Understanding 100G Adoption
Optical module packages have three common features: small form factor, low power consumption, and interoperability with all system vendors. Understanding the development of the 100G optical module market will help us understand the introduction of 400G technology. Service providers need pluggable optical modules to achieve long distances and specialized technologies such as coherent detection. Data center teams need a low-power, low-cost solution for short-distance (up to 2KM) applications.
The CFP form factor is the first 100G pluggable transceiver that supports short-distance and long-distance applications, but it is very large and consumes high power (12W). As technology and components improve in size and power consumption, smaller CFP2 and CFP4 are introduced to the market. Despite the continuous advancement of technology, embedded coherent technology for 100G and 200G is still only available on CFP and CFP2 form factors. At the same time, the huge demand for additional bandwidth capacity in hyperscale data centers has driven the use of the QSFP28 form factor for various short-reach applications (DAC, PSM4, CWDM4, and SR4). QSFP28 has been widely adopted and is smaller and consumes less power than the CFP form factor.
Understanding the issues behind 100G adoption is important to predict how 400G will be adopted. Who needs 400G pluggable optical modules? For which applications? What is the technology maturity? Is there interoperability with previous form factors?
Trends in 400G optical module Form Factors
Following the same logic as 100G, 400G is the preferred choice for large data centers, while smaller service providers prefer 400G. 400G requires PAM4 modulation for transmission, which makes coverage more challenging. Initial 400G optical module coverage will be limited to a few kilometers. Longer coverage will require coherent detection and its supporting technologies, including amplification and dispersion compensation. Like 100G, 400G will gain similar adoption based on the intended application. Most likely there will be a dedicated form factor for data centers and another for longer reach applications.
Early 400G technology development avoided the “intermediate” form factors (i.e. CFP2, CFP4) that 100G adoption followed. 400G will be introduced in two form factors, one for access networks and one for data centers. QSFP56-DD (also known as QSFP-DD, which stands for QSFP Double Density), OSFP (which stands for Octal SFP). Both form factors run 8x 50G PAM4 channels on the electrical side, while the optical side can be either 8x 50G PAM4 lasers or 4x 100G PAM4 lasers. In the quad laser design, we add a “gearbox” that converts the 8x 50G PAM4 electrically to 4x 100G PAM4. QSFP56-DD is defined by the QSFP-DD MSA Alliance (www.qsfp-dd.com) while OSFP is defined by the OSFP MSA Group (www.osfpmsa.org). The two form factors are similar, but have three key differences:
400G OSFP allows for more power (< 15W) than 400G QSFP-DD (<12W). OSFP allows for early adoption because it is easier to release technology designed for 15W instead of 12W.
QSFP-DD ports are backwards compatible with QSFP+ (40G), QSFP28 (100G), and QSFP56 (200G). OSFP ports require a QSFP to OSFP converter module.
OSFP integrates heat dissipation directly into the form factor, while QSFP-DD does not.
Both QSFP-DD and OSFP are designed for intra-DC applications, including DACs, AOCs, and fiber connections up to 2km. Other variants are in development to support Data Center Interconnects (DCI) with longer reach and other technologies such as DWDM. For instance, QDD-400G-FR4-S uses CWDM EML laser supports a maximum reach up to 2km. 400GBASE-ER8 uses LWDM EML laser supports a maximum reach up to 2km.
The CFP8 form factor, defined by the CFP MSA (www.cfp-msa.org), is distinct from QSFP-DD and OSFP. Allows up to 24W of power consumption. It has 16x channels of 25G NRZ on the electrical side (instead of 8x 50G PAM4 for QSFP-DD and OSFP) Provides MDIO management interface (instead of I2C for QSFP-DD and OSFP) Due to its large footprint and high power consumption (up to 24W), CFP8 is suitable for transmission applications. The initial version (CFP8 400GBASE-LR8) will support up to 10km, using 16x electrical channels of 25G NRZ, converted to 8x channels of 50G PAM4.
Other variants using coherent detection technology will support transmission distances up to 80KM. In addition to transmitting longer distances, CFP8 also opens the door to 800G. 800G is possible by combining 50Gbps PAM4 modulation DSP, coherent detection and multiplexing lasers to the 16x electrical channels of CFP8. Obviously, this technology is still far away. The era of 400G has arrived. Understanding the application of technology will help us better understand the application of optical module technology.
