Communication has always been an  important aspect of human life from stone age to this modern era. However the mode of communication gradually changed with the passage of time. Telecommunication came into existence  about 150 year ago. Initially only voice communication was part of telecommunication. Systems were designed keeping in view the voice transmission. It was low capacity, short distance, and purely frequency based analog system. This was know as FIRST generation  system. Use of data on this voice network started in SECOND generation systems. Internet,  private data transfer, Tele printer, Fax, 9.6 kbps data for defence, were application supported by second generation voice network. This system was designed for voice communication but data was also supported by it. Now in this modern era we had THIRD generation system which is design for Data communication and also support voice on it. It support high speed data communication such as Broad band, Teleconferencing, Video conference, Internet, GPRS, Edge, Core networking   and so on.

To meet  demand of very high  BANDWIDTH   due to exponential increase in telephone, mobile, and Data  networks, Time Division Multiplexing (TDM)  switching technique was used in Telecom Networks .Media  such as coaxial cable  and microwave were aving  their limitation such as limited capacity, disturbance due to atmospheric effect, cost and maintenance  problem. OPTICAL FIBER cable were having capabilities to over come hese limitations and has been proved as the best replacement. Use of optical fiber resulted in a revolution in telecommunication transmission.

In  analog  carrier  communication  system,  the  frequency  division multiplexing  (FDM) method is often adopted to make full use of bandwidth resources of cables and enhance the transmission capacity of the  system. It involves transmitting several channels of signal imultaneously in a single cable and, at the receiver end, utilizing band pass filter to filter the signal on each channel according to the frequency difference among the carriers. But this TDM/FDM was not utilizing the complete Bandwidth of optical fibers. This resulted in optical fiber frequency division multiplexing (OFDM). This method can also be used to nhance the transmission capacity of the system .In fact this multiplexing method is very effective  in optical  communication. Unlike  the frequency division multiplexing in analog arrier communication system, optical fiber communication utilizes optical wavelength as single carrier. It divides the  low attenuation window of optical fiber (Appendix A)   into several channels according to the frequency difference of each wavelength channel. Further, it implements multiplexed transmission of multi channel optical signals in a single fiber. Since some optical components (such as narrow-bandwidth optical filter and coherent laser) are currently not mature, it is difficult to implement ultra dense optical frequency division multiplexing of optical channel. However,  alternate channel optical frequency division multiplexing can be implemented based on current component technical level. Usually, multiplexing with larger channel spacing (even indifferent windows of optical fibers) is called  optical wavelength division multiplexing(WDM)   and WDM  in   the same  windows within a band (C,L…))  with smaller channel  spacing is called as dense wave division multiplexing(DWDM)  (Appendix  B) With the advancement in optical technologies, anometer level multiplexing can be implemented.  Multiplexing of 8,16,32 or more wavelengths  with smaller wavelength spacing is called DWDM.