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The MSA and IEEE bodies work to standardize different transceiver form factors and technical specifications so that the transceiver manufactured by various vendors can be a simple plug-and-play unit with universal interoperability.But with the rainbow of available 100G transceivers, standardized (for up to 40km) and non-standardized (metro distances), and their associated difficulties, many think 100G transceivers are a plug and pray module. As funny as it sounds, it might be a nightmare for the engineers working hard to make a 100G link up and running. Throw on top of that the probability of multi-vendor interoperability failure. Are we already heading to that point of time where we need to have a standard for the metro 100G transceivers as well?
The surge of the 100G links is nothing extraordinary but simply natural as the world and every part of our daily life get connected and service providers endeavor to push higher data rates throughout.
While having 100G link might be a requirement for some, others are simply staring at the array of available solutions and the technical difficulties therewith, and waiting for someone to rise up and give them a plug and play solution.
So where is the industry heading?
For metro distances, here defined as links > 40km distances, there aren’t any standard 100G transmitters. Few years back, the only way of achieving this was via a transponder that has a line card transmitting 100G services. But in the last two years, two pluggable metro 100G transceivers have gained popularity, viz. direct detect and coherent. Since these two transceivers are based in two completely different technologies, they are not interoperable.
Predominantly suited to trans-oceanic submarine or terrestrial long haul applications, the performance of the coherent detection is undeniably superior to that of direct detection. In the case of coherent detection, the opto-electric conversion process is linear. Thus the phase information embedded in the optical signal is preserved permitting the straight forward electrical compensation of fiber linear effects including chromatic dispersion (CD) and polarization mode dispersion (PMD).
However, the hardware required to perform coherent detection is somewhat more elaborate and comprises a local oscillator, a 90° Hybrid module necessary to discriminate the phase quadrature of the received optical signal, and four balanced photodiodes to detect the signal from a single polarization. The higher price per bit due to its digital signal processing (DSP), greater power dissipation and complexity are hindering the quick deployment of coherent CFPs.
The coherent technology is now already moving towards CFP2 form factors, with reduced footprint to attract hot-pluggable applications. The DSP being the most energy consuming part can then either be integrated into the transceiver (DCO-CFP2) or into the line card (ACO-CFP2). It is because of different flavors of DSPs from different DSP chip manufacturers and system vendors, the coherent transceivers are not yet interoperable.
A direct detection technique, on the other hand, requires only a delay interferometer, two single photodiode and no DSP chip translating into a device of much lower cost and complexity. The advantage of longer transmission distances (in the range of several 1000 km) enabled using a coherent approach is overkill for deployments of much shorter reaches - ranges of several 100 km.
The direct detection technique is thus seen as an attractive alternative for metro networks, enterprise and datacenters where economic viability plays a pivotal role. The direct detect DWDM 100G CFP(called the DWDM 100G CFP from this point on) transceiver uses four lasers, over the DWDM grid, such that the resulting link realizes 4 x 28 Gb/s with each 28 Gb/s optical duo-binary signal travelling in a separate single mode duplex fiber.
The DWDM channels are 50GHz full C-band tunable. The tunable CFP is designed to interoperate with DWDM services that support 50GHz or 100GHz spaced wavelengths per the ITU-T C-band grid for metro applications. The duo-binary signal is spectrally efficient and has better dispersion tolerance compared to NRZ (non-return to zero) binary modulation.
While these transceivers when purchased directly from the transponder/ switch/ router vendors might not show the problem of plug-and-pray, the accompanying CAPEX hike might be an economical hindrance. For those thinking about re-using 100G LR4 switches, it is good news that that DWDM CFP works well out of this port. But care has to be taken to reprogram the DWDM CFP to follow the internal signaling like LR4 optics such that the switch or the router would work faultlessly.
The DWDM CFP can thus, similar to 3rd party 10G optics, be converted to out of the box plug-and-play module and correspondingly interoperable. The coherent transceiver on the other hand would not work with the LR4 ports, because of its higher power consumption and DSP requirements. It can be used only with line cards designed for coherent optics either with or without the DSP.
As already mentioned the performance of coherent is supreme however the wait for many is for the prices to drastically drop and contest with its direct detect counterpart.