ight-hand side of Figure 5-2. At a glance, this seems to imply that DWDM trumps SONET because it requires only one fiber pair (quadrant D) instead of multiple fiber pairs (quadrant B). However, the true economics vary with demand and deployment situations. Assume that the carrier in quadrant A has a demand for increasing bandwidth. Clearly, its preferred migration path is to deploy DWDM, rather than the architecture of quadrant B, which would require additional fiber. On the other hand, assume that the carrier is in the situation depicted in quadrant B and already has several parallel systems deployed along with a demand for increasing bandwidth. A migration path Dense Wavelength Division Multiplexing (DWDM) Systems Figure 5-2 Fiber Fiber Design comparison: Left: increasing Fiber demand but with no Fiber multiple systems Fiber already deployed. Right: increasing Before Before demand but with multiple systems A. Traditional Ring B. Traditional Ring already deployed. Configuration Configuration C. Ring Configuration D. Ring Configuration with WDM After After with WDM Fiber Fiber Fiber Fiber Fiber to DWDM, as specifically shown in quadrant D (the favorite final stage of proponents of DWDM), is not that optimal because it would leave a number of assets already owned by the carrier stranded. Hence, in this case, the migration path would probably be more like the one shown in Figure 5-3, and the dramatic saving implied by Figure 5-2 may or may not be forth- coming in this case. DWDM is well suited for long-distance telecommunications operators that use either point-to-point or ring topologies. The availability of 16, 32, or more new transmission channels where there used to be 1 improves an operator's ability to expand capacity and simultaneously set aside backup bandwidth without installing new fiber. Proponents make the case that this large amount of capacity is critical to the development of self-healing rings. By deploying DWDM terminals, an operator can construct a protected 40-Gbps ring with 16 separate communica