Gbit/s exist and are used mainly for backhauling of remote local exchange in rural areas, more frequently CWDM wavelengths carry 2.5 Gbit/s SDH/SONET frame or directly application related formats line Fiber channel and FICON for data related applications or Gbit ethernet for small distance connections. Typical power budgets of 2.5 Gbit/s CWDM systems are 8 dB for short-haul applications, 12 dB for long-haul, and up to 16 dB for extremely long connections. An example of CWDM system used to backhaul three small local offices to a hub is shown in Figure 6.41. The adopted architecture is a protected bus that exploits the fibers already installed in the field. The first local exchange is reached by a complete path diver- sity, also using two diverse OADMs, since there were available two nearby but different sites on the ground. The second and the third local exchanges are reached with different paths but the OADM location is only one since two nearby locations were not available. In considering this architecture, it is to take into account that in rural areas, the points of presence not always are real central offices, but could also be big street cabinets where the local equipment is located. 6.4 Alternative Modulation Formats Albeit almost all commercial equipment for optical transmission uses either NRZ or RZ modulation; a great number of other modulation formats have been devised, either to par- tially relieve the impact of different transmission impairments or to assure better receiver sensitivity or better wavelength selectivity. Telecommunication Networks The most promising set of different modulation formats is coupled with the use of the so-called coherent detection and is constituted by different kinds of phase modulations. Even if in a few specific applications like single span systems, the greater sensitivity- provided by coherent detection of phase modulated signals could be useful, this kind of system never gave rise to commercial product due to their cost and to the success of IM-DD. In facing the chall