High capacity undersea cable goes live

High capacity undersea cable goes liveAdrian Morant The Gemini Submarine cable system has gone live doubling the transatlantic telecoms capacity between London and New York. The first leg entered service earlier this year with the full system scheduled for completion by the end of the year to provide a fully redundant self-healing network. Gemini is being built at a cost of over $500m by a Cable & Wireless and Worldcom joint venture. When completed the system will have a total capacity of 60Gbit/s i.e. 30Gbit/s working with a further 30Gbit/s protection. This will more than double the existing transatlantic capacity – currently estimated by C&W to be around 26Gbit/s. The installed cable, Gemini South, runs between Manasquan, New Jersey and Porthcurno in Cornwall. Gemini North will be installed between Charlestown, Rhode Island and Oxwich Bay in Wales. From these cable landing stations, the cable backhaul in each country extends to two separate termination points. In the UK, for example, they are at Aylesbury Street in London and at Bracknell. Constructed by Alcatel Submarine Systems, Gemini is the first of a series of new generation transatlantic systems that are being built – others including Atlantic Crossing 1 (AC-1) and TAT-14 – to meet the explosion in the demand for capacity caused by the Internet, commercial data traffic, and emergent multimedia applications. The Gemini group predicts that this demand is set to grow to 95 per cent in 5 years from today’s figure of just over 50 per cent. Even assuming no growth in voice traffic, this would amount to a roughly ten-fold increase in total demand. “The growth of Internet traffic makes prediction of future capacity requirements very difficult in absolute terms,” said a spokesman for C&W. “Data traffic has already overtaken voice traffic on the transatlantic route and there is every sign that, while there may be a short term glut as big new systems come into operation, demand will continue to grow strongly for the foreseeable future.” The system is entering service only eighteen months from when it was first announced in October 1997 – said to be an “unprecedented time from conception to service”. This has been a major advantage to Gemini as it has been able to offer capacity to, and to sign up, the new alternative telecommunications operators (being spawned by liberalisation) and Internet Service Providers (ISPs) before competitive cables are operational. In fact, Gemini claims that over 50 per cent of its capacity is already spoken for. Its standard unit of capacity is a 140Mbit/s VC-4 between London and New York. This will support up to 63 x 2Mbit/s paths each of which, in turn, supports typically 30 x 64kbit/s channels. As well as offering volume discounts for multiple VC-4 purchases, the company also offers sub-VC-4 rates. While providing a one-stop-shot between London and New York, the Gemini system is also able to provide easy onward connection to Continental Europe and beyond. This can offer the opportunity, as Gemini operations director Dave Shirt says, of “going East to go West”. Even though the full network is not installed, arrangements are in place for “mutual support and restoration” with the operators of other cables such as Cantat 3 and TAT-12/13. In fact, Gemini itself has already been used to provide a restoration path for one of the existing transatlantic cables. This was probably fortuitous as, when its full network is installed, it will be fully redundant and thus virtually immune from system breakdown. Technically speaking The Gemini transatlantic cable, the first leg of which has just been installed, will have a capacity of 60Gbit/s i.e. 30Gbit/s working with a further 30Gbit/s protection. However, according to Gemini operations director, Dave Shirt, the company expects to be able to increase this to meet future requirements. This high capacity is achieved by means of wavelength division multiplexing (WDM). The initial system specification called for 4 x 2.5Gbit/s paths (line rate 2.7Gbit/s including forward error correction). However, tests on the installed system have shown that the system can be operated at 6 wavelengths per fibre. An increase from 6 to 8 wavelengths per fibre would have the effect of raising the overall system capacity from 60 to 80Gbit/s. This will be achieved without making any changes to the submarine portion of the system and will thus be a cost-effective system upgrade. Gemini consists of two cables, each of which contains four non-dispersion shifted optical fibres (two pairs) operating in the 1560nm window. Erbium-doped fibre is used with duplicated pump lasers for optical amplification, spaced typically at 70km, and providing 12dBs optical amplification in this window. Electrical power is fed to these amplifiers using 1.2A constant-current generators in the terminal stations each side of the Atlantic. End-to-end system voltage is approximately 10kV. When the system is completed there will be SDH rings for the ‘dry’ portions of the network each side of the Atlantic as well as for the ‘wet’. The SDH ring topology provides a high degree of network resilience as there is no single point of electrical or physical failure. The paths in the rings will be widely separated physically to ensure that, should there be any problem, it will not affect both paths. Furthermore, over 500km of the 6,200km total length of the wet cable was plough buried into the sea bed at a depth of typically 0.6m so as to minimise the risk of damage due to external forces. Add-Drop multiplexers broadcast the signal from the entry point both ways around the ring. At both exit points, the better of the two received signals is transferred across to the next ring. As a result, the network is resilient and could be subjected to multiple cable and node failures without loss of customer traffic. The simplicity of the protection routing arrangements enables very fast switching (less than 50mS) without the need of complex path monitoring in the event of failure. The Gemini engineers calculate that the downtime due to electrical failure, totalled over the 25 year design life of the system, will be less than 1.5 minutes — an availability of better than 99.9999 per cent. High bandwidth, which is now the norm across the LAN, is increasingly becoming available across the campus, the metropolitan area network (MAN) and even the WAN. Gemini, and similar networks, will enable the same levels of transparent connectivity to span the Atlantic.
It is unfortunate that, with all this increase in bandwidth, no one has been able to reduce the five hours difference between the opposite sides of the pond and save us staying up late to use it.

Leave a Reply

Your email address will not be published. Required fields are marked *