Optical Fiber Basic.ppt
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Public Switched
Telephone Network (PSTN)
BY
M C KOLADIYA
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The PSTNArchitecture
PSTNPublic Switched Telephone Network
Uses digital trunks between Central Office switches (CO) Uses analog line from phones to CO
Digital Trunks
Analog line
Central
Office
(CO)
Analog DigitalAnalog
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The PSTNDigitization
Voice frequency is 100 - 5000 Hz, with the main portionfrom 3003400 Hz
Nyquist Theorem states that sampling must be done at
twice the highest frequency to recreate. 4000 Hz was
chosen as the maximum frequency, thus sampling at8000 Hz
PCM = 8kHz * 8 bits per sample = 64 kbit/s
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Public Switched Telephone Network
Three major components of PSTN:
Local loops
Trunks
Switching Offices
Local CO
408-527-xxxx
22,000 in US
1-10 Km
CAT 3
Tandem
(class 4)
Mostly fiber or
microwave running
ATM or IP
(MCI, Sprint, ATT)
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PSTN
A: Switching types
Connectionless/ connection oriented
Packet/circuit
B: PSTN exchanges and interfaces interface Q.512
using access and trunk networks
signaling
network mana ement
Node 1 Node 2
Node 3
Access
Trunk Network
TerminalsTerminals
Access
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Switching in public networks
CSPDN: Circuit switched public data net*
PSPDN: Packet switched public data net**
DQDB: Distributed queue dual bus
* Used by European Telecoms that use X.21 in circuit switched nets**Used by British Telecoms Packet-switched Service (PSS), Data Pac (Canada) ...
(fixed
length)
Cel l sw i tch ing- resembles packet switching
- difference: cells (packets) have a fixed size :
offers bounded delay guarantees
(QoS compatible, long packets wont stuck cells)
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Circuit switching
Time swi tch
- Makes switching between time slots
- In the figure incoming slot 3 is moved to
outgoing slot 3 for one voice direction
- Each coming timeslot stored in Speech Store (SS)
- Control store (CS) determines the order the slot
are read from SS
- The info in CS is determined during setup
phase of the call
Space swi tch
- makes switching between PCM lines
- works with electronic gates controlled by CS Cross-pointcontrolled
by CS
Circu i t swi tch ing
- dedicated path
- constant delay/bandwidth
- voice/data
- paid by time
- examples: PSTN, VPN
TDMA
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Time Division Multiplexing (E1)
The T1 frame is composed of 230 channelsThe frame is 193 bits = 1 framing bit + 8 bits * 32 Time Slots (TS)
Framing bit creates an additional channel of 8 kb/s
Frame rate is 8 KHz, each channel is 8K * 7 bits = 56 Kbps
The extra bit is used for CAS (Channel Associated Signaling)
E1 32 channels, 2.048 MHz = 32 ch * 8K * 8 bits/ch, CCS
PCM voice
or data
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Packet switching
Node structure
Packet structureSeq: sequence number
Op code: message/control
identifier
CRC: Cyclic Redundancy
Code
Note:
- source address
required for retransmission
in ARQ
- byte count could be also an
end flag
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Connection-oriented and connectionless
switching
Connect ion o r iented
- Applies same route
- QoS well defined
- Phases
- Connection setup
- Data transmission
- Release
- Packets received in same order
- Example: ATM, frame relay, PCM
Connect ion less
- Use of different routes foreach packet possible
- Each packet have address fields
- QoS not guaranteed
- Packets may come in different order
- Example: IP (Internet Protocol),
TCP takes care of cleaning the mess
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Transfer modes & connections
summarized
Circuit switching
Packet switching
Connection oriented
Connectionless
- hand-shaking
- strict error requirements
- for fast data transfer
- broadcasting- modest error rates
often accepted
- fast data in good channels
Transfer modes Connection types
- developed for data- nowadays also for voice
- Statistical multiplexing
- variable delays
- developed for voice
- nowadays also for data
- well-specified delays
- echo problems
PSTN
ISDN
PCM
IP, Frame-relay
ATM
X.25
ATM
Frame-relay
IP, UDP*
*User Datagram Protocol
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Packet Switching
A comparison of circuit switched and packet-switched
networks.
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The Local Loop (Last Mile)Modems, ADSL, and Wireless
Use both analog and digital transmissions (E1)Conversion is done by the modems and codecs.
Transmission problems :Attenuation (loss of energy), delay distortion (propagation speed vs freq), noise
(white noise, crosstalk, impulse noise)
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Integrated Services Digital Network(ISDN)
Telephone
network
Digital data
network
Facsimile
network
Telex
network
Services and network integrations
ISDNDigital technologiesComputer control
technologies
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I SDN Basic Rate I nter face (BRI )
B
B
D
64 kbit/s
64 kbit/s
16 kbit/s
Basic rate interface
(2B+D)
144 kbit/s
B ch for information transfer
D ch for signaling transfer
2B+D
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Primary Rate I nterface(PRI)
D
B
B
B
30B+D
64 kbit/s
64 kbit/s
64 kbit/s
64 kbit/s
Primary
Rate
Interface
(2.048 Mbit/s)30 B channels
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Broadband ISDN
Todays N-ISDN
Circuit switched
network
Packet switched network
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To be integrated by B-ISDN
ATM and
OpticalFiber
Network
Broadband ISDN (continued)
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ATM (Asynchronous Transfer Mode)
Information(48 Bytes)
Header(5Bytes)
ATM Cell Structure
Incoming data is chopped every 48 bytes, added a header thenan ATM cell is formed. Transmission and switching are
conducted by cell basis.
