Singlemode fiber (SMF) continues to be the dominant fiber type in use worldwide. Various types of premium-priced dispersion-shifted fiber (DSF) have entered the market over the past five years and captured almost 10% of fiber demand. These fibers have traditionally provided enhanced dispersion performance advantages for long-haul routes, although SMF is proving to be more economical and offer superior performance with many emerging high-capacity applications. Let's take a look at changes in system technologies and their economic and technical impacts on optical-fiber selection.
Optical-fiber systems have evolved rapidly over the last few years. New technologies continually appear to address the key system limiting factors in the design of optical networks. These limiting factors include:
Examples of such technologies are shown in Table 1 in the order of their appearance in the network. These new technologies have enhanced the capabilities of installed optical fibers as well as widened the choices for fiber selection for new routes.
In the process, this technological evolution has once again made standard singlemode the best economical and technical choice in most installations, including many long-distance applications with new 10-Gbit/sec systems. A review of how the capabilities of SMF and nonzero DSF (NZ-DSF) have changed as system technologies evolve illustrates this evolution:
Figure 1 shows the component requirements for 2.5- and 10-Gbit/sec systems operating over 600 km using SMF or NZ-DSF. The system design for 2.5-Gbit/sec systems is exactly the same for SMF or NZ-DSF. The only difference in a 10-Gbit/sec design between SMF and NZ-DSF is the amount of DCF used for compensation. While NZ-DSF requires external dispersion compensation at the terminal locations, SMF also requires compensation at the intermediate points since it has three times the dispersion along the fiber. With 10-Gbit/sec systems, this additional DCF requirement is the key difference between SMF and NZ-DSF networks. A cost analysis is required to evaluate which configuration is more economical, since NZ-DSF fiber is priced at a significant premium to SMF. Currently, NZ-DSF is priced at more than two times the price of SMF. With this cost difference, SMF will be the preferred choice unless there are other system costs or technical considerations.
For applications up to 2.5 Gbits/sec, system designs for SMF or NZ-DSF offer identical performance for DWDM applications. SMF is recommended because of significant fiber price savings.
For applications at 10 Gbits/sec, DCF costs need to be considered. At less than 80 km, SMF is clearly recommended again since the system requirements for SMF and NZ-DSF are identical. For routes over 80 km, a present value economic analysis is required. SMF requires DCF compensation for distances greater than 80 km, and NZ-DSF requires compensation at distances exceeding 300 km. However, NZ-DSF requires less compensation per kilometer than SMF.
Since DCF is only required when the fiber is upgraded or activated with a 10-Gbit/sec system, the DCF cost is likely deferred until it is activated with a 10-Gbit/sec system. Deferring the installation and cost of dispersion compensation has several benefits:
- The cost of capital or interest reduces the current value cost of DCF deployed in the future.
- DCF costs will likely drop in the future, enabling even greater savings.
- Alternative dispersion-compensating technologies will likely become available that will further reduce the cost of compensation or enhance performance.
An example of the expenditures and time value of money factors for a 240-km route length is shown in Figure 3. The true cost of DCF with SMF drops drastically for 10-Gbit/sec systems deployed in the future. These savings are even more significant if DCF price reductions or new technology costs are considered. For example, emerging innovations in fiber Bragg gratings or high-order mode conversion promise to improve external dispersion compensation in the next two to five years. As in Figure 2, this example shows savings with 10% of the fibers with 10-Gbit/sec systems at initial deployment and 10% of the remaining fibers are activated every two years assuming no reductions in DCF costs.
The network installer should evaluate the total system cost over the project lifetime to compare current value costs of alternatives for a true understanding of the financial impact.
With current system technologies, SMF and NZ-DSF have the same capabilities. SMF may require dispersion compensation with 10-Gbit/sec routes, depending on the route distance. This compensation investment is only required at the time the fiber is activated for 10-Gbit/sec transmission, thus enabling operators to defer spending.
As there have been numerous technical drivers in the past that change the key system design-limiting factors, there will be new technology changes that will continue to affect fiber choice.
The fiber choice for compatibility with future system drivers is dependent on the network technology outlook. If the future system growth is to be met with closer channel spacings or new amplifier bands (L or S), SMF best suppresses nonlinearity effects and will be capable of handling more channels. NZ-DSF applications with 50-GHz spacings are expected to be limited with 10-Gbit/sec systems. These limitations have been publicized in research findings by Alcatel and others at the recent Optical Fiber Communications Conference (OFC '99) in San Diego. In fact, several large network operators have recently selected SMF to futureproof their networks for emerging DWDM applications (see Fig. 4).With a migration toward 40-Gbit/sec time-division multiplexing, both SMF and NZ-DSF will require external dispersion compensation. At these data rates, dispersion will become a critical limiting parameter for both types of fiber. DWDM fiber transmission limitations from nonlinearities will not be completely known until the optimal future transmission format is chosen. This will determine how closely channels will be spaced. Further studies are being conducted by various manufacturers and network operators to characterize future fiber needs at 40 Gbits/sec and beyond.
Fiber selection for network installation requires an assessment of three key considerations. First, current system requirements must be evaluated. SMF and NZ-DSF are technically equivalent with current network transmission technologies. Both fiber types are capable of 700-km route spans with 2.5-Gbit/sec systems and up to 600 km with 10-Gbit/sec systems. Second, future upgrade requirements must be considered. SMF provides the best choice for future growth with narrower DWDM channel spacings due to suppression of nonlinearity effects. Current NZ-DSF products need to further optimize their dispersion and effective-area characteristics to compete with the close DWDM spacing performance of SMF. Finally, a project timeline of value cost analysis must be performed for 10-Gbit/sec applications to determine the cost tradeoffs over time.
Therefore, SMF is currently the cost-effective solution for applications under 2.5 Gbits/sec. SMF is also a lower-cost option for 10-Gbit/sec applications up to 80 km. For distances past 80 km, the most economical fiber depends on the timing of 10-Gbits/sec deployment due to different DCF needs between fiber types.
Jim Ryan is the new business development marketing manager for cable network products at Alcatel (Plano, TX). Questions on this article can be forwarded to [email protected].