9/9/2015© 2010 raymond p. jefferis iiilect 02 - 1 telephone communications
TRANSCRIPT
04/21/23 © 2010 Raymond P. Jefferis III Lect 02 - 1
Telephone Communications
04/21/23 © 2010 Raymond P. Jefferis III Lect 02 - 2
Internet Communications
• Largely carried on telephone network
• Lately quite a bit of privately owned fiber– communication carriers– electric companies– private organizations
• Some carried over television cables
• New conveyance by wireless providers
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Present Telephone Conveyance
• ATM backbone (long distance)
• ATM switching at core
• T1 data lines to businesses (1.544 Mb/s)
• Copper “last mile”– voice grade lines– DSL possible using residual bandwidth outside
the voice channel - has distance limitations
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Terms
• ATM - Asynchronous Transfer Mode
• DSL - Digital Subscriber Line
• DSLM - DSL Modem
• DSLAM - DSL Access Module
• POTS - Plain Ordinary Telephone System
• SONET - Synchronous Optical NETwork
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TelCo Network Interface
ATM
ATM
ATM
CS
DSLAM
R R
S
S
S
...
CoreSwitching
EdgeSwitching
Access(Local Loop)
OC 192 Backbone T1
Copper
Copper
DSLM
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Residential Data Lines• POTS line with modem
– 56 kb/s– rate depends on line quality
• DSL line with splitter/modem*– Advanced, up to 1.5M / 128K $59.95/month– Premium, up to 384K / 384K $69.95/month– Professional, up to 1.5M / 384K$79.95/month
-------------------------------------------------------------* Speed depends on distance to TelCo office
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Digital Data Transmission
• Binary data
• Transmitted as pulses
• Pulses shaped by line bandwidth
• Pulses have high frequency components
• Limiting bandwidth limits data rate
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Fourier Series of Bandlimited Pulse
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Fourier Transform of Pulse
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Notes:
• First crossover of spectral amplitude is at B
• 2B is effective bandwidth needed to transmit through noiseless channel.
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Nyquist Bit Rate
• Relates three transmission variables:– Channel Capacity (C)– Bandwidth (B)– Signal levels (L) – quantization levels– Formula:
C = 2Blog2[L]
• Noise-free channel assumed
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Nyquist Example:
• Formula: 2BLog2[L]
• Example:
L = 1 (binary signals)
B = 3000 Hz (300 – 3300 Hz)
C = 2*6000*1 = 6000 [bits/s]
Note: Applies to noise-free channel only
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Shannon Formula
• C = B log2(1 + SNR)
• Example:– B = 3000 Hz (300 – 3300 Hz)– SNR = 3163 (35 dB power ratio)– C = 34,882 [bits/s]
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All Digital Telephony
• Voice-to-Digital conversion coding at transmitter
• Digital transmission
• Digital-to-Voice code conversion at receiver
• Conversions performed by COder-DECoder (CODEC) module at each end of line
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Voice-Data Conversion
CODEC CODECVoice Data Voice
(8 bits)
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Digital Data Frame on T1 Line
• Voice lines are low-pass filtered to 3.1 KHz
• CODEC output is 8 bits wide
• Sampling rate is about 8000 samples/s
• Data rate is thus about 64000 bits/sec
• 24 lines carried on T1 link (1.544 Mb/s)
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T1 Time-Division Multiplexing• Data frame starts with framing bit
• Data samples (8 bits each)
• 24 lines each supply a data sample every 125 microseconds (0.000125 sec)
• Samples are sequentially multiplexed
• 193 bits per data frame
• 1.544 Mb/s total data rate
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Time Division Multiplexing
Multiplexer
Line 1
Line 2
Line n
...
Line 1
Line 2
Line n
...
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Time Division De-Multiplexing
De-Multiplexer
Line 1
Line 2
Line n
...
Line 1
Line 2
Line n
...
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De-Multiplexing
• Data samples are redistributed into lines
• Low-pass filter recovers analog voice
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High-Speed Backbone
• 28 T1 streams merged to T3 stream
• ATM cells repackage data at core– 53 octets/cell
• 5 octets of header information
• 48 octets of data
• SONET frames– 8 x 810 = 6480 bits sent 8000 times per second– 51.85 Mb/s data rate (some frame overhead)
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ATM Cell Transmission
Time
ATM Cell ATM Cell ... ATM Cell
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TelCo Standard Data Rates
• T1 1.544 Mb/s (24 voice circuits)• T3 44.736 Mb/s (672 voice circuits)• OC-3 155.52 Mb/s (2430 voice circuits)• OC-12 622.08 Mb/s (9720 voice circuits) • OC-48 2488.32 Mb/s (19440 voice circuits) • OC-192 9953.28 Mb/s (38880 voice circuits)
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Future Network Interface
l 1
l n
l -R
l -R R
S
S
S
...
CoreSwitching
EdgeSwitching
Access
l - BACKBONE
Fiber
Fiber
Copper
Copper...
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- Network
• Backbone fiber combines 16 OC-192 lines
• Each is given a different wavelength
• All data streams merged into single fiber
• Streams split by wavelength
• 16 OC-192 lines out
• Switches to TelCo customers
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Optical-Data Conversion
LASER DIODECopper Fiber Copper
()
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Optical Data Conversion
• Data on wire drives tunable laser
• Laser emits photon pulses
• Photons propagate down fiber
• Photon energy activates receiving diode
• Diode produces voltage or current
• Amplifier drives wire
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Transmission Problems
• Photons lost at fiber coupling
• Photons lost in fiber due to scattering
• Photons per pulse deteriorates with length
• Repeater amplifiers needed
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Optical-Data Link
LASER DIODECopper
FiberCopper
()Repeater
Fiber
()
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More Problems
• Repeaters require external power
• Photons need conversion to voltage, amplification, and then reconversion to photons - data rate bottleneck!
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Repeater for Fiber Optic Line
Diode LaserFiber
CopperFiber
()Amplifier
Copper
()
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Wave Division Multiplexing
LaserLine 1
Line 2
Line n
...
Laser
Laser
WDM Fiber
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Wave Division Multiplexing
• Incoming data converted to photons
• Photon streams have individual frequencies
• Streams can be merged onto single fiber
• Streams propagate without interference
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WDM Problems
• Limited number of “colors” of photons
• Repeaters must work on all “colors”
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Repeaters for WDM
• Must amplify all photon “colors”
• Must not cause interaction between photon streams
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Pumped Laser Repeater
Pumped LaserFiber Fiber
() ()
Pump
Photon Energy
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Pumped Laser Repeater
• Rare-earth doped glass
• Pumped by external light
• Photons receive excitation and are amplified
• Amplification of all photon “colors”
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WDM De-multiplexing
• Photon stream split by “colors”
• Separated streams may be converted to voltage pulses
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Wave Division De-Multiplexing
Line 1
Line 2
Line n
...WDM Fiber
DIODE
DIODE
DIODE
1
2
n
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WDM Routing
• Data streams must be switched
• Ideally this should be optical
• Optical switching of SONET frames?
• Electro-optics?
• State-of-the-art - developments taking place rapidly (Lucent, Nortel, others)