The main difference between QSFP28 and QSFP56 lies in their data transmission capacity.

QSFP28 is a form factor for transceivers that can support data transmission rates of up to 28 gigabits per second (Gbps) per channel, resulting in a total throughput of 100 Gbps with four channels.

On the other hand, QSFP56 is a form factor for transceivers that can support data transmission rates of up to 56 gigabits per second (Gbps) per channel, resulting in a total throughput of 200 Gbps with four channels.

25G SFP28 ER 1310nm 30/40KM Optical Transceiver Module 

In summary, QSFP28 supports data transmission rates of up to 100 Gbps, while QSFP56 supports data transmission rates of up to 200 Gbps.

What is QSFP56?

200G optical modules with PAM-4 technology

QSFP56 is a relatively new form factor for data communications, designed to support data rates up to 200Gbps. The QSFP56 transceiver is compliant with the QSFP56 Multi-Source Agreement (MSA). 200G QSFP56 transceiver is an upgraded version of QSFP28. It uses the same physical specifications as prior QSFP28, the biggest improvement is the modulation technology. QSFP56 upgrades the modulation technology by using PAM-4 instead of NRZ of 100G QSFP28. While NRZ needs 8x25Gbps to reach 200Gbps, PAM4 only needs 4×50Gbps to support 200Gbps, so as to save the optical fiber costs and reduce the link loss.

The common 200G QSFP56 transceivers include 200G QSFP56 SR4, DR4, FR4, LR4, and ER4. Their main specifications are shown below.

Comparison of QSFP28 vs QSFP-DD vs QSFP56

Differences Quick View

The following table concludes the main differences of QSFP28, QSFP-DD and QSFP56.

The following will give a detail illustration to their differences.

Differences 1: Data rate and channel count

Data rate is the most fundamental difference among them. QSFP-DD is the max data rate one, as its name implies, double density, that is its channel count has doubled from 4-channel of QSFP28 and QSFP56 to 8 channels. As a result, QSFP-DD supports a max data rate of 400G (8x 50Gb/s) based on PAM4 modulation, and also supports 200G (8x 25Gb/s) based on NRZ modulation. And with 4 channels modulated with NRZ results of 50Gbps/25Gbps per channel, QSFP56 supports 200Gbps whereas QSFP28 supports 100Gbps.

Differences 2: NRZ vs PAM4 Modulation

As mentioned above, QSFP28 and 200G QSFP-DD use NRZ modulation technology while QSFP56 and 400G QSFP-DD use PAM4 technology. Then what are the differences between NRZ and PAM4?