200G Data Centers: Choosing Between QSFP56 and QSFP-DD as the Dominant Standard

Posted on Dec 22, 2023 by

With the rapid advancement of optical communication and the Internet, there has been a corresponding increase in network demand, leading to a significant surge in telecom backbone network traffic at a rate of 50% to 80% annually. In order to cater to user needs, the transmission rate of optical communication has seen continuous development, progressing from 10G, 25G, and 40G to the current 100G, 200G, 400G, and beyond. While 100G optical transceivers have become the prevalent choice in the market, the requirements for bandwidth and port density continue to escalate, thereby propelling the advancement of technology towards 200G, 400G, and higher-speed systems.

What Are the Package Types of 200g Optical Modules?

At present, the market features two primary forms of 200G optical transceivers: the 200G QSFP56 and the 200G QSFP-DD. The QSFP56, introduced in 2017, signifies a notable design progression from earlier QSFP transceivers. In contrast, the QSFP-DD was still undergoing development during that timeframe. These transceivers are specifically crafted for high-performance computing and data centers, and they offer backward compatibility with earlier QSFP versions, including the QSFP28.

Engineered for 200G Ethernet applications, the QSFP56 features four channels for both transmitting and receiving, with each channel capable of operating at 53.125 Gbps, culminating in a total data rate of 212.5 Gbps. This transceiver operates over wavelengths of 850nm, 1310nm, CWDM, or LWDM. For optical connectivity, the QSFP56 utilizes an MPO interface, while electrically it connects through a 38-pin connector. In contrast to its QSFP predecessors, the QSFP56 employs the PAM4 digital modulation technique to enhance data transmission capabilities.

The QSFP-DD (Quad Small Form Factor, Pluggable Dual Density) optical transceiver is compliant with IEEE802.3bs and QSFP-DD MSA standards. The dual density architecture essentially increases the number of channelsof electrical interfaces. 200G QSFP-DD has an eight-channel electrical interface totaling 212.5Gb/s of total bit rate. The optical interface is either MPO or duplex LC. QSFP-DD is backward compatible with most QSFP formspecifications, including QSFP56. QSFP-DD electrical interfaces feature eight lanes of up to 25 Gbps each, using the NRZ modulation scheme.

Comparison of 200G QSFP56 and 200G QSFP-DD

NRZ and PAM4 are two different types of digital modulationn technology. NRZ( Non-Return-to-Zero)is a modulation method with two voltage levels representing logic 0 and 1 (also known as PAM2). In contrast, PAM4, or Pulse Amplitude Modulation, uses four voltage levels to represent four combinations of two bits: 11, 10, 01, and 00. This allows the PAM4 signal to be transmitted twice as fast as a conventional NRZ signal.


The main advantage of PAM4 over NRZ is the relatively faster transmission speed. 200G NRZ modulation also has the advantages of lower power consumption, lower latency and ease of deployment, despite the lack of speed. 200G NRZ enables low-cost interconnect solutions within the datacenter.

Pros and Cons of 200G QSFP56 and QSFP-DD

  • QSFP56 is designed to support 200G applications and cannot accommodate network upgrades to 400G and beyond.

  • QSFP-DD operating in 200G and 400G versions and allowing incremental network upgrades.

  • QSFP56 uses PAM4 modulation. QSFP-DD typically operates at 200G on the NRZ.

  • QSFP56 transceivers need to use only 4 channels for 200G transmission, compared to 8 channels for QSFP-DD, resulting in fiber cost savings and reduced link loss.

  • QSFP-DD offers lower maintenance costs, low power consumption, low latency, low BER (Pre-FEC=E-8, post FEC=E-12) and ease of deployment.

  • QSFP-DD provides greater flexibility for network and system engilneers to upgrade their networks, where high-speed QSFP-DD canbe split into lower-speed legacy form factors.

  • QSFP-DD is backward compatible with earlier iterations of QSFP transceivers, including QSFP56.

  • QSFP56 backward compatibility with QSFP-28 iterations, but not QSFP-DD.

  • QSFP-DD is more expensive than the QSFP56 in terms of price.

Not all switches and routers are configured to support QSFP-DD, making it very expensive to increase link rates. If the initial setup cost is considered, QSFP56 may be a better choice. If cost is not an issue, QSFP-DD can be easily upgraded for later expansion.

QSFP-DD transceivers can cost anywhere from 15% to 30% more than QSFP56 transceivers. However, the 200G QSFP-DD makes up for this initial cost through lower maintenance costs: it consumes less energy and electricity. In addition, it achieves lower latency.

FS 200G optical transceivers/AOC/DAC

FS offers a full range of next-generation 200G InfiniBand and 200G Ethernet Products,includingincluding 200G QSFP56 SR4, 200G QSFP56 FR4, 200G QSFP56 LR4, 200G QSFP-DD 2SR4, 200G QSFP56 AOC, 200G QSFP-DD AOC 200G QSFP56 DAC and 200G QSFP-DD AOC, with both DAC and AOC supporting"break-out" applications.

The 200G QSFP56 SR4 optical transceiver is designed for 200GBASE Ethernet using OM4 multimode fiber with a wavelength of 850nm up to 100 meters. It is suitable for data centers, high performance computing networks, enterprise core and distribution layer applications.

The 200G QSFP56 FR4 optical transceiver is designed for 200GBASE Ethernet throughput up to 2km over single-mode fiber using a wavelength of 1295nm to 1309nm. It is suitable for 200G Ethernet, Data Center, and Cloud Network.

The 200G QSFP56 LR4 optical transceiver is designed for 200GBASE Ethernet throughput up to 10km over single-mode fiber. It is suitable for 200G Ethernet, Data Center, and 5G backhaul.

The 200G QSFP-DD 2SR4 optical transceiver supports up to 100m link lengths over multimode fiber. It is suitable for 2×100GBASE-SR4 Ethernet, Data Centers and Switch & Router Connections.

The 200G AOC and DAC are commonly utilized in the connectivity between access switches and servers. In scenarios where the base-level interconnections between access switches and servers, branch DAC and AOC serve diverse needs beyond the standard direct-connection DAC and AOC. FS offers a range of 200G to 4x50G, 200G to 8x25G, and 200G to 2X100G DAC and AOC products, providing data centers with a more flexible and adaptable solution.

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