光学 MSA：QSFP-DD 使 PAM4 信号处于主导地位
19 February 2019 / by Matt Bolig
在 2018 年进行了大量演示和 alpha 模块测试之后，双密度版本的四通道小型可插拔 (QFSP) 多源协议 (MSA) 模块标准有望成为新线卡的规则。QSFP-DD 热潮的隐含意义不是模块本身的物理特性，而是转向先进信令方法的趋势。十年前，QSFP28 的问世预示着使用 NRZ 信令的 25Gbps 电通道即将占据主导地位。如今，QSFP-DD 及其更庞大的表亲 —— 八进制 SFP 或 OSFP ，都使用 PAM4 信令。其用得最多的是 50Gbps 通道。但 QSFP-DD 足够灵活，可以在各种实施中使用八通道接口：25G x 8 用于 200Gbit 以太网，50G x 8 用于 400Gbit 以太网，可能还有一些早期的 100G x 8 用于预标准 800Gbit 以太网。
The slow evolution of pluggable optics MSAs has all but eliminated larger WAN-style optical modules (CFP types), while providing interoperability in physical and electrical standards through informal agreements rather than mandates from standards bodies. The original Small Form-factor Pluggable, or SFP, helped standardize and drive 10Gbps networks. The quad-channel QSFP was useful in migrating 10Gbps to 40Gbps networks. Then the faster successor, QSFP28, proved even more useful in driving 28Gbps channels that used NRZ coding. Four of these 28Gbps channels could be used to implement an effective 100Gbps module.
In 2017, PAM4 still was the emerging signaling for both 50Gbps channels, and for single-lambda 100G Ethernet. The long-time use of NRZ for 10Gbps and 25Gbps channels certainly will not disappear overnight. But the maturation of PAM4 technology (analog and digital instantiations) has placed the new signaling method at the forefront of the newest higher-speed Ethernet implementations. In fact, the Institute of Electrical and Electronics Engineers (IEEE) has approved PAM4 as the preferred signaling for all 50Gbit, 100Gbit and 200Gbit and 400Gbit Ethernet standards within the umbrella 802.3bs, 802.3cd and 802.3ck standards. Additionally, MSAs such as the 100G per Lambda group are organizing to fill out PAM4 signaling for the entire data center ecosystem.
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