Preparing for the long haul

Oct. 1, 2001

The next step on the capacity maximization route will be 40 Gbits/sec, and service providers want 'build on what we have' technologies.

STEPHAN RETTENBERGER and WILL RUSS, Optisphere Networks Inc.

Service providers competing in today's Internet-driven marketplace increasingly find themselves caught between the pro verbial rock and a hard place. On one side is the growing demand, on the part of both business and residential subscribers, for high-bandwidth services that can support their voice, data, and video applications. Obviously, any service provider that wants to ensure long-term profitability and expand its customer base must invest in enough network capacity to keep up with customers' insatiable appetite for bandwidth.

On the other side, pricing pressures in the marketplace, combined with all the budgetary restrictions of today's tough economy, are forcing service providers to look for every possible way to reduce their operating costs. Given the intensity of competition and the current economic climate, they must be able to build a solid business case for every capital expenditure-and that includes investments in additional network capacity.

There are two basic strategies available to service providers trying to expand the transport capacity of their infrastructures. They can build larger networks, or they can use advanced technology to extract more capacity from the networks they've already built. Building larger infrastructures presents several problems, primarily be cause it typically means doing parallel overbuilds of the transport network.
Depending on network topology, a 40-Gbit/sec DWDM system can deliver configurable "multihaul" capabilities, such as short, fat pipes; thinner long-haul pipes; and ultra-long-haul pipes.

As they reach the capacity limits of their embedded DWDM platforms, service providers all too often discover those platforms do not scale readily or cost-effectively. Confronted by a rigid system architecture, they typically have to invest in a second platform in order to obtain more capacity. That means that service providers seeking to double the capacity of their existing platforms are forced to invest in double the amount of fiber and associated equipment.

In addition, because this capacity-expansion scenario affects several network locations, it slows down operations dramatically-a situation that no service provider can afford in today's industry, where time-to-market is a critical success factor. As if all that wasn't bad enough, the parallel-overbuild strategy also requires service providers to invest even more time and money in shifting traffic onto the new systems.

Yet perhaps the biggest problem with building larger networks to obtain more capacity is the fact that capacity demand, both current and future, varies dramatically from one route to another. As a result, it is virtually impossible for any service provider to cost-justify an investment in what could well turn out to be too much capacity. For most service providers operating in today's capital-constrained economy, building a larger network is not a practical solution to the capacity problem.

Instead, many service providers, in an effort to minimize their investments in additional capacity, are looking for technologies that enable them to use their existing fiber infrastructures more efficiently. Specifically, they are seeking configurable optical-networking solutions designed to get the maximum transport capacity from embedded fiber. From a service providers' perspective, "configurable" means systems that can be deployed where and when traffic volumes really do justify more capacity, without necessitating expensive forklift upgrades.

Ideally, what service providers want is a single DWDM platform that scales easily and cost-effectively to a higher bit rate per wavelength and a higher wavelength count.

In response to service providers' requirements for such a flexible, scalable system, some vendors now offer a 40-Gbit/sec "multihaul" optical transport solution. A modular upgrade to 1.6-Tbit/sec DWDM platforms, the 40-Gbit/sec solution basically consists of a compact 40-Gbit/sec multiplexer and regenerator system. Service providers can plug this "thin mux" system into an existing DWDM platform and then transport 10-Gbit/sec traffic transparently on 40-Gbit/sec wavelengths. This configurability effectively allows them to double the capacity of an existing DWDM platform on a route-by-route basis. In other words, they can use one system to create "fat" pipes on routes where traffic volumes require more bandwidth, while operating "thinner" pipes on routes that carry relatively lower traffic volumes.

