What you need to know about 100G
Transporting data was up to 16Gbps a matter of light on or light off, a stream of one's and zero's or as its official name said: "On Off Keying". Simply limited by physical laws, that doesn't work for 100Gbps.
To work around the limitations, two solutions were standardised, one still using the on/off keying but now using four different channels of up to 28Gbps, pretty much the maximum speed that could be reached using the on/off keying. Using four channels changed the design rules for WDM networks, especially where it comes to multiplexing and amplification. The standard for the modules containing the lasers is C form-factor pluggable (CFP).
The second technology that was created, is much more advanced but for a lot of network engineers a familiar one. Basically by modulating the signal the number of bits can be quadrupled, meaning at maximum physical speed, you can transport 4 times more data. The technology is much like the well known Quadrature Amplitude Modulation or QAM, in use in the cable television industry and wireless networks. In fiber optic networking, the technology is referred to as "Coherent" and the standard for the modules or optics is also CFP and CFP2 is on its way. This technology however brings us more than just 100Gbps, it brings also tools to calculate and compensate the effect of dispersion, which is one of the major limiting factors of 10Gbps optical transport and the coherent CFP's tunable, meaning the wavelength can be chosen at installation, limiting the number of spares. But as always, this comes at a cost too, for now, more amplification is needed for the same distance and the modulation also introduces additional latency.
The need for 100Gbps is obvious, the right choice, yet, is not that obvious. Due to the limitations and the current costs of 100Gbps modules, in much cases it is the most cost effective solution is to multiplex 10 times 10Gbps using Dense Wave Division Multiplexing (DWDM) technology. But starting from scratch, no doubt that 100Gbps is the better choice, management, maintenance and future expansion are all in favour of 100Gbps.
Relevant article: Building blocks of a 100Gbps network
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