What is Dark Fibre?: rawcompute.in Glossary

Dark fibre is fibre-optic cable that has been laid but is not currently carrying data. It is “dark” because no light pulses are being transmitted through it. Telecommunications companies and infrastructure providers lay excess fibre capacity during construction, and they lease unused strands to organisations that need dedicated, private connectivity. The lessee provides their own optical transceivers and networking equipment to “light” the fibre, giving them full control over the protocol, bandwidth, and encryption used on the link.

Dark fibre is commonly used for high-capacity connections between two data centres (e.g., linking a primary and disaster recovery site), between a data centre and an office campus, or to connect to an internet exchange point. Since the lessee controls the equipment at both ends, they can upgrade speeds (from 10GbE to 100GbE to 400GbE) by simply changing the transceivers, without needing to lease additional fibre or pay the carrier for a higher-bandwidth service.

Why it matters when buying hardware

For organisations deploying GPU clusters across multiple data-centre facilities or needing ultra-low-latency links between sites, dark fibre provides the highest-performance dedicated connectivity available. In Indian metros like Mumbai, Chennai, and Hyderabad, dark fibre is available from providers like Railtel, BSNL, Tata Communications, and Airtel. When planning multi-site deployments, factor in the cost and lead time for dark fibre procurement. It can take 4-8 weeks to provision in most Indian cities.