For example, using this flexible, scalable, optical transport solution, a service provider can deploy 80 wavelengths at 40 Gbits/sec per wavelength to obtain a total transmission capacity of 3.2 Tbits/sec on a single strand of fiber. By plugging the 40-Gbit/sec multiplexer/ regenerator into an existing DWDM platform, service providers can readily obtain multihaul flexibility, as well as the operational and financial advantages of 40-Gbit/sec transport. These advantages include:

  • The ability to optimize network capacity on a route-specific basis.
  • Simplified network design and stream lined operational tasks.
  • Significant savings from reductions in the incremental costs of providing bandwidth to end users.
Service providers are looking for ways not only to reduce their per-bit transport costs, but also to minimize their investments in client router interfaces. As more service providers have de ployed 2.5- and 10-Gbit/sec optical transport systems in the core optical network, router and switch vendors have brought their products up to those speeds as well. In fact, many vendors of Internet backbone routers are now developing 40-Gbit/sec interfaces. With the appearance of faster routers in the marketplace, more service providers will have to carry 40-Gbit/sec client signals across the core network. Consequently, they will need to scale their core bandwidth to handle those signals.
Transponder costs tend to dominate optical-networking costs. Multiplexing technology in general, and 4x10 Gbits/sec to 40 Gbits/sec in particular, can reduce the cost of optical networks by 20% or more.

Again, they have two possible options for doing so. The first involves the use of DWDM technology and a 40-Gbit/sec to 10-Gbit/sec inverse multiplexer to add four more 10-Gbit/sec channels. However, this option requires major investments in equipment and time, and it makes service provider operations even more complex. The second option, implementing a modular 40-Gbit/sec solution, gives service providers the flexibility to provide Internet backbone capacity precisely where and when they need it. Using this compact, plug-in solution, with its low power consumption, they can not only offer a 40-Gbit/sec point-to-point service, but also minimize their equipment investments and streamline their network operations.

Compared to a 160-channel 10-Gbit/sec solution, the 40-Gbit/sec optical transport solutions now appearing in the market can provide twice the capacity with only half the number of wavelengths. In addition to making the most efficient use of installed fiber, a 40-Gbit/sec solution offers service providers another distinct benefit-it can cover significant distance without requiring electrical regeneration. As a result, service providers can eliminate the major costs, in terms of money, power, and floor-space, typically associated with multiple reshape/retime/ regenerate (3R) sites.

Some 40-Gbit/sec solutions can reach 1,000 km without regeneration, given the appropriate fiber quality, amplifier spacing, and maturity of system components. Such a solution relies on several technologies to eliminate the major barriers to 40-Gbit/sec transport and thereby delivers both economical capacity and reach.

For example, a 40-Gbit/sec solution not only uses a higher bit rate per channel to deliver more capacity, but also achieves greater spectral efficiency. Specifically, a 10-Gbit/sec solution may be spaced in a 50-GHz grid, while a 40-Gbit/sec solution, which uses more spectrum per wavelength, can be accommodated only on a 100-GHz grid.

Aside from capacity considerations, service providers and vendors have long recognized that the biggest obstacle to 40-Gbit/sec transport up to now has been getting the systems to cover the longest possible distance before requiring regeneration. The greater the distance an optical signal must travel, the more that signal degrades, requiring expensive 3R overhaul.

Consequently, optical signal-to-noise ratio (OSNR), power-level ripples, and/or such non-linearities as four-wave mixing have limited network spans. To cover greater distances on specific routes, prior to needing regeneration, 40-Gbit/sec solutions now incorporate three technologies: forward error correction (FEC), high-power erbium-doped fiber amplifiers (EDFAs), and distributed Raman amplification.

FEC techniques basically fix corrupted data packets to reduce the OSNR requirement for a given bit-error rate. The optical signal can then travel much farther before needing regeneration. For example, the use of FEC techniques in submarine-system applications has quadrupled an optical network's reach prior to signal regeneration.

In addition to incorporating high-power EDFAs, some 40-Gbit/sec solutions also feature optional Raman amplification, which service providers can use to supplement the EDFAs. Raman amplification is a technique that, by directly connecting a laser pump to the fiber span, makes the span itself the amplifying medium.

