In this book different researches on communication networks will be studied: 1. The WAN protocols with their different ring topologies. 2. The limitations of bit rate and losses in single mode fiber. 3. Ultra wideband system that improved gain-flatness of a Raman amplifier while maintaining a wide gain-bandwidth and a high gain-output power in WDM systems. 4. An 50 nm gain-flattened ultra wide band erbium-doped silica fiber amplifier for high capacity WDM optical communication systems. 5. Suitable packet switching Metropolitan Area Backbone Network to interconnect twenty-six locations. ...