SFP (Small Form-factor Pluggable) and QSFP (Quad Small Form-factor Pluggable) are both types of optical transceivers used in networking equipment to transmit and receive data over fiber optic cables. The main difference between the two lies in their form factor and the number of channels they support:

1. Form Factor:    - SFP: SFP transceivers have a smaller form factor and support one channel. They are commonly used in networking equipment where space is limited.    - QSFP: QSFP transceivers have a larger form factor and support four channels. They are typically used in high-density applications where higher bandwidth and transmission speeds are required.

2. Number of Channels:    - SFP: SFP transceivers support a single channel, allowing for the transmission and reception of data over a single fiber optic cable.    - QSFP: QSFP transceivers support four channels, enabling higher data transmission speeds and greater bandwidth capacity over multiple fiber optic cables.

In summary, the main difference between SFP and QSFP transceivers lies in their form factor and the number of channels they support, with QSFP transceivers being larger and supporting more channels than SFP transceivers.

Comparison of SFP vs SFP+ vs SFP28 vs QSFP+ vs QSFP28

SFP vs SFP+

Same size with different speeds and compatibility. SFP+ is used in 10-Gigabit Ethernet applications while SFP is for 100BASE or 1000BASE applications. SFP complies with the standards of IEEE802.3 and SFF-8472 while SFP+ is based on SFF-8431. SFP+ ports can accept SFP optics but at a reduced speed of 1 Gbit/s. An SFP+ transceiver can not be plugged into an SFP port, otherwise, the product or port may be damaged. Normally, SFP+ is more expensive than SFP.

SFP+ vs SFP28

SFP+ is mainly linked to 10G, while SFP28 is commonly associated with 25G connections. They use the same form factor, and the pinouts of SFP28 and SFP+ connectors are mating compatible. So SFP28 will work with SFP+ optics but at a reduced speed of 10 Gbit/s. 10G SFP+ modules will work well with the SFP28 port on a network switch if the port can be set up for 10G transmission, otherwise, the SFP+ modules can not work.Learn more about SFP28 module here: 25G SFP28 Transceiver Module Overview.

SFP+ vs QSFP+

The primary difference between QSFP+ and SFP is the quad form. QSFP+ is an evolution of QSFP to support four 10 Gbit/s channels carrying 10-Gigabit Ethernet, 10G Fiber Channel, or InfiniBand, which allows for 4x 10G cables and stackable networking designs that achieve better throughput. QSFP+ can replace 4 standard SFP+ transceivers, resulting in greater port density and overall system cost savings over SFP+. Get details about QSFP here: What Is QSFP+ Module: QSFP+ Transceiver Wiki and Types.

SFP28 vs QSFP28

SFP28 and QSFP28 transceivers actually adopt different sizes and working principles. SFP28 supports only one channel with 25 Gbit/s, while QSFP28 supports 4 separate lanes, and each is 25 Gbit/s. Both of them can be used in 100G networks, but the SFP28 is applied in the form of QSFP28 to the SFP28 breakout solution. Read article 100G QSFP28 Optical Transceiver Comparison to gain more pieces of information.