At OFC 2022, OIF demonstrates products from several companies communicating with each other based on a standard for interoperable coherent optics. Other demonstrations include 112 Gb / sec electrical connection, flexible Ethernet, co-packaging and a 224 Gb / sec technology offering.
As the search for more data and faster data speeds never stops, optical network connections must be developed to deliver data. 400 Gb / sec connections are common within and between data centers, with 800 Gb / sec products under development and 1200 and 1600 Gb / sec connections under discussion. 2022 OFC conference I saw several demonstrations in the industry for the latest optical transport technologies, courtesy of Optical Internet Forum. The organization demonstrates the results of several standard efforts.
Interoperable coherent optics
Coherent optics is one of the techniques for increasing data speed. It modulates the amplitude, frequency and polarization as explained in How coherent optics increase data speed.
Unfortunately, coherent optical modules suffer from a lack of interoperability. This is according to the marketing director of the Optical Internetworking Forum (OIF). Nathan Tracy of TE Connectivity. “You had to use optics from the same company at both ends of the line,” Tracy said EE world after the conference. Describing the secret sauce in the DSP, Tracy said: “There are differences in modulation and coding. There is a need for low-cost interoperable optical connections that connect data centers. “
Tracy explained that the range required for low-cost interoperable connections is 120 km, long enough to connect most data centers. Because of this distance, the industry may make some performance concessions that are not possible over longer ranges. The OIF demonstrates 400 ZR a project using silicon, optical modules, routers and switches from Cisco, NeoPhotonics, Ciena, Fujitsu, Juniper and Marvel.
Figure 1. OIF demonstrates a network using interoperable coherent optics.
“The OIF’s goal was to write an interoperability specification, and this demonstration proved it,” Tracy said. The demonstration uses optical modules in several form factors such as QSFP, CFP-2 and QSFP-DD. The OIF is now developing an interoperability specification for 800G coherent optics.
Figure 1 shows a network diagram for demonstrating interoperability. In the video, Tracy takes you through the equipment and data paths of the demonstration.
Figure 2. OIF demonstrates an electrical interface with a long range of 112 Gb / sec.
112G electrical interface
Because all optical modules require electrical interfaces, the OIF demonstrates 112 Gb / sec electrical interfaces that cover several bands. In this demonstration, the companies provided silicon, test boards, cables, connectors and test equipment. Figure 2 shows a diagram of 112G-MR – medium range, 2 m copper cable – demonstration. The connections consist of eight 112 Gb / sec bands, making it an 800 Gb / sec connection. The actual data rate per bandwidth was 106.25 Gb / sec using the PRBS31Q model with PAM4 modulation.
Figure 3. Flexible Ethernet adds protocols to set data rates as needed.
IEEE defines Ethernet in its own 802.3 a series of specifications that include specifying data rates. Assume that the physical infrastructure cannot support the full data rate specified in the standards, but can support more than the next lower speed? For example, a network that can support 35 Gb / sec but not 40 Gb / sec? In this case it will drop to 25 Gb / sec. This leads to underutilized capacity. Flexible Ethernet (FlexE) allows the network to operate at the highest data rate it can support. According to Tracy, FlexE allows the operator to allocate the best resources as needed when using network sharing.
The OIF publishes a White paper which describes FlexE. Figure 3 shows a typical stack of FlexE protocols.
Figure 4. Co-packing move the optics to or inside the silicon switch to improve signal integrity.
Joint packaging
“Copacking has become a huge noise in the industry,” Tracy said, describing the demonstration. “We are moving the transceiver function to move it closer to the silicone of the mains switch” (Figure 4). Traditionally, electrical-optical connections appear on the edge of a network line card. At today’s data rates, PCB traces are simply too long to avoid signal loss and distortion, which requires significant processing power to equalize the transmission signals. Moving the optics shortens the distance and reduces signal integrity problems. “Packaging puts optics in a switch,” Tracy said. “This reduces the amount of power needed to equalize.”
Katie Liu
224G: The next electrical interface
While 800 Gb / sec optical connections are being developed, studies are already underway for 1.2 Tb / sec and 1.6 Tb / sec connections with test equipment already available. At 112 Gb / sec per bandwidth, this is too much bandwidth needed to maintain these speeds. Early 800 Gb / sec connections will work with 8 × 112 Gb / sec tapes, doubling the data speed and using half as many tapes improves efficiency and halves the number of PCB traces. This is where the following specification, covering 224 Gb / sec electrical strips, makes sense. In February 2022, the OIF published a White Paper Next generation frame CEI-224G.
president of the OIF Katie Liu on Broadcom discussed four 224 Gb / sec projects currently in progress. The projects cover the following electrical interfaces:
- Extremely short range (XSR), on the package;
- Very short range (VSR), chip-optical module;
- Medium range (MR), chip to chip;
- Long range (LR), rear board and copper cable.
There are still applications for long-range electrical interfaces with long-range and copper cable, according to Liu. “We are considering 224 Gb / sec connections over these long bands.” Project teams need to establish parameters such as modulation, baud rate, and bit rate (BER) to achieve data rates. These solutions will be based on the characterization of signal losses due to loss of insertion and other signal degradations. For modulation, the most likely choice will be PAM4, but there is still work to be done on receiver tests.
Figure 5. PAM4 signal (l) has four amplitude levels, each of which represents two bits. The PAM8 signal has eight levels, each of which represents three bits. Source: IEEE
Each time the data rate increases, a new challenge arises. These challenges often come from the physics of printed circuit boards, electromagnetic problems, or some other obstacle that was not previously a factor at lower speeds. “We need to double the PAM4 transmission rate to 112 Gbaud,” Liu said. This affects every aspect of the transmission channel, including IC packets, printed circuit boards, connectors, optical module design, etc. Problems such as reflections, loss of return and cross-disturbances must be overcome. In addition, SerDes will have to deal with doubling the baud rate. This affects its DSP and ADC architecture. Of course, price and power are always problems.
If the channels cannot support the bandwidth required for 112 Gbaud PAM4, then higher order modulations such as PAM6 or PAM8 should be considered. This means more and lower voltage levels and less signal eyeswho carry their own set of problems. Figure 5 compares PAM4 and PAM8 signaling. Smaller differences in voltage levels put more strain on the receivers, making it harder to distinguish between bit combinations. Doubling the transmission rate of PAM4 is preferable, just as it was with non-return to zero (NRZ), before PAM4 is finally accepted.
What is the timeline of 224 Gb / sec? According to Liu, the OIF plans to have a draft specification ready in 2023 with a published specification in 2024 or 2025.
