Need for Speed

Passive optical networks (PONs) connect multiple users to a single fibre from a broadband service provider using unpowered optical splitters — passive components used to avoid electromagnetic interference in the network. A PON system can terminate at multiple locations, delivering fibre to a subscriber’s home, an office building, a curb, a shared point in a residential neighbourhood or a business campus, making it a flexible network architecture for deploying a range of service options.

PON technology has been widely deployed: its flexibility and multiservice transmission means it can potentially deliver more than just Internet access. A shared PON fibre can also support voice and video services, allowing communications service providers to deliver all three services at a high-quality service level with one shared network.

The architecture of a PON includes three major elements. An optical network unit, which connects to a central office through a fibre optic cable and provides one or more Ethernet ports for users. It’s typically located on the customer’s premises. Next is the optical line terminal that serves as the service provider end-point of the network. These two devices are connected by an optical distribution network, a physical channel that completes the two-way transmission of optical signals.

These networks have many advantages, one of which is high bandwidth: they can transmit data at gigabit-per-second speeds. The data transmission is also symmetrical, meaning that it’s still fast whether the data is travelling upstream or downstream. This can be highly beneficial for businesses because of download and upload traffic demands.

Also, a PON is not only cost-effective in terms of cost per bit, but equipment costs too. Because of its point-to-multipoint topology, in which multiple optical network units are connected to the same PON port, the amount of fibre and centralized equipment required is lower than with point-to-point architectures.

Furthermore, the point-to-multipoint structure makes PONs flexible and scalable, as more optical network units can be added to existing PON ports in the optical line terminal. This means it can in be scaled up with new connections in line with customer demand — making the business model adaptable for changing bandwidth needs.

The reliability and adaptability of the network is also notable. The fibre cabling itself is smaller, lighter, more flexible and easier to route through homes and buildings than other types of wiring. Also, as fibre doesn’t use electricity to transmit data, the network is much less susceptible to electromagnetic interference than traditional copper-based broadband alternatives.

But how do PONs need to evolve to ensure that the technology remains an optimal solution for the broadband needs of the future?

Bandwidth is key. It’s generally believed that the next wave of optical access networks will need to increase capacity to 50 Gbps to accommodate the rapidly growing demands for home broadband, campus and enterprise access. So, how to upgrade system capacity from 10G-PON to 50G-PON in a simple and efficient way has become a vital research focus.

There are two potential methodologies to address the 50 Gbps bandwidth requirements. The first is to improve the single wavelength rate, and the second is to increase the total rate through multi-wavelength multiplexing and channel binding technology. This would provide two 25 Gbps channels to achieve a transmission rate of 50 Gbps.

The evolution of PON-based standards is very complex, but essentially, the ITU-T has chosen to focus on the former single wavelength method in formulating the next PON standard after 10G-PON, now known as 50G-PON. It uses the uplink for the service network and downlink for users through various user interfaces, supporting 50 Gbps in both directions over the single wavelength.

Single-wavelength 50G-PON supports a point-to-multipoint topology as well as video, data and voice services. It uses wavelength division multiplexing to implement two-way transmission for a single fibre, as well as time-division multiplexing for downstream traffic and time-division multiple access to modulate upstream traffic and enable point-to-multipoint communication. Other promised improvements include dynamic bandwidth allocation, quality of service and increased security and data encryption, supporting block lengths of 128 bits and key lengths of 128, 192 and 256 bits. It can also reach up to 256 optical network units per optical line terminal port.

With single-wavelength 50G-PON established as the next step, it’s essential to ensure a smooth path for communications service providers. At present, 10G-PON is reaching mass-market deployment. When its successor becomes available, the two will coexist for a long time to meet a variety of networking needs. Both PON standards use voice over IP and proprietary encapsulation for voice, and Ethernet for data, supporting existing infrastructure. This sharing of resources is critical — it saves deployment space in equipment rooms, reduces energy consumption of optical access equipment and reduces network construction costs for broadband providers.

Products that support the coexistence of different generations of technology are vital. One recent example is ZTE’s multimode optical transceiver module that can handle the transition to 50G-PON but continues to support 10G-PON and GPON standards. ZTE claims to be the first provider to deliver a PON transceiver module of this type, which is said to be more reliable as well as to lower latency, support virtual network slices and save energy. These capabilities support new services requiring more intelligence and optimization of resources in the network, as well as managing upgrade costs.

This feature set could make 50G-PON applicable to an increasingly broad range of market and service segments across industrial, mobile network and consumer domains, according to ZTE.

Developing broadband services to be fit for the future is complicated for providers, and network capabilities will be at the heart of their competitiveness and value. But the introduction of new technologies such as 50G-PON shouldn’t be solely based on hunger for more capacity or speed. They should also consider compatibility with existing networks and enhanced capabilities for a better user experience — such as quality of service, lower latency and better security — to drive adoption of a range of applications including gaming, video, extended reality and the metaverse.