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Recent discussions on fibre-optic transmission systems in Africa, particularly submarine systems,  have left me with the feeling that a good deal of confusion exists over the usage of the term ‘capacity’. This post attempts to provide some clarity…

Capacity is a measure of the traffic-carrying capability of a transmission system, and is normally quantified in multiples of bits per second (bps):

  • kbps (thousands of bits per second)
  • Mbps (millions of bits per second)
  • Gbps (billions of bits per second)
  • Tbps (trillions of bits per second)

It may be used in a variety of ways to describe transmission systems, including:

Design capacity – the maximum traffic-carrying capability of the system if it were fully equipped using today’s  technology

Lit capacitythe actual traffic-carrying capability of the system today, based on what has been equipped to date

Purchased capacity – the amount of traffic-carry capability that has been sold to customers

Used capacity – the amount of traffic actually being carried on the system

Fibre-optics and associated technologies evolve very rapidly, with manufacturers and equipment vendors continually incorporating the latest developments into their products. One reason why submarine systems are never fully equipped at launch is that this allows their owners to take advantage of the latest developments throughout the lifetime of the system (typically 25 years). Most of the fibre will remain ‘dark’ (i.e. not immediately usable), until it is ‘lit’ using appropriate equipment within the landing stations.

Submarine owners  only light sufficient capacity to meet their near-term requirements – what they will use themselves + what they have sold to other customers (the purchased capacity) + what they expect to sell in the short-term. As this initial lit capacity starts to fill up, they go back to the market to evaluate and procure the very latest technology to support the next ‘upgrade’, lighting more capacity.

At launch in July 2010, the initial design capacity of WIOCC’s EASSy system was 1.4Tbps. Since then, developments in Alcatel Lucent’s product offerings – including a migration from 10Gbps to 40Gbps technology – have resulted in the design capacity growing to 3.84Tbps then 4.72Tbps. With the rapid growth in purchased capacity, EASSy’s initial lit capacity of 30Gbps will be increased significantly with an upgrade later this year.

Part 2 in the series will explore ways to create resilience in submarine systems.

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