200G Optical Transceiver: QSFP56 vs QSFP-DD

NADDOD Jason Dec 12, 2022

The 200G optical transceiver is available in two technical directions: an economical 8x25G NRZ modulation in a QSFP-DD package; and a 4x50G (up to 4x56G) PAM4 modulation in a QSFP56 package.

200G QSFP-DD Optical Transceiver

200G (8x25G NRZ) QSFP-DD has the advantages of low power consumption, low latency, and easy deployment.

Low power consumption: The signal-to-noise ratio of NRZ is 9.54 db higher than that of PAM4. Compared with the transceiver of NRZ, a transceiver implementing PAM4 is more complex and more power consuming because of the need for a more advanced equalization function, NRZ power consumption will be lower, which is better for optimizing the data center OPEX and cooling efficiency.

Low latency: PAM4 is more sensitive to noise, and more likely to receive noise impact, the same transmission conditions receive a higher BER. Therefore, the FEC forward error correction mechanism needs to be added, which will increase the transmission delay while using NRZ can achieve a BER of 1E-15 with FEC turned off, and the delay is lower which has an excellent performance in some special application scenarios with demanding requirements on delay, such as “high-frequency trading” network scenarios.

Easy Deployment: For backward compatibility, PAM4 should be implemented at every level from PHY to the physical layer while retaining the existing NRZ circuit and logic while adopting PAM4, and the 25G NRZ optoelectronic chip and optical device technology industry chain is mature, related 200G port switches and 200G optical transceiver products are relatively easy to develop and fast to achieve commercial deployment.

Based on the above advantages, 200G QSFP-DD optical transceivers can realize low-cost optical interconnection within data centers, commonly available in 200G QSFP-DD SR8, PSM8, and LR8.

200g qsfp-dd parameter comparison

200G QSFP56 Optical Transceiver

200G QSFP56 applies a single channel 56G rate requirement, considering that the channel loss and reflection introduction cost of 56G signals is too high, the Nyquist frequency required by PAM4 at 56 Gbps is half of the Nyquist frequency required by NRZ (NRZ fNyquist = 56/2 = 28 GHz, PAM4 fNyquist = 56/4 = 14 GHz), the insertion loss on a sample IEEE 802.3-compliant backplane is about 33.35 dB at a Nyquist frequency of 14 GHz, and for a Nyquist frequency of 28 GHz, the same backplane shows an insertion loss of about 62 dB. These insertion loss numbers clearly indicate that, compared to PAM4, using the NRZ balanced backplane is more challenging than with PAM4.

The 4*50Gbps PAM4 technology enables 200G transmission, eliminating the need to use eight 25G lanes for 200G transmission as in NRZ. This saves fiber costs and reduces link loss.

200G QSFP56 optical transceivers are commonly available in 200G QSFP56 SR4, DR4, FR4, LR4, ER4.

200g qsfp56 parameter comparison

How to Choose a 200G Optical Transceiver?

With the rapid growth of data centers, the demand for data transmission is also increasing, driving the network to higher bandwidths and higher densities.

While the QSFP-DD package is currently the most popular package type in the 400G Ethernet market, this does not mean that the QSFP56 package type is obsolete. For some data centers that don’t have the same high speed requirements as 400G, deploying 200G Ethernet using 200G QSFP56 package optical transceivers is still the ideal solution. If you are considering upgrading to 400G, deploying 200G QSFP-DD package optical transceivers is a more cost effective solution.

NADDOD 200G products include optical transceivers and DAC/AOC. 200G QSFP56 SR4 is suitable for short distance data transmission, such as interconnection between aggregation switch and access switch; 200G QSFP56 FR4 can be used to interconnect data center Leaf switch with Spine switch; 200G QSFP-DD LR8 is usually used for Tier 1 and Tier 2 interconnection, taking up data transmission between core router, core switch and access switch; 200G DAC and AOC are usually deployed between core router, core switch and server.

In the underlying access switch and server interconnection solution, in addition to using direct-connect DAC and AOC, branch DAC and AOC can also be used for different needs. NADDOD’s 200G to 4x50G or 200G to 2X100G DAC and AOC products provide a more flexible solution for data centers.