Optical NetworkingBasic Engineering, Architectures, and Strategies.
(Take 2)
Tutorial
Presented at the
Internet2 Joint Techs Conference
February 5, 2003
Mark [email protected]
Jerry [email protected]
WARNING!!!Do not gaze into fiber with remaining eye!
Purpose
• Develop a basic familiarity with engineering design issues associated with emerging optical network technologies
• Communicate architectural and non-technical aspects of developing such infrastructure
Outline• Definition of scope
– For purpose of this tutorial: What is optical networking?
• Fiber characteristics– How does fiber affect the network?
• Optical components and systems architectures– Basic building blocks and how they fit together
• Case studies– Supercomputing 2002 WAN engineering– NCREN optical engineering
• Informational Sources– How to stay in the thick of it
What is “Optical Networking”• Lowest layer data transport is carried via light over fiber
optic cable.– I.e. Not electrical, not wireless, etc.
• For purposes of this tutorial, includes:– “Traditional” connections utilizing short reach, intermediate reach,
and long reach interfaces over multimode and singlemode fibers– Current technology using mono and multi-wavelength transport
techniques– Futures – Where is the optical networking headed?
• Other topics (not covered today)– Free space optics– Optical processing technologies
Pieces
• Whats on the “ends”– Optical transmission sources
• Characteristics – laser frequency, spectral width, modulation
– Receivers
• Whats in the “middle”– Fiber
• Optical characteristics and implications for network performance
Why are fibers what they are?• Most data communications fibers are silica based• Fibers are “pretty clear”, but not perfectly clear
– Impurities and construction limitations will constrain the optical transmission properties
• Many or the design properties of fibers are based on inherent technology capabilities/limitations of the light sources available at the time– LED sources were good for multimode fibers in the 850 nm range– Higher speed lasers at 1310 nm required lower attenuation and
dispersion in the fiber – and vice versa– Higher data rates required still further evolution into the 1550 nm
range
So whats up wit the fiber?• Fibers are “light guides”
– Almost clear, silica based– Use materials of different refractive indices to confine
and guide the light• Core
– Lowest refractive index– Primary light medium
• Cladding– Higher index of refraction than core– Bends escaping light back into core
• Jacket– Mechanically protects the fiber
Limiting factors of optical fiber
• Junctions– Splices– Connectors
• Linear effects – directly related to the length– Attenuation
• Absorption• Scattering
– Dispersion• Modal dispersion• Chromatic dispersion• Polarization Mode dispersion
Limiting Characteristics of Fiber• Linear effects – a function of the fiber length
– Attenuation – reduces power output of a fiber segment• Absorption – light is absorbed due to chemical properties of the fiber
so that less energy is emitted• Scattering – light is re-directed by the molecular properties of the
fiber resulting in leakage into the cladding, jacket, or lost at junctions
– Dispersion – broadens the optical pulse over length of a fiber segment
• Modal – differing “modes” traverse different paths in the fiber• Chromatic – different frequencies of light travel at different speeds
in a medium• Polarization – orthogonal light waves travel at different speeds in
the fiber
Limiting Characteristics of Fiber• Non-Linear effects
– Self phase modulation– Four wave mixing– Ramon scattering
Review of basic architecture:• Laser emits a light source • Modulator “blocks” according to electrical bit stream (Intensity
Modulation)– Direct modulations of laser typical in lower data rates– External mod more common in high speed data rates
• Receiver regenerates electrical bit stream from modulated optical signal
Connector
ModulatorLaser Receiver
Fiber Connector
The “Eye” Diagram• The analog representation of the digital
signal waveform– Overlays both “0” and “1” values
Time
Power
Logic “0”
Logic “1”
Rise/Fall Hold
Optical characteristics of fiber
• Low attenuation in 1310 nm range
• Low dispersion in the 1550 nm range
1310nm1550nm
1550nm Low-Loss Wavelength Band
At 1550nm, wide region of low-loss wavelengthsIs irresistable for WDM systems even with high dispersion.
