QSFP28 ZR4, The Best Choice for 100G Long-distance DCI

NADDOD Edward Jan 5, 2023

As the development of 100G Ethernet becomes a trend, the demand for 100G optical modules is increasing, and today 100G optical modules represent a significant portion of the network construction cost. Three solutions have emerged in the 100G 80km DCI space, namely 100G Coherent, 100G PAM4 DWDM and 100G QSFP28 ZR4.

In this article, we introduce 100G QSFP28 ZR4 optical module and compare it with 100G coherent and 100G PAM4 DWDM optical transceiver to explore why QSFP28 ZR4 is the best choice for 100G long-distance DCI.

A Brief Introduction to 100G QSFP28 ZR4

The 100G QSFP28 ZR4 optical module is used for connection between switches, routers, transmission equipment, etc. in the data center, and the transmission distance over single-mode fiber is up to 80km.

The 100G QSFP28 ZR4 optical transceiver fully complies with the QSFP28 industry standard and related MSA described in the latest SFF-8665/8636, and digital diagnostic functions are available through the I2C interface. It also complies with IEEE 802.3 100GBASE-ZR4 related specifications. In addition, the module supports the standard KR4 FEC (Forward Error Correction) baud rate, which will help the receiving side detect and correct bit errors and improve the overall quality of the link.

100G QSFP28 ZR4 Structure

At the transmitting end of 100G QSFP28 ZR4, the four-channel serial data (NRZ) is recovered by the CDR and passed to four laser drivers, which control four lasers with center wavelengths of 1296nm, 1300nm, 1305nm and 1309nm. Optical signals are multiplexed onto single-mode fiber through standard LC connectors. At the receiving end, the optical signals of four channels are amplified by the SOA, and then demultiplexed by the integrated optical demultiplexer. Each optical signal is recovered by a PIN detector and passed through a transimpedance amplifier and CDR to a CAUI-4 compliant output driver, as shown in Figure 1:

100G QSFP28 ZR4 thumbnail

100G 80km DCI Solution Comparison

Metro connectivity and DCI refer to very large data centers and large network switches connected within cities that require many high bandwidth connections at 100G+ data rates. This requires optical modules that simplify operations, reduce space, cost and power consumption.

100G Coherent

Traditionally, switch vendors have used CFP/CFP2 coherent optical modules in their DCI equipment. Coherent technology is typically used for long-haul metro optical transmission using more advanced modulation techniques such as QPSK, 8QAM or 16QAM, which can double, triple or quadruple the bit rate. However, such modulation requires an advanced technique of coherent detection at the receiver side and DSP to correct for dispersion. While cost per bit remains a concern, component costs are high, and optical modules require space and sufficient power budgets and are only available on CFP and CFP2 pluggable packages. Of course, it is possible to insert coherent modules into switch/router boards for IPoDWDM scenarios. However, the high power consumption and large size of the CFP/CFP2 modules require dedicated line cards.


The latest development of 100G QSFP28 PAM4 DWDM offers another option. However, PAM4 requires the use of amplification and dispersion compensation systems on the optical link when the transmission distance is greater than 5km, which results in higher end-to-end costs. DCI operators need to deploy Mux/Demux, EDFA and DCM that may not exist in the current network. and this solution is not compatible with 50GHz Mux/Demux.

QSFP28 PAM4 DWDM transceiver solution

100G QSFP28 ZR4

The QSFP28 ZR4 overcomes the shortcomings of both of these solutions with the 100G ZR4 QSFP28, which is cost-optimized to support 100G Ethernet for up to 80 km. The QSFP28 ZR4, in a QSFP28 package, is small in size and low in power consumption, providing a point-to-point solution in the data center without the need to deploy legacy CFP/CFP2 interfaces and capable of operating over point-to-point links up to 80 km point-to-point links without optical amplification and dispersion compensation. This is a true plug-and-play solution. As shown in Figure 3 below.

QSFP28 ZR4 solution

In summary, in 100G 80km DCI, 100G QSFP28 ZR4 has significant cost and power advantages over coherent solutions, and compared to 100G PAM4 DWDM solutions, there is no need to deploy redundant and complex equipment to achieve point-to-point optical transmission, so 100G QSFP28 ZR4 is most preferred in long-haul DCI applications.