As the speed moves from 100G to 400G, the words PAM4 and related appear more and more frequently in quotations and technical exchanges. What exactly are they? Should we care about ordinary data centers running 25G/40G/100G? This article explains it all in one go.

NRZ and PAM4: one symbol from 1bit to 2bit

Traditional NRZ signals use two levels of high and low levels to transmit 1 bit per symbol. 25G serial is a single channel of 25.78G. PAM4 uses four amplitude levels and one symbol distinguishes 2 bits. The net rate is nearly doubled under the same clock. Therefore, 100G single wave solutions and 400G modules with single wave 50G generally use PAM4. The price is stricter signal quality requirements: the eye diagram height is reduced to about one-third, it is more sensitive to insertion loss and noise, and usually requires DSP equalization and forward error correction (FEC).

Coherent Detection: Handing the Distance to the DSP

Coherent reception not only demodulates amplitude but also demodulates phase, and combined with DSP dispersion compensation, can greatly extend the transmission distance and improve sensitivity. Coherence is generally not used for 100G links in the data center computer room. The real main battlefield is DCI and metropolitan medium and long-distance scenarios; as 400G ZR and others put coherence into pluggable packaging, campus-level long links began to consider it. In ordinary computer rooms, the solution of 25G entering the server, 100G doing aggregation, and intensity modulation direct detection is completely sufficient.

Three reminders for existing network users

• Don't just look at the nominal rate, first check whether the single-channel baud rate and modulation format match the new switch;
• After FEC is turned on, the delay and error threshold characteristics will change. Financial low-latency services must be measured first before being released into volume;
• Short-distance interconnection button in the computer roomData center high-speed interconnection solutionThe conventional configuration is enough, and there is no need to pay extra budget for modulation methods that are not used.

To summarize in one sentence: PAM4 solves how to run a channel faster, and coherent solves how to send signals further; for computer rooms where the existing network is mainly 25G/40G/100G, first select the NRZ parallel solution based on distance and loss budget, and then study PAM4 and coherent for cross-campus or metropolitan links; EB-LINK tests 100G/400G high-speed modules one by one before leaving the factory, and cross-campus link solutions can be dialed in 0755-83179002 Check.