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Telecommunications Chapter 6. Telecommunications. From Chapter 1: Data communications Telecommunications: Voice and Video Communications. Technical Elements of the Public Switched Telephone Network. Figure 6-1: Elements of the Public Switched Telephone Network (PSTN). 1. Customer Premises - PowerPoint PPT Presentation
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Telecommunications
Chapter 6
6-2
Telecommunications
• From Chapter 1:
– Data communications
– Telecommunications: Voice and Video Communications
Technical Elements of the Public Switched Telephone Network
6-4
Figure 6-1: Elements of the Public Switched Telephone Network (PSTN)
1. Customer PremisesEquipment
1. Customer Premises Equipment
6-5
Figure 6-2: Customer Premises Equipment
PSTN
PBX
Handset
4-Pair UTPTelephone Wiring
Site
A typical business site.
The private branch exchange is an internal switch for the site.
4-pair UTP was created for business premises telephone wiring
Company is essentially its own telephone company that connects to the outside PSTN
6-6
Figure 6-1: Elements of the Public Switched Telephone Network (PSTN)
2. & 3. End OfficeSwitch (Class 5)
2.Access Line(Local Loop)
2.Access Line(Local Loop)
The Access System consists ofthe access line to the customer
(called the local loop)and termination equipment at the end office
(nearest telephone office switch).
6-7
Figure 6-1: Elements of the Public Switched Telephone Network (PSTN)
3. Transport Core
3. TrunkLine
3.Switch
The Transport Core connects end officeswitches and core switches.
Trunk lines connect switches.
6-8
Figure 6-1: Elements of the PSTN
• Telephone Company Switch
6-9
Figure 6-1: Elements of the Public Switched Telephone Network (PSTN)
4. Signaling System
Transport is the actual transmission of voice.
Signaling is the control of calling(setup, teardown, billing, etc.).
SS7 in the United StatesC7 in Europe
6-10
Figure 6-3: Points of Presence (POPs)
In the U.S., competingcarriers connect at
points of presence (POPs).
Local Access and Transport Area (LATA)
LocalCarrier 1Switch
LocalCarrier 2Switch
POP
LocalCarrier 1Customer
LocalCarrier 2Customer
Other Local Area
Other Country
POPLong-Distance
Carrier A
InternationalCarrier X
6-11
Figure 6-4: Circuit Switching
A circuit is an end-to-endconnection between two subscribers.
Capacity is reserved on alltrunk lines and switches along the way.
Capacity must be paid for even if it is not used.
The PSTNhas traditionally used
circuit switching.
6-12
Figure 6-5: Voice and Data Traffic
Full-Duplex (Two-Way) Circuit
Voice Traffic:Fairly Constant Use;Circuit Switching Is
Fairly Efficient
Data Traffic:Short Bursts,
Long Silences;Circuit Switching Is
Inefficient
Full-Duplex (Two-Way) Circuit
The reserved capacity of circuit switchingis OK for voice, but not for bursty data transmission.
6-13
Figure 6-6: Dial-Up Circuits Versus Leased Line Circuits
Dial-Up Circuits Leased Line Circuits
Operation Dial-Up. Separatecircuit for each call.
Permanent circuit,always on.
Speed for CarryingData
Up to 56 kbpsResidence can onlySend up to 33.6 kbps
56 kbps to gigabitspeeds
Number of VoiceCalls Multiplexed
One Several due to multiplexing
There are two types of circuits between customer premises:ordinary dial-up circuits and leased line circuits.
6-14
Figure 6-7: Local Loop Technologies
Technology Use Status
1-Pair Voice-GradeUTP
Residences Already installed
2-Pair Data-GradeUTP
Businesses forLowest-speedaccess lines
Must be pulled to thecustomer premises(this is expensive)
Optical Fiber Businesses forhigher-speedaccess lines
Must be pulled to thecustomer premises(this is expensive)
Residential 1-pair voice-grade UTP is already installed.This makes it inexpensive to use
Business 2-pair data-grade UTP and fiber for leased linesmust be installed; this is expensive.
