As we all know, common optical transceivers, like SFP+, SFP, normally require two fibers to achieve data transmission between switches, firewalls, servers, routers, etc. The first fiber is dedicated to receiving data from networking equipment, while the other is to transmit data to the equipment. With the development of technology, a new class of pluggable optical transceiver—BiDi fiber optic transceiver has been developed to combine the transmit and receive functions onto a single fiber (single-mode or multimode). The image below shows the differences between common optical transceiver and BiDi transceiver. How does BiDi transceiver achieve the transmission of optical channels on a fiber propagating simultaneously in both directions? What are the most commonly used BiDi transceivers available on the market? Is it worthwhile to use this kind of transceiver which is much more expensive than standard transceiver? The following test will provide more details from these three aspects.
BiDi fiber optic transceiver is also called as WDM transceiver, since it is fitted with WDM coupler, which helps to combine and separate data transmitted over a single fiber based on the wavelengths of light. Unlike common optical transceivers use two fibers for duplex transmission, BiDi transceiver uses two different wavelengths to carry the duplex signals separately. The optical signals for transmitting and receiving are separately converted into signals of specific wavelengths as shown in the following image. This is why BiDi transceiver module can achieve the transmission of optical channels on a fiber propagating simultaneously in both directions.
Currently, there are a lot of fiber optic transceivers designed with BiDi technology available on the market, but the most commonly used ones are BiDi SFP, BiDi SFP+ and BiDi QSFP+. The following part will introduce them one by one.
BiDi SFP: BiDi SFP (shown in image below), compliant with the SFP multi-source (MSA), is specially used for the high-performance integrated duplex data link over a single optical fiber. It uses a long wavelength DFB laser diode, enabling data transmission up to 80 km on a single fiber. Generally, BiDi transceiver can be produced with LC simplex port which is used both for transmitting and receiving. Nowadays, it is one of the most popular industry formats supported by many fiber optic component vendors.
BiDi SFP+: Like BiDi SFP, BiDi SFP+ (see following image) is also connected by simplex LC fiber cable. Currently, BiDi SFP+ transceivers using 1270 nm and 1330 nm for transmission are most commonly used for 10G transmission. And it can achieve 10G data rate for the link lengths of 10 km, 20 km, 40 km, and 80 km. When you select BiDi transceivers, please take these two factors into consideration.
BiDi QSFP+: BiDi QSFP+ transceiver is a latest product used for 40G short-reach data communication and interconnect applications. Compliant with the QSFP MSA specification, it is terminated with a duplex LC connector interface to transmit data over laser-optimized OM3 and OM4 multimode fiber cables for the length of up to 100 m and 150 m respectively, that is the same as the traditional QSFP-40G-SR4. This 40G transceiver has two 20G channels, and each channel transmits and receives two wavelengths over a single multimode fiber strand. The following picture is a BiDi QSFP+ transceiver.
Generally, BiDi fiber optic transceivers are much more expensive than common fiber optic transceivers. From Fiberstore, a Cisco SFP-10G-SR is 16 dollars, while a cheapest Cisco BiDi SFP+ transceiver is 50 dollars. Is it worthwhile to use this kind of transceiver? The answer is definitely yes.
As we have mentioned above, compared to traditional optical transceivers, BiDi transceivers uses fewer fibers (about 2 fibers) to support signal transmission, which can save an amount of money for you. Take BiDi QSFP+ transceiver as an example, if you are building a new 40G data center fabric in the traditional way, you would need to run 8-multimode fiber strands between your access and aggression layer: a cost of $2000 per port. However, with BiDi QSFP+ transceivers, you can get 40G performance using just 2 fiber strands: a quarter of cabling. For a standard server rack (384 ports), that translates to a savings of more than $550,000.
Although the label price of BiDi fiber optic transceiver is higher than standard optical transceiver, it is of much more value in practical applications. With the existence of BiDi fiber optic transceiver, the cost of fiber infrastructure will reduce, while the network capacity will increase. But keep in mind that BiDi transceiver is usually deployed in matched pairs to get the work most efficiently, since it uses two different wavelengths for transmission.