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Since 2015 coherent pluggable modules have been available in CFP MSA form-factor that specifies the power consumption to be less than 24W.The coherent CFP transceiver encompasses three blocks: optical transmitter, optical receiver and the signal processing unit as depicted in figure below. Since the analog-to-digital signal converter (ADC) and digital signal processor (DSP) are included inside the transceiver, these CFP transceivers are referred to as Digital Coherent Optics (DCO). The CFP-DCO can be plugged directly to 100G routers or switches equipped with client CFP ports. Additionally, the CFP-DCOs from the same vendor plugged into switches or routers from two different vendors can also be interoperable making CFP-DCO desirable.
However, the 100G client side optics market is already bursting with CFP2, CFP4 and QSFP28 form-factors as it is simply economic and attractive to implement high density switches together with these energy-efficient transceivers. The CFP2 MSA specifies the power to be less than 12W. Simultaneously, pluggable coherent market has conjectured the CFP2-DCO and CFP2-ACO. The CFP2-DCO, similar to CFP-DCO, incorporates the DSP inside the CFP2 module ever-while trying to keep the power consumption within the CFP2 limits, which is currently very challenging.
Since the DSP eats up a lot of energy, it would relax the transceiver requirements when the DSP is kicked out of the transceiver and placed on the line card/host board. The ADC is also moved to the host card, such that the electrical host interface is analog, which is why such a transceiver is termed as Analog Coherent Optics (ACO).
Since the DSP is out of the transceiver, CFP2-ACO modules cannot be plugged into any CFP2 client ports; it would require special host boards furnished with DSP. Interoperability between different switch vendors would also be an issue because of the use of proprietary DSP from the vendors. Similarly, an ACO would not be interoperable with DCO. Nonetheless, the CFP2-ACO module will be cheaper because of the market competition and additionally the same CFP2-ACO can be used for 100G and 200G transport as the module can support multiple modulation formats like DP-QPSK or DP-16QAM.
Though coherent pluggable optics brings the opportunity of pay-as-you-grow model, the nasty variety of coherent transceivers makes it difficult to understand the future-proof way to upgrade the network to 100G.