CMIS and C-CMIS: How to manage coherent modules without vendor lock-in

Optical interoperability means that modules from different vendors will establish a link. Management determines whether the router will recognize, configure, and monitor them at all. Without this second layer, a multi-vendor network remains a promise rather than a production-ready solution. We explain how CMIS and C-CMIS are changing this, what exactly they standardize, and where the limits of their promise lie.

The problem that CMIS solves

Before the advent of CMIS, the pluggable module industry struggled with the lack of a unified management framework. Each manufacturer implemented its own way of addressing module memory, its own set of commands, and its own initialization logic. Using a coherent module from one vendor in another vendor's router was a gamble: it might work, it might work partially, or it might not work at all.

The scale of the problem grew along with the complexity of the modules. A simple grey module has few parameters to manage. A tunable 400G QSFP-DD coherent module must handle wavelength, modulation mode, transmit power, FEC parameters, chromatic dispersion compensation, and a whole set of diagnostic data. The more advanced the module, the more functional components and configurations it has, and consequently, the more memory and better data organization it requires.

The answer to these challenges arrived in 2018, when the QSFP-DD MSA published a new system for handling modules: the Common Management Interface Specification.

Using a coherent module from one vendor in another vendor's router was a gamble.

What is CMIS

CMIS is a communication interface between a host and an optical module. It defines a precise mechanism for initializing and managing optical and copper modules in a standardized way, while still allowing for the provision of vendor-specific features. This duality is key: the standard covers what must be common and leaves room for what the manufacturer wants to add beyond the standard.

CMIS introduced architectural concepts that previous specifications lacked. This is not a cosmetic change to the register format, but a different model for describing what a module is and how it works.

Today, CMIS covers a wide spectrum of devices: passive and active copper cables, AOCs, client grey modules, DWDM modules, coherent modules, co-packaged modules, and ELSFP. On the host side, these include routers, switches, network interface cards, and line cards in optical transport equipment. The standard is now developed by the OIF, which took over its maintenance from the QSFP-DD MSA. Current versions 5.0, 5.2, and 5.3 include support for 400G and 800G modules, including registers specific to coherent optics: DSP status, optical parameter monitoring, and selection of modulation and FEC modes.

What is C-CMIS and why is it a separate document

C-CMIS stands for Coherent CMIS, an extension of the CMIS standard designed specifically for DCO (Digital Coherent Optics) modules. It does not replace CMIS but is used in conjunction with it. It defines additional management registers, CDB messages, and VDM monitors, along with new functionality and behaviors required by coherent modules. Appropriate address spaces have been reserved within CMIS itself for coherent applications, allowing for a clean separation of both layers.

The practical significance is this: C-CMIS extends CMIS with parameters specific to coherent optics. These are data points that simply do not exist in classic grey modules, but are essential in coherent modules for assessing link status and network planning.

C-CMIS provides monitoring of standard parameters in a normative way, while leveraging the flexibility of CMIS to monitor additional vendor-specific parameters. The first version of the standard covered 400ZR modules, and its scope is being successively expanded to include other types of DCO modules.

How CMIS translates into vendor independence

Vendor lock-in in optics has two levels that are easy to confuse, and they are solved by completely different standards.

Without standardized management, optical interoperability alone is not enough. A lack of visibility into optical parameters means no ability to proactively detect path degradation and longer diagnosis times during a failure.

A module that physically establishes a link but does not report telemetry is a problem in a production network, not a solution.

CMIS provides this second level. The standard allows hosts to write generic software for managing CMIS-compliant modules, which significantly shortens integration time and accelerates the deployment of new capabilities in operator networks. In practice, this means the ability to build a network where the router comes from one vendor, the coherent modules from another, and the optical line system from a third, all while maintaining unified multi-layer management.

What CMIS does not guarantee

An honest description of a standard must include its limitations, as excessive expectations are a common cause of implementation problems in this area.

Not every router will support every module

The standard defines how a host should communicate with a module, but the host-side implementation depends on the manufacturer and software version. A router with a QSFP-DD port and outdated firmware will not support a coherent module, regardless of how compliant the module itself is.

Proprietary features remain outside the standard

CMIS intentionally leaves room for proprietary extensions. This is an advantage in terms of innovation, but it means that the full set of diagnostic features for a specific module may only be available in an environment that understands those extensions.

The standard is evolving

The OIF is working on subsequent versions, including a transition from a register-based model to a message-based object model. This is intended to speed up management interactions and make software more portable, but it also means that the compliance landscape will continue to shift for some time.

What this means when purchasing modules: three questions

Before placing an order, there are three questions worth asking. You should direct the first and third to the module supplier, and the second to the hardware manufacturer.

CMIS standardizes how a host communicates with an optical module. C-CMIS extends this standardization to include parameters and mechanisms specific to coherent optics. Together, they create a management layer without which 400ZR and OpenZR+ optical interoperability would be incomplete in production networks.

For an operator, this means a real opportunity to build a multi-vendor network with unified management, provided that compliance is verified not only at the level of standard declarations but also at the level of the specific platform and software version.

Optical interoperability without a management layer remains a promise, not a production-ready solution.

‍