The Ultimate Guide to BiDi Transceivers

In the fast-paced world of modern networking, optical transceivers play a crucial role in the transmission of data in data centers, 5G, and enterprise networks, where their compact design and high data transfer rates make them indispensable components. In the pursuit of seamless connectivity, BiDi (Bidirectional) optical modules offer the advantage of conserving optical fiber resources in optical communication. This article delves into the intricacies of BiDi optical modules, their operational principles, and the critical role fiber optic choices play in achieving optimal performance.

What Is BiDi Transceiver?

BiDi transceivers have become synonymous with reliable and high-performance networking, which can achieve bidirectional fiber optic communication by operating on a single fiber. Understanding the role of BiDi optical modules requires recognizing their significance in facilitating streamlined connectivity. Unlike traditional optical modules that use separate fibers for transmitting and receiving data, BiDi modules accomplish this bidirectional data transfer on a single fiber, optimizing space utilization and paving the way for enhanced network efficiency.

How Does BiDi Transceiver Work?

Before delving into the importance of fiber optic choices, it's essential to understand how BiDi SFP+ operates. Traditional optical modules use separate fibers for transmitting and receiving data. In contrast, BiDi SFP+ must be used in pairs and it can utilize a single fiber for both functions. This bidirectional transmission is achieved through the use of wavelength division multiplexing (WDM), where different wavelengths are assigned for upstream and downstream data. The result is a more efficient use of available fiber resources and increased network capacity.

For example, from point A we have installed a BiDi transceiver that transmits TX on 1310nm towards point B on a single fiber. In point B there is a transceiver that receives RX on 1310nm and transmits back to point A on 1550nm on the same fiber, then in point A the RX frequency is on 1550nm. It also means that BiDi transceivers should be deployed in pairs—each part transmits at an opposite wavelength.

The Types of BiDi Transceiver

BiDi transceivers are available at a few different rates, reaches, and wavelengths. Refer to the detailed descriptions below:

1G BiDi Transceiver

The 1G BiDi SFP transceiver is one of the most common types used in today's networks. They are designed for 1G deployment. If the transmission distance is within 40km, the wavelength scheme for BiDi SFP optical modules is 1310nm/1550nm and 1310nm/1490nm. For distances exceeding 40km, the wavelength scheme for BiDi SFP optical modules is 1550nm/1490nm.

10G BiDi Transceiver

Similar to the 1G BiDi SFP transceiver, the 10G BiDi SFP transceiver also belongs to the most common types used in current networking. Designed for 10G deployment, they are utilized in bidirectional 10G serial optical communication with 1550/1490nm transmitters and 1490/1550nm receivers, enabling them to reach distances of up to 80km. BiDi variants are also available in the XFP form factor for 10G.

25G BiDi Transceiver

The 25G BiDi transceiver provides 25GBase-BX throughput up to 40km over single mode fiber (SMF) using a wavelength of 1310nm-TX/1270nm-RX via an LC simplex connector. This bidirectional unit must be used with another transceiver or network appliance of complimenting wavelengths. It provides 25GBASE Ethernet connectivity option for 25G Ethernet, telecom, and data centers.

40G BiDi Transceiver

The commonly used wavelength for the 40G QSFP+ BiDi module is 850nm, ensuring stable transmission in multimode fiber optic systems. Additionally, with the use of OM3 patch cables, the transmission distance reaches 300m, while OM4 patch cables extend the distance to 400m. The utilization of OM5 patch cables allows for a transmission distance of up to 500m.

100G BiDi Transceiver

The 100G BiDi Transceiver is ideal for Ethernet and Data Centers. With a duplex LC connector interface, the transmission distance for it using OM3 patch cables is 75m, 100m with OM4 patch cables, and up to 150m with OM5 patch cables.