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STM/ATM
# 1
# 4
# 3
# 2STM
MUX
ATM
MUX
# 1
# 4
# 3
# 2
STM
MUX
STM: Synchronous Transfer Mode
E l f ll it hi
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Example of cell switching:
Distributed queue dual buss (DQDB)
LAN
access
unit
accessunit
accessunit
LAN
accessunit
Function
- transport unitsconstant length
- access unitsaccess
info in the ring:
- know subscribers in access units
subnets and route packets for them
- access protocol applies token ringProperties
- distributed switching (Compare
to FDDI*)
- ATM compatible
- rates: 64 kb/s ... 45 Mb/s
- geographical limit up to 200 km
*FDDI: Fiber Distributed Data Interface
see: http://www.iol.unh.edu/training/fddi/htmls/
Transport Unit (same as in ATM)
C ti i t PSTN h
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Connecting into PSTN exchange:
Equipment in the access network
ISDN connection
example: 30B+D
(2.048 Mb/s)
Twisted pair - connection
Private Branch Exchange
Multiplexer
On-line subscriber
with several telephones
Business
subscriber
Wireless
access
(or radio access
point)
Distribution point
Cross connection point
ISDN 2B+D
144 kb/s
Q.512 specifiesexchange interfaces
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Switch
Local exchange
Subscriberstage
Groupswitch
ETC
Announcementequipment
Signalingequipment
Test/measure-ment equipment
Control system
to other exchanges
Third-partyequipment
PBX
ETC: Exchange terminal circuit
IN: Intelligent network
conference calls, call waiting,
broadcasting ...
Signaling (SS7) with users and
other exchanges
Traffic concentration- Operation & maintenance support (Q.513)- Charging
- Supplementary (IN) services
- Subscriber data, switch control Control
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Subscriber stage
To
ETC
ETC: Exchange terminal circuit
Speech store: shift registers storing bits for time switching
Control store: gates guiding speech store switches
Connects to: digit receivers, info
tones, test equipment
Control System: subscriberauthentication, routing, billing, O & M, ...
Concentrator
internet access
(DSLAM)
centrex serviceMUX
E h t l f ti
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Exchange control functions
Maintenancefunctions
supervision of subscriber lines and trunk circuits Operationalfunctions
adminis t rat ivedata as
subscriber database
routing database
stat ist icaldata as
from where and whom subscribers call
holding times for different equipment types
utilization of IN services
User servicesSample of IN services
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Exchange user services (examples)
Absent-subscriberservices as the answering
machine
Call booking: connection at the desired time
Person-to-person call: ensures that call goes to aright person
Serial call: setting up several calls
Telephone conferencing: several persons
participate to call in real-time (compare: tele-conferencing)
Directory inquiries: also speech recognition,
recorded messages
(many of these nowadays available in terminals)
Th it h
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The space-switch(used as a cross-switch and concentrator)
Number of cross-connections reduced compared if a simple
space division matrix of NxM (input x output) would be used
Usually performs concentration: Blocking possible
Same signal can be routed via different paths: increased
reliability
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The time-switch
One of the time slots of any full-duplexlines is connected tosome other line (at a time)
Thus two switches / time slot connect a line
For 100 full-duplex lines at 19.6 kbps a 1.92 Mbps bus is thusrequired for no blocking
If no fixed assignment of input lines to time slot but ondemand allocation -> blocking switch that reduces number ofswitches and switch clock frequency. For instance 200 lines of19.6 kbps with bus of 1.92 Mbps-> about half of the devices can connect at any time, egconcentration is 2:1
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The time-space-time (TST) switch
Works in local exchange and subscriber stage Performs PCM concentration, usually 10:1 3:1
Connects subscribers also to information tones
and test equipment
Time switch contains one bus for incoming and outgoingcalls (full-duplex)
Subscriberstage
Space switch
Time switch
Time switch
PSTN ISDN e change interfaces
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CN
CN:ConcentratorET:Exchange T.LT:Line T.AN:Access Net.
LT
NT ET
X
ET
ET
ET
LT
LTLT
ETLT
ETLTISDN PABX
ETLTAN
V1
V2
V3
V5
A
B
NT:NetworkT. (in ISDN)
T:Terminal
PSTN ISDN exchange interfaces
Q.512)
Peek to Q-recommendations
V4
http://localhost/var/www/apps/conversion/tmp/scratch_2/Q.512.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_2/Q.512.pdf -
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Exchange interfaces and tasks, V1
Purpose of exchange is to organizes connectionbetween exchange terminators!
