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.
Bundling of many channels
onto one optical fibre pair
over long distance
Data (IP, FR, ATM, PL)
Storage (FC, ESCON, iSCSI, outsourcing)
Legacy, IP voice. Video
Next Gen Metro Optical
IP/ Ethernet Switching
Resilient Packet Ring (DPT)
Switching and Aggregation
Dense Wavelength Division Multiplexing
Dark Fiber InfrastructureHierarchy of Optical Networks
DWDM is the foundation of all other services
A Reliable, Scaleable and Flexible solution is required !
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
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
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
Ensure scalability to minimum the C and L bands
Understand the excess cost of scaling the amplifiers
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
spanEquipment Pitfall #3: Span limitation of DWDM networks
Ensure that the SLA supports the BOL DWDM design !
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.
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