COBO acts to bring optics closer to the chip

The formation of the Consortium for On-Board Optics (COBO) highlights how, despite engineers putting high-speed optics into smaller and smaller pluggable modules, further progress in interface compactness is needed.

The goal of COBO, announced at the OFC 2015 show and backed by such companies as Microsoft, Cisco Systems, Finisar and Intel, is to develop a technology roadmap and common specifications for on-board optics to ensure interoperability.

“The Microsoft initiative is looking at the next wave of innovation as it relates to bringing optics closer to the CPU,” says Saeid Aramideh, co-founder and chief marketing and sales officer for start-up Ranovus, one of the founding members of COBO. “There are tremendous benefits for such an architecture in terms of reducing power dissipation and increasing the front panel density.”

On-board optics refers to optical engines or modules placed on the printed circuit board, close to a chip. The technology is not new; Avago Technologies and Finisar have been selling such products for years. But these products are custom and not interoperable.  

Placing the on-board optics nearer the chip - an Ethernet switch, network processor or a microprocessor for example - shortens the length of the board’s copper traces linking the two. The fibre from the on-board optics bridges the remaining distance to the equipment’s face plate connector. Moving the optics onto the board reduces the overall power consumption, especially as 25 Gigabit-per-second electrical lanes start to be used. The fibre connector also uses far less face plate area compared to pluggable modules, whether the CFP2, CFP4, QSFP28 or even an SFP+.  

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The founding members of the Consortium for On-Board Optics are Arista Networks, Broadcom, Cisco, Coriant, Dell, Finisar, Inphi, Intel, Juniper Networks, Luxtera, Mellanox Technologies, Microsoft, Oclaro, Ranovus, Source Photonics and TE Connectivity.

Given the breadth of companies and the different technologies they prefer,  will the COBO's initiative choose a specific fibre type and wavelength?

“COBO currently has no plans to specify a single medium or a single wavelength, but rather will reference existing standards,” Brad Booth, Chair for the Consortium for On-Board Optics told Gazettabyte.

“There has not been any discussion on the fibre type - single mode versus multi-mode - yet,” added Aramideh. “This will be one item among many interworking specification items for the consortium to define.”

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“The [COBO] initiative is going to be around defining the electrical interface, the mechanical interface, the power budget, the heat-sinking constraints and the like,” says Aramideh.

To understand why such on-board optics will be needed, Aramideh cites Broadcom’s StrataXGS Tomahawk switch chips used for top-of-rack and aggregation switches. The Tomahawk is Broadcom’s first switch family that use 25 Gbps serialiser/ deserialiser (serdes) and has an aggregate switch bandwidth of up to 3.2 terabit. And Broadcom is not alone. Cavium through its Xplaint acquisition has the CNX880xx line of Ethernet switch chips that also uses 25 Gbps lanes and has a switch capacity up to 3.2 terabit.

“You have 1.6 terabit going to the front panel and 1.6 terabit going to the back panel; that is a lot of traces,” says Aramideh. “If you make this into opex [operation expense], and put the optics close to the switch ASIC, the overall power consumption is reduced and you have connectivity to the front and the back.” 

This is the focus of Ranovus, with the OpenOptics MSA initiative. “Scaling into terabit connectivity over short distances and long distances,” he says.

OpenOptics MSA

At OFC, members of the OpenOptics MSA, of which Ranovus and Mellanox are founders, published its WDM specification for an interoperable 100 Gbps WDM standard that will have a two kilometer reach. 

The 100 Gigabit standard uses 4x25 Gbps wavelengths but Aramideh says the standard scales to 8, 16 and 32 lanes. In turn, there will also be a 50 Gbps lane version that will provide a total connectivity of 1.6 terabit (32x50 Gbps). 

Ranovus has not detailed what modulation scheme it will use to achieve 50 Gbps lanes, but Aramideh says that PAM-4 is one of the options and an attractive one at that. “There are also a lot of chipsets [supporting PAM-4] becoming available,” he says. 

Ranovus’s first products will be an OpenOptics MSA optical engine and an QSFP28 optical module. “We are not making any product announcements yet but there will be products available this year,” says Aramideh. 

Meanwhile, Ciena has become the sixth member to join the OpenOptics MSA.