d by environmental conditions, making it difficult to determine ways to offset its effect on high-bit-rate systems. Carriers are forced to test their constituent spans of fiber to determine their compatibility with STM-64/OC-192; in many cases, PMD will rule out its deployment altogether. Dispersion Figure 3-4 (psec/nm/km) Erbium Chromatic dispersion. Window Positive-dispersion Non-zero Dispersion-shifted Fiber Conventional SMF Dispersion-shifted Fiber Negative-dispersion Non-zero Dispersion-shifted Fiber 15475 16475 Wavelength (nm) Chapter 3 A Third Approach-DWDM DWDM (discussed in greater detail in Chapter 5, "Dense Wavelength Division Multiplexing (DWDM) Systems") is a technology that enables multiple information streams to be transmit- ted simultaneously over a single fiber. DWDM technology utilizes a com- posite optical signal carrying multiple information streams, which are each transmitted on a distinct optical wavelength. The DWDM approach multi- plies the baseline 2.5-Gbps system capacity by up to 80, 120, or 160 times, providing a major increase in throughput while utilizing the embedded fiber. Sixteen channel systems supporting 40 Gbps in each direction over a fiber pair are commercially available; systems are now entering the mar- ket that can support 32 channels operating at 10 Gbps each, for a total of 320 Gbps. To transmit 40 Gbps over 600 km using a traditional system would require 16 separate fiber pairs with regenerators placed every 35 km, for a total of 272 regenerators. A 16-channel DWDM system, on the other hand, uses a single fiber pair and four amplifiers positioned every 120 km, for a total of 16 regenerators to transmit over 600 km. The most com- mon form of DWDM uses a fiber pair: one for transmission and one for reception. Systems do exist in which a single fiber is used for bidirectional traffic, but these configurations must sacrifice performance and reliability. The benefits of DWDM for increasing capacity are clear; however, this increase does not come for free. The cost can be as 