COBO issues industry’s first on-board optics specification

• COBO modules supports 400-gigabit and 800-gigabit data rates   
• Two electrical interfaces have been specified: 8 and 16 lanes of 50-gigabit PAM-4 signals. 
• There are three module classes to support designs ranging from client-slide multi-mode to line-side coherent optics. 
• COBO on-board optics will be able to support 800 gigabits and 1.6 terabits once 100-gigabit PAM-4 electrical signals are specified. 

Source: COBO

Interoperable on-board optics has moved a step closer with the publication of the industry’s first specification by the Consortium for On-Board Optics (COBO).

COBO has specified modules capable of 400-gigabits and 800-gigabits rates. The designs will also support 800-gigabit and 1.6-terabit rates with the advent of 100-gigabit single-lane electrical signals. 

“Four hundred gigabits can be solved using pluggable optics,” says Brad Booth, chair of COBO and principal network architect for Microsoft’s Azure Infrastructure. “But if I have to solve 1.6 terabits in a module, there is nothing out there but COBO, and we are ready.”

Origins 

COBO was established three years ago to create a common specification for optics that reside on the motherboard. On-board optics is not a new technology but until now designs have been proprietary.

I have to solve 1.6 terabits in a module, there is nothing out there but COBO, and we are ready

Brad BoothSuch optics are needed to help address platform design challenges caused by continual traffic growth.

Getting data on and off switch chips that are doubling in capacity every two to three years is one such challenge. The input-output (I/O) circuitry of such chips consumes significant power and takes up valuable chip area.

There are also systems challenges such as routing the high-speed signals from the chip to the pluggable optics on the platform’s faceplate. The pluggable modules also occupy much of the faceplate area and that impedes the air flow needed to cool the platform. 

Using optics on the motherboard next to the chip instead of pluggables reduces the power consumed by shortening the electrical traces linking the two. Fibre rather than electrical signals then carries the data to the faceplate, benefiting signal integrity and freeing faceplate area for the cooling.    

Specification 1.0

COBO has specified two high-speed electrical interfaces. One is 8-lanes wide, each lane being a 50-gigabit 4-level pulse-amplitude modulation (PAM-4) signal. The interface is based on the IEEE’s 400GAUI-8, the eight-lane electrical specification developed for 400 Gigabit Ethernet. 

The second electrical interface is a 16-lane version for an 800-gigabit module. Using a 16-lane design reduces packaging costs by creating an 800-gigabit module instead using two separate 400-gigabit ones. Heat management is also simpler with one module.

There are also systems benefits using an 800-gigabit module.“As we go to higher and higher switch silicon bandwidths, I don’t have to populate as many modules on the motherboard,” says Booth.  

The latest switch chips announced by several companies have 12.8 terabits of capacity that will require 32, 400-gigabit on-board modules but only 16, 800-gigabit ones. Fewer modules simplify the board’s wiring and the fibre cabling to the faceplate.  

Designers have a choice of optical formats using the wider-lane module, such as 8x100 gigabits, 2x400 gigabits, and even 800 gigabits.

COBO has tested its design and shown it can support a 100-gigabit electrical interface. The design uses the same connector as the OSFP pluggable module. 

“In essence, with an 8-lane width, we could support an 800-gigabit module if that is what the IEEE decides to do next,” says Booth. “We could also support 1.6 terabits if that is the next speed hop.”  

It is very hard to move people from their standard operating model to something else until there is an extreme pain point

Form factor and module classes

The approach chosen by COBO differs from proprietary on-board optics designs in that the optics is not mounted directly onto the board. Instead, the COBO module resembles a pluggable in that once placed onto the board, it slides horizontally to connect to the electrical interface (see diagram, top).  

A second connector in the middle of the COBO module houses the power, ground and control signals. Separating these signals from the high-speed interface reduces the noise on the data signals. In turn, the two connectors act as pillars supporting the module. 

The robust design allows the modules to be mounted at the factory such that the platform is ready for operation once delivered at a site, says Booth. 

COBO has defined three module classes that differ in length. The shortest Class A modules are used for 400-gigabit multi-mode interfaces while Class B suits higher-power IEEE interfaces such as 400GBASE-DR4 and the 100G Lambda MSA’s 400G-FR4.

The largest Class C module is for the most demanding and power-hungry designs such as the coherent 400ZR standard. “Class C will be able to handle all the necessary components - the optics and the DSP - associated with that [coherent design],” says Booth. 

The advantage of the on-board optics is that it is not confined to a cage as pluggables are. “With an on-board optical module, you can control the heat dissipation by the height of the heat sink,” says Booth. “The modules sit flatter to the board and we can put larger heat sinks onto these devices.”  

We realised we needed something as a stepping stone [between pluggables and co-packaged optics] and that is where COBO sits    

Next steps

COBO will develop compliance-testing boards so that companies developing COBO modules can verify their designs. Booth hopes that by the ECOC 2018 show to be held in September, companies will be able to demonstrate COBO-based switches and even modules. 

COBO will also embrace 100-gigabit electrical work being undertaken by the OIF and the IEEE to determine what needs to be done to support 8-lane and 16-lane designs. For example, whether the forward-error correction needs to be modified or whether existing codes are sufficient.   

Booth admits that the industry remains rooted to using pluggables, while the move to co-packaged optics, where the optics and the chip are combined in the same module - remains a significant hurdle, both in terms of packaging technology and the need for vendors to change their business models to build such designs. 

“It is very hard to move people from their standard operating model to something else until there is an extreme pain point,” says Booth. 

Setting up COBO followed the realisation that a point would be reached when faceplate pluggables would no longer meet demands while in-packaged technology would not be ready. 

“We realised we needed something as a stepping stone and that is where COBO sits,” says Booth.     

Further information

For information on the COBO specification, click here.