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.

100G QSFP28 ZR4 Introduction

The 100G QSFP28 ZR4 optical module is used to connect switches, routers, transmission devices, etc. in data centers and over distances up to 80km on single-mode fiber.

The 100G QSFP28 ZR4 optical module is fully compliant with the QSFP28 industry standard and associated MSA as described in the latest SFF-8665/8636, and is available with digital diagnostics via the I2C interface. It is also compliant with the IEEE 802.3 100GBASE-ZR4 related specifications. In addition, the module supports the standard KR4 FEC (Forward Error Correction) baud rate, which will help detect and correct error codes on the receive side and improve the overall quality of the link.

100G QSFP28 ZR4 Architecture

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

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, requiring 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.

100G PAM4 DWDM

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.

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 can operate over point-to-point links up to 80km long without the need for optical amplification and dispersion compensation. This is a true plug-and-play solution. As shown in Figure 3 below.

Summary

In summary, in 100G 80km DCI, 100G ZR4 has significant cost and power advantages over coherent solutions, and compared to 100G PAM4 DWDM solutions, it eliminates the 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.