Technology advances spur growth in undersea festoon networks
S. HANSEN LONG, T Soja & Associates Inc.
Driven by the forces of market deregulation, telecommunications privatization, end-user delivery technology, and demand for bandwidth in currently underserved niche markets, the unrepeatered submarine-systems market is experiencing continued steady growth. Combining DWDM technology with more fiber pairs (up to 96) over longer distances at higher bit rates, it is conceivable that future unrepeatered systems will soon equal-if not overtake-the fiber capacity of long-haul repeatered technology.
The introduction of advanced high-power post amplifiers and Raman pumps also helped pace the technical evolution of unrepeatered systems over the past few years. During this time, unrepeatered systems moved from single-channel to multiple-wavelength systems, while bit rates increased from the once impressive rate of 622 Mbits/sec to 2.5 and 10 Gbits/sec. Today, significant research advances have moved toward achieving the 40-Gbit/sec threshold in the near future.
In addition to increased system capacity, system length or reach is also an important consideration in unrepeatered systems. But there is a tradeoff. For example, achieving a high channel count over long unrepeatered links (e.g., >300 km) requires high channel launch power to overcome the limitations imposed by fiber loss. But launching high signal power into a system to overcome fiber loss results in an upper limit on launch power per channel caused by nonlinear effects.
This power limit depends on different fiber types such as singlemode fiber (SMF) and dispersion-shifted fiber (DSF), high effective area fibers (e.g., 110-micron fiber), and WDM system configurations such as number of channels, channel spacing, and modulation formats. The channel power limit as well as the combination of various receiver technologies determines the channel power budget achievable-thus, the maximum system length.1
Concurrent with advances in technology, unrepeatered submarine-cable systems have evolved within recent years in support of two basic architectures: point-to-point and festoon. Point-to-point systems typically consist of undersea links providing connectivity from mainland-to-island and island-to-island spans. Festoon systems, on the other hand, typically link coastal population centers and, along with providing an efficient interconnectivity of coastal cities, provide a diverse route to terrestrial networks (see Figure 1).
The enormous quantities of telecom traffic that can be carried over a single cable today exceed that carried in the early 1990s by more than 4,500 times. With such large capacity vested in a single cable, the risk of a major communications disruption is unacceptable should the cable suffer damage. Thus, route diversity is not only important, it is a vital element in today's cable-planning environment.
Fiber-optic festoon networks have been operational since 1989, when Telecom Italia's Italian Festoni became the first nationwide coastal fiber-optic network ever built. The festoon was ideal for Italy's geography, with its long coastline and multiple population centers. It was so successful that it lent its name to all of the coastal fiber-optic networks that followed.
Close to 40 such systems, spanning more than 26,000 route-km, may be found today in almost every region of the world. The Table provides a breakdown by geographical region of the total route-kilometers of festoon systems in place since 1989.
Since their inception, festoon systems have provided an attractive option for rapidly increasing network capacity and connectivity-and the growth rate of established systems has kept pace at an average annual compound growth rate in excess of 25%. Announced new festoon systems scheduled for completion between 2002 and 2005 continue this upward growth trend, increasing the total for in-place and planned systems to more than 36,000 km by 2005 (see Figure 2).
Early festoon systems typically incorporated 12-fiber single-channel and migrated to 48-fiber multiwavelength by the mid-1990s. Subsequently, demand for higher fiber counts in submarine cables was created by a number of factors, including increased customer capacity needs on both undersea and terrestrial routes, the elasticity effects of rapidly declining bandwidth prices, and the need to directly match undersea-cable fiber counts to those of terrestrial cables.
With increased fiber counts and the application of DWDM technology, festoon systems soon experienced dramatic increases in design transmission capacity per individual system. In the early '90s, this capacity was fixed at an average of 3.4 Gbits/sec per system. By 2000, installed festoon cable systems featured total design capacities in the terabit range, although initial capacity for these systems averaged 25-30 Gbits/sec per system (see Figure 3).