6-15
Figure 6-8: Analog Telephone Transmission
SoundWave
Analog(Analogous)
Electrical Signal
Analog signals rise and fall in intensity with the human voice.No resistance to errors as there is in digital transmission.
Initially, the entire PSTN was analog.
6-16
Figure 6-9: The PSTN: Mostly Digital with Analog Local Loops
Trunk Line(Digital)
LocalLoop
(Analog)
LocalLoop
(Digital)
PBX(Digital)
ResidentialTelephone(Analog)
Today's Telephone Network: Predominantly Digital
Switch(Digital)
Switch(Digital)
Switch(Digital)
Today, everything is digital except for thelocal loop access line and residential telephones.
The actual local loop line can carry either analog or digital signals,but the equipment at both ends is analog.
6-17
Figure 6-10: Codec at the End Office Switch
Analog Signal
Local LoopTelephone
Home
Codec
DAC Digital Switch
ADC
End Office
DigitalSignal
A codec at the end office translates betweenresidential analog and PSTN digital signaling.
ADC = analog to digital conversionDAC = digital to analog conversion
6-18
Figure 6-11: Frequency Division Multiplexing (FDM) in Microwave Transmission
Channel 1 / Circuit A
Channel 2 / Circuit D
Channel 3 / Circuit C
Channel 4 / Unused
Channel 5 / Circuit E
Each circuit is sent in a separate channel.If channel bandwidth is large,there will be fewer channels.
Voice uses 4 kHz-wide channelsto allow more channels.
Microwave usesradio transmission
for PSTN trunk lines
Box:Codec Operation
6-19
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Subscriber
Analog ElectricalSignal
Analog VoiceSignal
Filter atEnd Office Switch
Step 1: Bandpass Filtering
At the end office, the voice signal is bandpass-filteredto limit its bandwidth to 4 MHz.
This permits more calls to be multiplexed on trunk lines
Filter atEnd Office Switch
Box:Codec
Operation
6-20
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
0 Hz
Signal
300 Hz 3,400 Hz (3.4 kHz) 20 kHz
FrequencyBandwidth (3.1 kHz)
Energy Distribution ofHuman Speech Along theFrequency Spectrum
Step 1: Bandpass Filtering
Actually, to provide a safety margin, the signalis filtered to between about 300 Hz and 3.4 kHz
instead of from 0 Hz to 4 kHz.
Box:Codec Operation
6-21
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
SignalAmplitude
Time
AnalogSignal
Sample
Intensity of Sample(125/255 or 01111101)
Durationof Sample
(1/8000 sec.)
0
255 (maximum)
Step 2: Pulse Code Modulation (PCM) Sampling
Nyquist found that signals must besampled at twice their highest frequency.
For a top frequency of 4 kHz,there must be 8,000 samples per second.
Each sample is 1/8000 second.
Box:Codec Operation
6-22
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
SignalAmplitude
Time
AnalogSignal
Sample
Intensity of Sample(125/255 or 01111101)
Durationof Sample
(1/8000 sec.)
0
255 (maximum)
Step 2: Pulse Code Modulation (PCM) Sampling
In each samplingperiod, the intensity
of the signal ismeasured.
In pulse codemodulation, the
signal is measuredas one of 256
intensity levels.
One byte storesone sample.
Box:Codec Operation
6-23
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
SignalAmplitude
Time
AnalogSignal
Sample
Intensity of Sample(125/255 or 01111101)
Durationof Sample
(1/8000 sec.)
0
255 (maximum)
Step 2: Pulse Code Modulation (PCM) Sampling
Pulse CodeModulation (PCM)
produces8,000 one-byte
samples per second.
This is 64 kbpsof data.
