What is an SFP and How Does It Work?

SFP Overview

The SFP or Small Form-Factor Pluggable is an innovative, hot-swappable interface. It’s used primarily in network and storage switches. The SFP ports on a switch, and SFP modules like the native 100G laser QSFP+, allow you to connect to fiber cables of different types and speeds.

Starting with the basics, SFP stands for small form-factor pluggable. It’s also commonly called mini-GBIC (gigabit interface converter). SFP is a popular transceiver for three main reasons. First is the small form factor. Its size allows it to be used in tight networking spaces to provide fast communication between switches and important networking components. The second reason for its proliferation is the variety of SFP connection options. SFP works with copper or fiber optics. The networks that can’t utilize SFP are scarce. Finally, SFP is hot-swappable. That makes it ideal for expanding or adjusting existing networks without having to redesign the entire cable infrastructure.

SFP Classification

Rate classification

According to the rate, there are 155M/622M/1.25G/2.125G/4.25G/8G/10G, 155M and 1.25G are more used in the market, 10G technology is maturing and the demand is developing in an upward trend.

Wavelength classification

According to the wavelength, there are 850nm/1310nm/1550nm/1490nm/1530nm/1610nm, the wavelength is 850nm for SFP multimode, the transmission distance is below 2KM, the wavelength is 1310/1550nm for single mode, the transmission distance is above 2KM, relatively speaking, the price of these three wavelengths is cheaper than the other three.

Bare modules are easy to be confused if there is no logo, generally, manufacturers will distinguish on the color of the pull ring, for example, a black pull ring is multimode, the wavelength is 850nm; blue is a wavelength of 1310nm module; yellow is a wavelength 1550nm module; purple is wavelength 1490nm module, etc.

Mode Classification


Almost all multimode fibers are 50/125um or 62.5/125um in size and have a bandwidth (the amount of information transmitted by the fiber) of typically 200 MHz to 2 GHz. multimode optical terminals can transmit up to 5 km over multimode fiber. Light-emitting diodes or lasers are used as the light source. The pull ring or body color is black.


Single-mode fiber has a size of 9-10/125µm and has unlimited bandwidth and lower loss than multimode fiber. And single-mode optical terminations are mostly used for long-distance transmission, sometimes up to 150 to 200 km. LD or LED with narrower spectral lines are used as the light source. The pull ring or in-body color is blue, yellow, or purple.

Difference and connection

Single-mode fiber is inexpensive, but single-mode devices are much more expensive than their multimode counterparts. Single-mode devices typically operate on both single-mode and multimode fibers, while multimode devices are limited to operating on multimode fibers.

The 10G module has undergone development from 300Pin, XENPAK, X2, and XFP, and finally achieved the same size as SFP to transmit 10G signals, which is SFP+. SFP, with its miniaturization and low-cost advantages to meet the demand for high-density optical modules, has replaced XFP as the mainstream of the 10G market since the standard was pushed from 2002 to 2010.

Which cable or plug types can be connected?

SFP and QSFP modules are differentiated according to their connection type in Singlemode and multimode fibers. The diameter of the fiber optic cable core differs for Singlemode with 9 µm and multimode with 50 µm to 62.5 µm. The direct visual distinction is marked by the color coding of the yoke for most manufacturers. The types used for the connectors differ mainly in the type of transceiver. SFP modules are usually provided with an LC port. QSFP modules with an LC port or MPO port. The so-called GBIC transceivers typically offer the connection of an SC connector. This should be taken into account when choosing transceivers and connector types.

Are the different SFPs compatible with each other?

SFP modules and SFP+ modules have identical sizes. Depending on the SFP+ port, they can also support the normal SFP modules. However, this does not apply to every SFP+ port. Conversely, an SFP+ module cannot be used in an SFP port.

SFP28 ports can also control SFP+ modules. However, the data rate must be reduced to 10 GBit/s at the port for communication to take place.

QSFP ports are usually backward compatible. A QSFP module could therefore also be used on a QSFP28 port. In this combination, the QSFP module naturally only supports its own possible data rate of four times 1.25 GBit/s and thus limits the possible data rate of the QSFP28 port.

Due to their dimensions, QSFP and SFP are not compatible with each other.

How can the right SFP be selected?

The transceiver modules are selected to suit the application. Typical applications can be found in the area of Ethernet, Fibre Channel, or SONET. Based on the application, the SFP must be used for the appropriate cable type “single mode or multimode” and the necessary data rate. When selecting, there should therefore be no “bottleneck” in the communication chain.