What is an Optical Network Unit: Understanding Principles & Applications

With the rapid development of modern communication technology, fiber optic networks have become the mainstream choice for high-speed internet access. In this "last mile" of the information superhighway, the Optical Network Unit (ONU) plays a crucial role. As an essential node in Passive Optical Networks (PON), the ONU not only handles the conversion between optical and electrical signals but also supports various services such as data, IPTV, and voice. This article will provide a detailed explanation of the working principles of ONUs and their specific applications in home and enterprise networks.

What is an ONU?

An Optical Network Unit, as a key node in a passive optical network, is responsible for the "last mile" of fiber optic network access to homes and buildings. In a PON system, a single fiber typically connects the OLT device to the ONU to achieve Fiber to the x.

The ONU can support services such as data, IPTV, and voice, enabling true triple-play applications. In a PON network, the ONU has two main functions: selectively receiving information sent by the Optical Line Terminal and responding after receiving data and collecting and caching data that users need to send, then transmitting it back to the OLT according to the corresponding transmission window.

An ONU usually includes an integrated optical port, electrical signal processing module, network interface module, and power management module. Each component plays a different role in the data transmission and decoding process, forming a multifunctional terminal device.

How Does an ONU Work?

For an ONU to achieve complete data reception and transmission, it first needs to convert the optical signals sent by the OLT into electrical signals, so that household electronic devices can receive the relevant data and perform their tasks.

Further data transmission relies on the network Transport Protocol, which specifies the seven nodes for transmission, also known as the OSI 7-Layer Model.

From OSI 7-Layer Model

From the perspective of network transmission protocols, the working principle of an ONU is divided into two main steps:

Signal Conversion in the ONU

The first step involves converting optical signals to electrical signals, which includes both downstream and upstream processes.

• 1. Downstream Signal Processing: This refers to downloading data to the devices we use. The downstream signal processing converts the optical signals from the fiber line into electrical signals. This process includes optical detection, amplification, and signal conditioning, ensuring even weak optical signals can be identified and processed.

• 2. Upstream Signal Processing: This involves uploading data signals generated at the user end to the OLT, such as internet requests and voice call data. During this process, the ONU needs to convert electrical signals into optical signals and then send them back to the OLT. This conversion process corresponds to the first layer of the OSI network protocol: the physical layer. In this layer, the ONU converts the mechanical, electrical, functional, and procedural characteristics of data signals into forms suitable for further transmission.

Data Link Layer and Upper Layer Protocol Conversion

After converting data at the physical layer, the next step involves the data link layer, which controls communication between the network layer and the physical layer. Its main function is to provide reliable transmission over an unreliable physical medium, ensuring data is transmitted according to network requirements with minimal loss and packet drop, thus completing the connection and configuration functions of the ONU and OLT.

From Reception & Transmission

In addition to analyzing network transmission protocols, we can also look at working principles from the perspective of signal transmission and reception.

Reception

The reception part of the ONU consists of ROSA and the main amplifier. ROSA includes an APD and a preamplifier to convert optical signals into electrical signals.

When the ONU receives optical signals from the OLT, the APD first converts the input light of varying power levels into corresponding electrical signals, which are then sent to the preamplifier for amplification. Low-power electrical signals undergo high-gain amplification, while high-power signals undergo low-gain amplification.

This ensures that the amplitude fluctuation of the electrical signal is significantly less than the power fluctuation of the optical signal.

The main amplifier receives the initially amplified electrical signals and performs secondary amplification, ensuring that the electrical data output remains constant and does not fluctuate with changes in the input optical signal's power.

Transmission

The transmission part mainly includes the LD laser diode, PD photodiode, and APC automatic power control.

The laser diode (LD) converts electrical signals into laser light, which is the core function of the transmission end.

Similar to the reception, controlling the power of the optical signal at the transmission end is necessary. This process is usually managed by a monitoring photodiode (MPD) and automatic power control (APC). The MPD collects real-time optical output information and provides feedback to the APC, which ensures stable optical output power.

Through signal conversion and amplified transmission, the ONU completes its reception and transmission functions. However, it should be noted that not only the ONU device has the function of photoelectric conversion, but the ONU optical module can also achieve this function and complete efficient signal conversion.

Additional Functions of the ONU

Beyond its core functions of data transmission and reception, the ONU is equipped with several built-in functions:

• Built-in Routing Function: Many modern ONUs have integrated routing capabilities, allowing for network address translation (NAT) and stateful packet inspection (SPI), enabling multiple devices to share a public IP address and providing network security.

• Built-in DHCP Service: This automatically assigns IP addresses to connected devices, simplifying the home network configuration process.

• WiFi Function: Some ONUs now support wireless access points (AP), providing Wi-Fi connectivity. For example, the FS ONU1910-4G-W supports 3000M Wi-Fi in addition to its core functions.

• Network Management Function: Through various network management protocols, suppliers can configure, diagnose faults, and monitor the performance of ONUs. Common management protocols include TR-069, OMCI, and HTTP.

These rich built-in features make ONU not only play a role in the home network but also can be deployed in the campus, office, etc. FS is a global solution provider to create a series of all-optical network solutions, to provide users with flat, convenient network deployment solutions.

Conclusion

As fiber optic networks continue to proliferate globally, the importance of ONUs is becoming increasingly apparent. They not only provide fast and stable internet connections for home and enterprise users but also play a crucial role in triple-play services. From simple optical-electrical conversion to complex signal processing, from basic transmission functions to integrated built-in functions, ONUs meet the high-performance network connection needs of users at multiple levels. In the future, with the continuous development of new technologies such as 5G and IoT, ONUs will continue to evolve and improve, adapting to more complex and diverse network requirements, and providing users with an enhanced network experience.