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Forms of Communication

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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

Telephone on Internet (VOIP)

• Voice messaging using IP packets– Session layer setup and closing of “call”– Voice is digitized using CODEC– Encoded and packaged into frames– Encryption can be arranged (SRTP)– Transmitted over packet network (e.g. Internet)

• Uses Internet transport (IP)

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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

ATM

• Asynchronous Transfer Mode– Bakbone of telephone system– Data in packets (48-bit cells)– Uses time-division multiplexing

• 8000 samples/second (CODEC)

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DSL

• Digital transmission over phone lines

• Uses frequency channels above phone

• Channel filters separate Up/Down bands– Uses DSL Modem (DSLM) for this

• Speed depends on distance to TelCo station– 256 kb/s to 24 Mb/s data rate

• Becoming obsolete due to fiber and cable

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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 = 2 (binary signals), (Log2[L] = 1)

B = 3000 Hz (300 – 3300 Hz)

C = 2*3000*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– Why? (Anti-Aliasing)

• 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– Fiber attenuation “windows”– Laser limitations (separation practicality)

• 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 (intermodulation)

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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 many 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