QSFP28 and QSFP56 are both types of transceivers used for high-speed data transmission in networking and telecommunication systems. The main difference between the two lies in their data transmission capabilities.

QSFP28 is a 4-channel transceiver that supports data rates of up to 28 gigabits per second (Gbps) per channel, allowing for a total data rate of up to 100 Gbps. It is commonly used for 100G Ethernet and InfiniBand EDR applications.

On the other hand, QSFP56 is a 4-channel transceiver that supports data rates of up to 56 Gbps per channel, allowing for a total data rate of up to 200 Gbps. It is used for 200G and 400G Ethernet, InfiniBand HDR, and other high-speed networking applications.

In summary, the main difference between QSFP28 and QSFP56 lies in their supported data rates, with QSFP56 offering higher data transmission capabilities compared to QSFP28.

The QSFP56 package type can support 4x50G transmission to achieve a 200G transmission rate, while the QSFP+ module is an upgraded version of the QSFP module and can support 4x10G transmission for 10G Ethernet, 10G Fibre Channel and QDR InfiniBand. It can multiplex four channels to increase bandwidth and achieve 40Gbps transmission rates (10GBaud NRZ per channel). QSFP28 optical modules, like QSFP+ optical modules, contain 4 optical transmitters and 4 optical receivers to support 4x25G transmission, resulting in 100G transmission rates.

The main difference between the QSFP56 package and the QSFP+ and QSFP28 packages is that the QSFP56 no longer adopts the previous NRZ modulation technology, but instead adopts PAM4 modulation technology, which can achieve transmission rates of up to 50G per channel, allowing more data to be transmitted over existing fibre, making it more suitable for applications in hyperscale data centre networks. 

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