Summer 2018 Optical Connections Magazine


UPCOMING 100G/400G TRANSITION AND 100G PAM4 TECHNOLOGY Current hyperscale data centre networking is characterised by a much faster pace of interconnect speed transition, typically occurring at three-year intervals. Innovative 100G interconnects are going mainstream and have been in deployment for the last two years, while the next speed transition is looming on the horizon. Although 200 Gbps is being considered, there is industry consensus that 400 Gbps will be the natural next step. Current 4x25G based 100G technology is complex to package and is not scalable to 400G. In order to reduce 100G cost and support 400G optics economically, the industry is moving to adopt a new technology with optics encoded with PAM-4 (4-level Pulse Amplitude Modulation) at 50 GBaud, enabling 100G per channel and later 400G with 4x100G aggregation. The 100G Lambda MSA (Multi-Source Agreement) was formed to define this new industry standard and supported by 23 promoting companies, representing a broad industry ecosystem including companies making semiconductor integrated circuits, optical transceiver modules and networking systems, as well as end-user web companies. The strong benefit of adopting single channel 100G optics includes a much- reduced optics element count for lower cost, thereby laying a foundation for economical 400G and the elimination of an inverse gearbox when the electrical interface migrates to 100G serial interface in the future. According to one estimate, PAM-4 100G delivers a 60% reduction in component count and a 33% reduction in power requirements. The 100G Lambda MSA has recently released to the public a first draft of specifications that define 100G FR (2km), 100G LR (10km) and 400G FR4 (2km) and potentially also defines 400G LR4 (10km). With advances in digital signal processing and high speed optoelectronic device technologies such as high-speed silicon photonics, we anticipate rapid industry adoption and implementation with field deployments possibly starting as early as 2019. Hyperscale data centres are deployed near population centres globally and interconnected with ultra-high bandwidth. While many ultra-high speed optical fibre links are deployed across continents and oceans, the majority of these links are between data centre buildings within a data centre campus, or data centres within the same metropolitan area. These data INTERDATA CENTRE DCI SOLUTIONS

centre buildings are interconnected with massive bandwidth, which can be up to tens of terabits per second. For interconnected data centres within a few kilometres on one another, an operator may choose to deploy simple 100G CWDM4 (2km) or 100G LR4 (10km) type optical transceivers and migrate to 100G FR/LR (utilising PAM-4 technology), with several hundred pairs of fibre. If fibre is not adequate and adding more fibre becomes too costly, operators may choose to deploy DWDM (Dense Wavelength Division Multiplexing) optical transceiver solutions in order to aggregate up to 40x100G per pair of fibre. For these on-campus short distance interconnections, single channel 100G PAM-4 with direct detection is a much more economical and attractive solution versus more complex coherent transmission technology, which requires both amplitude and phase modulation/ decoding with polarisation multiplexing/ demultiplexing and coherent detection using a precisely controlled optical local oscillator. For interconnect data centres up to 80km apart, 100G PAM-4 DWDM with advanced digital signal processing technology may still be cost favourable, even with the added tunable dispersion compensation requirement which can however be shared among all DWDM channels. Coherent detection will be used to cover distances greater than 80km distance. When data centres transition to 400G, DCI solutions will scale accordingly, but 4x100G PAM-4 can still be used to cover relatively short reach DCI applications and coherent 400G will extend coverage for the remaining inter-data centre connections.

adoption. The leading candidate is the QSFP-DD (Quad Small Form-factor Pluggable Double Density), which is derived from the QSFP28 and has twice the electrical data connections and an only slightly longer mechanical length, thereby preserving compatibility with the QSFP28. In conjunction with an improved thermal design, a QSFP-DD transceiver module and cage configuration can support power dissipation beyond 12W. The second contender is the OSFP (Octal Small Form-factor Pluggable) optical transceiver, which is slightly larger and longer than the QSFP-DD interface. The key advantage of the OSFP module is the larger form factor which allows for higher power dissipation of up to 16W. The disadvantages are the lack of backward compatibility with the QSFP28 and the slightly larger size which translates into a lower faceplate density. A third MSA called COBO (Consortium of On-Board Optics) has defined a form factor with the electrical interface located away from the system faceplate and directly onto the system PCB. The advantage of this configuration is the placement flexibility of the transceiver module which can be closer to a switch IC higher speed interface, making it easier to deal with signal integrity issues. Since the COBO transceiver module is mounted on a 2D PCB surface, there is also more room for heat sink implementation, thereby supporting a potentially higher power dissipation rating. Hyperscale data centres are quickly becoming critical for major web companies, which are investing heavily and at a fast pace to keep up with technology developments and service innovations. The development of faster electrical and optical signalling technologies will continue to accelerate massive data aggregation at hyperscale data centres globally. The latest 100G and 400G optical technology developments provide a broad range of ecient hyperscale data centre connectivity solutions to support an ever-increasingly rich mix of data- intensive applications.


For 100G data centre applications, the industry has overwhelmingly adopted the QSFP28 (Quad Small Form-factor Pluggable) transceiver module. As the industry is preparing to transition from 100G to 400G, several emerging MSA form factors are contending for

Oplink 100G Tranceiver


ISSUE 13 | Q2 2018

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