How does the latest optical module management standard work?

The demand for data collection is constantly increasing. As 400G transmission slowly becomes the standard, 800G is in the testing phase, our plans run towards 1.6 TG.

‍Memory management and data organization in optical modules with such high bit rates forced the creation of a new system: the Common Management Interface Specification (CMIS).

‍‍What differentiates modules up to 100G from high-throughput ones (200G, 400G and more)? The first answer is obvious: the speed of data flow. However, the differences in the way these devices work are much more.

100G is the contractual limit of the technological leap. Up to this bit rate value, the modules were managed through the control interface, using the basic command system mapped in memory SFF-8636. As speed increased, this historical system had increasing problems keeping up.

‍High Throughput Modules QSFP-DD/QSFP112G/QSFP-DD800 are much more technologically advanced than lower bit rate modules such as 100G. They have up to 8 electrical paths and 8 optical paths. In addition, they use the PAM4 signal modulation technique, which requires complex error correction.

For example, 400G QSFP-DD SR8 Module fully utilizes both 8 electrical tracks and 8 optical tracks. It is even more demanding 400G QSFP-DD ZR tunable coherent module. This protocol is prepared to handle additional parameters such as wavelength or chromatic dispersion.

As you can see, the more advanced the module, the more functional components and flexible configurations that the internal system must manage. So we needed more memory space and better data organization. The answer to these challenges came in 2018. QSFP-DD MSA has released a new Common Management Interface Specification (CMIS) module support system.

What is CMIS?

CMIS is an interface for communication between the host and the optical module. As we have already mentioned, CMIS includes modules such as QSFP-DD/OSFP/ QSFP112/QSFP-DD800. It is also fully prepared to support technologies that are yet to appear on the market, such as a module with bitrates of up to 1.6 TB.

The first, official specification of CMIS appeared on September 18, 2018 in revision 3.0. This revision is designated 3.0 because previous publications on this system were not an official document.

*Development of the Common Management Interface Specification (CMIS)*

Work on CMIS was initiated by the QSFP-DD MSA in 2017. Since 2022, the development of the system is continued by the Optical Internet working Forum. OIF is an organization working on interoperability in optical networks, which has an impact on facilitating connectivity in the world of networks. Specifications are published on the OIF website OIF-C-CMIS, which is an extension of the “regular” CMIS with characteristics for coherent modules.

How does CMIS work?

CMIS introduces completely new concepts such as Application, Data Path, State Machine, Control Sets, Signal Integrity. The CMIS document also contains the entire EEPROM memory map, which clearly describes the firmware (firmware) updates of the module.

The memory map of QSFP28 modules complies with the SFF-8636 standard. It contains only four memory pages, which is enough to handle all the functionalities of the QSFP28 modules.

*Memory map for QSFP28 modules compliant with SFF-8636.*

The new generation 400G/800G modules are much more complex than the standard QSFP28, hence the map itself is more extensive. It contains much more information compared to older versions.

In the software, there were also areas of memory called “Bank Pages”, which provide support for 8 data transmission lines and their multiplicity.

*CMIS compliant memory map for next-generation modules*

In the case QSFP-DD modules we have the ability to customize the module to the required work configuration. These are the so-called applications made available by the module. The most advanced modules can provide up to 15 applications. For example: 1x400G, 2x200G, 1x100G, 2x100G, 3x100G, 4x100G.

The application specifies many conditions such as: the number of electrical tracks between the module and the host, the number of optical tracks, the types of modulation used on the electrical side and on the optical side, and the mapping of analog tracks on the electrical and optical side.

If the hardware resources of the module allow it, it can share several instances of the same application at the same time. For example, the module can work as 1 x 400GBASE-DR4 (8 electrical tracks and 4 optical tracks), but also as 4 x 100GBASE-DR independent of each other (2 electrical tracks; 1 optical track).

The vastness of these possibilities has led to the introduction of the concept of the so-called Data Path. It is a way of aggregating and mapping the transmission paths (lane) on the optical and electrical side and the hardware resources of the module intended for one application. Below, we present an example of four independent Data Paths for the module considered above, working as 4 independent 100GBASE-DR application instances, each using 2 electrical tracks, 1 optical path and resources located between the electrical and optical sides.

‍*Example of path assignment for 4 x 100GBASE-DR applications.*

In order to manage the work of the Data Path well, the so-called Data Path has been introduced. state machine, that is, the algorithm responsible for the configuration of individual elements of the module. Before starting work, the module receives a command from the host to initialize them. If the initialization was successful, the Data Path can then be activated, which means, among other things, the inclusion of a laser or lasers.

The host controls the operation of the state machine using a set of registers called Control Set. In addition to selecting a specific application (s), before activating Data Path, you must also configure the electrical tracks they use. For their correct operation, it is required to specify for each of them the configuration of the Tx CDR, Rx CDR blocks and the parameters of the so-called. Intergrity signal. At such high transmission speeds, it is necessary to compensate for the frequency characteristics of the attenuation of electrical signals between the module and the host, i.e. the so-called equalization.

GBC Photonics Simple Recording Device (SRD)

The degree of complexity of the memory structure of QSFP-DD modules and the number of changeable parameters of their operation is enormous. However, we have good news! The user of GBC Photonics modules does not need to worry about this. With the GBC Photonics SRD, we will provide him with an easy way to configure these modules in a few clicks, just as we do so far with all other generations of modules.

Development-oriented

With actual unification and standardization, 400G QSFP-DD modules are entering the market much faster than 100G modules. In this case, there is no concern about whether modules matching the ports in the currently purchased equipment will still be available during the development of the network, because there is one standardized type of interface. In addition, its specification is flexible enough that it will also work with new modules that may appear on the market.