Fundamentals of Optical Networking

Mark E. Allen, Ph.D.

mark.allen@ieee.org

Agenda

• Part I: Component overview– Wavelength division multiplexing– Filter technologies– Amplifiers– Fiber and switch technologies

• Part II: Design considerations– Span design– Restorability– Cost optimization in the metro and wide area– Wavelength routing

SONET and Optical Communications

Digital data transmission

• All forms of information will soon be carried on an optical infrastructure

MPEG II

Internet

MP3

Images

OpticalNetwork

Voice

Transmitter

InformationCoding Modulator

Timing source

CommunicationsMedium

•Voice•Video •Data

Bits

Voice over IPMPEG IIEthernet

ATMPacket over SONET

SONET

CopperCoax cableFiber opticsFree space

Carrier: RF, Laser, etc.

Receiver

Medium

Demodulator Decoding

Timing information

Bits Information

Representing bits: NRZ vs. RZ

• RZ pulse have better timing information and dispersion tolerance, but are more complicated to process

Return to Zero (RZ) Pulse stream

Non Return to Zero (NRZ) Pulse stream

1 1 1 10

1 1 1 10

Modulation: FSK

• FSK – Frequency shift keying. Different carrier frequencies represent different data symbols.

"ONE" "ONE" "ONE""ZERO" "ZERO"

Modulation: PSK

• PSK – Phase shift keying. Different phases of the carrier represent different data symbols.

"ONE" "ONE" "ONE""ZERO" "ZERO"

Modulation: ASK

• ASK – Amplitude shift keying. Different amplitudes of the carrier represent different data symbols. This is the most common technique for modulating a laser source.

"ONE" "ONE" "ONE""ZERO" "ZERO"

Examples of digital signals

• 10/100 Ethernet

• Gigabit Ethernet

• FDDI

• T1/DS3

• SONET/SDH– OC3 (STM1), OC12(STM4), OC48

(STM16), OC192 (STM64)

Phase diagrams

• Phase diagrams show the phase and amplitude for different symbols

O

90

180

270

O

90

180

270

ASK PSK

Modulation bandwidth

Freq

Power

carrier

Unmodulatedcarrier

Power

carrier Freq

Modulatedcarrier

width = 2X bit rateFor ASK modulated signals,

bandwidth is usually more than twice the bandwidth.

i.e. 10Gbps would occupy more than 20GHz

Optical Fiber

• Single mode

• Multimode

• Attenuation characteristics– Definition of dB– Power in dBm– Loss vs. wavelength– Wavelength vs. frequency

Optical fiber

core

cladding buffer coating

Optical source

• Typical low cost optical transmitter– 850nm or 1310 nm – Modest power –5 to -10dBm (how many

milliwatts is this?)– Uses a laser diode– The current level is modulated to create

ASK “on-off” light signal for 1’s and 0’s

Higher quality source(more $)

• May use 1550nm wavelength or “ITU” optics (15XX where exact wavelength is specified)– ITU optics makes it WDM capable– High power ~ 0dBm for 100km + reach

• Laser diode with external modulator for cleaner pulses (faster speeds)

• 10Gbps bit rate capable• $10K or more for transmitter

Detector

• Detectors are typically semiconductor based photodiodes– Generate current based on detecting photons– Low-cost :: PIN Diodes– Higher cost : Avalanche Photodiodes (APD)

• Include some amplification within the detector based on the Avalanche process

• Cost, reach and speed are all considerations in receiver designs.

Single mode vs. multi-mode• Multimode fiber

allows light many possible paths down the fiber. Different paths have different distances.

• Single mode fiber has a small core and allows only one ‘mode.’

Varying delays in the path length can result in dispersion when the fiber is long and high bit rates are transmitted

Low-loss regions of fiber

0.1

0.2

0.3

0.4

0.5

1100 1300 1500 1700

Wavelength ()

1550window

Att

enu

atio

n (

dB

/km

)

1310 nm

1550 nm

Wavelength vs. frequency

• In the neighborhood of 1550 nm, 0.8nm is 100 GHz, 0.4nm is 50 GHz, etc.

c

f

2

Wavelength plans

• The ITU grid– Standard wavelength spaced 100 GHz

apart. 40 channels currently specified.

• WDM block diagram

Amp

FiberWDM Filters

SONET NE

Filter technologies

• Thin-film

• AWG

• Bragg-gratings

WDM Operation• Current technologies allow 50GHz (.4nm) spacing• Dielectric thin-film• Array wave guide (AWG)• Bragg grating

Thin film operation

Array waveguide

Bragg grating

opticalcirculator

Bragg grating

Port 1Port 3

Port 2

passes through the Bragg grating, but and are reflected by it.