Box:Codec Operation
6-24
ADC Recap
• First, Bandpass-Filter the Incoming Signal to 4 kHz
– Really about 300 Hz to 3.4 kHz
– To reduce transmission requirements
• The Codec then Uses PCM for the Conversion
– Samples at twice the highest frequency (4 kHz so 8,000 samples/second)
– Loudness is recorded with 8 bits per sample (to give 256 loudness levels)
– Generates 64 kbps of traffic (8 bits/sample times 8,000 samples per second)
Box:Codec Operation
6-25
Figure 6-13: Digital-to-Analog Conversion (DAC)
To Customer:Generated “analog” signal(Sounds smooth because
the sampling rateis very high)
From digital PSTN network:Arriving digital signalfrom the PSTN Core
(8,000 Samples/Second)
00000100 00000011 00000111
DACat EndOfficeSwitch
One 8-BitSample
One 8-BitSample
Box:Codec Operation
6-26
Figure 6-14: Cellular Telephony
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel47
Mobile Telephone Switching Office
Cellsite
PSTN
HandoffIn cellular technology, the regionis divided into smaller cells.
In each cell, a cellsite servescellphones in the cell.
6-27
Figure 6-14: Cellular Telephony
• Cellsites
6-28
Figure 6-14: Cellular Telephony
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel47
Mobile Telephone Switching Office
Cellsite
PSTN
Handoff
Channels can be reused in different cells.Channel reuse supports more customers.
This is the reason for using cells.(Channel 47 is reused in cells A, D, and F)
6-29
Figure 6-14: Cellular Telephony
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel47
Mobile Telephone Switching Office
Cellsite
PSTN
Handoff
When a subscriber moves from onecell to another in a cellular system,
this is called a handoff.
When a subscriber moves fromone city to another, this is roaming.
(In WLANs, handoffs and roamingmean the same thing.)
6-30
Figure 6-14: Cellular Telephony
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel47
Mobile Telephone Switching Office
Cellsite
PSTN
Handoff
The Mobile TelephoneSwitching Office (MTSO)
coordinates the cellsites andimplements signaling and handoffs.
The MTSO also connectscellphones to the PSTN
(called the wireline network).
6-31
Cellular Technologies
• GSM is the worldwide standard for cellular voice– Uses time division multiplexing (TDM)
– Uses 200 kHz channels
– Divides each second into many frame periods
– Divides each frame into 8 slots
– Gives same slot in each frame to a conversation
Slot 1Conversation
A
Slot 2Conversation
B
Slot 8Conversation
H
……Slot 1
ConversationA
Time Frame 1 Frame 2
6-32
Cellular Technologies
• Cannot use the same channel in adjacent cells
– So can only reuse a channel about every 7 cells
– For example, suppose there are 50 cells
• Channel can be reused 50 / 7 times
• This is 7 (not precise, so round things off)
• So each channel can support 7 simultaneous customers in these 7 cells
6-33
Cellular Technologies
• Code Division Multiple Access (CDMA)
– Also used in the United States
– A form of spread spectrum transmission
– Unlike traditional spread spectrum technology, multiple users can transmit simultaneously
– 1.25 MHz channels
– Can support many users per channel
• Can use the same channel in adjacent cells– So can only reuse a channel in every cell
6-34
Figure 6-15: Voice over IP (VoIP)
PC withMultimedia Hardwareand VoIP Software
IP Telephonewith
Codec andTCP/IP Functionality
MediaGateway
PSTN
Internet
VoIP carries telephone calls overLANs and the Internet
With IP, there is no wasted capacityas there is with circuit switching.
This reduces cost.
6-35
Figure 6-15: Voice over IP (VoIP)
PC withMultimedia Hardwareand VoIP Software
IP Telephonewith
Codec andTCP/IP Functionality
MediaGateway
PSTN
Internet
Stations can be special IP telephoneswith IP functionality
Or a PC with multimedia hardwareand VoIP software
IP phones need a codec to convertvoice analog signals from the microphone
into digital IP signals
6-36
Figure 6-15: Voice over IP (VoIP)
PC withMultimedia Hardwareand VoIP Software
IP Telephonewith
Codec andTCP/IP Functionality
MediaGateway
PSTN
Internet
A media gateway connects aVoIP network to the PSTN
Handles transport and signaling differences
6-37
Figure 6-16: Speech Codes
Codec Transmission Rate
G.711 64 kbps (pulse code modulation)
G.721 32 kbps (adaptive PCM)
G.722 46, 56, or 64 kbps
G.722.1 24, 32 kbps
G.723.1A 5.3, 6.3 kbps
There are several codec standards.They differ in transmission rate, sound quality, and latency.
