Undersea cable design capacities
Before July last year South Africa had only one major undersea cable serving its international bandwidth needs: SAT3/SAFE.
In early 2009 the SAT3/SAFE cable system had a capacity of 120 Gigabits per second (Gbps), which was upgraded to 340 Gbps in late 2009.
SEACOM, South Africa’s first alternative to the Telkom-controlled SAT3/SAFE system, launched on 23 July 2009 with a design capacity of 1.28 Terabits per second (Tbps).
Another cable system, EASSy (East Africa Submarine Cable System), launched early last month with a design capacity of 3.84 Tbps.
Even more cables promise to bolster South Africa’s available international data transmission capacity with WACS and ACE set to launch with capacities of 5.12 Tbps. WACS is due to go live in 2011, whereas ACE has announced a 2012 launch.
Main One, a privately owned cable running at 1.92 Tbps between London and Nigeria, has also announced their intent to extend the cable to South Africa.
That’s a lot of bandwidth, but how is it calculated?
These reported design capacities suggest a promising future for Internet access in South Africa, but how exactly are they calculated?
Kobus Stroeder, chairman of the WACS Management Committee said that design capacities are an academic exercise. He qualified this statement by saying that the capacities of cable systems can be increased as technology improves.
Suveer Ramdhani, Head of Product Strategy at SEACOM South Africa, explained that the design capacity on the SEACOM cable could already be improved with the introduction of “40 Gbps per wavelength” technology.
Both Stroeder and Ramdhani explained the basic calculation used to determine the design capacity of the WACS and SEACOM cables.
Calculating design capacity
Design capacity is determined from the number of fibre pairs in a cable, the number of wavelengths per fibre pair, and the amount of capacity per wavelength. Multiplying these numbers together yields the design capacity.
For example, Stroeder said that the WACS cable is set to have 4 fibre pairs, 128 wavelengths per pair and runs at 10 Gbps per wavelength between Portugal and South Africa. This yields a total design capacity of 5120 Gbps, or 5.12 Tbps.
For the more technically inclined, bandwidth on these cables is measured in standard SI units and not binary units, so a kilobit per second (kbps) is 1000 kbps rather than 1024 kbps.
Ramdhani said that the SEACOM cable has two fibre pairs, 64 wavelengths per pair and also currently runs at 10 Gbps per wavelength.
He also explained that the wavelengths per pair is dependent on repeater spacing as well as the generation of the repeaters. Repeaters are devices used to boost the signals at various places along the cables.
In simplified terms, the smaller the spacing between repeaters the stronger the signal. The tradeoff is that this requires more repeaters along the cable which translates to increased costs.
Increasing design capacity
When Ramdhani mentioned the introduction of 40 Gbps per wavelength technology he referred to increasing the speed per wavelength from 10 Gbps to 40 Gbps.
Alcatel-Lucent recently announced that it would be using their 40 Gbps technology for the extension of WACS from Portugal to the UK.
Ramdhani said that upgrading SEACOM’s infrastructure to make use of 40 Gbps wavelengths would be a relatively painless exercise. The process is complicated by the need to migrate traffic from old 10 Gbps wavelengths to 40 Gbps wavelengths however.
He said that such a migration wouldn’t result in service disruption, and may be easier if all the wavelengths on the cable haven’t been used up.
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