FREE SHIPPING on Orders Over US$79
United States

PAM4 in 400G Ethernet application and solutions

GeorgeUpdated at Jul 8th 20221 min read

400G PAM4 (4-Level Pulse Amplitude Modulation) is the modulation technology that fits for high-speed signal interconnection in the next-generation data center, paving the way to 400G Ethernet in data centers. What is 400G PAM4? Why is it chosen to be applied to 400G Ethernet? Find answers here.
What Is PAM4 for 400G Ethernet?
Looking further into PAM4, each clock cycle can be sent at four different voltages – 0, 1, 2, or 3, which respectively represent 00, 01, 11, and 10 respectively. Compared to NRZ, the signal 101100100100 was transmitted in half the time with PAM4. Therefore, it is feasible to increase bandwidth by using advanced PAM4 modulation technology, which effectively doubles the bandwidth without having to configure the data center with more fibers. 400G Ethernet can be realized with 8x 50G PAM4 or 4x 100G PAM4.
Why Does 400G Ethernet Need to Use PAM4 Technology?
As the global bandwidth market grows, the transmission rate of NRZ at 25G per channel has reached its limit in supporting 400G Ethernet speeds, making it unsuitable for the demands of modern high-density data centers. PAM4 addresses the limitations of NRZ signal transmission efficiency, meeting the increasing bandwidth requirements while maintaining low construction costs, making it the most cost-effective solution.
Higher Transmission Efficiency
NRZ signaling uses two signal levels in which positive voltage defines bit 1 and the zero voltage defines bit 0. 1 bit signal is transmitted during a clock cycle.
Double Bit Rate:
Since PAM4 signals utilize four signal levels for transmission compared to the two levels in traditional NRZ signals, the bit rate of PAM4 is twice that of NRZ signals within the same symbol period, offering higher transmission efficiency.
Less Signal Loss:
PAM4 achieves the same data rate with only half the baud rate of NRZ signals, significantly reducing signal loss during channel transmission.
Figure: PAM4 vs NRZ
Lower Deployment Costs
Cost-effective:
Generally, optical modules achieve higher transmission speeds by increasing the number of channels or improving the single-channel speed, with the former spending higher deployment costs. PAM4 improves transmission efficiency per UI on a single channel by increasing signal levels and upgrades internal electrical chips in modules to double the network interface rate without changing channels or optical devices.
Low construction costs:
PAM4 signals offer higher bit rates, and in 5G transport networks, they can achieve greater transmission efficiency with fewer mature optical devices, eliminating the need for additional fiber optic equipment and significantly reducing construction and R&D costs.
Mature technology
The 100G backplane IEEE 802.3bj standard, released in 2014, defined two signal transmission technologies: NRZ (4×25.78Gbps baud rate) and PAM4 (4×13.6Gbps baud rate). With the development of chip, PCB, and connector, NRZ quickly became commercially viable, while PAM4 was not adopted in 100G Ethernet due to its immaturity and higher cost.
In developing the 200G/400G interface standards, the requirement was to increase the rate of each differential pair to 50G or higher. If NRZ had been used, the strict timing margin requirements for transceiver chips and transmission links, due to symbol periods being under 20ps, would have been too challenging. As a result, PAM4 became the optimal choice.
400G PAM4 Transceivers: Multimode and Singlemode
The 400G transceivers primarily utilize PAM4 technology and are available in both multimode and singlemode options. Based on PAM4 modulation, these modules support 8x50G PAM4 on the electrical interface, and 8x50G PAM4 or 4x100G PAM4 on the optical interface.
As a relatively low-cost solution for 400G and data centers, PAM4 has been widely adopted by the transceiver industry, enabling high-speed data rates and facilitating the transition to 400G and beyond. At present, all FS 400G transceivers, featuring 4×100G PAM4 or 8×50G PAM4 technology standardized by the IEEE working group, have undergone rigorous testing on target devices to ensure perfect compatibility and seamless interoperability.
Form Factor
Product
Wavelength
Connector
Modulation (Electrical)
Modulation (Optical)
Power Consumption
OSFP
850nm
MPO-16/APC
8x 50G PAM4
8x 50G PAM4
≤10W
1310nm
MPO-12/APC
8x 50G PAM4
4x 100G PAM4
≤10W
850nm
MPO-16/APC
8x 50G PAM4
8x 50G PAM4
≤10W
QSFP112
850nm
MPO-12/APC
4x 100G PMA4
4x 100G PAM4
≤9W
1310nm
MPO-12/APC
4x 100G PMA4
4x 100G PAM4
≤10W
1310nm
MPO-12/APC
4x 100G PMA4
4x 100G PAM4
≤10W
1310nm
Duplex LC/UPC
4x 100G PMA4
4x 100G PAM4
≤10W
1310nm
Duplex LC/UPC
4x 100G PMA4
4x 100G PAM4
≤10W
QSFP-DD
850nm
MPO-12/APC
8x 50G PAM4
4x 100G PAM4
≤8W
850nm,910nm
MPO-12
8x 50G PAM4
8x 50G PAM4
≤12W
1310nm
MPO-12/APC
8x 50G PAM4
4x 100G PAM4
≤9W
1271nm,1291nm,1311nm,1331nm
Duplex LC/UPC
8x 50G PAM4
4x 100G PAM4
≤9W
1310nm
MPO-12/APC
8x 50G PAM4
4x 100G PAM4
≤9W
1310nm
MPO-12
8x 50G PAM4
4x 100G PAM4
≤11W
1271nm,1291nm,1311nm,1331nm
Duplex LC UPC
8x 50G PAM4
4x 100G PAM4
≤10W
1271nm,1291nm,1311nm,1331nm
Duplex LC/UPC
8x 50G PAM4
4x 100G PAM4
≤9W
1310nm
Duplex LC/UPC
8x 50G PAM4
8x 50G PAM4
≤10W
1310nm
MPO-12/APC
8x 50G PAM4
4x 100G PAM4
≤9W
1310nm
Duplex LC/UPC
8x 50G PAM4
8x 50G PAM4
≤10W
Conclusion
As the market moves to PAM4-based modulation, more and more chip makers and transceiver vendors are manufacturing new 400G products using PAM4, transferring 400G PAM4 from theory to practice. PAM4 400G based on 50G PAM4 or 100G PAM4 will certainly become the basic rate of the next-generation Ethernet and stand out with its high performance and potential.