What Is PAM4 for 400G Ethernet?

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.

Why Does 400G Ethernet Need to Use PAM4 Technology?

In terms of supporting 400G Ethernet speed, the transmission rate of NRZ 25Gbps single channel has reached its limit, which cannot be adapted to the development of current high-density data centers. When discussing the 400GE IEEE 802.3bs standard, it was proposed to replace NRZ with PAM4 technology. So why is 400G PAM4 technology a viable alternative to NRZ?

Benefits of 400G PAM4

PAM4 modulation replaces 400G Ethernet using 16×25G baud rate NRZ and provides a path from 100G Ethernet using 4×25G baud rate to 400G Ethernet through 8×25G baud rate architecture. It is called 400G Ethernet The link adopts the 8×50G bit rate solution, reducing not only the fiber cost but also the link loss. For hyper-scale data centers, it's time for them to transition from the previous 100G or Gigabit networks to 400G PAM4 Ethernet for faster transmission efficiency.

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.

Both 400G SR8 and 400G SR4.2 multimode optical modules support 8x50G PAM4. 400G SR8 optical modules can use MPO-16 connectors or MPO-24 connectors to connect 8 pairs of fibers. The 400G SR4.2 modules use MPO-12 connectors, and the wavelengths are bidirectional and multiplexed.

400G optical modules modulated by 4x100G PAM4 are the focus of the current market, including 400G DR4, 400G FR4, and LR4, and their line-side uses four channels of 100G PAM4. In the 400G DR4 optical module, the DSP converts the 8x50G PAM4 electrical signal into 4x100G PAM4 and then transmits it to the optical engine.

<p class="blog_p padding_top_none community_blog_detail_p community_blog_detail_p1" background-color:#ffffff;"="" style="margin-top: 20px;margin-bottom: 0px;padding: 0px;text-size-adjust: none;font-size: medium;color: rgb(25, 25, 26);text-align: justify">Transceiver Solution Based on 400G PAM4

PAM4 is a relatively low-cost solution for 400GbE and data centers that has been adopted by the transceiver industry, enabling high-speed data rates, moving toward 400G and beyond. 3Coptics 400G transceivers apply 4×100G PAM4 or 8×50G PAM4 technology, which have been standardized by the IEEE working group, including 400GBASE-SR8, DR4, DR4+, FR4, LR8, ER8 as below.

400GBASE-SR8 100m MMF 8× 50G PAM4

400GBASE-DR4 500m SMF 4×100G PAM4

400GBASE-DR4+ 2km SMF 8× 50G PAM4

400GBASE-FR4 2km SMF 4× 100G PAM4

400GBASE-LR8 10km SMF 8× 50G PAM4

400GBASE-ER8 40km SMF 8× 50G PAM4