Fiber L
oss (dB/km
)
1550nmwindow
-30
-20
-10
0
10
20
30
1250 1350 1450 1550 1650
Wavelength (nm)
Dis
per
sion
(ps/
nm
) 1300nm
(Courtesy Celion Networks)
Conventional Single-Mode Fiber (SMF)
First single-channel systems operated at 1310nm (good laser materials)WDM systems moved to 1550nm: wider loss-window, but higher dispersion
Disp.-Limit = 1000 km at 2.5Gb/s in SMF, so not really a problem
S C L
-30
-20
-10
0
10
20
30
1250 1350 1450 1550 1650
Wavelength (nm)
Dis
pe
rsio
n (
ps
/nm
)
D(1530-1565nm) = 16 - 19 ps/nm*km
D = 0.065 ps/nm2km
Aeff = 85 um2
(Courtesy Celion Networks)
Dispersion-Shifted Fiber –Oops!
DSF: Zero dispersion at 1550nm, so no compensation required. However, FWM severely limits optical power levels. Substantial
amounts in some U.S. networks. Small Effective Core Area, So very nonlinear
S C L
-30
-20
-10
0
10
20
30
1250 1350 1450 1550 1650
Wavelength (nm)
Dis
per
sion
(ps/
nm
)
(Courtesy Celion Networks)
NZ-DSF• Move dispersion zero outside
bands of interest
• Various types available
• Increased effective core area to equal SMF
SMF-28SMF-28DSFDSFTrueWave ClassicTrueWave ClassicTrueWave Reduced SlopeTrueWave Reduced SlopeE-LEAFE-LEAF
S-Band C-Band L-Band
-4
0
4
8
12
16
20
1510 1530 1550 1570 1590 1610
Wavelength (nm)
Dis
pe
rsio
n (
ps
/nm
)
(Courtesy Celion Networks)
Attenuation
• Absorption– Chemical properties of the fiber absorb some of
the energy
• Scattering– Molecular properties cause the light to be re-
directed – portions of it are lost in the cladding or are reflected back to the source
Dispersion• Dispersion causes the digital waveform to
be “smeared” – Rise/fall time expands over the length of the
fiber
• Modal dispersion only present in multi-mode fibers
• Chromatic dispersion arises from spectral width
Modal dispersion• Each “mode” travels along a different path.
– Light enters the guide from different insertion angles
– Each path has a different length and so arrives at different times
• Primary limiting factor of multi-mode fiber for high speed communications
m0
m1
Modal Dispersion• Multimode fibers have a core diameter of 50 microns to
62.5 microns– Less rigorous tolerances make construction easier– Splicing and connectors are more easily engineered– Typically under 2 kilometer distances (less at high data rates)
• By sizing the diameter of the core properly as a function of wavelength and refractive indices of core and cladding, the wave guide can be constrained to carry only a single “mode” of the incident laser signal.– Single mode fiber has a core diameter of approximately 8-11
microns– SM fiber does not exhibit modal dispersion
Chromatic Dispersion• Lasers do not emit a single wavelength
– Spectral width
• Different wavelengths of light travel at different velocities in a given medium.– Index of refraction
• Tails of the laser spectral distribution travel at different speeds down the wave guide
Frequency domain
Chromatic Dispersion• Chromatic dispersion is sum of wave-guide dispersion (+)
and material dispersion (-)– Fiber design can vary the amount of wave-guide dispersion in
order to cancel the material dispersion at a desired wavelength– Zero Dispersion-Shifted Fiber (ZDSF)
• Non-linear effects are dampened by dispersion, so…– Shift the zero dispersion point a bit past the operating
wavelengths..– Non-Zero Dispersion Shifted Fiber (NZ-DSF)
• Dispersion can be positive or negative– Negative dispersion fiber can counter effects of normal fiber…
Dispersion Compensating Fiber (DCF)
Chromatic Dispersion• Measured in ps/(nm*km)
– E.g. 5ps/(nanometer kilometer)• How would chromatic dispersion affect an OC48 link with laser at +/-1nm spectral line over a 20 km
NZ-DSF fiber link?– Bit period = 416ps– 2 nm spectral band * 5 ps/(nm km) * 20 km = 200 ps– Result: rise/fall time is 50% of bit period – The link is on the edge (may see excessive bit errors)– Possible adjustments:
• Reduce span (add a regen point)• .Find an interface with better source laser, better receiver parameters, or both – I.e. may
mean a more expensive XL interface• Reduce the link bandwidth – GigEthernet would likely work comfortably.
