With the development of science and technology, the requirements for data transmission speed are increasingly higher. The old modulation scheme based on NRZ type coding has been difficult to meet the requirements of high rate. Whether it is the chip on the PC board or one end to the other end of the long-distance optical fiber, we need to obtain data from point A to point B as effectively as possible. Therefore, the more popular modulation scheme is PAM4. In short, PAM4 signal is a kind of line coding using pulse amplitude modulation technology. Each symbol of PAM4 signal has 2 bits. If the symbol rate is 28Gbaud, it means that 56Gb is transmitted per second. Therefore, it can be used for 400G Ethernet transmission and is widely used in 400G QSFP-DD optical module.
QSFP-DD (Quad Small Form Factor Pluggable Interface Double Density) is the latest optical module package for 400G data rate. Compared with the traditional single channel or four channel interface, the package provides eight channel electrical interface, which increases the bandwidth, channel capacity and port density. It is defined by QSFP-DD MSA group. The module meets the needs of high-density and high-speed network solutions in the form of backward compatibility.
All 400G QSFP-DD optical modules are either 8x53Gbps PAM4 or 4x106Gbps PAM4 modulated, except for the 400G SR16 which is 16×26.5Gbps NRZ modulated. The following is a brief introduction and comparison of the four types of 400G optical modules currently in use in the market: 400G FR4, DR4, SR8 and FR8.
·400G FR4
The 400G FR4 optical module is modulated with 4x106Gbps PAM4 and it supports SMF links up to 2km long via dual LC connectors. It is compliant with the IEEE 802.3bs, 100G Lambda MSA Group’s 400GBASE-FR4 specification and the 400GAUI-8/CEI-56G-VSR-PAM4 standard. It enables 400G Ethernet signals to be transmitted over four CWDM mesh optical wavelengths. The multiplexing and demultiplexing of the four wavelengths is managed within the device.
·400G DR4
The 400G DR4 optical module also features 4x106Gbps PAM4 modulation and it supports parallel SMF links of up to 500m via MPO-12 connectors. It is compliant with the IEEE 802.3bs protocol and the 400GAUI-8/CEI-56G-VSR-PAM4 standard. It uses the 1310nm EML transmitter type. It enables 400G Ethernet signals to be transmitted on four parallel channels, one wavelength per channel. 400G DR4 can be connected via MPO/MTP-8 single-mode fibre patch cables for 400G transmission. It can also be connected to a 100G QSFP28 DR/FR optical module at the opposite end via an MPO/MTP-8 to 4DLC singlemode fibre patch cable. QSFP28-100G-DR/FR optical module refers to a 100G serial DR/FR optical module for data center, transport and router applications within a 2km range. The module utilises a QSFP28 slot that plugs into the standard 4x25G electrical interface and uses only one serial 100G PAM4 optical channel.
·400G SR8
The 400G QSFP-DD SR8 optical module adopts 8x53Gbps PAM4 modulation and uses standard MPO-16 connectors to transmit up to 70m (OM3) or 100m (OM4) over multimode fiber. It uses 25G VCSEL laser chip, and the signal is modulated by PAM4 pulse amplitude modulation. 25G VCSEL laser chip is very mature now, so the 400G SR8 optical module has the lowest cost. Due to the short transmission distance, it is more suitable for short distance data communication. It can be connected to a 200G QSFP56 SR4 optical module at the opposite end via MPO/MTP-16 to 2xMPO/MTP-8 fiber patch cable.
·400G FR8
The 400G FR8 optical module also uses the 8x53Gbps PAM4 modulation method. In fact, the 400G FR8 is a variant of the 400G LR8, which first appeared to provide data transmission distances of up to 10km over duplex single-mode fiber, while the 400G FR8 is the result of improvements made to reduce costs, and is primarily used inside data centers to enable data transmission within 2km. Both the 400G LR8 and FR8 modules utilize direct modulation laser (DML) transmitter technology to provide a low-power, low-risk, cost-effective solution for 400G.
Through the introduction of the above four types of 400G optical modules, we can see that the four types of 400G QSFP-DD transceivers actually have very little difference. In detail, their electric port side are all 8x53Gbps PAM4 signals. For 400G-SR8/FR8 modules, the optical module only does CDR (clock recovery) and electrical/optical or optical/electrical conversion internally, so the optical port side is the same as the electrical port side, which is also 8x53Gbps PAM4 signal. For modules such as 400G-DR4/FR4, there is a Gearbox chip inside the optical module to multiplex the two electrical inputs into one signal and then modulate it to the optical, so the optical side is twice as fast as the electrical side, i.e. 4x106Gbps PAM4 signals.
Because of the cost advantage of multimode fiber in short-distance transmission, the IEEE also set up the 802.3cm working group in March 2018 to include the previously discussed in the industry with 8-way (SR8) or even 4-way (SR4.2) multimode fiber to achieve 400G transmission technology in the specification discussion.
In the future, for cost reasons, the 4-channel optical signal for 400G transmission may become mainstream; meanwhile, the electrical interface of the optical module may also be gradually upgraded to 4x106Gbps PAM4 to save power and cost by eliminating the Gearbox chip. In the ultra-long distance (>80km) transmission, the 400G-ZR specification being developed by OIF, for example, will be realized by coherent communication and DWDM technologies.
