 at datarates that do not comply with SDH standards. In some instances, they can be mapped onto the SDH frame structure but in others this cannot be performed efficiently. One example is a specific transatlantic subsea network called TAT- 12/TAT-13 (Bergano, 1997), which operates at 5 Gbit/s. However, owing to the need for expensive customized terminal equipment, it is unlikely that future cables will be designed to this rate. A more general example is a protocol commonly used for storage area networks, called "Fibre Channel" (this spelling is used even in the USA) (Keiser, 2002; Fibre Channel Industry Association, 2008). Fibre Channel rates include 1.06, 2.12, and 4.24 Gbit/s. A final example is "Gigabit Ethernet," widely used in optical LANs, which operates at 1.25 Gbit/s (Keiser, 2002). Wavelength Division Multiplexing Wavelength division multiplexing (WDM) allows us to augment the capacity of a point-to- point optical fiber by launching M wavelengths, each of which is modulated to form a channel (Ramaswami & Sivarajan, 2002). In this way, modern systems have demonstrated up to multi-Tbit/s data transport. This section explains the various categories of WDM and where they are applied. The Concept The primary justification for point-to-point WDM is financial: It is usually cheaper to operate with a small number of relatively high-capacity fibers than with many single-channel fibers. Wavelength Division Multiplexing In a WDM system, the fiber's total capacity is the sum of the channels' individual datarates. Laboratory prototypes of WDM systems have been demonstrated with M up to ~1000, but at the time of writing M is usually less than ~100 in commercial systems. All channels are commonly transmitted at the same rate, but this does not necessarily have to be the case (Ito, Sekiya, & Ono, 2002). Indeed, the channels do not need to use the same data protocols, and some can even be analogue. WDM allows operators to mix various combinations of SDH, Gigabit Ethernet, Fibre Channel, Internet Protocol (IP), Frame Relay