The QSFP28 (Quad Small Form-factor Pluggable 28) transceiver is commonly used for 100 Gigabit Ethernet (100GbE) applications. There are different types or variants of QSFP28 transceivers, each designed for specific applications or requirements. Here are some of the common types: 

1. QSFP28 SR4 (Short Range 4):

• QSFP28 SR4 is designed for short-distance connections within a data center.

• It uses multimode fiber (MMF) and supports a transmission distance of up to 100 meters.

2. QSFP28 LR4 (Long Range 4):

• QSFP28 LR4 is designed for longer-distance connections within a data center or for connections between data centers.

• It uses single-mode fiber (SMF) and supports a transmission distance of up to 10 kilometers.

3. QSFP28 ER4 (Extended Range 4):

• QSFP28 ER4 is an extended-range transceiver designed for even longer-distance connections, typically used for connections between data centers.

• It also uses single-mode fiber (SMF) and supports a transmission distance of up to 40 kilometers.

4. QSFP28 PSM4 (Parallel Single Mode 4):

• QSFP28 PSM4 is designed for connections using parallel single-mode fiber.

• It supports a transmission distance of up to 500 meters and uses four independent parallel channels.

5. QSFP28 CWDM4 (Coarse Wavelength Division Multiplexing 4):

• QSFP28 CWDM4 uses coarse wavelength division multiplexing technology to transmit signals over separate wavelengths.

• It is designed for longer-distance connections and typically supports a transmission distance of up to 10 kilometers.

6. QSFP28 CLR4 (Coherent LR4):

• QSFP28 CLR4 is designed for coherent communication applications, often used in long-haul and metro networks.

• It typically supports a transmission distance of up to 20 kilometers.

These are some of the common types of QSFP28 transceivers, and the appropriate type to use depends on the specific networking requirements, such as the distance of the connection and the type of fiber optic cable being used.