V1: Access to basic ISDN (This is users ISDN-u
interface that can be used to connect small PBX also)
Basic ISDN V1-functions: 2 B + D (2x64 kbps + 16 kbps) channeling structure
timing and frame synchronization
activate and deactivate terminator
operation and maintenance feeding power supply
ISDN basic access parameters defined in G.961
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Exchange interfaces and tasks, V2-V4
V2: Interface serves typically concentrators 2048 kbit/s eg
30 B + D
Electrical standard G.704
V3: Resembles V2 but intended for interface otherexchanges (PABX)
Electrical standard G.703
30 B + D at 2048 kb/s
also 23 B +D at 1544 kb/s (I.431)
V4:Interface to private networks (not ITU-T specified), for
instance DSLAM (ADSL-interface)
http://localhost/var/www/apps/conversion/tmp/scratch_2/G.704%20Synchronous%20frame%20structures%20used%20at%201544,%206312%20%202048%20%208448%20and%2044%20736%20kbit%20s%20hierarchical%20levels.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_2/G.704%20Synchronous%20frame%20structures%20used%20at%201544,%206312%20%202048%20%208448%20and%2044%20736%20kbit%20s%20hierarchical%20levels.pdf -
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Exchange interfaces and tasks, V5
Between access network and exchange 2048 kbit/s
Specifies basic interfaces for
Analog access
ISDN-access
Electrical interface G.703
Channel control and signaling
V5 supports interface rates 2048 kbit/s
8448 kbit/s
C ti th l l l
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Connecting the local loop:
Line interface circuit (LIC)
Used for sign al ing in certaincoin -operated pay-phones and PBX
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Line interface circuit components
Over-voltage protection
Test equipment to connect to monitor the line condition
faults
Voltage feed
ringing
telephone current supply
Detection of
hook stage, pulse generated, or dual-tone receiver The hybrid junction (2 wire - 4 wire interface)
An A/D converter (uses PCM techniques at 64 kbps)
The hybrid circuit
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The hybrid circuit
4-wire connection is used between
exchanges and 2-wire connections fromexchange to subscribers
Amplifier
Amplifier
Bridge Bridge
Two-wire
Exchange A Exhange B
Two-wire
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The hybrid-circuit
If the impedance Zbequals the line impedance noincoming voice (down right) leaks to outgoing voice (up right)
but the signal goes via the two wire connection on the left
To exchange
From exchange
Local loop
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The hybrid circuit summarized
The hybrid circuit transformstwo-wire connection into4-wire connection.
If the hybrid is unbalanced echowill result
Hybrid is balancedwhen no own voice is leaked into
own loudspeaker Hybrid unbalance can result from line impedance
changes due to weather conditions
Unbalance results echo
Echo cancellationcircuits are harmful in dataconnections
Now a days realized by operational amplifierbasedcircuitry that automatically monitors line impedancechanges
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Network echo suppressor
R: transmission gate, A: attenuator, L: logic circuit
When the signal is present on the receiving line thetransmitting line is cut-off
A kind of semi-duplex approach to solve the echo
problem
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Network echo canceller (NEC)
Signal echo is extracted and subtractedfrom the
received signal
More effective than echo suppressor. Often NEC and
NES are however both used
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PSTN operation and maintenance
Different alarm classes
Vital functions and circuits(as SS7 and group switch)
use secured paths andbackups
Procedures provided for: troubleshooting
fault diagnostics
hardware faults canbe isolated
Supervision is realized alsoby connecting maintenanceunits to the network
Important switches haveextensive backup equipment
A supervision plan bynetwork levels:
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Modern PSTN hierarchy
to international level
Regional transport level
Local transport level
Access transport level
Local exchange
Transit exchange
PSTN Hi h t
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PSTN Hierarchy cont.
Local(example, within a city)
Subscriber connections
Switching within the local exchange
Switching to other exchanges
Transit(county level, say between Tampere and
Helsinki)
Switching traffic between different geographical areas
within one country
International
Gateway-type traffic between
different countries
DWDM (Dense Wavelength Division Multiplexing) routes
Rates can follow SONET or SDH standard
Subscriber signaling for local calls
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Subscriber signaling for local calls
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Inter-exchange signaling
Channel associated signaling (CAS) as No.5, R1, R2
analog and digital connections
Modern ISDN exchanges apply SS7(digital), that is a
common channel signaling method (CSS) that is
discussed later in its own lecture
CASis divided into line and register signaling:
Linesignaling:
line state between the trunk-links as
answer, clear-forward*, clear-back
Registersignaling:
routing information as
B-number, A-category, B-status
*A-subscriber on-hook message transmitted to B exchange
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Inter exchange signaling (cont.)
Three categories of information is
transmitted:
setup, supervision clearing
servicerelated information as
forwarding, callback, charging
statuschange information
transmission network congestion neighborhood exchange congestion
E l f i t h i li
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Example of inter-exchange signaling
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Inter-exchange signaling (cont.)