This article examines 50G transceiver signaling technology, different types of 50G optical modules and why 50G is key for upgrading to higher data rates.
In the past when data networks were limited to 100G data rates, networking systems utilized NRZ modulation. The building blocks of 100G networks are 10G and 25G modules which are forward compatible with 100G networks. When bandwidth demands escalated, networks began to upgrade to 400G data rates. One way to upgrade to 400G was to increase the size of the building block to 50G. PAM4 50G modules are forward compatible with 100G Single lambda transceivers and 400G and future 800G PAM4 modules.
Types of 50G Transceiver Modules
50G transceiver modules are available in the SFP56 and QSFP form factors. A 50G QSFP28 uses 2 out of the 4 available electrical lanes on a QSFP connector, with each lane running at 25Gbs NRZ. A 50G SFP56 uses 1 x 50Gbs PAM-4 lanes. SFP56 module is slightly smaller in size than a QSFP28 and allows for higher port density.
Data Rate and Form Factor
|Parameters||50G SFP56||50G QSFP28|
|Electrical I/O||1x 26GB PAM4||4x 25Gbps NRZ|
|Optical I/O Aggregated||50Gbps PAM4||50Gbps NRZ|
50G Transceiver Signaling Technology
Like mentioned above, SFP56 leverages PAM4 modulation. “PAM4” stands for Pulse Amplitude Modulation 4-level. PAM4 packs in 2 bits per symbol thus doubling the data rate and halving the bandwidth requirements but it also requires using FEC which leads to latency. PAM4 transceivers may not be ideal for certain latency sensitive applications like high performance computing.
Non-Return-to-Zero (NRZ) is a binary code using low and high signal levels to represent the 1/0 information of a digital logic signal. NRZ can only transmit 1 bit, i.e. a 0 or 1, of information per signal symbol period.
Primary applications for 50G transceivers include server-to-switch and switch-to-switch 50G Ethernet connections, and future 5G wireless applications. With 400G module data rates becoming more mainstream and 800G deployments not far behind, 50G transceivers are predicted to become the new backbone in the market.
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