Both sides must use the same codec standard.
6-38
Figure 6-17: VoIP Protocols
Transport is the transmission of voice(carries codec data).
Signaling is call supervision.
Signaling: SIP or H.323(Call setup, breakdown, accounting, and other supervisory tasks)
IPHdr
UDPHdr
RTPHdr
Codec DataStream
Transport(Voice transmission)PC with Multimedia and
VoIP SoftwareIP Telephone
VoIP Transport Packet
6-39
Figure 6-17: VoIP Protocols
Signaling: SIP or H.323(Call setup, breakdown, accounting, and other supervisory tasks)
IPHdr
UDPHdr
RTPHdr
Codec DataStream
Transport(Voice transmission)PC with Multimedia and
VoIP SoftwareIP Telephone
VoIP Transport Packet
2. The UDP header is followed by aReal Time Protocol (RTP) header, which contains
a sequence number and timing information.Receiver uses timing information to smooth out sound playback.
1. VoIP transport packets use UDP at the transport layer.(There is no time for retransmissions to repair errors.)
The receiver puts in fill sounds for lost packets.
3.The applicationmessage is acodec data
stream
6-40
Figure 6-17: VoIP Protocols
Signaling: SIP or H.323(Call setup, breakdown, accounting, and other supervisory tasks)
IPHdr
UDPHdr
RTPHdr
Codec DataStream
Transport(Voice transmission)PC with Multimedia and
VoIP SoftwareIP Telephone
VoIP Transport Packet
Signaling is call supervision.The H.323 signaling standard came first for VoIP signaling.
SIP is simpler and now dominates VoIP signaling
6-41
Video over IP
• The Other VoIP
– It’s not just voice over IP
– Video Telephones
– Video Conferencing
• PC to PC
• Multiparty
• Sometimes room-to-room
– Video Downloads on Demand
6-42
Figure 6-18: Residential Internet Access Services
• Telephone Modems
• Broadband Internet Access
• Asymmetric Digital Subscriber Line (ADSL)
• Cable Modem Service
• 3G Cellular Data Service
• WiMAX (802.16d and 802.16e)
• Broadband over Power Lines
• Fiber to the Home (FTTH)
Note:Speeds and Prices
Change Rapidly
6-43
Figure 6-19: Telephone Modem Connection to an ISP
Client A
TelephoneModem Telephone
PSTN (Digital)
33.6 kbpsDigital
AnalogAccess
Line
Analog
56 kbps
Telephone modemsconvert digital computer
signals to analogtelephone signals.
6-44
Figure 6-19: Telephone Modem Connection to an ISP
Digital Leased Line(No Modem)
ISP
PSTN (Digital)
56 kbpsDigital
33.6 kbps
ISP does not have a modem.
It has a digital leased line socan send at 56 kbps.
(There is no bandpassfiltering on digital leased lines.)
6-45
Figure 6-19: Telephone Modem Connection to an ISP
Client A
TelephoneModem
Digital Access Line(No Modem)
TelephoneISP
PSTN (Digital)
33.6 kbps56 kbps
DigitalAnalogAccess
Line
Digital
Analog
56 kbps
Circuit
Dial-up circuits connect the client with the ISP.56 kbps downstream, 33.6 kbps upstream
33.6 kbps
6-46
Telephone Modem Limitations
• Very low transmission speeds
– Long delays in downloading webpages
• Subscriber cannot simultaneously use the telephone line for voice calls
• Still used by 30% to 40% of Internet users.
6-47
Figure 6-20: Amplitude Modulation
Client A
Binary Data
Modem
Amplitude Modulation
Telephone
Modulated AnalogSignal
1 0 1 1
PSTNSerialCable
TelephoneCable
Modulation is the conversion of binary computer signalsinto analog signals that can travel over an ordinary access line.
Demodulation, at the other ends, converts the modulatedsignals back to digital computer signals.