Wavelength Division Multiplexing (WDM)

Economics of long-haul WDM: Amplifiers replace regenerators

Terminal TerminalConventional Networks

37 km

Terminal Terminal

100 km

Optically Amplified 4 x 25dB

Number of spans Loss per span

1310nm

1550 nm

WDM equipment savingsThe Optical to electronic compromise

•Reduce regeneration costs

•Reduce fiber costs

•Quicker turn-up time for new bandwidth

•TDM only•80 regens•8 fiber pairs

•WDM + TDM•3 amplifiers•1 fiber pair

Equipment savings with Optical Add/Drop

Dropped traffic

All trafficmust be regenerated

Dropped traffic

Pass-through trafficis all-optical

After

Optical Spectrum Analyzer (OSA) output

How much bandwidth in a fiber?

• The 1550 nm window has more than 10 THz of bandwidth.

• Current systems exploit less than 1% of this bandwidth.

Amplifiers

• Erbium doped fiber amplifiers (EDFAs)

• Extended band amplifiers

• Raman amplification

Erbium Doped Fiber Amplifier (EDFA)

• Pump source operates at 980 nm or 1480 nm– These wavelength are matched to characteristics of erbium– Stimulated emission occurs around 1530 nm– New photons at the same wavelength are created

Doped fiberPump source

Weak signal Amplified signal

Extended band amplification

(nm)

(THz)199.0

1505 1510 1570

192.0193.0194.0196.0 195.0

15551530 1535 1540 1545 1550 1560 1565

ITU Grid Reference Point (193.1THz)

C-Band OA Flat Gain Region

191.0 190.0 186.0

L-Band OA Flat Gain Region

1610

S-Band OA Flat Gain Region

ITUChannel 20

ITU Channel 60

Raman amplification

• Raman is a phenomenon where a fiber pumped at a certain wavelength exhibits gain 100 nm away. – Doesn’t require specially doped fiber

• Raman amplifiers can be made by pumping the fiber in the ground– Acts as a distributed amplifier compensating for loss

along the fiber– Normal EDFA is a lump source amplifier

• Effective noise figure for Raman can be lower than EDFAs

Fiber types

• Dispersion– Chromatic dispersion– Polarization mode dispersion (PMD)

• Dispersion management techniques– Lower bit rate– More frequent regeneration– Dispersion compensation– Advanced fiber types

What is dispersion?

• Dispersion causes pulses to be smeared together as they travel through the fiber.

1 1 1 10

1 1 1 10

Eye patterns and SNR

• Overlay plotting a 3 symbol sequence (randomly either 000,001, 010,… or 111) yields an ‘eye’ pattern.

• The eye pattern can be used to measure signal quality in terms of dispersion and SNR.

`

`

Two examples of eye patterns. The lower Figure has more dispersion and noise.

Single mode fiber (SMF) dispersion

Dispersion coefficient vs. wavelength

1012

1416

1820

22

1500 1520 1540 1560 1580 1600

wavelength (nm)

D p

s/(n

m*k

m)

Dispersion for DS fibers

1530 1540 1550 1560

+2

+4

- 2

- 4

LucentTrueWave

Corning LS

DSF

Dis

pers

ion

(ps/

nm -

km)

Corning LEAF

Characteristics for common fibers

Fiber type 0, (nm) S0

(ps/nm2*km)

D (ps/nm*km)

Comments

Corning SMF-28 1312 0.09 17 @ 1550 nm

Standard single mode fiber.

Corning SMF/DF 1535-1565 0.075 <=2.7 Dispersion shifted or dispersion compensated fiber.

Corning SMF/LS >=1560 0.08 -0.1>=D>=3.5 Lambda-shifted Non Zero Dispersion Shifted Fiber (NZDSF)

Lucent TrueWave 1518 0.08 1<D<5.5 NZDSF

Polarization mode dispersion (PMD)

• PMD is caused when different polarizations of the signal experience different amount of dispersion.

• PMD is most prominent when using older fiber that is not perfectly round.

• PMD is most common at 10 Gbps and above.• New PMD compensators are being

developed.

Optical time domain reflectometer (OTDR)

• OTDR plot shows where reflections occur– Location and loss of splices– Location of Fiber cuts– Overall span loss

Splice 1

Splice 2Cable end

Distance (km)

Lo

ss (

dB

)

Switch technologies

• Takes us to real optical networking

• What are the obstacles?– Attenuation management– Dispersion management– Performance monitoring– Scalable switches– Wavelength conversion

Design considerations

Data traffic is driving network growth

0

20

40

60

80

100

1997 1998 1999 2000 2001 2002 2003 2004

Time

Per

cent

Assumptions - 10% growth in voice traffic per year - Sidgemore’s law for data growth (data demand doubles every 6 months)

