Pitfalls in fibre network design

Ole Saunte-BoldtIndependent Consultantsaunte-boldt@mail.dk

Introductive guide towards a stronger DWDM network design

What is an Optical Network?

Bundling of many channels onto one optical fibre pair

over long distance

Traffic Pipeline

Hierarchy of Optical Networks

DWDM is the foundation of all other servicesA Reliable, Scaleable and Flexible solution is required !

Dark Fiber InfrastructureDark Fiber Infrastructure

Resilient Resilient Packet Packet

Ring (DPT)Ring (DPT)

IP/ IP/ Ethernet Ethernet

SwitchingSwitching

Dense Wavelength Division MultiplexingDense Wavelength Division Multiplexing (DWDM)(DWDM)

Next Gen Metro Next Gen Metro Optical Optical

TransportTransport(SONET/SDH)(SONET/SDH)

SERVICESSERVICESData (IP, FR, ATM, PL) Data (IP, FR, ATM, PL)

Storage (FC, ESCON, iSCSI, outsourcing)Storage (FC, ESCON, iSCSI, outsourcing)Legacy, IP voice. VideoLegacy, IP voice. Video

Grid applicationGrid application

Switching and AggregationSwitching and AggregationL

ease

d L

ines

Lea

sed

Lin

es

The Future ?The Future ?

Factors influencing theReliability, Scalability and Flexibility

Dark Fibre DWDM Equipment

Tech

no

log

y

Attenuation

Chromatic Dispersion

Polarisation Mode Dispersion

Type of fibre

Age of fibre installation

(Distance between POP’s)

Data rate

Filter size

Amplification bands

Transmission range

Forward Error Correction

Dispersion Compensation

Laser Tuneability

Filter Tuneability

Management System

(DWDM Road Map)

Services

Service Level Agreement

Managed Network Services

First Level Services

Next Business Day Service

Next Day Service

24x7 Service

Second Level Services

Third Level Service

Managed Spared part service

Pre-Design Considerations

Make a visionary decision with no regards to cost (initially)

Know you current network requirementsBest guess on future requirements

Consider router consolidation and other high bandwidth university applicationsDWDM investments are for 15-20 years !

Select Scalability through the 1-10-100 rule1: DWDM solutions supports current requirement, BOL10: DWDM system scales to 10 times the BOL capacity without extra basic investments100: The installed network can support 100 times the BOL traffic with extra investment.

Select Reliability through a Pure Optical NetworkAvoid O-E-O regeneration

Select Flexibility through a Tuneable Optical Network

Select the correct platform!CWDM – p2p city platform with limited capacityMetro DWDM – p2p/ring city platform with high capacityLong Haul DWDM – Intercity platform with high capacity

Fibre Pitfall #1: Attenuation

Fibre attenuation is basically defined by the fibre impuritiesA = 0,20 dB/km @ 1550nm

Excess losses that adds to the all over fibre attenuation are Bend loss, splicing and connector losses0,05 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 DispersionThere are mainly three fibre groups on the marked:

1. Non–dispersion-shifted fibre (NDSF), standard single-mode fibre (SMF)- zero dispersion point around 1550 nm

2. Dispersion-shifted fibre (DSF)- zero dispersion point around 1550 nm

3. 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 fibre40Gb/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 fibrecontains 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 networkadding 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 !

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

sectionspan

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 dispersion10-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/s2.5Gb/s signals tolerate up to 12,200 ps/nm

40Gb/s transmission is 16 times more sensitive than 10 Gb/s40Gb/s signals tolerate up to 50 ps/nm

Equipment Pitfall #4: Channel Growth ModelCost 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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λ6 λ9 λ12

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S$]

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CAPEX OPEX year 1 OPEX year 5 OPEX year 10 OPEX year 15

Cost Parameter

Fibre

Equipment

Service

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

Statements

Q&A