Things You Should Know About BiDi SFP Modules

For the past few years, SFP transceiver modules have received much attention and widely used in telecommunication or communications applications. It is known that almost all modern SFP modules use two fibers to transmit data. One is to receive data from networking equipment, and the other is to transmit data to the networking equipment. However, in recent years, a new type of SFP transceiver is available—Bi-Directional SFP (BiDi SFP) transceiver, which can both transmit and receive data to/from interconnected equipment through a single optical fiber. This post tends to introduce BiDi SFP from the perspectives of definition, working principle, and differences from common SFP modules.

What Is BiDi SFP?

BiDi SFP transceiver is a compact optical transceiver module which uses WDM (wavelength division multiplexing) technology and is compliant with the SFP multi-source agreement (MSA). It is specially designed for the high performance integrated duplex data link over a single optical fiber. BiDi SFP interfaces a network device mother board (for a switch, router or similar device) to a fiber optic or unshielded twisted pair networking cable. Nowadays, it is one of the most popular industry formats supported by many fiber optic component vendors. Here is a Cisco BiDi SFP produced by FS.COM.

Working Principle of BiDi SFP

Compared to traditional SFP transceiver, BiDi SFP are deployed in matched pairs. One for the upstream (“U”) direction, and the other for the downstream (“D”), each part is transmitting at a different wavelength. We could get intuitive understanding of how BiDi SFP works from the image below. The two wavelengths utilized in this example are 1310 nm and 1550 nm. (1310 nm/1550 nm or 1550 nm/1310 nm are commonly used wavelengths in BiDi SFP). Typically, the upstream or “U” transmits at the shorter of the two wavelength—1310 nm, while the downstream or “D” the longer wavelength—1550 nm.

Main Differences Between BiDi SFP and Traditional SFP

The primary difference between BiDi SFP and traditional two-fiber SFP transceiver is that BiDi SFP is fitted with wavelength division multiplexing (WDM) couplers, also known as diplexer, which combine and separate data transmitted over a single fiber based on the wavelengths of the light. That’s why BiDi SFP has only one port, while the traditional SFP module is generally with two ports. One is TX for the transmitting port, and the other is RX for receiving port. The following picture shows different port structures of common Cisco SFP and BiDi SFP.

Another difference between common SFP and BiDi SFP is upon optical sub-assemblies, major cost components of a fiber optic transceiver. As we all know, a common SFP usually has two optical sub-assemblies—the transmitter optical sub-assembly (TOSA) and the receiver optical sub-assembly (ROSA), but BiDi SFP only contains one optical sub-assemblies—BOSA (bi-directional optical sub-assembly), which acts the role of TOSA and ROSA, but with different working principle.

Applications of BiDi SFP

At present, the BiDi SFP is usually used in FTTx deployment P2P (point to point) connection. For example, in a FTTH deployment, optical fibers are used directly to connect the central office and the customer premises equipment. But because the use of P2P architecture, the customer premises equipment has to be connected to the central office on a dedicated fiber. Thanks to BiDi SFP, which allows a bi-directional communication on a single fiber by using wavelength division multiplexing (WDM), the connection between central office and customer premises equipment becomes more simple. Besides this application, BiDi SFP can be also applied in WDM fast Ethernet links, metropolitan area network, and inter-system communication between servers, switches, routers, OADM, etc.

Conclusion

BiDi SFP breaks the rules of traditional SFP modules in many aspects, which have been discussed in the above part. After reading this article, we could get some understanding of BiDi SFP including its definition, working principle and differences from traditional SFP modules. The biggest valuable advantage of BiDi SFP is that it can save cost on optical fiber, especially in long-haul fiber optic transmission. And believe it or not, with the development of technology, it will have broad market prospect.