The main difference between QSFP56 and QSFP+ is the data transfer rate and the number of channels.

QSFP56, also known as QSFP56-DD, supports data transfer rates of 200 Gbps per channel, allowing for a total data transfer rate of 400 Gbps when using double density (DD) cables and connectors. It has eight high-speed PAM4 lanes, each operating at 50 Gbps, resulting in a total of 400 Gbps.

QSFP+ supports data transfer rates of up to 40 Gbps per channel, allowing for a total data transfer rate of 40 Gbps or 100 Gbps using quadruple data rate (QDR) cables. It has four high-speed lanes, each operating at 10 Gbps, resulting in a total of 40 Gbps or 100 Gbps.

In summary, QSFP56 offers a higher data transfer rate and more channels compared to QSFP+, making it suitable for high-performance data center and networking applications.

QSFP56 vs QSFP28 vs QSFP+

By their industry names, QSFP56, QSFP28, and QSFP+ are very similar because they share the same QSFP form factor, as indicated by their suffixes, and they have the same dimensions. However, their data center and connectivity capabilities are different. The following table lists the basic parameters of QSFP56, QSFP28 and QSFP+.

It is clear from the comparison chart that the QSFP56 form factor offers higher network speeds than the 200G QSFP supporting 4 x 50G channels when compared to the QSFP+ and QSFP28. QSFP+ is an evolution of QSFP, supporting 4×10G channels carrying 10G Ethernet, 10G Fibre Channel, or QDR InfiniBand. it introduces the concept of multiplexing four channels to increase bandwidth, and is capable of handling a 40Gbps line rate at 10GBaud NRZ per channel. the QSFP28 supports 4×25G channels, and like the QSFP+, contains The QSFP28 supports 4 x 25G channels and, like the QSFP+, includes a 4-channel optical transmitter and a 4-channel optical receiver.

The most significant change from QSFP+ and QSFP28 to QSFP56 is that QSFP56 makes a change from NRZ encoding to PAM4 encoding. While QSFP56 still uses 4 channels as QSFP28, the modulation is doubled to 50G per channel, allowing for more data over existing fiber, making it more suitable for hyperscale data center networks.

Moving from QSFP56 to QSFP56-DD (400G QSFP-DD)

With the rapid growth of data centers, the need for ever-increasing amounts of data is driving network components to support higher bandwidth and higher density. The latest iteration of the optical module form factor is from the QSFP56 to the QSFP56-DD, also known as the 400G QSFP-DD. DD in this case refers to double density, denoting that 400G is achieved by doubling the data channels of the QSFP56 from 4 to 8 (using 50G PAM4).

The QSFP56-DD is similar in size to the QSFP56, although it has double density. 400G QSFP56-DD ports are backwards compatible with QSFP transceivers, which means that the QSFP56 will work on QSFP56-DD ports as long as the switch supports them. When using a QSFP56 module in a QSFP56-DD port, the port will be configured for 200G data rates instead of 400G.

The QSFP56-DD form factor is now recognized by the 400G market as the most popular 400G form factor. While 400G Ethernet is now seen as a future-proof solution for next-generation data centers, some organizations deploying 200G Ethernet still require 200G QSFP56.