• OC192 with a .2 nm spectral width over 50 km– Bit = 104 ps– .2 nm spectral band * 4ps/nmkm * 50 km = 40 ps (+/- 20 ps)– 40% of duty cycle – will probably work – The finer the laser line, the less chromatic dispersion affects the emitted signal.
Polarization Mode Dispersion• “Single mode” fiber actually allows light consisting of orthogonal
poloraizations (the electric and magnetic fields of different photons are not aligned.) “Bimodal” fiber…
• Due to construction methods, installation, environmental conditions, etc., the effective area of the core varies along the axis of the fiber.
• This variance if EA causes subtle differences in propagation speed of the light wave based upon the polarization of the component photons.– Result: Dispersion
• Not well understood – Typically only of concern at data rate in excess of 2.4 Gbs– Measured in ps/sqrt(km)– Of most concern in fiber manufactured and installed prior to early
1990s.
Optical Networking Components
• Optical Multiplexor
• Optical Demultiplexor
Optical Network Components• Splitters
– Splits off some portion of the optical signal – Splitters do not demultiplex the optical signals
• Wavelength Converters– Often require electrical intermediate step– New devices allow conversion in optical domain
50%
100%
50%
80%
20%
i o
Opto-electronic Conversion• Wavelength conversion is typically required to interface
traditional optical interfaces to ITU “grid compliant” wavelengths used in DWDM systems– CPE typically at 1310nm with relatively broad spectral
band– Optical Channel Modules (OCM) take the 1310 optical
signal, convert it to its electrical equivalent, and then re-transmit it with the assigned ITU wavelength
– This is generally referred to as O-E-O• This OEO process can be employed mid-span to perform
some or all of the 3Rs – Retiming, Reshaping, Re-generation.
The Three “R”s
• Re-timing– Verify and compensate for clocking drift
• Re-shaping– Compensate for attenuation and/or dispersion
– Sharpen the “eye”
• Re-transmission– Completely decode and re-create the digital bit stream.
– Often includes intelligent processing of the framing headers for O&M purposes.
Simple Two Example
Router A
Router B
WavelengthConverter
1310 nm ->1550 nm
1310
1550 1310
Mux
Router C
Router D1550
Dmux1310
Note: Wavelength conversion back to 1310 at Router D is not necessary because the optical receiver is actually sensitive to a broad range of optical wavelengths – including 1550.
Optical Add/Drop Multiplexor
Mux Dmux
Dmux Mux
Channel Modules
• Two fiber example
• Possibly from a ring configuration
OADM
Building the ARL OC48 for SC02
• Provision OC48c Sonet wave from Army Research Lab (White Oak, MD) to Supercomputing 2002 at the Baltimore Convention Center
• Segments: – 11 km Truewave(RS) from ARL to CLPK
– MAX Lambda from CLPK to DCNE (Qwest pop)
– SC02 Lambda from ECK to BCC (via MAR)
– SMF from BCC(noc) to booth
Building the ARL OC48 for SC02
ARL
CLPK
ECK
MAR
BCC
BCC
Before
MAR
DCNE ECK
CLPK
11 km TW(rs)
19 km AW
50 km
5 km
<1km
CPE
CPE
CPE
NOC
Calculating Network Limits Building the ARL OC48 for SC02
CLPK (Univ. of Md)
Army Research Lab
Connectors(Patch panels, interface connections, etc) = .5dB
Tx = -3 dBm Rx = -28 dbMPOSISMF ZD=1310, .32dB/kma) OC48 interfaces 1310 nm, =20nm
11 km Truewave RS
Attenuation = connectors+ fiber
= (6 * -.5dB) + (11km * -.25db) = -3dB – 2.75 dB = -5.75 dB Power is fine!
