As optical transmission accessories, transceiver modules are widely used in high-bandwidth data communications applications. The most typical usage scenario of an SFP module is a bridge between data equipment (such as Ethernet switches) with transmission cables. However, there are different fiber transceiver types and device ports. In mixed usage, compatibility issues may occur. Thus there come lots of related problems. Such as will SFP optics work in SFP+ switch ports? or Is SFP+ backward compatible with SFPs? This post will introduce the compatibility among the most commonly used transceiver form factors SFP vs SFP+ vs SFP28 vs QSFP+ vs QSFP28 and illustrate the typical transceiver compatibility issue cases.

SFP vs SFP+ Transceiver Compatibility

SFP and SFP+ compatibility may refer to different issues due to different usage scenarios. Here we clarify SFP vs SFP+ compatibility from three aspects.

SFP Module into SFP+ Port

SFP+ is also called SFP plus, which is the enhanced version of SFP. It shares the same appearance with SFP whereas speeding the data rate up to 10Gbps. Is it fully functional to plug SFP optics into SFP+ ports? In most cases, SFPs can work in SFP+ ports but at the cost of sacrificing speed to 1Gb/s. This is because SFP+ is backward compatible with SFP but not vice versa. However, there is no conclusion for this question since it depends on SFP models, function equipment, and vendors. The author suggests you consult your vendors for exact answers. 

Left: Plug a 1000BASE-SX SFP transceiver into the SFP ports on a gigabit switch

Right: Insert a 1000BASE-SX SFP transceiver into the SFP+ ports on a 10G switch

Middle: Connect the two modules with an OM3 multimode fiber optic cable to achieve 1Gbps data rate connectivity

QT Product Lists:

1. Cisco GLC-SX-MM Compatible 1000BASE-SX SFP Transceiver Module

2. LC UPC Duplex OM3 Multimode Fiber Patch Cable

SFP+ Module into SFP Port

What about the other way around? Can the SFP port take the SFP+ module? Seriously? I doubt no one will do this. Even if it can, let's imagine. Now that It is impossible for 10G SFP+ in an SFP port to run at 10Gbps. So what is the point of a 10G module inserted into an SFP port? It is more economical and worry-free to use the 1G module directly.

SFP+ Module and SFP Module Connectivity

Again, how about the SFP+ module and SFP module interconnectivity? Does SFP+ work with SFP? Can it work in the cases where an SFP+ module plugged in a 10Gb switch connects to an SFP module plugged in another gigabit switch? It depends on the types of module and transmission link. As we know, most fiber 10G SFP+ optics can only run at fixed 10G/s, which explains why 10G SFP+ fiber transceivers can’t auto-negotiate down to 1Gbps or less to support the SFP module. However, generally, two cases are supported. 

First, the transmission between two switches is over Ethernet over copper cabling. As we know, the 10GBASE-T SFP+ copper transceiver supports auto-negotiate 10/100/1000Mbps. So both ends of the transmission link maintain the same speed. For instance, there is a 10GBASE-T SFP+ module on a 10-gigabit switch and a copper SFP on a gigabit SFP switch. If you want to connect the two modules, pick an Ethernet Cat5e cable for connection. 

Second, the SFP+ optics here is a dual-rate 1G/10G SFP+ fiber module, which supports 1G and 10G Ethernet links over SMF or MMF. Many vendors provide this kind of dual-rate module in the market. If you need, please contact QSFPTEK to get a quote for requesting a cost-effective solution for you.

SFP28 vs SFP+ Transceiver Compatibility

For the SFP28 SFP+ compatibility issue, the most commonly asked question is: Is SFP28 compatible with SFP+? Generally speaking, yes SFPs work in SFP+ ports, but not vice versa. To understand this point, let’s review the relationships between SFP28 vs SFP+. SFP28 (small form-factor pluggable 28) is the enhanced version of SFP+, which is because SFP28 has upgraded data rates while inheriting the same package type with SFP+. Though SFP28 is designed for 25Gb/s data rate transmission, it has backward compatibility with SFP+. Taking transceiver usage with Ethernet switches as an example, an SFP+ module can be plugged into an SFP28 port. In this case, the SFP+ module works well but the max transmission speed will auto-negotiate down to 10Gb/s. 

QSFP28 vs QSFP+ Transceiver Compatibility

Now we step into the high-speed era. Is QSFP28 compatible with QSFP+? Through the previous knowledge, you may already have the answer yourself. Let’s go over the previous to find the answer. In most situations, SFP optics can be plugged into SFP+ ports optics whereas SFP+ optics cannot be plugged into SFP ports. Similarly, QSFP+ optics works in QSFP28 ports but QSFP28 optics cannot work in QSFP+ ports. Let's go back to the reason behind it. The size of the 100G QSFP28 optical module and QSFP+ module is identical. The former also has 4 parallel data channels, but the transmission rate of each channel has been increased from 10 Gbit/s to 25 Gbit/s, thus reaching a transmission rate of 100Gbps. QSFP28 supports not only full speed 1x 100GbE but also down to 4x 10GbE, 4x 25GbE mode. Thus it is feasible to plug QSFP+ optics into QSFP28 ports to run at 40Gbps. 

Conclusion

In summary, the puzzles of transceiver compatibility often focus on connectivity with network equipment, typically network switches. SFP compatibility with SFP+, SFP28 compatibility with SFP+, and QSFP28 compatibility with QSFP+ are the common topics. In brief, the new-gen optical transceiver is backward compatible with the previous version in most situations. In other words, SFP+ ports can take SFPs, it is possible to plug SFP+ into SFP28 ports to run 10G, and QSFP+ optics work fine in QSFP28 ports, but not vice versa. No matter what the compatible issue is between different transceiver modules, it will be easy to handle if you understand the working principle of each one. To avoid errors caused by compatibility issues, remember to ensure the same data rates at both ends of the link and ask your supplier to make sure there will be no exceptions.