Data demand

Voice demand

Characteristics of data traffic

miles

Number of calls

Voice traffic

miles

Number of flows

IP Traffic

• Voice– Slow steady growth– Predictable growth

pattern– Low bandwidth

consumption– Most calls terminate

within the local area

• Data– Rapid, unpredictable

growth– Huge bandwidth

consumption– Distance insensitive

Ring inefficiencies

ADM ADM

ADM

ADM

ADM ADM

ADM

ADM

ADM ADM

ADM

ADM

ADM ADM

ADM

ADM

Wasted protectioncapacity

Bottlenecks due tolow drop capacity

ADM Interconnections with Switch

ADMwp

wp

ADMwp

wp

Switch Local drop traffic

Switch

Local droptraffic

Top view Multi-ring scenario

Interconnecting 8 OC192 rings requires about 640 Gbps switch capacity320 Gbps (line) + 320 Gbps (local and drop)

Span design

• Signal loss of ~.25dB per km – ~100 km is limit without amplification

• Noise accumulates degrading SNR– Eventually, 3R regeneration required to

clean the signal.

• Dispersion accumulates – Dispersion compensation and 3R to

correct.

Non linear effects in fiber

• These non-linear effect result because the signals traveling through the fiber slightly change the index of refraction of the fiber– Four-wave mixing (FWM) – When three frequencies in

the fiber: w1, w2 and w3 interact to create for example w4 = w1+w3-w2. W4 might interfere with a desirable wavelength.

– Cross phase modulation (XPM) – When the intensity variations of a signal modulate the phase of other signals in the fiber.

– Self phase modulation (SPM) – When the intensity variations of a signal modulate the phase of the signal.

Other non-linear effects

• SBS – Stimulated Brillouin Scattering is produced by acoustic waves in the fiber.– Backscattered light depletes power from the forward

traveling lightwaves. – This can be minimized by reducing the signal power

and dithering the wavelengths

• SRS – Stimulated Raman is an interaction between light waves and silica molecules. Power is transferred to wavelengths several nm away.– This can be used for amplifiers

IP over SONET

• Well-known technology• Provides for ~50ms

restoration• Useful but expensive and not

needed when building an IP network.

ProvisionedConnectionProvisionedConnection ProtectionProtection

SONETSONETADMADM

SONETSONETADMADM

SONETSONETADMADM

SONETSONETADMADM

SONETSONETADMADM

SONETSONETADMADM

WorkingWorkingSONETSONET

ADMADMSONETSONET

ADMADM

SONETRing

Packet over SONET

• Packet over SONET is the serial transmission of data using SONET framing.

• RFC 1619, “PPP over SONET/SDH” • RFC 1662, “PPP in HDLC-like Framing”• ITU-T G.703 / ANSI T1X1

fla

g

fla

g

PPP header IP packets CRC 16/32

1 4 2/4 1

address (1)control (1)protocol (2)

data

Reducing the number of boxes

Lower Cost, Complexity, and OverheadLower Cost, Complexity, and OverheadLower Cost, Complexity, and OverheadLower Cost, Complexity, and Overhead

IPIP

ATMATM

OpticalOptical

B-ISDN

IPIP

OpticalOptical

IPIP

SONET/SDHSONET/SDH

OpticalOptical

ATMATM

SONET/SDHSONET/SDH

IPIP

OpticalOptical

Multiplexing, Protection, and Management at Every LayerMultiplexing, Protection, and Management at Every Layer

IP over ATM

IP over SONET/SDH

IP over Optical

Options when building backbone transport

• High speed OC-192 (10G) backbone– Wavelength specific optics on SONET ADMs– Cutting edge

• OC48 (2.5G) backbone with “Open Interfaces”– ADMs use short-reach optics– Transponders have wavelength specific lasers– “Tried-and-true” technology– Commodity components– Access to full protect bandwidth

Advantages of each

• OC192– Fewer wavelengths to

manage– Lower cost for 3R

regeneration– Filter technologies match

spectrum of signal• Wavelength drift not

issue• Filter drift not an issue

– 100 GHz thin-film are passive

– Arguably maximum capacity method today

• OC48– Dispersion less of an

issue– PMD not an issue– Fewer 3Rs required– Off-the-shelf technology– Common rate of ATM

and IP switches– Open interfaces– Less expensive

electronics– Runs anywhere

OC192 economics

10 20 30 40 50 60 Gbps

cost Higher up front cost

when lighting

fiber

Lower average cost per bps for

fully-loaded systems

OC48 economics

10 20 30 40 50 60 Gbps

cost Lower up front cost

when lighting

fiber Higher average cost per bps for

high capacity routes

Next fibers must be lit

sooner

Repeater spacing

• Increasing the repeater spacing– reduces the construction costs– reduces the electronics costs for the first few

systems

• But .. WDM system performance will suffer– WDM adds additional losses in the system– Total power must be divided over the number of

waves– Non-linearities are a function of the launch power

The impact of limited launch power

Number of waves (with limited peak power)

0

50

100

150

200

50 60 70 80 90 100 110

Distance (km)

Nu

mb

er

of

wa

ves

12 km reduction in spacing allows twice the number of waves if launchpower is the limiting factor!!