Optical Layer planning requires flexibility
Carriers pondering expansion into the Optical Layer can start with today`s technology--provided an avenue remains for future advancements.
THOMAS FUERST, Alcatel Telecom
In much the same way that Synchronous Optical Network (Sonet) technology brought efficiencies to the Transport Layer, the emerging Optical Layer will provide better use of fiber bandwidth in tomorrow`s network (see Fig. 1). Already, carriers faced with a shortage of available fiber have turned to dense wavelength-division multiplexing (dwdm) to provide additional capacity on their existing fiber plant. The Optical Layer evolution will continue as technology matures and optical add/drop multiplexers (oadms), optical bidirectional line-switched rings (oblsrs), and eventually optical crossconnect systems (oxcs) make it into the network. It becomes critical, however, that carriers select technologies today that will not diminish the efficiencies promised by tomorrow`s all-optical network.
Carriers that are ready to begin the exploration of intangible concepts such as the all-optical network must often begin with a very tangible question: What fiber plant do I need to support this evolution? For some, the concern is with the embedded fiber plant and whether years of investment will be lost. For others, new fiber builds are planned, and the concern is that today`s investment must be sufficient for many years to come.
There are three basic types of singlemode optical fiber that are used in networks today--non-dispersion-shifted fiber (dsf), dispersion-shifted fiber, and non-zero dispersion-shifted fiber (nzdsf). All three types incur similar optical attenuation in the singlemode transmission windows of 1310 and 1550 nm, differing only in their chromatic-dispersion characteristics. Because chromatic dispersion reduces feasible span distances, the natural inclination is to choose a fiber with as low a chromatic dispersion value as possible.
This is true for single-wavelength transmission; however, dwdm dictates a different rationale. The problem is that as chromatic dispersion approaches zero, nonlinear effects such as four-wave mixing are intensified. As such, carriers must understand the specific advantages and disadvantages of each type of fiber in the evolution to the all-optical network.
Non-dispersion-shifted fiber, also known as standard fiber, was the first type to be commercialized and is the most prevalent type found in the network today. Standard fiber`s performance was optimized for first-generation 1310-nm transmitter technology because it offered relatively low chromatic dispersion and attenuation in this window. The erbium-doped fiber amplifier (edfa), however, offers the greatest gain in the 1550-nm window, which is where the evolution to the all-optical network will occur.
Standard fiber exhibits relatively high chromatic dispersion in the 1550-nm window (see Fig. 2), limiting the span distance benefits afforded by an edfa. Even with today`s tight-tolerance transmitters, OC-48 2.5-Gbit/sec transmission is limited to 540 km before electrical regeneration is required. Dispersion-compensating fiber, which provides a finite amount of negative dispersion, is available. However, its use adds loss, which in turn, limits system gain.
dsf, developed in the mid-1980s, reduces the amount of chromatic dispersion in the 1550-nm window and increases the dispersion-limited distance of 1550-nm transmission. The problem, however, is that the zero-dispersion point falls squarely within the edfa passband. dwdm channels transmitted near this zero-dispersion point have an increased susceptibility to nonlinear effects, effectively reducing the available bandwidth for multichannel dwdm applications.
The third type of singlemode fiber, nzdsf, was developed in the early 1990s to overcome this drawback of dsf. Also known as lambda-shifted fiber, it features a zero-dispersion point outside the erbium passband. This reduces the susceptibility to nonlinear effects and enhances its claim as the fiber of choice for dwdm applications. Lambda-shifted fiber has one drawback that carriers must consider: It can cost as much as 2.5 times as much as standard fiber. For this reason, carriers building a new network must consider both the technology and economic factors in making a fiber-plant decision.
Each of the two types of lambda-shifted fiber available today--Lucent Technologies` TrueWave and Corning`s smf-ls--provides a finite amount of dispersion throughout the entire erbium passband. TrueWave provides positive dispersion with a zero-dispersion point around 1523 nm, while smf-ls provides negative dispersion with a zero-dispersion point slightly higher than 1560 nm.