6-48
Figure 6-20: Amplitude Modulation
Client A
Binary Data
Modem
Amplitude Modulation
Telephone
Modulated AnalogSignal
1 0 1 1
PSTNSerialCable
TelephoneCable
In amplitude modulation, there aretwo amplitude (loudness levels)—
one for 1 and one for 0
1011 is loud-soft-loud-loud
6-49
Figure 6-21: Asymmetric Digital Subscriber Line (ADSL)
PCADSL
Modem Single Pair ofVoice-GradeUTP Wires
Splitter DSLAM
Telephone
Telephone CompanyEnd Office Switch
SubscriberPremises
DataWAN
PSTN
ADSL ALSO uses the existing residential local loop technology.Inexpensive because no need to pull new wires, but
1-pair voice-grade UTP is not designed for high-speed transmission.
6-50
Figure 6-21: Asymmetric Digital Subscriber Line (ADSL)
PCADSL
Modem Single Pair ofVoice-GradeUTP Wires
Splitter DSLAM
Telephone
Telephone CompanyEnd Office Switch
SubscriberPremises
DataWAN
PSTN
1.Subscriber needs an ADSL modem.
Also needs a splitter for each telephone wall outlet.
2.Telephone carrier needs a digital subscriber line
access multiplexer (DSLAM) to separate the two signals.
6-51
Figure 6-21: Asymmetric Digital Subscriber Line (ADSL)
Unlike telephone modems, ADSL serviceprovides simultaneous voice and data transmission.
PCADSL
Modem
Ordinary TelephoneService
Single Pair ofVoice-GradeUTP Wires
Downstream DataUp to 1.5 Mbps
Upstream DataUp to 512 kbps
Splitter DSLAM
Telephone
Telephone CompanyEnd Office Switch
SubscriberPremises
DataWAN
PSTN
Downstream DataUp to 3 Mbps
6-52
Figure 6-21: Asymmetric Digital Subscriber Line (ADSL)
PCADSL
Modem
Ordinary TelephoneService
Single Pair ofVoice-GradeUTP Wires
Downstream DataUp to 1.5 Mbps
Upstream DataUp to 512 kbps
Splitter DSLAM
Telephone
Telephone CompanyEnd Office Switch
SubscriberPremises
DataWAN
PSTN
Downstream DataUp to 3 Mbps
Speed is asymmetricFaster downstream than upstream
(Up to 3 Mbps versus up to 512 kbps)Ideal for Web accessAcceptable for e-mail
Good for residential use
6-53
PCCable
Modem
Subscriber Premises
NeighborhoodSplitter Cable Television
Head End
OpticalFiber to
Neighborhoods
ISP
Coaxial Cablein Neighborhood
(Shared Throughput)
Coaxial CableDrop Cable
UTPor
USB
Maximum downloadthroughput is about 5 Mbps
Figure 6-22: Cable Modem Service
Cable modem service brings high-speedoptical fiber lines to the neighborhood.
6-54
PCCable
Modem
Subscriber Premises
NeighborhoodSplitter Cable Television
Head End
OpticalFiber to
Neighborhoods
ISP
Thick Coaxial Cablein Neighborhood
(Shared Throughput)
ThinCoaxial CableDrop Cable
UTPor
USB
Figure 6-22: Cable Modem Service
In the neighborhood,thick coaxial cablebrings service to
households.
This bandwidth isshared by
everyone in theneighborhood.
A thin coax linegoes to each
home’scable modem.
6-55
Figure 6-22: Cable Modem Service
PCCable
Modem
Subscriber Premises
NeighborhoodSplitter Cable Television
Head End
OpticalFiber to
Neighborhoods
ISP
Thick Coaxial Cablein Neighborhood
(Shared Throughput)
ThinCoaxial CableDrop Cable
UTPor
USB
Maximum downloadthroughput is about 5 Mbps
Downstream speeds up to 5 Mbps.Upstream speeds up to about 1 Mbps.
