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Optoelectronics


Figure 2: Comparing a traditional network approach (left) with an SDN/NFC approach (right).


Figure 3: Simplifi ed diagram of a network link with forward error correction (FEC). The FEC encoder adds a series of redundant bits (called over- head) to the transmitted data stream. The receiver can use this overhead to check for errors without asking the transmitter to resend the data.


to allocate network resources more intelligently and dynamically.


EFFECT Photonics believes that handling all the new devices and use cases of 5G networks will require self-managed, zero-touch automated networks. Realizing this full network automation requires not only SDN, NFV, and AI algorithms but also more sensor and control data from the physical layer. To increase the fl ow of data from the physical layer devices to the network management layer, optical networks require smart equipment and components that can be diagnosed and managed remotely. This is where smart pluggable transceivers with reconfi gurable DSPs come into play.


The importance of standardized error correction


Forward error correction (FEC) algorithms allow the DSP to enhance the link performance without changing the


hardware. This enhancement is analogous to imaging cameras: image processing algorithms allow the lenses inside your phone camera to produce a higher-quality image.


FEC makes the coherent link even more tolerant to noise and enables much longer reach and higher capacity. Thanks to FEC, coherent links can handle bit error rates that are literally a million times higher than a typical direct detect link.


Originally, FEC algorithms were proprietary. Equipment and component manufacturers closely guarded their FEC as a critical competitive advantage. Therefore, coherent transceivers from different vendors could not operate with each other, and a single vendor had to be used for the entire network deployment.


When coherent transceivers moved into the datacom industry, cloud providers demanded interoperability. These demands led the Optical Internetworking Forum (OIF)


Table 1: Summary of different use modes for a 400G pluggable transceiver. 400ZR


Target Application Target Reach @ 400G Form Factor FEC


Standards / MSA www.cieonline.co.uk


Edge data center interconnect 120km


QSFP-DD/OSFP CFEC OIF


Open ZR+


Metro, Regional data center interconnect 500km


QSFP-DD/OSFP oFEC


OpenZR+ MSA


to include an algorithm called concatenated FEC (CFEC) in their 400ZR standard for data center interconnects. Meanwhile, 400ZR+ telecom sector standards also agreed to use open FEC (oFEC) algorithms, which provide extended reach at the cost of more bandwidth and energy consumption. For the longest link lengths (500+ kilometers), proprietary FECs are still necessary, but at least a large segment of the 400G transceiver market can now use public, standardized FEC.


A smart DSP to rule all network links


A smart pluggable transceiver that adapts to all the use cases we mentioned before—data centers, carrier networks, SDNs—requires an equally versatile DSP that can be reconfigured via software to adapt to different network conditions and upgrades.


For example, let’s look at the case of


Proprietary Long Haul Long-Haul Carrier 1000 km


QSFP-DD/OSFP Proprietary Proprietary


upgrading a long metro link of 650km running at 100 Gbps with open FEC. The operator needs to increase that link capacity to 400 Gbps, but open FEC could struggle to provide the necessary link performance. However, if the DSP can be reconfigured to use a proprietary FEC standard, the transceiver could handle this upgraded link speed. Reconfi gurable DSPs can also react to


specifi c network conditions. For example, the DSP could adapt to transmit at a higher baud rate if the link has a good quality fi ber. However, the DSP could also adapt and scale down the baud rate to avoid bit errors and save energy if the fi ber quality is poor.


Takeaways


A versatile pluggable that can handle different use cases – data center links, long metro links, and dynamic network management – must have an equally versatile DSP. This DSP should switch among several operating modes – 400ZR, 400ZR+, proprietary – and error correction methods – cFEC, oFEC, and proprietary. These different operating modes can allow next-generation pluggable transceivers to handle most of what the telecom and datacom industries can throw at them.


https://www.effectphotonics.com Components in Electronics September 2022 37


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