The optical signal travels down the fiber and encounters "light" from the Raman pump. That light, operating on a shorter wavelength, transfers some of its energy to the longer, traffic-bearing wavelengths and, in doing so, amplifies the signal. With the fiber span itself functioning as the gain medium, the signal does not have to go through extra fiber as it would in an EDFA. Therefore, it "sees" a shorter span. Raman amplification, combined with traditional EDFA technology, delivers even more powerful performance and significantly extends the reach of an optical transport system. Further, by incorporating an advanced link-control mechanism with the EDFAs, vendors of 40-Gbit/sec solutions can eliminate power fluctuations and gain ripples.

Another major limitation on the traditional reach of optical networks is dispersion, or "smudging," of the optical signal. There are two basic types: chromatic dispersion and polarization-mode dispersion (PMD). The different wavelengths produced by a single laser (within a single pulse) travel down the fiber at different speeds. As a result, they spread, meaning they undergo chromatic dispersion. By contrast, PMD occurs when variations in the fiber itself cause the two polarization modes of a signal to spread as they travel down the fiber. Leading vendors tackle both types of dispersion-and ensure greater signal reach-by using additional fine-tuning techniques on the wavelengths carrying the 40-Gbit/sec signal.

The emergence of 40-Gbit/sec transmission technologies effectively resolves the capacity and cost-reduction challenges confronting service providers. However, it obviously does not address another equally tough problem-how to engineer a 40-Gbit/sec route in a cost-effective way and integrate that route with the rest of the network. As noted earlier, time-to-market is a crucial success factor in today's industry. That, combined with the difficulties service providers face in hiring skilled personnel, means service providers do not have the luxury of spending months of engineering time and expertise in getting a 40-Gbit/sec solution operational in their networks.

Recognizing the difficulty of this engineering-and-integration issue, some vendors include network-design tools with their 40-Gbit/sec solutions. For example, a PC-based modeling tool with a graphical user interface enables service providers to key in data related to their respective networks, simplifying and speeding up network design, engineering, and cost analysis. The modeling tool then allows service providers to test different configurations and choose one that optimizes the 40-Gbit/sec solution in their individual networks. Such a tool can model a 40-Gbit/sec route and "place" the equipment in minutes, rather than months, boosting operating efficiencies and reducing cost.

Caught between end-user demands for more bandwidth and the need to reduce the costs of delivering that additional bandwidth, service providers must find an economical way out of the dilemma. As both corporate and consumer subscribers adopt more bandwidth-intensive applications, competing service providers obviously cannot afford capacity shortfalls anywhere in their networks. Neither can they afford, however, to make network-wide investments when more capacity is needed only on certain network "hauls."

The ideal fix is a multihaul solution-a single optical transport platform that allows service providers to create "fat" pipes where and when actual traffic volumes call for fat pipes, while simultaneously running "thinner" pipes on routes with lighter traffic volumes.

An advanced 40-Gbit/sec solution effectively solves the capacity-versus-cost dilemma by allowing service providers to deliver additional bandwidth on an as-needed basis. As a result, they can continue to satisfy the subscribers' growing demands for bandwidth while, at the same time, trim the cost-per-bit of transport.

Some optical transport vendors, recognizing the different requirements of new market entrants and incumbent providers, have designed 40-Gbit/sec solutions to satisfy both. New competitors now have a complete multihaul solution available that delivers 2.5- and 10-Gbit/sec channel rates-and any combination thereof. Such a platform not only scales readily in a multivendor network, but also supports diverse network architectures.

Established service providers, like their newer competitors, can configure a 40-Gbit/sec solution to accommodate their evolving business strategies. Similarly, both new and incumbent providers can take advantage of the network-management solutions available with 40-Gbit/sec systems-integrating additional functionality with the network and element layers. In short, a multihaul optical transport system resolves today's capacity-versus-cost dilemma and ensures it will not recur any time soon.

Stephan Rettenberger is director of product-line management and Will Russ is senior manager for solutions marketing at Optisphere Networks Inc. (Reston, VA). They can be reached via the company's Website,