QSFP56 and QSFP28 refer to different generations or variants of the QSFP (Quad Small Form-Factor Pluggable) transceiver module, each supporting different data rates and technologies. Here are the key differences between QSFP56 and QSFP28: 

1. Data Rates:

• QSFP28: QSFP28 supports data rates of up to 28 gigabits per second (Gbps) per lane. It is commonly used for 100 Gigabit Ethernet (100GbE) applications, where four lanes are combined for a total data rate of 100 Gbps.

• QSFP56: QSFP56 supports higher data rates of up to 56 Gbps per lane. It is used for 200 Gigabit Ethernet (200GbE) and 400 Gigabit Ethernet (400GbE) applications, where either four lanes (for 200GbE) or eight lanes (for 400GbE) are combined.

2. Applications:

• QSFP28: Primarily used for 100GbE applications, but can also be used for 25GbE and 50GbE applications by utilizing fewer lanes.

• QSFP56: Used for 200GbE and 400GbE applications, providing higher bandwidth for more demanding network requirements.

3. Lane Configuration:

• QSFP28: Typically configured with four lanes for a total of 100 Gbps.

• QSFP56: Can be configured with either four lanes for 200 Gbps or eight lanes for 400 Gbps.

4. Form Factor:

• Both QSFP28 and QSFP56 share the same physical form factor, which is a compact and high-density design that allows for multiple transceivers to be installed on a single switch or router.

5. Interoperability:

• QSFP28 and QSFP56 modules are generally not interoperable due to differences in data rates and lane configurations. The electrical and optical specifications are designed to match the specific generation.

6. Backward Compatibility:

• QSFP28 modules are not backward compatible with QSFP56 ports, and vice versa. The hardware must support the specific QSFP variant to ensure proper functionality.

In summary, the main difference between QSFP28 and QSFP56 lies in the supported data rates, with QSFP56 providing higher bandwidth to meet the demands of evolving networking standards, such as 200GbE and 400GbE. When selecting transceivers for a network infrastructure, it's crucial to match the module type to the intended data rate and network standards.