9.1 Dark Fibre
Dark fibre are strands of fiber optic cables that are deployed underground, but are not currently being used. Because there is no light being transmitted across them, they are considered dark. While any type of fiber (single or multi-mode) can technically be dark, the term dark fibre typically refers to 9 µ single mode fiber. Dark fibre can be privately owned, typically across a campus-style network, or can be leased from a provider to create a metropolitan area network (MAN).
Figure 4. Dark Fibre
This example implementation is based on using dark fibre as a physical medium—that is, as a direct connection between two switch points. Other solutions that can use dark fibre as an underlying medium, such as CWDM and DWDM, are addressed later.
The maximum distance for dark fibre depends on a number of different factors, including signal attenuation and switch optics. However, 10 km is generally considered the maximum distance for a standard dark fibre link. In some special cases, it is possible to achieve distances up to 80 km for 10 GbE and 40 km for 8 GB Fibre Channel traffic, but these types of links must be designed to meet very stringent performance criteria and require specialized optics. Currently, dark fibre can support bandwidth up to 100 Gbps.
Because each fiber pair is dedicated to a pair of switch ports, different types of Layer 2 traffic cannot be mixed. This means, for example, Ethernet and Fibre Channel traffic cannot be mixed. To send both types of traffic across dark fibre, at least two pairs of fiber strands must be leased, one for each traffic type.
In summary, dark fibre has the following key design considerations:
• 9 µ single mode fiber
• Viable for campuses and extended distances
• Used in pairs
• Can support any available bandwidth
• Typical distance is up to 10 km
• Up to 80 km possible with 10 GbE and specialized components and media
• Up to 40 km possible with 8 GB Fibre Channel and specialized components and media
