100G 80KM DCI Solutions Comparison: Coherent, PAM4 or QSFP28 ZR4

With the rapid development of artificial intelligence, virtual reality, and the Internet of Things, data traffic is exploding, leading to the widespread adoption of data centers. For data center resources sharing, business continuity, and disaster recovery, data center interconnect (DCI) is required and is essential. There are three solutions for 100G 80km DCI, using 100G coherent DWDM technology, 100G PAM4 DWDM transmission, and 100G QSFP28 ZR4 optics. We will explore these three solutions and compare them to determine the best choice for 100G long-haul DCI.

100G 80km DCI Solution Options

100G Coherent

100G coherent DWDM technology enables transport of 100G capacity over a single wavelength across long distances. Coherent optical devices can transmit signals even over distances of up to 1000km. Coherent optics use advanced modulation techniques, such as QPSK, 8QAM, or 16QAM, to transmit multiple symbols per bit, condensing signals for speed benefits. However, this kind of modulation leads to demanding technical requirements. Coherent detection and DSP technology are needed to compensate for various transmission impairments in the electrical domain and simplify the transmission link.

Below is the connectivity diagram of 100G coherent DWDM solution, in which the M6500 platform seamlessly integrates with muxponder/transponder to deliver flexibility and scalability for data center interconnect. The FS M6500 OTN platform provides a managed, flexible and scalable architecture to construct a 100G high capacity optical transport network over a single wavelength.

100G PAM4 DWDM

PAM4 is the key technology of this solution which transmits two bits in one symbol slot, halving the signal bandwidth. Also, PAM4 can minimize signal degradation and double the data rate. In addition, PAM4 optical modules can be used directly in the switches of embedded DWDM networks, which is a cost-effective and simple solution for customers willing to build embedded DWDM data networks.

The following figure shows how 100G PAM4 DWDM solution is used for 80km DCI. The FS 100G PAM4 DWDM open line system is highly integrated with a 40CH DWDM mux/demux, optical amplifiers, dispersion compensation modules, and an optical supervisory channel, providing significant space savings and improved operational and maintenance efficiency.

100G QSFP28 ZR4

The 100G QSFP28 ZR4 supports 100G Ethernet for extended reach up to 80km transmission that provides point-to-point network connectivity solutions between data centers without the need to deploy either complicated and costly open-line systems or legacy CFP/CFP2 interfaces. The module supports the standard KR4 FEC (Forward Error Correction) baud rates, which will help receive-side detect and correct bit errors and improve the overall quality of the link. Therefore, the 100G ZR4 QSFP28 transceiver can operate on a point-to-point link up to 80km without optical amplification and dispersion compensation.

100G DCI Solutions Comparison

• 100G Coherent: Pros and Cons

The coherent optics are ideal for long-haul distance transmission (80km-1000km) but have limitations on cost and power consumption. The coherent transceivers are expensive and only available on the packages of CFP and CFP2 with large power consumption. However, the 100G coherent DCI solution discussed above is scalable to meet high-capacity DWDM requirements. The highly integrated M6500 platform allows the addition of 200G transponder/muxponder units as required, providing flexibility for the entire optical transmission network.

• 100G PAM4 DWDM: Pros and Cons

100G PAM4 DWDM solution is a highly integrated platform, making it more economical and convenient than 100G coherent DWDM solution, especially in 80km DCI networks. The 100G PAM4 DWDM open line system incorporates a DWDM multiplexer, an EDFA, and a Dispersion Compensator, bringing plug-and-play simplicity to operation and management. However, this solution is only suitable for data center interconnect within 80km.

• 100G QSFP28 ZR4: Pros and Cons

Compared to the two 100G 80km DCI solutions mentioned above, QSFP28 ZR4 is a cost-optimized and low-power solution without deploying redundant and complex equipment. However, 100G QSFP28 ZR4 is also only available for 0-80km DCI solutions. If the whole optical link loss is too high, transmission up to 80km can't be achieved. And without the use of DWDM technology, more fiber optic cables are required.