Download - Intro Tele
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telephony Slide 1
ear
T
R
mouthDTMF
ringer
C
dial switch
SLIC
Telephone
Subscriber
Line
CO
(Local Exchange)
cradle switch
off-hookon-hook
tip (+)
ring (-)
Basic Telephone
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telephony Slide 2
hybrid
TTelephone
current
detector
~
ring switch
ring
generator
(100Vrms
25 Hz)-48 VDC
Subscriber
Line
crossconnectswitch
Subscriber Line Interface Circuit
Processor
control channel
Call Statesidle on hook
dialing dialing in progress
calling after dialing
ringing incoming call
called call in progress
tip (+)
ring (-)
Subscriber Line Interface
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telephony Slide 3
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
PSTN
Review
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telephony Slide 4
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
PSTN
Review
f
channels
t
timeslots
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telephony Slide 5
Cross-connect switch
Complexity increases rapidly with size
1 2 4 5 6 7 83
1
2
3
45
6
7
Analog Crossbar switch Digital Cross-connect (DXC)
processor
t1 2 3 4 5
t2 1 5 4 3
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telephony Slide 6
SLIC
PCM
MUX
Hybrid
SLIC
TDM
port
TDM
port
TDM
port
TDM
port
Tone
&
Ann
DTMF
decode
Processor
Subscriber
Lines
Hybrid
2w 4w
DXC
E1/T1 E1/T1
Basic TDM CO
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PSTN Topology
Many local telephone exchanges had sprung up
Bell Telephone acquired them
and interconnected them for long distance
Local
Exchange LocalExchange
LocalExchange
local loop
Long distance
network
trunkcircuit
subscriber line
PSTN
Review
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Old US PSTN
Class 3 Class 3
Class 2 Class 2
Class 1
Class 3
Regional centers
Sectional centers
Primary centers
Toll (tandem) offices
Central (end) offices
Class 4
last milesubscriber lineslocal loop
circuits,trunks
Class 5 switch is the sole interface to the subscriber lines
PSTN
Review
Class 5 Class 5 Class 5 Class 5 Class 5
Class 4 Class 4 Class 4
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Numbering Plans
An E.164 International Numberhas the format :
Country-Code Area-Code Exchange-Code Line-Number
EX: 972 2 588 9159
Country-Codes1 : N America (US, CA, Caribbean) 1 digit
2 : Africa 2 or 3 digits 20 Egypt 27 South Africa 235 Chad3 : Europe 2 or 3 digits 31 Netherlands 354 Iceland4 : Europe 2 or 3 digits 44 UK 49 Germany 420 Czech Republic5 : S America 2 or 3 digits 54 Argentina 595 Paraguay6 : Australia & S Pacific 2 or 3 digits 61 Australia 675 Papua7 : Russia 1 digit8 : China & N Pacific 2 or 3 digits 86 PR China 855 Cambodia9 : Middle East 2 or 3 digits 90 Turkey 972 Israel
MAXIMUM 15 DIGITS
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Signaling
PSTN with automatic switching requires signaling
The present PSTN has thousands offeatures
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
PSTN
Review
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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
PSTN
Review
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Subscriber - ExchangeSignaling*
On/off hook DC current flow
Dial-tone 350+440 continuous
Pulse Dialing
DTMFL1 697 L2 770 L3 852 L4 941
H1 1209 H2 1336 H3 1477 H4 1633
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) numberannouncement, SIT-tone
* US cadences - national differences
H1 H2 H3 H4
L11 2 3 A
L24 5 6 B
L37 8 9 C
L4* 0 # D
make
break
>700ms>100ms
(60:40)off-hook
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Subscriber - SubscriberSubscriber - Exchange
Signaling Hook-flash on-hook for 0.1 - 1 sec
Echo suppressor disabler2100 continuous
FAX CED2100 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 ringsRING RINGCLI>300 ms > 475 ms
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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)
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Interexchange Signaling
CAS - R2
CAS - R1
CCS - SS7
PSTN
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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 routingalgorithms are used
Call duration consists ofset-up, voiceand tear-downphases
PSTN
Review
PSTN
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The PSTN circa 1960
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)
trunks
circuits
PSTN
Review
PSTN
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The Digitalization of the PSTN
Shannon (Bell Labs) proved
is better than
and the PSTN became digital
Bettermeans
More efficient use of resources (e.g. more channels on trunks)
Higher voice quality (less noise, less distortion)
Added features
Digital
Communications
Analog
Communications
PSTN
Review
PSTN
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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
PSTN
Review
PSTN
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The Present PSTN
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 linksT1/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)
PSTN Network
core
backbone
PSTN
Review
PSTN
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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
PSTN
VoP course
PSTN
Review
capabilities negotiation
mutual identification
end of page/document
modem recognition
modem training
data compression
PSTN
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Digital Loop Carrier
Pushes the digital PSTN closer to customerAT&T SLC-40, SLC-96, Nortel DMS P-phone, pair-gain
TR-08 multiplex 96 lines on:Mode 1: 4 T1sMode 2: 2 T1s (2:1 concentration)
GR303/V5.1/V5.2 multiplex up to 2048 lines
CLASS
5 StreetcabinetUTP/coax/fiber
UTP
pedestal
TR-08Mode 1pair-gain:Replace 96 pairs with 5 T1s(one spare for span protection)
9610 = 86
TR-08Mode 2pair-gain:Replace 96 pairs with 2 T1s(without span protection)
964 = 92
Access Network
CPEFTTB/FTTC
PSTN
Review