-start,
  ground-start, EM
• 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 350440 continuous
• Pulse Dialing
• DTMF
• L1 697 L2 770 L3 852 L4 941
• H1 1209 H2 1336 H3 1477 H4 1633
• Ring AC with cadence
• Ring-back 440480 with 24 cadence
• Busy 480620 with 1/21/2 cadence
• Trunk busy 480620 but 0.20.3 cadence
• Receiver off-hook 1400206024502600 with
  ).10.1 cadence
• Invalid (nonworking, unobtainable) number
  announcement, SIT-tone
• US cadences - national differences

13
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 0.53 cadence
• Modem 1300 with 0.52 cadence
• Caller Line Identification (CLI/CND)
• 1200 bps FSK (V.23) data signal 1300 1 2100
  0
• Between 1st and 2nd rings

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
• EM (wink)

15
Interexchange Signaling

• CAS - R2
• CAS - R1
• CCS - SS7

16
Optimized Telephony Routing
PSTN Review
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

• 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

• Analog
• Communications

• Digital
• Communications

19
Timing
PSTN Review

• 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 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
Nonvoice services
PSTN Review

• 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

PSTN

• capabilities negotiation
• mutual identification
• end of page/document

• modem recognition
• modem training
• data compression

22
Digital Loop Carrier
PSTN Review

• Pushes the digital PSTN closer to customer
• ATT SLC-40, SLC-96, Nortel DMS P-phone,
  pair-gain

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
Access Network
CLASS 5
Street cabinet
UTP/coax/fiber
CPE
FTTB/FTTC
pedestal
UTP
TR-08 multiplex 96 lines on
Mode 1 4 T1s Mode 2 2 T1s (21
concentration) GR303/V5.1/V5.2 multiplex up to
2048 lines