Over the past two decades, Ethernet technology has been widely used in the business park, home broadband, industrial control, security monitoring and other fields, the future more bandwidth, lower time delay of Ethernet technology will further penetrate the intelligent manufacturing, intelligent city, autopilot, 5G bearing, cloud computing, data center, such as scene, may affect our life all the time.
Ethernet is also growing in speed for new applications, from 10M and 100M initially to 400G recently standardized. Further responding to the need for data centers to double their switch capacity every two years, in 2018, the Ethernet alliance has made it clear that the next generation of Ethernet rates, 800G and 1.6t will be available in the next few years.
In order to support the corresponding interface rate, the corresponding optical module technology must be regulated. Current Ethernet interface specifications correspond to the optical module rate, transmission distance and electrical interface. Currently, the incomplete standards are mainly focused on 25G/50G EPON, 100G FR/LR, 400G FR4/ lr4-6, and 100G/400G 80km ZR. Different PMD specification different distance, actually on the optical module technology is roughly corresponding to the laser/modulator, general use VCSEL of multimode, long-distance generally use EML, ZR may need to use coherent IQ modulation, obviously with the increase of transmission distance, modulation technique is more and more complex, also means that the cost is higher and higher.
Among these standards, 50G PAM4 modulation is the key, becoming the basis of 50G to 400G interface standards.
As for the recently focused 80km optical interface standards for DCI and CATV applications, IEEE established the 802.3 ct working group as early as November 2018 to start standard formulation. The DCI is 400G/80km, and the CATV is 100G/80km. In these two ZR applications, the current industry believes that only through digital coherence technology can achieve the 80km level of high-speed transmission, and also need to use WDM to improve the capacity of single fiber. In addition, regarding FR/LR, a 2km/10km level interface standard, IEEE 802.3cu launched 100GBASE FR/LR and 400GBASE FR4/LR4 last march. The focus of this series of standards is the introduction of 100G PAM4 modulation and CWDM multiplexed wavelength grids. Compared with 50G PAM4, the higher single wave rate has the advantage of reducing the number of transceiver devices and reducing the cost. Because CWDM wavelengths are spaced 20nm apart, uncooled lasers are allowed, further reducing costs. Obviously, the introduction of single channel 100G technology is beneficial to the implementation of high-speed optical modules to reduce costs and effectively enhance manufacturability (fewer channels, optical modules are easier to do). In addition, the 802.3bs and CD working groups also adopted the LAN WDM wavelength assignment scheme. Obviously, the wavelength interval of LAN WDM is only 800GHz (4.5nm), so it needs to use TEC to control the wavelength shift. However, it works near the zero dispersion of o-band, and is less affected by the dispersion during high-speed transmission. In contrast, CWDM transmission may be affected by large dispersion, especially compared with MZM; EML still has the influence of chirp, which may be a challenge for 400GBASE LR. 802.3 also thinks that this 400G can only support up to 6km, namely 400bbase-lr4-6. However, for the 100G/lamda MSA working group, they adopted different wavelengths to solve the dispersion problem, so MSA defined the 400gbase-lr4-6 and 400gbase-lr4-10 specifications.
For 800G optical interfaces, two MSA working groups were established in 2019, one qsfp-dd800 MSA and the other 800G Pluggable MSA. In the newly released 800G Pluggable white paper, it is considered that single channel 100G PAM4 can be used to achieve 800G SR, and single channel 100G or 200G can be used to achieve DR and FR scenarios. For subsequent 1.6t, single channel 200G May be required. For LR/ER/ZR and other long-distance 800G applications, digital coherence technology will be a more suitable choice.
At present, in the interface with rates below 400G, single channel 50G PAM4 and 100G PAM4 are the mainstream modulation modes, while for rates above 800G, single channel 200G PAM4 and even coherent technology will probably dominate, maybe three or four years, this demand will emerge.
Overall, IEEE802.3 only defines the overall photoelectric performance of the optical transmitter and receiver. Specific parameters such as mechanical size, PIN definition, management interface definition, etc. are specified by the industry's multi-source protocol MSA. Currently, a variety of MSA specifications for hot plug optical modules are widely used. For 100G, CFP/CFP2/CFP4 and OSFP are the most popular, while for more than 100G (200G/400G), the industry is more inclined to QSFP-DD, OSFP.
It has to be said that with the rapid growth of internal data center traffic, the switch capacity, port density and interface rate will face severe challenges. In particular, the PCB routing between the optical module's port and the switch's internal switching chip will affect the signal integrity, and the power consumption on the switch's panel will also become a bottleneck. To address both, the industry is also exploring new opportunities to replace the current pluggable optical modules.