400G BiDi Transceiver

As for the 400G BiDi module, it is a quad version of the 100G BiDi approach utilizing four fiber pairs (consistent with commonly installed infrastructure such as a 4+4 fiber MPO). Cisco already offers a range of Digital Coherent CFP2 transceivers capable of supporting up to a 400-Gbps wavelength. Thanks to technology miniaturization, Cisco is now able to offer a CFP2 DCO supporting up to 400 Gbps of line rate that can cope with single-fiber bidirectional transmission thanks to the availability of a duaI-laser approach between Tx and Rx.

How to Choose BiDi Fiber Optical Cables for BiDi Transceiver?

Single-fiber or Dual Fiber

How to choose BiDi fiber for BiDi transceivers is not an easy thing. Different types of BiDi transceivers need to use different types of optical cables.

Here take 10G BiDi SFP+ transceivers for example. Different from the common SFP+ transceivers, which are typically designed with two ports, one for transmitting signals and the other one for receiving signals, BiDi SFP+ has only one port for both transmitting and receiving based on WDM technology. Consequently, BiDi SFP+ uses only one single fiber while the common SFP+ optical transceivers need duplex fiber to achieve fiber optic communication.

But 40G QSFP+ BiDi transceivers, such as the QSFP-40G-SR-BD use two wavelengths to transmit and receive simultaneously over a single MMF strand. The optical cables for the 40G BiDi transceiver are duplex MMF. And 100G QSFP28 BiDi transceivers also use duplex MMF cables.

In the Single-fiber configuration, both upstream and downstream data travel bi-directionally on a single optical strand. This approach optimizes space utilization and is easily scalable as additional channels can be added without extensive cabling, making it ideal for scenarios with limited infrastructure space.

Single-mode Fiber or Multimode Fiber

Another critical decision lies in choosing between Single-mode Fiber (SMF) and Multimode Fiber (MMF).

Generally, BiDi SFP+ is designed to support link lengths of 10km to 100km. For fiber optic cable, the signal is transmitted as a light through the fiber and reflects back from the fiber cladding. Signal loss occurs partially during this reflection process. The smaller core helps minimize this bounce signal loss, allowing for longer transmission distances, so single-mode fiber is typically used in BiDi optics links for its smaller diameter core of 9um compared with 50um of multimode fiber (OM3/OM4).

Additionally, the bidirectional transmission over a single strand reduces physical space requirements and allows for easy scalability, as additional channels can be added without extensive cabling. However, can the existing multimode be used to connect BiDi transceivers during the transmission process?

The answer is yes, they could operate well over short-distance via multimode OM2 cable but ended up with a bit error rate in the longer distance. Consequently, BiDi SFP+ can be used in multimode fiber connections, but it is recommended for short-distance transmissions. Signal attenuation in long-distance multimode fiber may result in performance degradation.

Applications of BiDi Transceivers

BiDi optics revolutionizes bidirectional fiber optic communication over a single fiber, significantly saving fiber resources and cabling costs. Widely employed in Packet Transport Network (PTN) deployments, BiDi transceivers enhance network link capacity, expedite network maintenance, and provide cost-effective solutions for expanding optical cable resources. The application of the transceiver can be concluded as follows:

Data Centers

BiDi optical modules optimize connectivity in data centers, where high data transfer rates and efficient use of space are paramount.

Telecommunications

In telecommunications networks, BiDi technology enables cost-effective and high-performance data transmission over long distances.

5G Point-to-point fiber Direct Drive Solution

BiDi optical transceivers also have been used in 5G front haul, especially the use of 25G BiDi SFP28 transceivers. The choice of deployment solution has a significant impact on the resource occupation and construction period of each deployment point. FS currently offers point-to-point fiber direct drive solutions (single-fiber bidirectional), which can meet the performance requirements of 5G forward transmission, such as bandwidth, delay, and transmission distance.

Conclusion

In summary, the adoption of BiDi optical modules and appropriate fiber optic selections significantly enhances overall network performance. BiDi technology optimizes bidirectional data transmission on a single fiber, thereby improving network efficiency, and it has demonstrated success across diverse industries, showcasing its versatility and reliability. FS offers a comprehensive range of BiDi transceivers to cater to various applications.