Rollout SURFnet6 progress report

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Roll-out SURFnet6 progress report
May 17th, 2005
Roeland Nuijts Network Services
Department SURFnet BV roeland.nuijts_at_surfnet.nl
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Outline

• Introduction
• SURFnet6 WDM network
• Dark fiber requirements testing
• Loss
• Optical reflections
• Dispersion
• PMD
• Experiences
• System tests of SURFnet6
• Timeline SURFnet6 build progress

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Dark fibers in SURFnet6

• five DWDM subnetworks for transporting the IP
  traffic and lightpaths and connecting the various
  add/drop sites in SURFnet6
• all transmission fiber is leased DFs (Dark
  Fibers), mix of standard SMF (G.652) and NZ-DSF
  (G.655)
• 40 DF pairs for DWDM
• 37 DF pairs for TDM
• 8 suppliers
• one CfP
• one subnetwork finished (subnetwork 3) and
  tested, four under construction
• OADM sites for adding and dropping
  wavelengths/lightpaths
• DWDM system
• maximum 72 wavelengths _at_ 10Gb/s, total capacity
  of 720Gb/s per subnetwork
• modular design for adding wavelengths

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Requirements for Dark Fibers

• Loss lt 0.25dB/km (_at_ 1550nm)
• splice loss lt 0.15 dB, max no. splices 1 per
  2km
• connector loss lt 0.5 dB, generally accepted but
  properly treated connector should easily meet
  0.2dB
• Optical reflections below -30dB
• Fiber type G.655 (preferred) or G.652
• PMD lt 0.5 ps/?km
• SC-PC (flat) connectorized
• practical, easy to use
• sufficient performance if well treated

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Loss

• Fiber loss is wavelength dependent, minimum is
  around 1550nm
• Current fiber loss is close to fundamental limit
  which arises from Rayleigh scattering (scattering
  caused by local variations in density and
  therefore refractive index), proportional to ?-4
  therefore dominant at short wavelengths
• Loss at long wavelengths (? gt 1625nm) dominated
  by infra-red absorption
• Peak at 1400nm arises from OH impurities, can be
  removed (AllWave fiber)

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Reflections

• Main source of reflections, mismatch in
  refractive index

n1
n2
Pinc
Ptrans
Preflc
Example reflection of a glass-air interface is
about 2.8 which corresponds to -15.5dB
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Reflections

• Sources of reflections, mismatch in refractive
  index
• Airgaps in connections
• Offset in alignment
• Bad or dirty connections
• Glas-air interface (end of fiber)
• MPI (Multi Path Interference)
• Optical receiver receives desired signal plus
  undesired delayed copies of signal, deteriorates
  BER (Bit Error Rate)
• Acceptable reflection for transmission equipment
  is -27dB
• Specification for maximum reflection in fibers is
  -30dB

fiber1
fiber2
ncladding
ncore
ncladding
ncladding
ncore
ncladding
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Measurement of loss and reflections- OTDRs
(Optical Time Domain Reflectometer)
pre-spool (1km)
post-spool (1km)
OTDR
fiber under test
pulse source
Glass-air interface 15.5dB ? easy sanity check
of OTDR equipment!!
photo detector
digital signal processing
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OTDR- example of measured data

• Trade of between resolution and distance
• Length of pulse 1 ?s x 2.998 108 m/s /1.5 200m
• Short pulse 10 ns ? 2m ? small enough to check
  patches in remote central office

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Fiber dispersion G.655 versus G.652

• Refractive index varies with wavelength which
  leads to a wavelength dependence of the group
  delay, ?g, (delay for different wavelengths) in
  ps
• Dispersion coefficient, D, is the derivative of
  the group delay, ?g , with respect to wavelength
  per unit length (ps/nm km)

?g (ps)
? (nm)
Optical signal at Tx output in time domain
D (ps/nm km)
0
? (nm)
?0
?T

• No distortion at zero-dispersion wavelength, ?0
• Distortion at other wavelengths

Optical signal in frequency domain
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Historical perspective

• Early fibers extremely lossy 1000dB/km (1960)
• Progress in fabrication lead to low loss fibers
  (0.2dB/km at 1550nm wavelength, limited by
  fundamental limit of Rayleigh scattering) around
  1979
• Initial (80s) devices for transmission through
  single mode fiber operated in the 1310nm
  wavelength region, 1550nm devices did not exist
• Installed fiber base largely comprised of
  standard (1.3µm zero-dispersion wavelength) SMF
  (Single Mode Fiber)
• This embedded base represents an enormous
  investment, strong incentive to use it
• Development of commercially available 1550nm
  sources and detectors started in 80s
• Invention of EDFAs (Erbium Doped Fiber
  Amplifiers) for amplification of optical signals
  in 80s

