Name that tune

Jan 30, 2002 12:00 PM

International Data Corp. (IDC), a well-known research organization, estimates that the number of Internet users will increase from 150 million to more than 500 million in four years. Today's extraordinary bandwidth requirements necessitate that providers look for new solutions that maximize the capacity and flexibility of their networks, reduce their networks’ complexity and lower the costs of delivering service. Fortunately, the breakthrough technology of dense wavelength division multiplexing (DWDM) and the emergence of tunable lasers hold out the potential for pioneering service providers to increase their bandwidth and flexibility while simultaneously cutting costs.

Optical networks: Past and present

Historically, fiber-optic networks have been static and were able to accommodate long-distance networks only by using lasers fixed on a given wavelength with no flexibility to dynamically provision traffic to other wavelengths. Service providers cannot forecast how much capacity their customers will require. Therefore, the commitments made to purchasing lasers with a certain fixed wavelength can result in delays in turning up services. These delays can be especially serious when last-minute changes in network deployments change the wavelength requirements, leaving the provider stranded with a fixed-wavelength laser that it can't use.

Today, thanks to DWDM technology, each optical fiber can now carry in excess of 100 channels, delivering a huge increase in network traffic capacity, for both long-haul and metropolitan networks, without the massive infrastructure investment of laying new fiber-optic cable.

At first glance, the innovation of DWDM would seem to make a simple and perfect story of supply and demand, problem and resolution--but the reality is neither so simple nor so perfect. Most implementations of DWDM cannot consistently operate at their full theoretical capacity. Businesses move, people change ISPs, traffic patterns shift and problems arise because, although a DWDM network can carry many channels, it can't be reconfigured simply and quickly. The lack of flexibility in supplying capacity, inability to meet demand fluctuations, requirement for mountains of spare lasers and wavelength blocking at optical cross-connects are limitations with today's DWDM systems that are broadly accepted.

Enabling bandwidth on demand

Most of the limitations of DWDM can be addressed through the widespread deployment of tunable lasers, an emerging technology with the potential to enable bandwidth on demand, for the benefit of Internet users around the globe.

For example, when a provider purchases a tunable laser, it's not committing to just one wavelength, so it no longer needs to forecast demand for more than 100 wavelengths. Instead, the provider can purchase a single widely tunable laser to cover the whole C-band (1525 nm to 1565 nm) and another to cover the L-band  (1565 nm to 1610 nm).

Likewise, traffic fluctuations no longer result in lasers having to stand idle and fiber-optic cable having to carry less than its capacity. Tunable lasers enable providers to use this untapped bandwidth, via pre-programmed software that rapidly changes laser channels from one wavelength to another to avoid wavelength roadblocks. Dynamic wavelength provisioning enables the provider to use all lasers at all times, leveraging the full capacity of the network to meet peaks in demand.

Leveraging available fiber cost-effectively

While service providers will not often disclose details on the amount of time that they are unable to use bandwidth because of channel conflicts, industry watchers estimate that fixed-wavelength lasers are being sidelined for a significant and growing portion of their potential operating time. Networks cannot be planned for the average demand; Internet traffic is sporadic in nature, depending on the time of day.

More frequently, last-minute changes in network deployments are leaving these providers stranded with fixed-wavelength lasers they cannot use, which translates to valuable bandwidth lying idle. Offering greater excess capacity and provisioning new services through electrical cross-connects can address this issue, but the delivery time can stretch to months. Ultimately, this increases the per-unit cost of bandwidth the provider is able to sell.

Reaping the benefits of tunable lasers

Optical networking will benefit even more from tunable lasers when systems companies evolve their infrastructures to leverage the dynamic capabilities of this new technology.

Today, in the absence of such architectures, most service providers using tunable lasers do so to ease the planning problems associated with forecasting thousands of product codes and to reduce the number of lasers they have to hold as spares. As a result, only a fraction of the cost savings associated with deploying tunable lasers throughout their networks is realized. 

However, companies with foresight in both the long haul and the metropolitan markets are building new systems with racks of equipment and software controllers that are designed for lasers that can tune to any frequency. As these systems come online, and as organizations replace fixed-wavelength lasers with widely tunable lasers that can tune dynamically, fiber-optic networks will be able to fully realize the potential of dramatically increasing bandwidth supply for metropolitan and long-haul traffic and will realize significant cost savings as a result.

Going beyond narrowly tunable lasers

Narrowly tunable lasers, which typically tune across 20 to 30 channels, offer a reasonable approach to cost-effective sparing and product code management, but they don't go far enough. As bandwidth demands explode and networks increase in size, these lasers aren't sufficiently flexible to ensure maximum use of capacity. Providers need a more powerful and versatile laser technology that can support long distances and an almost unlimited number of channels.

Optical networking infrastructure companies are using widely tunable lasers to help communications providers maximize existing network resources, streamline the planning process for network growth and dynamically provision bandwidth to millions of Internet users around the globe. With widely tunable lasers, communications providers can get more bandwidth out of the same network by instantly moving traffic from overcrowded channels into unused channels on the fly.

Widely tunable lasers simplify the planning process. For example, when a provider purchases a tunable laser, it only needs to forecast the total volume of demand because a single widely tunable laser can currently cover the whole C-band, and in time, another will cover the L-band. Forecasting aggregate demand is challenging but much easier than the current situation, which requires providers to estimate demand for each of more than 100 distinct wavelengths.

Likewise, with just one widely tunable laser transmitter in reserve, a provider can ensure uninterrupted transmission on the wavelength of any failed transmitter, fixed or tunable. Using widely tunable lasers throughout the network delivers the maximum economies of scale. This approach to sparing enables companies to eliminate an inventory that could number several thousand spare lasers, replacing them with just a handful of widely tunable lasers. 

Widely tunable lasers also enable flexible add/drop multiplexing, allowing carriers to change traffic capacity and patterns at the network pinch points on the fly. 

Choosing the right tunable laser

Communications providers need to choose their lasers carefully because manufacturers and models have substantial differences. Some lasers tune over relatively few channels--typically 10 nm to 15 nm--but those will quickly become obsolete with the availability of lasers that tune to more than a hundred channels. Such widely tunable lasers are expected to come into the market in early 2001.  

One of the most promising tunable laser technologies leverages a dual sampled-grating design to achieve ranges of several tens of nanometers from a single waveguide output. Linewidth and modal purity of these devices can rival the best commercial DFB lasers. Some manufacturers are also choosing to monolithically integrate high bit rate modulators and amplifier functions on the same platform.  With a wavelength range of up to 40 nm, these lasers can cover either the full C-band or full L-band with support for channels on 50 GHz spacing and high output power.

Tunable lasers are increasingly deployed in networks throughout the world, but they are still relatively new. As a result, manufacturers are going through a learning curve: Tunable lasers have not yet demonstrated the same reliability as fixed-wavelength lasers. Nor do communications providers have many manufacturers to choose from.

Industry watchers are excited about the advent of widely tunable lasers. However, the challenge for vendors will be to break through the difficulties of manufacturing reliable products in high volume. Demand for tunable lasers is high--ElectroniCast, Corp., a technology-based independent forecasting firm, forecasts that the global DWDM optical component market will grow from $2.85 billion in 2000 to more than $8.4 billion by 2004.

Ultimately, demand will drive quality up and prices down, delivering communications providers and their customers more bandwidth for the buck.

Arlon Martin is Vice President of Marketing for Agility Communications. You can reach him at amartin@agilitycom.com

Visit Agility Communications online.

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