Today, carriers are taking the first steps into the Optical Layer with the deployment of fixed-wavelength dwdm systems. Yet carriers have already begun to realize the limitations of fixed-wavelength dwdm, which has led them to explore the technologies available to realize true optical add/drop capability. The oadm (see Fig. 3) will perform a role in tomorrow`s network similar to that which Sonet add/drop multiplexers perform in today`s--routing wavelengths with time-slot assignment and time-slot interchange functionality. The key difference, however, is that the oadm will add/drop wavelengths irrespective of their content. Carriers will be able to more efficiently manage traffic distribution and ensure network survivability by intelligently routing individual wavelengths.
Just as efficiency in the Sonet layer comes from grooming across the entire time-division-multiplexing (tdm) bandwidth, efficiency in the Optical Layer will come from using as much of the optical bandwidth as possible and providing unrestricted wavelength add/drop. With the flexibility to add/drop any wavelength, a whole new set of capabilities emerges for carriers exploiting this new layer. Carriers will now be able to sell individual wavelengths as the market demands and be able to control and manage each wavelength to ensure quality of service. Another is that carriers will be able to accept any bit-rate signal and efficiently route it through the Optical Layer. This universality of the Optical Layer will allow carriers to market "virtual fibers" to a wide range of customers, all from a single infrastructure.
Today`s investment in fixed-wavelength dwdm won`t be lost as the network evolves, however, because dwdm products can be designed for easy upgradability to an oadm. This product migration path will allow carriers to build incrementally upon the investments that they are beginning to make today. One of the keys to successfully navigating this evolution is using a flat-gain edfa as the backbone of the dwdm product. This will allow the same edfa and multiplexing equipment to be used to handle the wavelength add/drop that comes with oadm. In addition, because every optical channel is available for wavelength add/drop, restrictions on where the oadm is used are minimized.
Enhanced survivability
Increases in the amount of traffic carried in the Optical Layer demand survivable architectures. Carriers will desire millisecond restoration of traffic-bearing wavelengths in the event of a failure. A familiar restoration technique, the bidirectional line-switched ring (blsr), will be optimized for the Optical Layer to provide the answer.
An oblsr has several advantages over traditional electrical blsrs. Using dwdm, multiple electrical blsrs can be stacked on a single physical ring. However this can be a cumbersome network to administer and maintain. oblsrs provide increased network efficiency because each wavelength on the ring can be optimized for the traffic demands (see Fig. 4).
In this example, traffic requirements dictate an OC-192 connection between sites A and B, whereas only OC-48 is required between other sites on the ring. If this were an electrical blsr, every site on the ring would need to be an OC-192 10-Gbit/sec adm, leading to capacity inefficiencies for a large part of the ring. The oblsr allows survivable bandwidth to be deployed in the network only when and where it is needed, providing significant network cost advantages.
The oblsr also increases the survivability of other traffic types in a carrier`s network. For instance, linear point-to-point traffic that lacks the ring protection offered by an electrical blsr can be routed over the oblsr, greatly enhancing its survivability without requiring additional tdm investment.
Once core technologies sufficiently mature, oxcs will emerge as crucial bandwidth management tools in the Optical Layer just as Sonet crossconnects fulfill that role in today`s network. The oxc will provide wavelength management between optical rings and high-level restoration in mesh networks. Technologies required to build a true oxc are beginning to see commercialization. Much work, however, is yet to be done to improve its size and cost-effectiveness. The majority of carriers today should be more concerned that the oadm architecture they deploy provides an upgrade path to the oxc. Because today`s oadm is similar to a small crossconnect, the eventual upgrade to the oxc requires only an increase in the crossconnect matrix.
Optical access strategies
Eventually, the Optical Layer will stretch all the way to the access environment. Demand for wavelengths will continue to grow, and carriers will want to route traffic into the Optical Layer as near to the customer as possible. In addition, carriers will realize efficiencies from the Optical Layer in interoffice and interexchange networks and will desire similar efficiencies in the access network. Today`s understanding of optical access evolution is sketchy at best. However, one thing is certain: The requirement to maximize the available optical bandwidth will remain.
The technologies available today provide carriers with the ability to begin exploring the Optical Layer. There are, however, many variables that must be considered in successfully navigating this evolution, such as fiber type, edfa technology, and optical component maturity. Above all, it is imperative that carriers select technologies that provide as flat a gain as possible, ensuring a smooth product evolution from today`s dwdm to tomorrow`s oxc.u
Thomas Fuerst is a lightwave product marketing manager for Alcatel Telecom in Richardson, TX.