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Dispersion limited distance, LD

• Chromatic dispersion places a limit on the
  maximum transmission distance
• LD scales with square of the bitrate

B 10 Gb/s D 17 ps/nm km LD 50km ??
0.112 nm

• 800km at 2.5Gb/s
• 50km at 10Gb/s, dispersion compensation is
  needed in case of transmission beyond 50km

3dB bandwidth of 7GHz assumed, doublesided
spectrum ? 14GHz ? 0.112nm
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Example of impact of dispersion on transmission
systemperformance - optical pulse shape of
10Gb/s signal after 120km
120km standard SMF D17.8 ps/nm km
Tx
Rx
OA
OA
rb 10Gb/s ?c 1557nm
120km-10Gb/s system configuration
Optical pulse shape at transmitter output

• Pulse shape after 120km transmission completely
  distorted due to pulse broadening, error-free
  transmission not possible

After 120km transmission
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Dispersion Compensating Fiber
Dispersion compensating fiber
Standard fiber
D (ps/nm km)
0
?0, SMF
?T
?0, DCF
? (nm)

• Advantages
• Wide band compensation (one DCF compensates for
  all channels in C-band)
• Feasible for DWDM systems
• Easy to use, reliable
• Disadvantages
• bulky
• DCF loss, requires additional optical amplifier
• smaller core area, hence nonlinear effects

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Optical pulse shape after 120km and after
DCF(Dispersion Compensating Fiber)
Optical pulse shape at transmitter output

• Pulse shape after 120km transmission completely
  distorted due to pulse broadening, error-free
  transmission not possible
• Optical pulse shape recovered after passing
  through DCF with negative dispersion
• More residual dispersion if optical power level
  at the input of the DCF is high due to the
  (undesirable) nonlinear effect in the DCF

After 120km transmission
After compensation
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NZ-DSF (Non-Zero Dispersion ShiftedFiber)

• DSF (zero-dispersion at 1550nm), good for single
  channel transmission but causes FWM (Four Wave
  Mixing)
• NZDSF, finite dispersion in 1550nm DWDM window to
  avoid FWM, dispersion slope needs to be
  compensated

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PMD (Polarization Mode Dispersion)

• Orthogonal polarizations propagate at different
  speeds through fiber which leads to signal
  distortion and limits the achievable transmission
  distance
• More prominent at higher speeds 10Gb/s
• More prominent in old fibers (non-cylindrical
  core)
• PMD coefficient is given in ps/?km
• PMD coefficient measured by interferometric
  method

• Example PMD coefficient 0.5 ps/?km, maximum
  differential
• delay, ?PMD, is 10 of bit period. In case of
  10Gb/s this is 10ps.
• Maximum transmission distance is 10ps/(0.5
  ps/?km) 20?km
• maximum transmission distance 400km

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Measured DF data for DWDM spans
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Summary of experiences with DFs

• Deviation in delivered lengths prompted
  re-simulation of dispersion compensation of all
  subnetworks
• Measurement crews unaware of proper specs
• High reflections unnoticed
• High connector losses unnoticed
• One of reasons is outsourcing
• Traditional telcos deliver worst DF service
• PMD passed (PMD coefficient lt 0.5 ps/?km) in all
  but one span
• G.653 (DSF, Dispersion Shifted Fiber), supplier
  did not notice

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SURFnet6 Subnetwork 3
OSNR (0.1nm) gt 40dB

• Subnetwork 3 specific
• eight spans
• total distance 528km
• dispersion optimized to allow traffic between
  any pair of sites

OSNR (0.1nm) gt 28dB
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Test results

• Long term BER-test error-free for 1 day 25 min,
  corresponds to BERlt1.1 10-15
• Power supply interruption, no traffic hits
• Simulation of fiber cut, surviving traffic
  remained error-free

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Simple formula for OSNR calculation
repeater
SMF
SMF
SMF
DCF
DCF
Tx
Rx
NF2
NF3
NFN-1
NFN
NF1
Pin,1
Pin,2
Pin,3
Pin,N-1
Pin,N
OSNR
Simple formula, accurate to a few tenth of dBs
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General timelines and planning

• September 2004 blueprint version 1.0 finalized
• November 2004 - June 2005 installation by
  Telindus
• Ring 3 has been completed
• H2 2005 transition connected organizations to
  SURFnet6

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Thanks!roeland.nuijts_at_surfnet.nl