Transcript Document

Optical Networking
Basic Engineering, Architectures, and Strategies.
(Take 2)
Tutorial
Presented at the
Internet2 Joint Techs Conference
February 5, 2003
Mark Johnson
[email protected]
Jerry Sobieski
[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 nontechnical 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 l
• Modulator “blocks” l 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
Laser
Modulator
Connector
Receiver
Fiber
Connector
The “Eye” Diagram
• The analog representation of the digital
signal waveform
– Overlays both “0” and “1” values
Rise/
Fall
Hold
Logic “1”
Power
Logic “0”
Time
Optical characteristics of fiber
• Low attenuation in 1310 nm range
• Low dispersion in the 1550 nm range
1550nm
1310nm
1550nm Low-Loss Wavelength
Band
30
Dispersion (ps/nm)
Fiber Loss (dB/km)
1300nm
20
1550nm
window
10
0
-10
-20
-30
1250
1350
1450
1550
1650
Wavelength (nm)
At 1550nm, wide region of low-loss wavelengths
Is irresistable for WDM systems even with high dispersion.
(Courtesy Celion Networks)
Conventional Single-Mode Fiber
(SMF)
30
S C L
Dispersion (ps/nm)
20
D(1530-1565nm)
= 16 - 19 ps/nm*km
10
0
DD = 0.065 ps/nm2km
-10
Aeff = 85 um2
-20
-30
1250
1350
1450
1550
1650
Wavelength (nm)
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
(Courtesy Celion Networks)
Dispersion-Shifted Fiber –Oops!
30
S C L
Dispersion (ps/nm)
20
10
0
-10
-20
-30
1250
1350
1450
1550
1650
Wavelength (nm)
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
(Courtesy Celion Networks)
S-Band
C-Band
NZ-DSF
L-Band
Dispersion (ps/nm)
20
16
• Move dispersion zero outside
bands of interest
12
• Various types available
• Increased effective core area to
equal SMF
8
4
0
-4
1510
1530
1550
1570
1590
1610
Wavelength (nm)
SMF-28
DSF
TrueWave Classic
TrueWave Reduced Slope
E-LEAF
(Courtesy Celion Networks)
Attenuation
• Absorption
– Chemical properties of the fiber absorb some of
the energy
• Scattering
– Molecular properties cause the light to be redirected – 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 multimode 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
l0
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
50%
80%
50%
20%
100%
• Wavelength Converters
– Often require electrical intermediate step
– New devices allow conversion in optical domain
li
lo
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, Regeneration.
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 l Example
Mux
Router A
Router B
1310
1310
Dmux
1310
1550
1550
Router C
Router D
Wavelength
Converter
1310 nm ->1550 nm
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
• Two fiber example
• Possibly from a ring configuration
Mux
Dmux
Dmux
Mux
Channel Modules
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
BCC
MAR
ARL
CLPK
ECK
Before
<1km
NOC
CPE
BCC
5 km
MAR
11 km TW(rs)
CPE
50 km
CPE
CLPK
19 km AW
DCNE
ECK
Calculating Network Limits
Building the ARL OC48 for SC02
CLPK (Univ. of Md)
Tx = -3 dBm Rx = -28 dbM
POSISMF ZD=1310, .32dB/km
a) OC48 interfaces l=1310 nm, Dl =20nm
11 km Truewave RS
Connectors
(Patch panels,
interface connections,
etc) = .5dB
Army Research Lab
Tx = -3 dBm Rx = -28 dBm 11 km
tw(rs) Attenuation= .25 dbm/km
Dispersion ~5ps/nmkm @1550…but –8ps/nmkm @ 1310
OC48 interfaces l=1310 nm, Dl = 20nm
Link Budget = -3 – (-28) = 25dBm
Attenuation = aconnectors+ afiber
= (6 * -.5dB) + (11km * -.25db)
= -3dB – 2.75 dB = -5.75 dB Power is fine!
Dispersion:
Dt = sqrt( Dt2chromatic + Dt2polarization )
= -8ps/nm.km * 11 km * 20 nm + 0
= -1760 ps  not good (given a 400 ps bit period)
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
After
Add inverted transponder!
<1km
NOC
CPE
CPE
BCC
5 km
11 km
MAR
Line
Inverted Transponder
CPE
50 km
CPE
CLPK
19 km
DCNE
ECK
Tx = -3 dBm Rx = -28 dBm 11 km
tw(rs) Attenuation= .25 dbm/km
Dispersion ~5ps/nmkm @1550…but –8ps/nmkm @ 1310
OC48 interfaces l=1550 nm, Dl = .2 nm
Link Budget = -3 – (-28) = 25dBm
Attenuation = aconnectors+ afiber
= (6 * -.5dB) + (11km * -.25db)
= -3dB – 2.75 dB = -5.75 dB Power is still fine!
Dispersion:
Dt = sqrt( Dt2chromatic + Dt2polarization )
= 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
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
CLPK
44.7 km
-14 dB
17 km
-4 dB
DCNE
DCGW
9.8 km
-2.25 dB
20 km
-5 dB
ARLG
MAX Lambda Provisioning
CLPK
NGIX
l5
l3
DCGW
l2
l1
l4
l1 = ITU 33
l2 = ITU 35
l3 = ITU 37
l4 = ITU 39
l5 = ITU 33
l6 = ITU 35
ABIL
DCNE
l6
ARLG
IP (oc48 sonet)
GigE
NGIX (oc12 atm/sonet)