ITU‐T's Technical Reports and Specifications 237 Dense Wavelength Division Multiplexing (DWDM) This is an optical multiplexing technology used to increase bandwidth over existing fiber networks. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. The technology creates multiple virtual fibers, thus multiplying the capacity of the physical medium. WDM has revolutionized the cost per bit of transport. Owing to DWDM, fiber networks are capable of carrying multiple Terabits of data per second over thousands of kilometers – at cost points unimaginable less than a decade ago. State‐of‐the‐art DWDM systems support up to 192 wavelengths on a single pair of fiber, with each wavelength transporting up to 100Gbit/s capacity – 400Gbit/s and one Terabit/s on the horizon. DWDM provides ultimate scalability and reach for fiber networks. Without the capacity and reach of DWDM systems, most Web 2.0 and cloud‐computing solutions today would not be feasible. Establishing transport connections as short as tens of kilometers to enabling nationwide and transoceanic transport networks, DWDM is the workhorse of all the bit‐pipes keeping the data highway alive and expanding. Figure 22 – Optical transport to optical networking: evolution of the photonics layer Use of DWDM allows providers to offer services such as e‐mail, video, and multimedia carried protocol (IP) data over asynchronous transfer mode (ATM) and voice carried over SONET/SDH. Despite the fact that these formats‐IP, ATM, and SONET/SDH‐provide unique bandwidth management capabilities, all there can be transported over the optical layer using DWDM. This unifying capability allows the service provider the flexibility to respond to customer demands over one network. Ethernet data link In a world that is moving to a packet‐based future, Ethernet is the dominant data‐link protocol for today's networks, supporting a multitude of communication applications. Also, Ethernet is one of the key protocols used to interconnect routers and to carry applications in high‐speed optical networks to backhaul access traffic.