Which is the Mainstream for 200G Data Centers, 200G QSFP56 or 200G QSFP-DD?

NADDOD Jason Data Center Architect Dec 12, 2022

With the rapid development of optical communication and the Internet, the demand for the network has also grown in line with the trend, resulting in the rapid growth of traffic in telecom backbone networks at the rate of 50% to 80% per year. To meet user demand, the transmission rate of optical communication has increased its development, from 10G, 25G, 40G before to 100G, 200G, 400G, etc. today. Although 100G optical transceivers have become the mainstream of the market, the bandwidth and port density requirements are still increasing, which further drives the technology to 200G, 400G or higher speed systems.

What Are the Package Types of 200g Optical Modules?

Currently, there are two main types of 200G optical transceivers: 200G QSFP56 and 200G QSFP-DD. The QSFP56 was introduced in 2017 and by its very nature represents a design advancement from earlier QSFP transceivers, while the QSFP-DD was still in the development stage during the same time frame. Both are designed for high-performance computing and data centers, and both are backward compatible with earlier QSFP versions, including the QSFP28.

Specifically designed for 200G Ethernet, QSFP56 has four transmit and receive channels, each capable of operating at 53.125 Gb/s with a total bit rate of 212.5 GB/s. It transmits data at 850nm, 1310nm, CWDM or LWDM wavelengths. The QSFP56 optical transceiver uses MPO as an optical interface and a 38-pin connector as an electrical interface. Unlike the previous the QSFP optical transceiver, the QSFP56 optical transceiver utilizes PAM4 digital modulation scheme.

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 channels of 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 form specifications, 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 modulation technology. NRZ stands for non-return-to-zero code and is known as PAM2, a modulation method with two voltage levels for logic 0 and logic 1, while PAM4 pulse amplitude modulation uses four voltage levels to represent four combinations of two logics 11, 10, 01 and 00. This allows the PAM4 signal to be transmitted twice as fast as a conventional NRZ signal.

NRZ and PAM4 Modulation

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 data center.

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 engineers to upgrade their networks, where high-speed QSFP-DD can be 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 a consideration, 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.

NADDOD 200G optical transceivers/AOC/DAC

As a leading provider of optical networking solutions, NADDOD offers a full range of next-generation 200G data center interconnect products, including 200G QSFP56 SR4, 200G QSFP56 FR4, 200G QSFP-DD LR4, 200G QSFP56 AOC and 200G QSFP56 DAC, with both DAC and AOC supporting “break-out” applications.

The 200G QSFP56 SR4 optical transceiver transmits up to 100m over multimode fiber and is suitable for short distance data transmission, such as interconnecting aggregation switches and access switches.

The 200G QSFP56 FR4 optical transceiver transmits up to 2km over single-mode fiber, which can be used for data center leaf switch and spine switch interconnection.

The 200G QSFP-DD LR4 optical transceiver can transmit up to 10km over single-mode fiber and is typically used for Tier 1 and Tier 2 interconnections, undertaking data transmission between core routers, core switches and aggregation switches.

The 200G AOC and DAC are typically deployed between access switches and servers. In the underlying access switch and server interconnection solution, the branch DAC and AOC can be used for different requirements in addition to directly connected DAC and AOC. NADDOD’s 200G to 4x50G or 200G to 2X100G DAC and AOC products provide a more flexible solution for data centers.