The festoon systems in place subsequent to 1999 and planned between 2002 and 2005 are rated at 10 Gbits/sec per fiber pair with a maximum of 24 fiber pairs and between 16 and 64 wavelengths per channel. As bandwidth needs grow beyond their initial installed capacity, these cable systems offer great potential for subsequent upgrading at competitive incremental costs.
The major advantage of festoon systems is that short-haul links of several hundred kilometers can be constructed without the need for repeaters, electrical power-feeding equipment (PFE), or submerged electrically active components found in submarine networks-nor do they need optical inline amplification facilities as required in terrestrial networks. Accordingly, a festoon system "can evolve more easily and at lower cost since it is just a question of changing the optical fibers. This is the major advantage over the previous generation of submarine networks that cannot evolve because of the excessive cost of changing the equipment."2 Achieving expandability sole ly through high fiber counts in submarine-cable systems could be viewed as a disadvantage, however, in view of higher implementation costs, capital and operational expenses, and complicated maintenance issues.
From an operational perspective, festoon systems offer the advantage of high reliability, based on low calculated failure rates, low installation timelines, and the provision of efficient backhaul transport networks directly connecting existing and planned transoceanic cable systems. By providing an efficient cable system from an international gateway directly into major coastal cities that terminate significant international traffic volume, festoon systems significantly reduce the cost of international communications. And "by selecting landing points that coincide with those of the international systems, nearly direct connection to coastal telecommunications hubs can be achieved through the festoon system."3
From a maintenance perspective, festoon systems contain fewer components such as repeaters and PFE, which are susceptible to failures that would require maintenance. Festoon-system installation procedures requiring cable burial and judicious seabed routing to avoid major fishing activity are also key to low maintenance exposure of these cable systems.
The permitting process has evolved in recent years into one of the most important considerations in the implementation of both festoon and terrestrial cable-system projects. The environmental impact of cable landings on environmentally sensitive locations is vigorously assessed and monitored by governmental authorities, and system developers must factor the time impact of the permitting process into their project completion timelines.
The technical improvements and price declines witnessed in unrepeatered systems have had a positive impact on the array of options available to customers, allowing them to optimize an offshore festoon-system route to their capacity needs within the business and economic constraints imposed by their particular business plans. A few years ago, such options were greatly limited. But today, with the availability of multiwavelength systems, high fiber counts, and increased bit rates per fiber pair, customers have greater flexibility in selecting system parameters and choosing the option that best meets their system capability and cable routing needs.
Furthermore, the system optimization process now allows customers to upgrade their systems within their budgetary limitations as their capacity requirements increase. In addition, customers now have an option to evaluate whether to upgrade or acquire new construction based on tools currently available from the industry. These tools do the following:
- Aid in the comparison of upgrading an existing undersea fiber-optic system or building a new one.
- Analyze and sensitize the internal and external variables of either option.
- Help the customer understand the effects of conditions on investment returns, while offering them the option of upgrading or purchasing.
The future of festoon systems is assured as they continue to serve the rapidly growing capacity needs of regional niche markets, interconnect international gateways to coastal population centers, and provide an efficient, viable, and diverse route to traditional terrestrial networks.
S. Hansen Long is a senior consultant for T Soja & Associates Inc., a telecommunications market analysis firm in Boston, and can be reached via the company's Website, www.tsoja.com.
- Y.R. Zhou and A. Lord, "System Design Considerations for Future Unrepeatered Systems," SubOptic 2001 Conference Proceedings, May 20-24, 1997, p. P3.9.
- J-M. Evanghelou, "The Mediterranean Has a New Generation Optical Network," France.internet.com, Jan. 1, 2001.
- A. Ducreux, L. Frame, J. Mariano, and T. Stamnitz, "Today's Unrepeatered Technology Yields a Viable Business Alternative to Traditional Terrestrial Systems," SubOptic 2001 Conference Proceedings, May 20-24, 1997, p. P3.3.