Optical Amplifiers--Key to WDM Cost Savings

WDM technology provides obvious benefits by increasing the capacity of existing optical fiber and thereby reducing or avoiding the need to install additional fiber. Another important benefit is the ability of optical amplifiers to reduce the need for costly electrical regeneration of optical signals in long-haul networks.

Optical signals become attenuated as they travel through fiber and must be periodically regenerated in long-haul networks. In SONET/SDH optical networks prior to the introduction of WDM, each separate fiber carrying a single optical signal, typically at 2.5 Gbps, required a separate electrical regenerator every 60 to 100 kilometers. As additional fibers were "lit up" in a long-haul network, the total cost of regenerators could become very large, and included not only the cost of the regenerators themselves, but also facilities to house and power them. The need to add regenerators also increased the time required to light new fibers.

The introduction of optical amplifiers in conjunction with WDM systems has significantly reduced the total cost of high-capacity, long-haul networks. A single optical amplifier is able to reamplify all of the channels on a WDM fiber without the need to de-multiplex and process them individually, with a cost approaching that of a single regenerator. The optical amplifier merely amplifies the signals, and does not reshape, retime, or retransmit them as a regenerator does, so the signals may still need to be regenerated periodically. But this can be done every 1000 kilometers or so. One optical amplifier on a 40-channel WDM system can potentially replace 40 separate regenerators (potentially more since longer spans may be possible between optical amplifiers than regenerators).

In addition to dramatically reducing the cost of regenerators, optical amplifiers and WDM systems greatly simplify the process of turning up additional channels. All that is required is to install additional transponders in the WDM systems at either end of the WDM links. The existing optical amplifiers simply amplify the new channel along with the others, without the need for additional regenerators.

These cost savings generally more than offset the cost of WDM systems themselves, and consequently virtually all long-haul network operators are deploying WDM technology. Even new operators with a large number of fibers available will generally use WDM to increase their capacity 2 or one fiber pair before lighting additional fibers.

Metro versus Long-Haul WDM

The economic trade-offs for the deployment of WDM are significantly different on long spans (say more than 100 kilometers) versus the shorter distances between switching equipment that are typical of metropolitan fiber networks (on the order of tens of kilometers). In long-haul networks, the cost of installing additional fiber is prohibitive and provides a strong incentive to increase the capacity of existing fibers via WDM before contemplating new fiber builds. The elimination of large numbers of electrical regenerators creates incentive to deploy WDM even when additional fibers are available. These savings more than offset the cost of WDM system hardware.

For distances of less than 100 kilometers, there is no need for regeneration or amplification. Typical long-reach optics currently available on SONET/SDH terminals and ADMs, and increasingly available on ATM switches and routers, can transmit over these distances. As a result, there are no regenerator savings to offset the cost of WDM system hardware. When it becomes necessary to add network capacity and additional fibers are available, it may make sense to light them directly with intermediate-reach or long-reach interfaces on ADMs, switches, or routers, rather than incur the additional cost of WDM hardware.

The cost of installing additional fiber also obviously differs in short-haul versus long-haul networks, and this affects the decision to deploy WDM or add fiber capacity when existing capacity is exhausted. Given the current cost of WDM systems and the ability to install a very large number of additional fibers with a new build, economics often favor installing and lighting additional fiber. However, 3rd generation DWDM Metro Systems, specifically designed for Metro applications, with a combination of low cost, small footprint and flexible topologies are starting to penetrate Metro area applications.

Implications for Optical Networking

The above analysis of long-haul versus short-haul WDM highlights some of the requirements that can be anticipated for optical internetworking equipment in the short to medium term. In optical internetworks, the data equipment is connected directly via fiber or WDM systems, without the use of separate SONET/SDH terminals or ADMs. In the long-haul network, high-speed data equipment is likely to interface to colocated WDM systems, requiring cost-effective short-reach interfaces to connect the data equipment to one or more channels provided by the WDM system. In metropolitan areas and other short-haul applications, WDM systems are beginning to appear as the newer technology better fits the service/ economic & phisically requirements of the Metro space. It is likely that data equipment will be required to interface directly to fiber, with resulting requirements for intermediate and long-reach optics, and to integrate all of the functions required to build a robust fiber-based infrastructure.

Want to Buy a Wavelength?

In the past, service providers wanting to procure large amounts of network capacity from another service provider have had two basic options: lease a SONET/SDH circuit (or perhaps an ATM virtual circuit), or lease a dark fiber. The advent of WDM has opened up a new possibility: the ability to lease one or more channels on a DWDM system. From the point of view of the buyer, the DWDM channel is much the same as a dark fiber, to which the buyer can attach his own ADMs, switches, or routers. There may, however, be some restrictions. For example, the DWDM system may require that the information transmitted by the client is SONET/SDH framed to facilitate performance monitoring by the provider of the channel.

Given the cost savings that can be provided by WDM, especially on long fiber routes, and the ability to easily add channels to an existing fiber as WDM technology advances, leasing wholesale WDM channels is becoming a cost-effective alternative to leasing SONET/SDH circuits. This is especially true for 2.5 Gbps and 10 Gbps circuit requirements, where SONET/SDH circuits may not be available. Thus WDM is starting to play yet another role in decreasing the cost of procuring the capacity required to meet growing data traffic requirements.