The Essential Role of N+1 Redundancy in RF Over Fiber Communication Systems (2024)

by Meir Bartur, CEO, Optical Zonu

The ability to communicate over long distances has been crucial since the beginning of modern civilization. However, today’s communications have never been more sophisticated and critical to innovation and national security. Between 24/7 data centers routinely transmitting exabytes of data to and from the cloud, the growth of high-frequency (and easily disrupted) military communications, as well as public safety radio systems, telecom infrastructure cannot afford even nanoseconds of downtime without placing data and operations at serious risk.

This is why businesses and government organizations can only afford to implement some type of network redundancy in radio frequency over fiber (RFoF) systems. Protection against a single point of failure can be achieved by duplication, but it is expensive, recovery is not necessarily automatic, and operational complexities make it impractical. Where full or even half redundancy may not be economically plausible for most organizations, N+1 redundancy is perhaps the most efficient and reasonable way to protect communication systems.

What is RFoF and Why Do Organizations Avoid Redundancies?

RFoF is typically the answer to a network operator’s challenge of providing high-speed, resilient transport of RF signals over long distances in a way that traditional telecom infrastructure cannot support. It is a technology that transmits RF signals over optical fiber cables instead of coaxial cables. This method leverages the advantages of fiber optics, such as low signal loss, high bandwidth, and immunity to electromagnetic interference to transport RF signals over long distances with minimal degradation.

However, these hardware components, which often include direct modulation laser diodes, photodetectors, integrated amplifiers, fibers, transmitters, receivers, and optical switches, can be costly. This is especially true for Ku and Ka bands and other high frequencies applications. The fact that most RFoF links stand the test of time without any intervention (i.e., are highly reliable) is why most institutions do not invest in redundancy. But every mission-critical business should have at least one backup because the downtime from issues that may occur could be significant and costly. N+1 redundancy requiring signal switching becomes highly attractive to provide system backup and appropriately manage the rising costs of high-frequency RFoF hardware.

N+1 Redundancy and its Different Configurations

N+1 redundancy refers to a system in which multiple primary components (the “N” components) are backed up by a single additional component (the “+1” component) that serves as a spare. This approach enhances the reliability and availability of the system by ensuring that if one of the primary components fails, the backup can take over without interrupting the service. This vastly differs from 2N redundancy, which creates a mirror image backup of the links network to ensure every link and component has a replacement. With 2N redundancy, you are essentially investing in two RFoF networks but only using one. Examples of the full redundancy spectrum can be found in the reference at the end of this article.

There are multiple ways to configure N+1 redundancy at various costs, and each configuration makes sense for different applications. An operator can choose to add a backup to an entire communication link or just certain components that compose a link, such as optical transceivers, switches, amplifiers, and additional fiber. To employ N+1 redundancy, the system components must first be capable of detecting faults in specific areas, such as a missing optical signal, a lack of RF power, or a system alarm related to component functionality.

This is because it would be impossible to reroute the components around the point of failure without that level of precision. The technique to reroute at a point of failure is called bypass switching, which uses a 2×2 optical switch to transport the signal to the spare component on a backup channel and return to its proper route. The various configurations are described below:

Transceiver N+1 Redundancy: In this configuration (Figure 1), multiple RFoF transceivers (N) are active, each converting RF signals into optical signals for transmission over fiber. A single backup transceiver (+1) is on standby if one of the primary transceivers fails, and then the backup automatically takes over. Bad transmitters are usually detected by system electronics, such as improper laser bias, current consumption, etc. This is commonly used when multiple RF sources, such as distributed antenna systems (DAS) or broadcasting, must be transmitted over fiber.

The Essential Role of N+1 Redundancy in RF Over Fiber Communication Systems (1)

Receiver N+1 Redundancy: This configuration (Figure 2) involves multiple RFoF receivers (N) that convert the optical signals back into RF signals at the destination, with a single backup receiver (+1) available to replace any failed receiver. Just like the transceivers, bad receivers are also detected by system electronics. This type of redundancy is most useful when RF signals are received at many different locations, including large-scale communication hubs or satellite ground stations.

The Essential Role of N+1 Redundancy in RF Over Fiber Communication Systems (2)

Link N+1 Redundancy: In this case, multiple optical fiber links (N) carry bidirectional RF signals over each fiber (e.g. to multiple antennas) between one location and another, with one additional fiber link reserved as a backup. This is common in critical communication networks where continuous operation is essential, and the likelihood of fiber damage may disrupt a critical link, such as aerospace and defense or public safety networks. The fibers are routed via different passes (not bundled)—and a cut or disconnect of a single fiber is detected. The 2×2 optical switching re-routes the signals to the spare fiber.

In practice, any combination of the N+1 configurations mentioned above (as opposed to each individually) can be used. For example, a network operator may want to deploy a second fiber and receiver but not the transceiver. However, the fewer redundancies an operator invests in, the more the probability of catastrophic failure increases. Certain premium RFoF hardware with smart optical switches can expertly reroute frequencies to adjust to the complexities of replacing just a singular transceiver, amplifier or switch. This will provide the necessary protection of these systems in the case of a fiber cut or component failure without the need for redundancy at every component.

Summary

As seamless and uninterrupted communication is essential for innovation and national security, N+1 redundancy in RFoF systems offers a pragmatic solution to safeguarding critical telecom infrastructure without incurring prohibitive costs. By providing strategically placed backup components, N+1 redundancy ensures that essential communications remain uninterrupted even in the face of network failures.

This approach balances the high cost of full redundancy and the need for reliable, continuous service, making it an ideal choice for organizations that cannot afford downtime but must also manage resources efficiently. Despite a belief that telecom hardware is a commodity and a race to the bottom line, fiber-based system managers should work with the most skilled integrators and best RFoF products to ensure cost-effective N+1 redundancy.

Reference

1. What is Data Center Redundancy? N, N+1, 2N, 2N+1, CoreSight

https://www.coresite.com/blog/data-center-redundancy-n-1-vs-2n-1

Optical Zonu Enhances Its Network Management System

Optical Zonu has enhanced its CloudView Network Management System (NMS) with fiber fault detection and localization capabilities for its ZONUConnect base transceiver station to distributed antenna system (DAS) fiber optic transport solution. The updates give telecommunication operators and enterprises greater visibility into RF over Fiber (RFoF) network performance, allowing for quicker and more precise detection of fiber faults. This helps address network issues more effectively and prevent downtime.

The Essential Role of N+1 Redundancy in RF Over Fiber Communication Systems (3)

The ZONUConnect platform incorporates a proprietary micro-optical time domain reflectometer (uOTDR) within its pluggable modules, which can detect fiber faults within a few meters. However, accessing and utilizing uOTDR data for network issue resolution was not fully realized in the previous NMS. With the latest CloudView NMS upgrades, operators can now view a visual representation of the fiber path on terrain maps, with OTDR data overlaid on the management panel. This allows for pinpointing fiber faults and reflection events with precise map locations.

The NMS now also visualizes preventive fiber events, such as connector or patch-panel imperfections, rather than just fiber breaks. Addressing these issues is crucial for ensuring RFoF network reliability, as even minor imperfections in components can introduce significant loss, equivalent to a kilometer or more of fiber. Identifying and correcting these imperfections helps reduce total fiber loss and enhances BTS to DAS link performance.

For more information, visit Optical Zonu at opticalzonu.com.

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The Essential Role of N+1 Redundancy in RF Over Fiber Communication Systems (2024)

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