Obtaining bandwidth and connectivity from traditional communication service providers gets more expensive as organisations grow. The accessibility of dark fibre now offers an affordable alternative for smaller organisations to create a private network with almost unlimited bandwidth using cost-effective and scalable DWDM technology.

Metro network pricing varies intra-city, city-to-city and country-to-country due to variables including digging trenches or accessing ducts to place multiple fibre optic cables, and access to leased fibre or wavelengths to counter upfront costs.
Long-haul networks are more consistent and incur fewer regulations. When leasing dark fibre, it’s common to sign an IRU (infeasible rights of use), which is a contract that cannot be undone of a period from 10 to 30 years and maintenance contracts renewable every five years with cost based on route miles and not how many strands of cable.

DWDM aids in mitigating these upfront and ongoing costs by combining multiple wavelength channels on a fibre pair, extending the reach of client-side signals like Ethernet and Fibre Channel. As a signee of the contract, the customer will be able negotiate maintenance options, leaving them to control their network, minimising business disruption.

Transport Mediums

The physical media, often called Layer 0, comes in two media types. The first to be released commercially is multi-mode fibre and is usually used within a local area network (LAN), intra-building connectivity and other shorter distance applications. The other is singlemode fibre that is used in dark fibre networks where SMF28 is the most laid fibre optic cable on the planet with LEAF fibre leading the way for long distance applications.

Multimode fibre, thanks to its large core, is less expensive and cheaper to implement than singlemode but is limited to a maximum distance of 2KM, 100BASE-FX, an instance of Fast Ethernet for optical fibre and requires relatively inexpensive instruments to splice and singlemode much more challenging with specialised tools needed.

Reaching Out

In the previous post, we saw how Ethernet has a limited capacity and as we’ve seen so far here, multimode fibre has a limited reach. But DWDM technology lends a hand to combine multiple client-side signals to send over a fibre pair using a transponder, thereby extending its reach from metres of Ethernet and multi-mode fibre that now extends to many tens of kilometres in a metro network on singlemode fibre.

On the road to dark fibre are two impeding factors. Firstly, attenuation causes a loss in strength of a signal over distance that could prevent a receiver from distinguishing between a digital one or zero as shown in Figure 1. Optical amplifiers are deployed to combat this thereby amplifying the signal but reaches a point of OSNR (optical signal to noise ratio) as amplifiers also amplify noise and thus cannot be added incessantly.


The second factor is dispersion, with chromatic and polarisation mode as two types of dispersion that subsequently impact data with errors. This dispersion happens due to meagre geometry on the fibre (oval than a healthy circular), the fact frequencies do not travel at precisely the same time, mechanical stresses and temperature changes on the fibre. These can cause substantial problems on longer distance systems but can be alleviated against using DCMs (dispersion compensation modules), a special reel of fibre optic cable. A balance is needed here too as DCMs have inherent attenuation, requiring additional amplification but a price worth paying for the wealth of benefits dark fibre delivers.

Fibre Dispersion

Here, we can see that although dark fibre does present challenges to the goal of virtually unlimited bandwidth, there are astute techniques developed to mitigate those issues. Dark fibre is the clear winner for meeting future bandwidth demands for decades to come, these challenges provide valuable rewards, with significant reductions at a cost-per-bit ratio.