Understand the types of optical modules

Before understanding the differences between these modules, it is necessary to understand what SFP, SFP+, SFP28, QSFP, and QSFP28 modules are. The following will introduce these modules separately.

SFP

SFP can be understood as an upgraded version of GBIC, with a volume of only half that of GBIC modules, greatly increasing the port density of network devices. In addition, the data transfer rates of SFP range from 100Mbps to 4Gbps.

SFP+

SFP+is an enhanced version of SFP, supporting 8Gbit/s fiber channel, 10G Ethernet, and optical transmission network standard OTU2. In addition, SFP+direct cables (i.e. SFP+DAC high-speed cables and AOC active optical cables) can connect two SFP+ports without adding additional optical modules and cables (network cables or fiber optic jumpers), which is a good choice for direct connection of two adjacent short distance network switches.

SFP28

SFP28 is an enhanced version of SFP+, with the same size as SFP+, but it can support a single channel speed of 25Gb/s. SFP28 provides an efficient solution for upgrading 10G-25G-100G networks, which can meet the continuous growth needs of next-generation data center networks.

QSFP+

QSFP+is an upgraded version of QSFP. QSFP can support 4 channels of transmission simultaneously, with a data rate of 1Gbit/s per channel. However, QSFP+is different from QSFP in that it supports 4 x 10Gbit/s channel transmission and can achieve a 40Gbps transmission rate through 4 channels. Compared with SFP+, the transmission rate of QSFP+optical modules can reach four times that of SFP+optical modules. When deploying a 40G network, QSFP+optical modules can be directly used, thereby effectively saving costs and improving port density.

QSFP28

QSFP28 was born to meet the requirements of 100G applications, with a total of 4 high-speed differential signal channels, each with transmission rates ranging from 25Gbps to possible 40Gbps, which can meet the application requirements of 100G Ethernet (4x25Gbps) and EDR InfiniBand. There are many types of QSFP28 products, and the ways to achieve 100G transmission are different, such as 100G direct connection, 100G to 4-way 25G branch link, or 100G to 2-way 50G branch link.

 Differences and Similarities of SFP, SFP+, SFP28, QSFP+, and QSFP28

After understanding what SFP, SFP+, SFP28, QSFP+, and QSFP28 are, the following will introduce the specific similarities and differences between them.

SFP and SFP+: Same size, different rates, and compatibility

SFP and SFP+modules have the same size and appearance, so device manufacturers can adopt SFP physical design on switches with SFP+ports. Due to the same size, many customers use SFP modules on the SFP+ports of switches. This operation is feasible, but the speed will decrease to 1Gbit/s. Also, it should be noted that SFP+modules cannot be used in SFP slots, otherwise it may cause port or module damage. In addition to differences in compatibility, the transmission rates and standards of SFP and SFP+are also different. SFP can transmit up to 4Gbit/s, while SFP+can transmit up to 10Gbit/s; SFP is based on the SFF-8472 protocol, while SFP+is based on the SFF-8431 and SFF-8432 protocols.

SFP28 and SFP+: SFP28 optical modules can be used on SFP+ports

As mentioned earlier, SFP28 is an upgraded version of SFP+, which has the same size but different transmission rates. SFP+has a transmission rate of 10Gbit/s, while SFP28 has a transmission rate of 25Gbit/s. If the SFP+optical module is inserted into the SFP28 port, the link transmission rate is 10Gbit/s, and vice versa. In addition, SFP28 direct connected copper cables have a higher bandwidth and lower losses than SFP+direct connected copper cables.

SFP28 and QSFP28: Different protocol standards

Although both SFP28 and QSFP28 have the number "28", their sizes and protocol standards are different. SFP28 supports 25Gbit/s single channel, while QSFP28 supports four 25Gbit/s channels. Both can be used in 100G networks, but their usage is different. QSFP28 can achieve 100G transmission through the three methods mentioned earlier, but SFP28 requires high-speed cables from QSFP28 to SFP28 branches. The following figure shows the direct connection of 100G QSFP28 to 4xSFP28 DAC.

QSFP and QSFP28: Different rates, different applications

The QSFP+and QSFP28 optical modules have the same size and four integrated transmission and reception channels. In addition, although both QSFP+and QSFP28 series products have optical modules and DAC/AOC high-speed cables, their speeds are different. The QSFP+module supports a single channel rate of 40Gbit/s, while the QSFP+DAC/AOC supports a transmission rate of 4x10Gbit/s. The QSFP28 module transmits data at a rate of 100Gbit/s, and the QSFP28 DAC/AOC supports 4x25Gbit/s or 2x50Gbit/s. It should be noted that the QSFP28 module cannot be used for 10G branch links, but if a switch with QSFP28 port supports QSFP+modules, QSFP+modules can be inserted into the QSFP28 port to achieve 4x10G branch links. The specific implementation method can refer to the article on how to use QSFP28 port to transmit 10G/25G/40G. 

conclusion

The previous text has provided a detailed explanation of the differences between SFP and SFP+, SFP28 and SFP+, and QSFP+and QSFP28 optical modules. Although some of them have similar sizes and shapes, their working rates are not entirely the same. It is not difficult to see from the comparison that the main driving force for the development of optical modules is to pursue smaller sizes while achieving higher bandwidth, as evidenced by the fact that QSFP28 and QSFP+have the same size but higher bandwidth.