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Three 80km 100G Optical Solutions for Long-Haul Data Center Interconnect (DCI)



Three optical transceiver solutions can be used for up to 80km long-haul 100G data center interconnect (DCI): 100G DWDM coherent optics, 100G PAM4 DWDM QSFP28, and 100G ZR4 QSFP28.

Three optical transceiver solutions can be used for up to 80km long-haul 100G data center interconnect (DCI): 100G DWDM coherent optics, 100G PAM4 DWDM QSFP28, and 100G ZR4 QSFP28. This article will introduce the three solutions and analyze their pros and cons. Hope you will have a clear picture of how to get the optimal choice for a long-haul 100G DCI solution.


100G DWDM Coherent Optics

Using coherent optical modules in a DWDM platform system is the most widely used solution for 100G long-haul DCI. It offers many benefits in terms of transmission distance, scalability, and flexibility. Coherent transceivers can be used in both single-mode and multimode fiber networks. They support a wide range of modulation formats and data rates, from 25G to 400G. Coherent optics is also backward-compatible with older technologies such as 10G and 40G. 


Coherent 100G optics are restricted with CFP and CFP2 form factors, therefore coherent optics can not be directly inserted into most data center switch ports. A series of DWDM equipment for networking and technical support is required to maintain and simplify transmission links. 



PAM4 DWDM QSFP28 is another solution for 100G long-haul DCI. PAM4 is a newer modulation format that allows four bits to be encoded in a single symbol. This enables higher data rates and longer distances to be reached with PAM4 than with other modulation formats. For example, the QSFPTEK 100G PAM4 QSFP28 transceiver support 40-channel 100G links for reaching up to 80km with a total capacity of 4Tbps. 


The PAM4 QSFP28 optics can directly interoperate with existing switches, which makes it a cost-effective and simple solution to build an embedded DWDM network. Using DWDM PAM4 transceivers for 80km 100G DCI solution, Tunable Dispersion Compensation Module (TDCM) and erbium-doped fiber amplifier (EDFA) are deployed to amplify optical signals and compensate for dispersion.


100G ZR4 QSFP28

Similar to 100G PAM4 QSFP28, 100GBASE-ZR4 QSP28 shares the same form factor, therefore ZR4 QSFP28 can be compatible with the existing Ethernet infrastructure. Also, it supports a max transmission distance of up to 80km 100G link over single-mode fiber (SMF).

The major difference is that ZR4 QSFP28 is not designed for DWDM connectivity. Using 100G ZR4 QSFP28 for 80km data center switch interconnect, only matched SMF fiber optic cables are required, and no extra conversion modules, DCM, and EDFA equipment is required for networking. This is a real point-to-point (P2P) high-speed and long-distance optical connection solution.


ZR4 QSFP28 transceivers are frequently used to directly connect two servers, switches, routers, and other data center equipment for up to 80km reach.


Coherent vs PAM4 vs ZR4, Advantages and Disadvantages

The table below lists the major advantages and disadvantages of each module. When choosing the best solution for your circumstance, you should take factors such as the budget, transmission distance, networking complexity, existing architecture and equipment, DWDM capability, and future upgrade into consideration.


100G 80km DCI Solution



100G Coherent

• Ideal for long-haul transmission even over 80km (3000km is possible)

• High scalability & flexibility for future upgrade

• Costly

• Power Consuming

• CFP and CFP2 Form Factors


• Cost-effective

• Simpler: integrate DWDM directly into equipment

• within 80km application

100G QSFP28 ZR4

• Most Cost & power saving

• Simple P2P connection; plug-n-play

• within 80km application

• without DWDM capability

Compared to the 100G coherent solution, 100GBASE-ZR4 QSFP28 has original advantages in cost and power savings. 


Compared with 100G coherent and 100G PAM4, 100G QSFP28 ZR4 does not need any WDM system equipment. For long-haul transmission within 80km and non-WDM networks, 100G ZR4 QSFP28 is the optimal solution to go.


For a highly converged DWDM network, 100G coherent makes over 80km transmission possible. It is optimized for longer-distance optical communication (Up to 3,000 km is capable by Cisco coherent DWDM solution). However, it requires deploying complex DWDM equipment such as DWDM mux/demux, and Muxponder/transponder. 




Coherent 100G CFP/CFP2, 100G PAM4 DWDM QSFP28, and 100G ZR4 QSFP28 are three optical modules that support 100G Ethernet for up to 80km transmission. However, they have their pros and some limitations in real applications. The coherent solution is specially designed for long-haul metropolitan optical transmission, 80km is not a restriction. PAM4 and ZR4 QSFP28 are applied within an 80km link. 100G PAM4 is a cost-effective and much simpler solution for building an embedded DWDM network. ZR4 has advantages over the previous two in cost, power, and networking simplicity. 


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