400G PAM4 (4 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.
Pulse Amplitude Modulation 4-level (PAM4) is a technology that uses four different signal levels for signal transmission and each symbol period represents 2 bits of logic information (0, 1, 2, 3). By transmitting two bits in one symbol slot, PAM4 halves the signal bandwidth. Therefore, it is feasible to increase bandwidth by using advanced modulation PAM4 technology to increase the data rate without having to configure the data center with more fibers. 400 Gbps Ethernet can be realized with four lanes of PAM4 (8× 50 Gbps). This effectively doubles a network’s data rate, enabling 400G PAM4 for short-haul and long-haul transmission.
Since the bandwidth of NRZ requires two times as much as PAM4 for the same data throughout and the data rate of 25Gbps per lane using NRZ signaling was already reaching its limit, when the 400GE IEEE 802.3bs standard was discussed, PAM4 technology was proposed to replace NRZ. The proposal was finally passed after analysis and certification.
400G LR8/FR8 standard is the first 400G PAM4 interface standard and then PAM4 modulation is widely applied in 400G transceivers. Instead of using 16× 25G baud rate NRZ for 400G Ethernet, PAM4 modulation provides a path from 100G Ethernet using 4× 25G baud rate to 400G Ethernet via 8× 25G baud rate architecture, which means 400G Ethernet links via 8× 50G bit rate solution, both decreasing the cost of fibers and link loss.
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.
Figure: PAM4 vs NRZ
Double Bit Rate - PAM4 doubles the bit rate for a given baud rate over NRZ. Thus, a 28 Gbaud PAM4 signal can deliver the same bit rate as a 56 Gbaud NRZ signal.
Less Signal Loss - PAM4 should let you develop 56 Gbps data lanes with less signal loss than would occur by simply doubling the NRZ (sometimes called NRZ-PAM2) bit rate. Exotic PCB materials can compensate for the deficiencies, but at a cost few are willing to pay.
PAM4, a relatively low-cost solution for 400GbE and data centers, has been adopted in the transceiver industry, enabling high-speed data rates, toward 400G and beyond. FS 400G optical interfaces using either 4× 100G PAM4 or 8× 50G PAM4 that have been standardized by IEEE task forces are in the following table:
|Standard||Transceiver Types||Link Distance||Media Type||Lanes||Power Consumption|
|IEEE P802.3cm||400GBASE-SR8||100m||MMF||8× 50G PAM4||<10W|
|IEEE 802.3bs||400GBASE-DR4||500m||SMF||4× 100G PAM4||<10W|
|400GBASE-LR8||10km||SMF||8× 50G PAM4||<14W|
|IEEE P802.3cn||400GBASE-ER8||40km||SMF||8× 50G PAM4||<14W|
|100G Lambda MSA||400GBASE-XDR4||2km||SMF||8× 50G PAM4||<12W|
|400GBASE-FR4||2km||SMF||4× 100G PAM4||<12W|
|400GBASE-LR4||10km||SMF||4× 100G PAM4||<12W|
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 potentials.