400G offers a welcome upgrade to relieve network bottlenecks for data center and large-scale networks. Data center and network operators must understand both the practical challenges brought about by the underlying technology introduced by 400G and alternatives to minimize upgrade costs across the network.
400G data rates are enabled by applying PAM-4 modulation to combine higher signal rates and baudrates, ultimately increasing data rates by four times over current Non-Return to Zero (NRZ) signaling. The introduction of PAM-4 signaling creates new interoperability challenges between these 400G ports and legacy networking gear. Existing NRZ transceivers, switch ports, or NICs found in the network are not interoperable with PAM-4.
The most common use case where PAM-4/NRZ interoperability challenges will be experienced is in the use of network breakout connections. Widely deployed breakout connections are used to aggregate connections between multiple network elements such as servers, storage, and other appliances with a single top-of-rack or leaf switch port. In the recent 40G and 100G upgrade cycles, modulation was not a concern. Configuring breakout ports and aligning data rates were the primary complexities to consider.
Utilizing existing 100G switches and transceivers to aggregate to 400G via breakout connections will need to consider new options. 400G transceivers typically transmit and receive with four lanes of 100G or eight lanes of 50G with PAM-4 signaling on both the optical and electrical interfaces. Legacy 100G transceivers are built on four lanes of 25G NRZ formatted signaling both on the optical and electrical side. 100G-SR4, 100G-SWDM4, 100G-CWDM4 and 100G-LR4 are among the common NRZ transceivers commonly found in the data center and will not be interoperable with 400G transceivers in a 4x100 application.
Data center and network operators have transceiver-based solutions to avoid the unneeded request of upgrading existing 100G switches to support PAM-4. A new class of 100G transceivers has been introduced that will support 100G PAM-4 at the optical interface and 4x25G NRZ at the electrical interface. These transceivers perform the re-timing between PAM-4 and NRZ modulation within the transceiver gearbox. The QSFP28 DR and FR (also referred to as DR1 and FR1) transceivers are the first of these transceivers, are fully interoperable with not only legacy 100G network gear, but with QSFP-DD DR4 and DR4+ breakout transceivers. The QSFP-DD DR4 and DR4+ are parallel series modules, accepting a MPO-12 connector, with breakouts to LC connectors to interface with the DR or FR transceivers. DR4 to 4xDR connections are up to 500M, DR4+ to 4xFR connections are up to 2KM over single-mode fiber.
The dawn of 400G requires careful consideration and planning to avoid unwanted network gear and overhead investment. Understanding the transceiver solutions available is key to data center and network operators to minimize network hardware costs and increase port density.