The relatively new and complex PAM4 modulation scheme has been adopted for some aspects of the 400 Gbit/second Ethernet standard, setting a direction for new silicon development.
Modulation formats have been hotly debated over the last 6-12 months in the IEEE P802.3bs group setting a standard for for 400 Gigabit Ethernet (400GbE). Decisions to adopt PAM4 optical modulation schemes at the recent IEEE May meeting will have significant implications on the technology direction of the industry moving forward.
The 400 GbE project includes electrical interfaces to address chip-to-chip and chip-to-module applications as well as optical interfaces for 100 meter multimode fiber (MMF), 500m single-mode fiber (SMF), 2km SMF, and 10km SMF. Initially, the group was able to make fast decisions on some 16-wide interfaces for both electrical and optical MMF, adopting 16x25 Gbit/second with traditional NRZ modulation. These proposals strongly leverage the existing 25 Gbit/s signaling technology developed initially for the 100 GbE market which is seeing wide development and implementation today.
Numerous proposals for interfaces narrower than the 16-lane approaches were submitted for consideration to respond to market pressure for cost effective interconnects. Such interfaces use fewer pins on chips or optical devices and therefore reduce power, lower cost, and increase use. Both 8-wide and 4-wide proposals were made to the group.
A 50 Gbit/s NRZ approach was shown to be viable for 8-wide optical and electrical interfaces. Alternate modulation schemes were also proposed to further lower costs reasons or the baud rate to leverage existing technologies and know-how around 25 Gbaud components. Alternate modulation schemes considered included PAM4 modulation for both electrical (8-wide) and optical (4- and 8-wide) and discrete-multitone (DMT) for 4-wide optical.
We saw how decisions about electrical and optical solutions could be made separately, however there was clearly strong perception that a common ecosystem could develop if one modulation scheme was adopted. After much debate, the electrical decision was made in March 2015 that a second 8-wide electrical interface would be defined based on 50 Gb/s PAM4 modulation. This was widely supported by many of the major silicon suppliers with test results and simulations showing good performance with known connector and channels.
With the electrical decision out of the way, at the May 2015 meeting the 400 GbE group focused on resolving the SMF optical interface decisions. After much consideration and deliberation, the group decided to support PAM4 modulation for at least two of the SMF solutions. An 8x50G PAM4 scheme was adopted for duplex 10km SMF and a 4x100G PAM4 scheme was adopted for 500m parallel fiber (4 fibers per direction).
Technical decisions require at least a 75% approval, which indicates the wide support in the group for both of these decisions. The 8-wide solution for 10km provided the group confidence that they would be able to address the early telecom adopters of 400GbE in a timely manner. The 4-wide 500m solution for 500m provided the group with confidence that a narrower, lower cost solution would be developed for early adopter data center applications that supported PSM fiber.
The decision around the 2km duplex SMF solution remains open as the group continues to study the risks and merits around extending the 4-wide solution or 8-wide PAM4 solution. While NRZ modulation is still a possibility, the momentum of the ecosystem shift towards PAM4 modulation may be too much to overcome (see chart below).
The adoption of PAM4 for parts of the 400GbE standard likely will set a trend toward use of the relatively new modulation scheme.
The implications of these decisions are significant for the industry. It is expected that a converged solution set will allow developers to focus technology development towards optimizations for this scheme and we see a good balance between the solutions, meaning the industry will be well served in the near term and longer term. While other groups, like the OIF, continue to pursue the definition of 50 Gb/s NRZ interfaces, the market adoption of such efforts is unclear, given the decisions made by the IEEE P802.3bs Task Force. Nonetheless, the technology development will definitely add value into the PAM4 technologies as baud rates rise.
-- Mark Nowell is on the board of directors of the Ethernet Alliance, chairs the IEEE 802.3 25 Gbit/s Study Group and is a senior director of engineering at Cisco Systems.