Dispersion:t = sqrt( t2
chromatic + t2polarization )
= -8ps/nm.km * 11 km * 20 nm + 0 = -1760 ps not good (given a 400 ps bit period)
Link Budget = -3 – (-28) = 25dBm
Tx = -3 dBm Rx = -28 dBm 11 kmtw(rs) Attenuation= .25 dbm/km
Dispersion ~5ps/nmkm @1550…but –8ps/nmkm @ 1310OC48 interfaces 1310 nm, = 20nm
So how do we correct it?
Building the ARL OC48 for SC02
• Situation: @1310 (or at 1550) power is good, but…
• At 1310 dispersion, 1760 ps, is too high to support an OC48.
• Options:– Reduce bandwidth: OC3 duty cycle is 6400 ps and
would work fine – but not adequate for application
– Find a long reach interface, hopefully with a SW less than 2nm and at 1550
AfterAdd inverted transponder!
MAR
DCNE ECK
11 km
19 km
50 km
Line
CPE
CPECLPK
CPE
BCC5 km
<1km
CPENOC
Inverted Transponder
Attenuation = connectors+ fiber
= (6 * -.5dB) + (11km * -.25db) = -3dB – 2.75 dB = -5.75 dB Power is still fine!
Dispersion:t = sqrt( t2
chromatic + t2polarization )
= 5 ps/nm.km * 11 km * 0.2 nm + 0 = 11 ps Dispersion is no longer a problem – in fact would be fine for OC192
Link Budget = -3 – (-28) = 25dBm
Tx = -3 dBm Rx = -28 dBm 11 kmtw(rs) Attenuation= .25 dbm/km
Dispersion ~5ps/nmkm @1550…but –8ps/nmkm @ 1310OC48 interfaces 1550 nm, = .2 nm
Why does the Inverted Transponder solve the problem?
• The transponder has broadband receiver(s) on both the line side and CPE side
• The CPE xmit was 1550 with broadband recv.
• By inverting the transponder we send a 1550 signal with a very narrow SW towards the CPE – dispersion is reduced
MAX Fiber Engineering
• Needed POPs in several locations• Spoke to Carriers in those locations
– Looked at available fiber routes
– Discussed available fiber types
– Iteratively identified a set of specifc locations
• Contracted for fiber– Tried to move quickly – needed the capacity urgently
– Contract based upon a relatively short 3 yr lease
MAX Primary Ring Details• Two strands Lucent AllWave
• Four points of presence• 49 miles total circumference
• Provisioning Trade-offs– Where/when are additional lamdas useful
• Layer 3 protection between routers• Backhaul access circuits to routers• “PVN”s – Parallel Virtual Networks
– E.g. IPv6, transient applications• Non-L3 service – e.g. NGIX access
– Routers are more expensive than switches– Lambdas cost ~$75,000 incremental cost
• But have ammortized cost of fiber, wdm nodes, support, sparing, etc that need to be included• Hard laser wavelengths limit re-application of OCMs.
Fiber Engineering Specs
17 km -4 dB
20 km -5 dB
44.7 km-14 dB
9.8 km -2.25 dB
ARLG
DCGW DCNE
CLPK
MAX Lambda Provisioning
ARLG
DCGW DCNE
CLPK
NGIXABIL
IP (oc48 sonet)GigENGIX (oc12 atm/sonet)
1 = ITU 332 = ITU 353 = ITU 374 = ITU 395 = ITU 336 = ITU 35
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3
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