What is a Passive Optical Network (PON)?

Passive Optical Network (PON) is a telecommunications technology that uses fiber-optic cables and optical splitters to provide broadband internet access and other telecommunications services to multiple subscribers. In a PON, a single fiber-optic cable is used to connect a central office or head-end to multiple subscribers, with optical splitters used to split the signal and send it to individual homes or businesses.

The network is called "passive" because it does not require active components like amplifiers or repeaters to relay signals from the central office to the subscriber premises.

Dissecting Passive Optical Network (PON)

The first PON system was introduced by Bell Labs in 1991, based on a technology called Fiber To The Curb (FTTC). FTTC was a precursor to PON technology that used fiber optic cables to deliver broadband services to a central location near the customer's premises, and then used copper wires to deliver the service the rest of the way to the customer's premises.

This demonstrated the potential of using fiber optics for broadband delivery and paved the way for the development of true PON technology, which uses a passive optical infrastructure to distribute signals directly to the customer's premises. The first true PON system, which used passive optical splitters to share the signal among multiple customers, was developed by NTT in Japan in the mid-1990s.

PON technology was developed to address the limitations of traditional copper wire and coaxial cable networks for delivering high-speed broadband services. Its development was primarily aimed at providing a more cost-effective, reliable, and scalable solution for delivering broadband services to customers while also simplifying the network architecture.

PON Key Components and Architecture

In a PON, a single optical fiber is split into multiple fibers to serve multiple customers. The central office or head-end equipment sends data downstream to the customer's premises, and the customer sends data upstream to the central office.

PON Primary Components

To enable communication between the central office and the customer's premises in a PON architecture, there are three key components: the Optical Line Terminal (OLT) at the central office, the Optical Network Unit (ONU) at the customer's premises, and the Optical Distribution Network (ODN) that connects them.

Optical Line Terminal

The OLT is the central node of the PON network, located in the service provider's central office. It is responsible for receiving and transmitting data to and from the end-users. The OLT converts the electrical signal received from the service provider into an optical signal and sends it over the fiber optic cable. It also receives optical signals from the ONU and converts them back into an electrical signal that is sent to the service provider.

Optical Network Unit

The ONU is the end-user device that connects to the PON network. It is typically installed at the user's premises, and each ONU serves a single user or a group of users. The ONU receives the optical signal from the OLT, converts it into an electrical signal, and then sends it to the user's equipment such as a computer, router, or phone.

Optical Distribution Network

The ODN is the passive component of the PON network that connects the OLT to the ONUs. It consists of optical fiber cables and passive optical splitters that split the optical signal from the OLT and distribute it to multiple ONUs. The ODN does not require any power supply, making it low-maintenance and reliable.

PON Architecture

The PON infrastructure operates on a Point-to-Multipoint architecture, where a single optical fiber is shared among multiple ONUs. The optical signal is distributed from the OLT to multiple ONUs using optical splitters. PONs employ two main types of optical splitters:

• Passive Optical Splitters: made of materials that split the light waves into multiple fibers without the use of any electrical or electronic components. They work by using the principles of light reflection and refraction.
• Active Optical Splitters: use electronic components to split the light waves, and they can amplify the signals, allowing them to travel over longer distances.

These divide the optical signal into multiple paths, each of which is connected to an Optical Network Unit (ONU). The Optical Line Terminal (OLT) at the central office is responsible for sending data downstream to the customer's premises. The OLT converts electrical signals into optical signals that are sent over the fiber-optic cable. At the customer's premises, the ONU receives the optical signals and converts them back into electrical signals that can be used by the customer's devices.

Similarly, when the customer sends data upstream to the central office, the ONU converts electrical signals into optical signals that are sent over the fiber-optic cable. The OLT at the central office receives the optical signals and converts them back into electrical signals that can be processed by the central office equipment.

Data Transmission in Passive Optical Networks

Data transmission in Passive Optical Networks (PONs) occurs through the use of light waves that travel over fiber-optic cables. In a PON, the central office or head-end equipment sends data downstream to the customer's premises, and the customer sends data upstream to the central office.

Time-division Multiplexing (TDM) and Wavelength-division Multiplexing (WDM) are two techniques commonly used in PONsto increase the capacity and efficiency of data transmission over the fiber-optic cable.

Time-division Multiplexing

TDM is used in PONs to allocate time slots to multiple customers sharing the same fiber-optic cable. The downstream data from the central office is divided into time slots, with each time slot dedicated to a specific customer's Optical Network Unit (ONU). This allows multiple customers to share the same fiber-optic cable while ensuring that each customer receives their data at the appropriate time.

Wavelength-division Multiplexing

WDM employs different wavelengths of light to carry multiple data streams over a single fiber-optic cable. This technique divides the optical spectrum into multiple channels, each of which can carry a different data stream, and allows multiple data streams to be transmitted simultaneously over the same fiber-optic cable.

PONs also support Quality of Service (QoS) mechanisms, which allow service providers to prioritize traffic and ensure that critical services, such as voice and video, receive the necessary bandwidth and performance.