# Fiber-Ring: BiDi (Bi-directional) or WDM Technology

- **Language:** EN
- **Published on:** 2025-12-16

In industrial automation networks, reliability and operational continuity are crucial. The star topology, common in traditional IT environments, presents a central point of failure (the aggregation switch). To overcome this limitation, the **fiber optic ring topology**, which offers **link redundancy** essential for critical applications, is often used.

The use of **DIN Rail industrial switches**, such as those offered by specialized industrial networking brands, is the standard for these implementations, thanks to their robustness, resistance to harsh environments, and easy integration into electrical cabinets.

## Fiber Ring: In General

### Key Components

To build a redundant fiber ring, the choice of switch is fundamental:

1. **Managed Switch:**

- This is the crucial element. The **Managed** switch must support an industrial-specific ring redundancy protocol, such as **MRP (Media Redundancy Protocol)**, **RSTP (Rapid Spanning Tree Protocol)** or a manufacturer's proprietary protocol (e.g., P-Ring, Turbo Ring, etc.).
- It will be configured as the **Root** or **Manager** of the ring, managing the dynamic opening and closing of the redundant link to prevent network loops.
2. **A Manager Switch (Managed or Unmanaged):**

- The other switches in the ring can also be **Managed** (and configured as **Client** or **Client Node**) to offer advanced diagnostic and management features, or **Unmanaged** if compatible protocols are used for the ring or if they are cascaded outside the main ring.

### How Ring Topology Works

In a ring topology, each switch is connected to its neighbor so that the first and last switches in the chain connect to each other, closing the ring.

Fiber optics are preferred over copper in this context for two main reasons:

- **Interference Immunity:** Essential in industrial environments rich in electromagnetic interference (EMI) generated by motors and machinery.
- **Greater Distance:** Allows connecting switches hundreds or thousands of meters apart.

### The Principle of Redundancy

An unmanaged ring would create a **network loop**, causing a *broadcast storm* and network outage. For this reason, the **redundancy protocol** must:

1. **Normal State:** Keep a link ("backup link") blocked (in "blocked" or "blocking" state) to prevent the loop. Traffic follows a primary path through the ring.
2. **Failover State:** If the primary link breaks (e.g., fiber cut), the **Manager/Root** switch detects the fault and **immediately unblocks** the backup link.
3. **Reconfiguration Time:** Advanced industrial protocols (such as P-Ring or MRP) ensure a very fast reconfiguration time, often less than **20-50 milliseconds** (ms), guaranteeing almost instantaneous communication recovery.

### Advantages of a Fiber Optic Ring Network

| **Advantage** | **Description** |
|---|---|
| **Redundancy** | The main advantage. A fault anywhere in the ring does not interrupt the network; traffic is redirected in a few milliseconds. |
| **Low Latency** | Fiber optics guarantee high transmission speed and minimal latency, essential for real-time control data. |
| **Simple Cabling** | Requires simpler point-to-point cabling compared to a star topology with cabling returning to a central point. |
| **Scalability** | Easy expansion by adding new switches as "nodes" between two existing switches in the ring. |
| **Industrial Robustness** | DIN Rail switches are designed to operate in extreme conditions (temperature, vibration, humidity), ensuring the longevity of the infrastructure. |

Creating a fiber optic ring using managed DIN Rail industrial switches is the most effective method to build a **highly available and resilient** network backbone in industrial automation and control.

## The Bi-Directional Revolution: SFP BX for Long-Distance Fiber Optic Rings

Modern networking demands ever-increasing efficiency, especially when it comes to long-distance fiber optic links. For architectures such as ring topologies or point-to-point links that need to extend up to **20 km**, traditional two-fiber duplex SFP (Small Form-factor Pluggable) transceivers can become costly in terms of cabling.

The solution to this challenge is represented by **BX-type SFP** (Bi-Directional) transceivers, which leverage single-mode fiber to optimize the infrastructure, halving fiber usage while maintaining high-speed full-duplex communication.

