1. Pitfalls in fibre network design
Introductive guide towards
a stronger DWDM network design
Ole Saunte-Boldt
Independent Consultant

2. What is an Optical Network?
Bundling of many channels
onto one optical fibre pair
over long distance
Traffic Pipeline

3. Hierarchy of Optical Networks
DWDM is the foundation of all other services
A Reliable, Scaleable and Flexible solution is required !
Dark Fiber Infrastructure
Resilient Packet Ring (DPT)
IP/ Ethernet Switching
Dense Wavelength Division Multiplexing
(DWDM)
Next Gen Metro Optical
Transport
(SONET/SDH)
SERVICES
Data (IP, FR, ATM, PL)
Storage (FC, ESCON, iSCSI, outsourcing)
Legacy, IP voice. Video
Grid application
Switching and Aggregation
Leased Lines
The Future ?

4. Factors influencing the Reliability, Scalability and Flexibility
Dark Fibre
DWDM Equipment
Technology
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

5. 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 years !
Select Scalability through the 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

6. Fibre Pitfall #1: Attenuation
Fibre attenuation is basically defined by the fibre impurities
A = 0, nm
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

7. 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)
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.

8. PMD: Polarisation Mode Dispersion
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

9. 0,25 dB/km attenuation and 0,5 ps/km of PMD
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

10. Equipment Pitfall #1: Amplification bands
Ensure scalability to minimum the C and L bands
Understand the excess cost of scaling the amplifiers

11. 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

12. 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 !
section
span

13. 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

14. 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.

15. 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 !

16. 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

17. Q&A