Need for deployment of advanced fibre technologies

It is universally recognised that countries with higher broadband penetration usually have higher GDPs. Thus, broadband penetration has been a priority on the national agenda of many countries besides India.

Studies have predicted that a 10 per cent increase in internet and broadband penetration would lead to a 1.08 per cent increase in India’s GDP. Thus, the country’s National Telecom Policy redefines broadband speeds and outlines aggressive targets for increased broadband penetration in the country, which currently stands at only 1 per cent

Currently, broadband is available to users through a variety of high speed wireline and wireless options including VDSL, fibre-to-the-home (FTTH), fibre-to-the-building (FTTB) and 4G/long term evolution (LTE). VDSL uses optical fibre feeder cable to connect the local exchange and the access network, from where data reaches subscribers over traditional copper cables. On the other hand, FTTH and FTTB connect the local exchange directly with subscribers’ homes or buildings. The latter set-up can also enhance the performance of 4G/LTE networks.

The proliferation of high speed communication devices and data-rich applications has led to an increasing demand for bandwidth. As a result, the capacity requirements of access and last mile networks have increased dramatically, and thus there is greater pressure on the core networks. Even as operators are deploying 100G systems to address core network capacity requirements, 400G prototypes are being tested or are under development. Typically, commercially available 100G systems, deployed on existing infrastructure have a limited signal reach spanning a few thousand kilometres. 400G systems are expected to have an even more limited reach of a few hundred km due to the higher optical signal to noise ratio (OSNR) requirement. To minimise the constraints on signal reach, it is imperative that advancements in fibre technology are incorporated into the system. The industry is also developing a new generation of low-attenuation fibres for enhancing signal reach.

Low-attenuation fibres

Most service providers in India utilise the standard G.652.D fibre, with performance as high as 0.21 dB per km, in their core and access networks. In comparison, a next-generation low-loss G.652.D fibre delivers about 0.18 dB per km at an operating wavelength of 1550 nanometres (nm). Over a span of 100 km, the 0.03 dB per km decrease in performance translates into an additional system margin of 3 dB, which can be leveraged by operators in the following ways...

Extending the operational life of OFC

Optical fibre cable (OFC) cuts are a major concern in developing countries like India. In some areas, cables have to be replaced every five to six years.  Each repair splice may reduce up to 0.2 dB of the OFC’s lifespan. The 3 dB of additional system margin allows 15 extra cuts to be accommodated on next-generation low-loss fibre across a 100 km span as compared to standard fibre over the same span. Hence, even at a conservative estimate of five cuts per year across a 100 km span, next-generation low-loss fibre will have an additional three years of repair resilience.

Future-proofing networks

In the absence of any transmission system upgrade, transition from 10G to 100G requires 10 dB of additional system OSNR. Through the introduction of advanced modulation formats and coherent detection, system providers have reduced this OSNR requirement by about 5 dB. However, due to the remaining OSNR deficit of 5 dB, the signal reach of commercially available 100G systems is limited to a few thousand km. Indian operators suggest that this distance is much lower (approximately 600-800 km) due to high fibre attenuation in existing cables.

OSNR challenge in transition to higher data rates

For fibres spanning 600-800 km, transition to 100G will entail the deployment of expensive additional optical-electrical-optical (OEO) mid-links in the networks. Low-attenuation fibres can help eliminate or minimise this need for OEO links by extending system reach and thus help reduce the capex required for migration to higher data rates, particularly for migration from 10G to 100G, but more so for migration to 400G.

Extending signal reach/reducing active equipment

In long-haul and metro networks, low-attenuation cables enhance signal strength and reduce the need for amplification, which, in turn, leads to lower capex and opex due to savings on active components and sites. This also applies to access networks where next-generation low-loss fibre helps improve performance despite shorter link distances.

A reduction of 0.03 dB in attenuation on a G.652.D fibre, from 0.35 dB per km to 0.32 dB per km at wavelengths of 1310 nm, can deliver 10 per cent of additional signal reach. This would lead to an increase of up to 20 per cent in subscriber, cabinet and antenna coverage areas.

Extended reach

In the case of gigabit passive optical network (GPON) technology, using the same transmission equipment, low-loss fibre will increase the maximum link length from 18 km to 19.7 km. This extended reach translates into an incremental coverage of 198 square km.

Transitioning to 10GPON with the existing OFC infrastructure

GPON systems use wavelengths of 1290-1330 nm for upstream data transmission, and 1480-1500 nm for downstream data transmission. GPON networks are designed taking into account attenuation at these wavelengths, particularly at 1310 nm.

In the future, when operators consider upgrading GPON systems to 10GPON or wide division multiplexing (WDM)-PON to enhance capacity, they will find that the transmission wavelength windows for these technologies exceed the fibre transmission spectrum. In particular, the upstream signals for 10GPON require wavelengths of 1260-1280 nm, while for WDM-PON, they would require wavelengths of up to 1610 nm. This implies that:

The spectrum used by the system is being broadened and hence there is a need for fibres that can operate effectively over broader wavelengths of 1260-1610 nm.

10GPON upstream signals can be limited due to high optical fibre attenuation encountered at shorter wavelengths. Low-loss fibres deliver equivalent attenuation at wavelengths of 1260 nm as standard fibre delivers at wavelengths of 1290 nm. Thus, migration to 10GPON on standard fibre will pose additional power costs, limit reach and require expensive reconfiguration of network architecture. The transition can be achieved without compromise on a low-loss fibre network.

10GPON also defines downstream transmission at longer wavelengths than PON, at about 1580 nm. Wavelengths above 1570 nm face high attenuation on standard fibres. Next-generation low-loss fibres offer low attenuation at longer wavelengths and can support transmission beyond 1600 nm.

Thus, deployment of next-generation low-loss fibres can enable subsequent introduction of 10GPON or WDM-PON, with minimal compromise on network architecture or reach.

Conclusion

Upgrading existing fire networks can reduce expenditure on and enable better utilisation of the active components of the passive fibre infrastructure. Service providers, system houses and fibre manufacturers can address today’s network challenges with the help of advanced system technology and next-generation low-loss optical fibres.