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Pitfalls in fibre network design. Introductive guide towards a stronger DWDM network design. Ole Saunte-Boldt Independent Consultant [email protected] What is an Optical Network?. Bundling of many channels onto one optical fibre pair over long distance Traffic Pipeline.

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Pitfalls in fibre network design
Pitfalls in fibre network design

Introductive guide towards

a stronger DWDM network design

Ole Saunte-Boldt

Independent Consultant

[email protected]

What is an optical network
What is an Optical Network?

Bundling of many channels

onto one optical fibre pair

over long distance

Traffic Pipeline

Hierarchy of optical networks

The Future ?


Data (IP, FR, ATM, PL)

Storage (FC, ESCON, iSCSI, outsourcing)

Legacy, IP voice. Video

Grid application

Next Gen Metro Optical



IP/ Ethernet Switching

Resilient Packet Ring (DPT)

Switching and Aggregation

Leased Lines

Dense Wavelength Division Multiplexing


Dark Fiber Infrastructure

Hierarchy of Optical Networks

DWDM is the foundation of all other services

A Reliable, Scaleable and Flexible solution is required !

Factors influencing the reliability scalability and flexibility
Factors influencing theReliability, Scalability and Flexibility

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

Fibre Pitfall #1: Attenuation

Fibre attenuation is basically defined by the fibre impurities

A = 0,20 dB/km @ 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

Fibre Pitfall #2: Chromatic Dispersion

  • There are mainly three fibre groups on the marked:

  • Non–dispersion-shifted fibre (NDSF), standard single-mode fibre (SMF)

    • - zero dispersion point around 1550 nm

  • Dispersion-shifted fibre (DSF)

    • - zero dispersion point around 1550 nm

  • Non–zero dispersion-shifted fibre (NZ-DSF)

    • - zero point around other λ

  • 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.

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

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

Equipment Pitfall #1:

Amplification bands

Ensure scalability to minimum the C and L bands

Understand the excess cost of scaling the amplifiers

Equipment pitfall 2 filters
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

Equipment pitfall 3 span limitation of dwdm networks



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 !

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

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.

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 !


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