Pitfalls in fibre network design

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Pitfalls in fibre network design
Introductive guide towards a stronger DWDM
network design
Ole Saunte-Boldt Independent Consultant saunte-bol
dt_at_mail.dk
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What is an Optical Network?
Bundling of many channels onto one optical fibre
pair over long distance Traffic Pipeline
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Hierarchy of Optical Networks
DWDM is the foundation of all other services A
Reliable, Scaleable and Flexible solution is
required !
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Factors influencing theReliability, Scalability
and Flexibility
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Pre-Design Considerations
Make a visionary decision with no regards to cost
(initially) Know you current network
requirements Best guess on future
requirements Consider router consolidation and
other high bandwidth university applications DWDM
investments are for 15-20 years ! Select
Scalability through the 1-10-100 rule 1 DWDM
solutions supports current requirement, BOL 10
DWDM system scales to 10 times the BOL capacity
without extra basic investments 100 The
installed network can support 100 times the BOL
traffic with extra investment. Select
Reliability through a Pure Optical Network Avoid
O-E-O regeneration Select Flexibility through a
Tuneable Optical Network Select the correct
platform! CWDM p2p city platform with limited
capacity Metro DWDM p2p/ring city platform with
high capacity Long Haul DWDM Intercity platform
with high capacity
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Fibre Pitfall 1 Attenuation
Fibre attenuation is basically defined by the
fibre impurities A 0,20 dB/km _at_ 1550nm Excess
losses that adds to the all over fibre
attenuation are Bend loss, splicing and
connector losses 0,05 dB/km excess loss is not
rare !
Losses at 0,20 dB/km signifies a newly installed
fibre with limited excess loss Losses at
0,25dB/km signifies an older installation with
excess losses
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Fibre Pitfall 2 Chromatic Dispersion

• There are mainly three fibre groups on the
  marked
• Nondispersion-shifted fibre (NDSF), standard
  single-mode fibre (SMF)
• - zero dispersion point around 1550 nm
• Dispersion-shifted fibre (DSF)
• - zero dispersion point around 1550 nm
• Nonzero dispersion-shifted fibre (NZ-DSF)
• - zero point around other ?

• FWM limits the channel capacity of a DWDM system.
• FWM cannot be filtered out
• FWM is significant for DSF (2) which is
  unsuitable for WDM applications.

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Fibre Pitfall 3 PMD
PMD Polarisation Mode Dispersion Stochastic
variation of the fibre dispersion with time,
temperature and pressure.
The new fibre types have less than 0,5
ps/?km 10Gb/s signals tolerate 10ps of PMD which
permits 400km fibre 40Gb/s signals tolerate 2.5ps
of PMD which permits 25 km fibre
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Fibre Pitfall 4 SLA of fibre
Standard text in a Service Level Agreement for
dark fibre contains options for fibre replacement
if the values exceed 0,25 dB/km attenuation and
0,5 ps/?km of PMD
This could be very damaging to the quality of the
network adding high extra costs to the DWDM system
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Equipment Pitfall 1 Amplification bands
Ensure scalability to minimum the C and L
bands Understand the excess cost of scaling the
amplifiers
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Equipment Pitfall 2 Filters !
Smaller filters equal higher channel number but
reduced data rate Smaller filters puts
constraint on the system tolerance A good
balance is achieved by using the 50 GHz filters
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Equipment Pitfall 3 Span limitation of DWDM
networks
Rule of thumb Distance of spans are dependant on
the number of spans in a section e.g. 1 span in
the section maximum loss in span is 40 dB (182
km/span) 3 spans in the section maximum loss in
span is 34 dB (155 km/span) 25 spans in the
section maximum loss in a span is 19 dB (86
km/span)
Ensure that the SLA supports the BOL DWDM design !
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Equipment Pitfall 4 Dispersive effects
Standard SMF fiber has 17 ps/nm/km of chromatic
dispersion 10-Gb/s receivers can tolerate about
800 ps/nm of dispersion 500-km systems generates
8500 ps/nm of dispersion
2.5Gb/s transmission is 16 times less sensitive
than 10 Gb/s 2.5Gb/s signals tolerate up to
12,200 ps/nm 40Gb/s transmission is 16 times
more sensitive than 10 Gb/s 40Gb/s signals
tolerate up to 50 ps/nm
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Equipment Pitfall 4 Channel Growth Model Cost
pr channel upgrade
DWDM solutions are known to incorporate a
build-as-you-grow strategy hiding costs related
to upgrades of channels over time. The graph is
an attempt to uncover hidden cost elements and
major cost jumps as the system grow over time.
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Services Pitfall The high OPEX
The NBD (next business day) service below will
increase significantly if a more strict
response policy is required. Build a strong
protection into the DWDM platform and avoid high
service costs !
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Statements
AMBITION Build a DWDM network for the Future
FLEXIBILITY Build a state of the art
reconfigurable network
SCALABILITY Scalable to minimum 64 channels in
the C-band
RELIABILITY Absolute min. of regeneration
ENABLED 40 Gbit/s ready
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QA