6-56
ADSL versus Cable Modem Service
• Do Not Over-Stress the Importance of Sharing– Cable modem service usually is still faster than ADSL
service
– DSLAM sharing can slow ADSL service too
• The Bottom Line Today:– Cable modem service usually is faster
– ADSL service usually is cheaper• ADSL offers more speed-price options
• Both are improving rapidly in terms of speed and (sometimes) price
6-57
Figure 6-23: Third-Generation (3G) Cellular Data Services
• Cellphone connects to computer via a cellphone modem or USB
• Traditional GSM and CDMA
– Limited to only about 10 kbps
– Far too slow for usability
6-58
Figure 6-23: Third-Generation (3G) Cellular Data Services
• Both GSM and CDMA are evolving
• Second Generation (now dominant)– Only 10 kbps data transmission
• Third Generation– Low end: comparable to telephone modem service
– High end: comparable to low-speed DSL service
• Future– Speeds comparable to high-end DSL or cable modem
service
– 100 Mbps or more (fast enough for good video)
6-59
Figure 6-18: Residential Internet Access Services
• WiMax (802.16)
– Wireless Internet access for metropolitan areas
– Basic 802.16d standard: ADSL speeds to fixed locations
• Will use dish antennas
• Just reaching the market
– 802.16e will extend the service to mobile users
• Will use omnidirectional antennas
6-60
Figure 6-18: Residential Internet Access Services
• Satellite InternetAccess
– Very expensive
– Often needed to serve rural areas
New
6-61
Figure 6-18: Residential Internet Access Services
• Broadband over Power Lines
– Broadband data from your electrical company
– It already has transmission wires and access to residences and businesses
– It can modulates data signals over electrical power lines
– It works, but has very limited availability and is slow
– Especially promising for rural areas
6-62
Figure 6-18: Residential Internet Access Services
• Fiber to the Home (FTTH)
– Carrier runs fiber to the home
– Provides speeds of tens of megabits per second for high-speed video, etc.
• Less if fiber only goes to the curb (FTTC)
• Or to the neighborhood (FTTN)
– Much faster than other residential internet access services
– Could dominate residential (and business) Internet access in the future
6-63
Internet Access and VoIP
• Most ISPs are Planning to or Already Provide VoIP Telephone Service
– An alternative to the local telephone company service
– Media gateways will interconnect with the PSTN
– Should be less expensive that traditional phone service
– Questions remain• Voice quality and reliability• 911 and 911 location discovery• Regulation and taxation• Laws that require wiretapping with warrants
Topics Covered
6-65
Telecommunications
• Data Communications versus Telecommunications
• The PSTN’s Technical Elements
– Customer premises equipment (PBX and 4-pair UTP)
– Access system (local loop)
– Transport core
– Signaling (call setup and management)
• POP to interconnect carriers
6-66
Telecommunications
• Access Lines
– For residences, 1-pair voice-grade UTP
• DSL uses existing residential access lines to carry data by changing the electronics at each end (DSL modem in the home and DSLAM at the end office switch)
• DSL is cheap because 1-p VG UTP is already in place
– For businesses,
• 2-pair data-grade UTP for speeds up to a few Mbps
• Optical fiber for faster speeds
• Usually must be pulled into place, so expensive
– Eventually, fiber to the home (FTTH), FTTC, FTTN
6-67
PSTN Transmission
• Circuit Switching
– Reserved capacity end-to-end
– Acceptable for voice, but not for bursty data transmission
– Dial-up and leased line circuits
• Analog and Digital Transmission
– Analog signals on the local loop
– ADC and DAC at the end office switch
– ADC: bandpass filtering and sampling for 64 kbps
– DAC: sample values are converted to sound levels
6-68
Cellular Telephony
• Cells Allow Channel Reuse
– Channel reuse allows more customers to be served with a limited number of channels
• GSM: most widely used technology for cellular telephony
• CDMA for greater channel reuse
• Handoffs and Roaming
6-69
VoIP
• To allow voice to be carried over data networks
• Converge voice and data networks
• Phone needs a codec
• Transport: UDP header followed by RTP header
• Signaling: H.323 and SIP
• Video over IP
6-70
Residential Internet Access Services
• Telephone Modems
• Asymmetric Digital Subscriber Line (ADSL)
• Cable Modem Service
• 3G Cellular Data Service
• WiMAX (802.16 and 802.16e)
• Broadband Over Power Lines
• Fiber to the Home (FTTH)
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