Pitfalls in fibre network design Introductive guide towards a stronger DWDM network design Ole Saunte-Boldt Independent Consultant saunte-boldt@mail.dk

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 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 ?

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

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) 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.

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

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

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 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

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 !

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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