### The Heart of the Technology: BiDi SFP (BX)

A standard SFP requires two single-mode optical fibers (SMF) for a full-duplex connection: one for the transmit data stream (TX) and one for the receive stream (RX).

**BiDi (Bi-directional) SFP modules**, commercially identified as **BX**, revolutionize this scheme. They are designed to operate on a **single optical fiber (Simplex)**, allowing simultaneous transmission and reception of data on the same cable.

#### The Essence of BX Type

The key to this efficiency is the **WDM (Wavelength Division Multiplexing)** technique, which uses two distinct and non-interfering light wavelengths to separate data flows in both directions:

1. **Transmission (TX) and Reception (RX) occur on different wavelengths.**
2. A filter inside the module separates incoming from outgoing light.

#### The Mandatory Pair

For communication to function, BiDi modules must always be installed in **complementary pairs** at both ends of the link:

| **SFP Module** | **Transmits (TX)** | **Receives (RX)** | **Example Code** |
|---|---|---|---|
| **Side A (Upstream)** | 1310 nm | 1550 nm | 1000BASE-**BX10-U** |
| **Side B (Downstream)** | 1550 nm | 1310 nm | 1000BASE-**BX10-D** |

*Note: The most common wavelength for the Upstream signal is 1310 nm, and for the Downstream signal it is 1550 nm, or vice versa, depending on the manufacturer.*

### Application in Long-Distance Rings (Up to 20 km)

The characteristic of these SFP BX modules is that they are almost always designed for **single-mode fiber (SMF)**. SMF is the only practical choice for network links exceeding 300 meters, easily meeting the **20 km** (or even more) specifications required for metropolitan area networks or extended distribution rings.

#### Strategic Advantages

- **Cost Reduction:** The primary benefit is **50% savings on fiber infrastructure**. In a metropolitan ring or a data center where cabling is expensive, this translates into a significant reduction in implementation and maintenance costs.
- **Cabling Simplification:** Fewer fibers to manage and track, reducing the chances of connection errors (a single fiber cannot be reversed).
- **Spatial Efficiency:** Using only one fiber in a bundle or conduit increases its overall density, a crucial advantage in congested environments.

### Key Technical Specifications

When selecting an SFP BX for a 20 km ring, it is essential to ensure that the technical specifications are correct:

- **Standard:** Usually 1000BASE-BX10, 1000BASE-BX20, or 1000BASE-BX40, where the number indicates the supported distance (10, 20, or 40 km). For 20 km, the **BX20** model is ideal.
- **Fiber:** Necessarily **Single Mode Fiber (SMF)**, often identified as OS1 or OS2.
- **Connector:** The module uses a single **LC Simplex** connector.
- **Optical Power (TX/RX):** Power and sensitivity specifications must be suitable to cover the planned 20 km, ensuring a sufficient *optical budget* for signal attenuation.

In conclusion, BX-type SFPs represent a powerful and economical solution for long-distance fiber networks, allowing for the construction of robust, future-proof full-duplex infrastructures with half the cabling.

Discover these products on our online shop:

- [ISM5012D Industrial Gigabit Managed Ethernet Switch with 8 Eth ports and 4 SFP ports](/en/fibra-ottica/8-port-eth-and-4-port-sfp-managed-industrial-gigabit-ethernet-switch)
- [ISE5010D Industrial Gigabit Unmanaged Ethernet Switch with 8 Eth ports and 2 SFP ports](/it/shop-online/hardware/fibra-ottica/switch-ethernet-gigabit-industriale-non-gestito-a-8-porte-eth-e-2-porte-sfp-dettagli.html)
- [S-3553LC20D Kit of two SFP modules (1.25G), Single-Mode](/en/sfp/s-3553lc20d-kit-of-two-sfp-modules-125g-singlemode)

![Image](https://shop2.marcomweb.it/storage/blocks/images/Ksl6wudVmsJExS8vIoKxCRNThZgbbux83Yfy0riw.webp)

---

Source: https://shop2.marcomweb.it/en/news/fiber-ring-bidi-bi-directional-or-wdm-technology

MARCOM S.r.l. — a trusted supplier for industrial automation and building automation.