ries implementing the input queues and of the associated processors. Also this trend calls for a line bit-rate increase SO that useless multiplexing and demulti- plexing stages in the line cards need not be forcefully implemented. If all the above reasons push the evolution of transmission systems toward an increase of the bit-rate, designing systems at 100 Gbit/s is a formidable challenge. As a matter of fact, the presence of a great number of deployed transmission lines designed for 10 and 40 Gbit/s imposes a set of quite stringent requirements to practical 100 Gbit/s transmission systems. In particular, 100 Gbit/s channels should be transmitted for distances of the order of magnitude of 1000 km on existing lines and should coexist with 10 and 40 Gbit/s channels without requiring disruption of present services. Meeting these requirements is surely not possible with the same transmission technolo- gies adopted at lower rates, due to the way in which various transmission impairments affects their performances; thus a complete redesign of transmission systems is needed. In this chapter we will analyze the various alternatives for 100 Gbit/s transmission, tak- ing into account different applications in the telecommunication network. 9.2 100 Gbit/s Transmission Issues The first step to do in order to approach the problem of 100 Gbit/s long haul transmission is to understand impairments thwarting conventional transmission at a such a high speed to individuate potential solutions. In general, solutions to transmission impairments can be attained either via the use of suitable devices to eliminate or compensate the impairing effect or shaping the transmis- sion format and the detection strategy to be less sensible to the considered effect. We will consider compensation technique in this section while advanced modulation formats and detection techniques will be reviewed in the next two sections. 9.2.1 Optical Signal to Noise Ratio Reduction The first effect of the bit rate increase is the SNo reduction due to the increa