June 15, 2013
In a recent article, Will 400Gbps ever be relevant to Africa?
, we advised African network owners to build their networks with the future in mind. Optical fibre is supposed to be in the ground for at least twenty years if installed properly. Those who are rolling out networks today should ask themselves if the fibre they are installing will be useable throughout the life of the fibre as requirements rapidly evolve.
When some Western operators installed optical fibre in the 90’s, the requirements were primarily to transport voice traffic at transmission rates of 100Mbps-150Mbps in the backbone. Those with some foresight were able to install fibre cable with good enough attributes that they can use the same fibre for today’s 100Gbps.
We promised to outline the key attributes that network owners should pay particular attenuation to if the fibre they install today should adapt to the ever changing transmission requirements. The three key attributes are attenuation, chromatic dispersion (or simply dispersion) and polarisation mode dispersion (PMD).
Fibre attenuation determines the network span length, which is the distance you can go before you have to amplify the signal with Erbium doped fibre amplifier (EDFA).
When designing long spans (longer than 100km) between EDFAs, low fibre attenuation and special amplifiers, such as Raman amplifiers, may be necessary. Long spans are unavoidable if the network owner wants to avoid costly amplifier huts in deserts and jungles. Spans of up to 300km without inline EDFAs have been implemented in Africa using low attenuation fibre and end to end Raman amplifiers. Typical spans at 10Gbps using inline EDFAs and standard optical fibre are typically 80 to 100km.
When designing very long links, Lubumbashi to Mbandaka (about 3000km) for example, then Optical Signal to Noise Ratio (OSNR) becomes a critical parameter. Because the EDFAs generate amplified spontaneous emission (ASE) noise, OSNR is severely degraded when too many amplifiers are used in such a long link. The network will typically require expensive electronic regenerators but a good designer conversant with all systems and optical fibre attributes in the market may be able to preclude the use of costly regenerators by selecting the right transmission equipment and low fibre attenuation.
If designing long spans and links is a challenge at 10Gbps today because of attenuation and OSNR, it will become a nightmare if the networks have to be upgraded to 100Gbps. Higher data rates require significantly higher OSNR, an additional 10dB from 10Gbps to 100Gbps, for example. Spans and links that are possible today with conventional fibre may no longer be possible at 100Gbps.
Transmission systems and tier one fibre optic manufactures are cognisant of the challenge of migrating to higher data rates and are taking steps to develop systems and optical fibre that will enable higher OSNR. Fibre attenuation specifications for terrestrial applications have gone down from 0.4dB/km at 1550nm in the 80s to as low as 0.17dB/km at 1550nm today. Unfortunately, there are still some operators in Africa who are being manipulated into buying optical fibre with legacy attenuation that belongs in the 90’s. These fibres will not effectively handle today’s 10Gbps requirements, let alone future higher data rates.
Dispersion limits the distance signals can be transmitted because of the so called inter-symbol interference. For 10Gbps systems, dispersion is compensated by the use of dispersion compensating modules at every 80 to 100km if standard single mode fibre is used. Dispersion has also been mitigated by the use of non-zero dispersion shifted fibre (NZDSF) which complies with the ITU-T G.655 standard. Because G.655 fibre has low dispersion in the 1550nm transmission window, fewer dispersion compensating modules and mid stage amplifiers are required.
Most 100Gbps transmission system manufacturers have come up with new digital signal processing schemes that will eliminate dispersion impairments. This will make NZDSF less relevant for those who plan to migrate to 100Gbps in their long haul networks. For operators who are planning to deploy NZDSF today for 10Gbps long haul transmission, it is advisable to deploy hybrid cable consisting of both NZDSF and ITU-T G.652 compliant fibre just in case they have to migrate to 100Gbps and switch from NZDSF to G.652 fibre.
PMD, a phenomenon caused by asymmetries in the optical fibre structure has almost no effect at 2.5Gbps but becomes a serious impairment at higher data rates of 10Gbps and more. The higher the transmission rate, the higher will be the impact of PMD impairment. PMD cannot be easily compensated for because it is nondeterministic – it’s value changes randomly and cannot be predicted. The best way to deal with PMD is to use fibre with low PMD. Most tier 1 fibre manufacturers have reduced their fibre PMD to manageable levels.
For those interested in more technical insights on this topic, please contact
us for more detailed reference materials.