 2000s. As WDM capability grew further it became clear that better performance monitoring and finer switching granularity were required. Thus, ITU-T defined OTN (Optical Transport Network) which is getting deployed in many networks around the world. The transport network consists of both physical layer and protocol layer technologies as shown in Figure 7.8. The physical layer consists of technologies like SONET/SDH of top of the DWDM. The protocol layer (mainly layer 2) consists of ATM, Ethernet, and SO on, based technologies. Some of these key transport options for metro regional networks are discussed in this section. 7.4.1 Physical Layer Technologies The physical layer is the first and lowest layer in the seven-layer OSI (Open System Interconnection) model of computer networking. The physical layer supports the electrical mechanical, radio or optical interface to the physical medium. It does not include as such the physical media (copper, fiber, wireless). Examples include DSL, SONET/SDH, T1/E1, and SO on. Transmission Networks 7.4.1.1 Optical Fiber (Medium) The key advantage of optical fiber is its huge available bandwidth. In addition to this, one central characteristic of optics is that signals at different wavelengths do not interfere with each other. This allows the propagation of a multiple of signals at different wavelengths in parallel in the same fiber. This way of multiplexing signals is called Wavelength Division Multiplexing (WDM) and leads to a transmission capacity of several Tbps in commercial systems today. Typically, signals can be transmitted without amplification over a distance of around 80-90 km. This may suffice for so-called metro applications, that is, transmission within a city and the surrounding regional area [15]. Fiber to the X (FTTx) is a generic term for any network architecture that uses optical fiber to replace all or part of the usual copper local loop used for last mile telecommunications. The four technologies, in order of an increasingly longer fiber loop are shown in Figur