Passive Optical Networking (PON) Fundamentals

Passive Optical Networking (PON) was originally developed in the 1990s to enable Internet Service Providers (ISPs) to deliver data, voice and video services to residential customers. Essentially, PON Networks reduced the number of fibers needed to connect to homes without the need for any active equipment between the central office and end-users, making it a low-cost, easy to install and requiring little maintenance.

With the continuing need for access to higher bandwidths for residential users, especially during the recent pandemic and with the increase in home working, PON continues to be important for Fiber to the Home (FTTH) and Fiber to the Building (FTTB) for businesses. PON technology is continuously being improved to meet the needs for ever higher bandwidths and PON technology is used in a host of applications in addition to FTTH, in networks on business and university campuses, as part of metro network rollouts, smart IoT and in mobile backhaul in support of 5G services.

What is PON?

PPON networks are used primarily by network operators, metro network carriers and ISPs to deliver the “last mile” of broadband access to end-customers via FTTH and FTTB. PON technology uses a single optical fiber which uses a passive fiber optic splitter to deliver data to multiple endpoints or end-users using Time Division Multiplexing (TDM) or Wavelength Division Multiplexing (WDM). Passive Optical Networking is “passive” as power is not used by the splitter, but only at the source and delivery point of the network.

PON networks offer lower operational costs, with less equipment and fiber than would otherwise have been used with a point-to-point network that requires a dedicated fiber and a powered router to distribute data to individual customers.

History of PON – from APON to XGS-PON and beyond…

Passive Optical Networking (PON) standards have been developed by both the International Telecommunications Union (ITU) and the IEEE.

ITU PON Standards

APON – The first PON solution developed in the 1990s to the International Telecommunications Union (ITU) standard for PON networks, was Asynchronous Transfer Mode PON (ATM-PON), also known as APON, with a speed of 622 Mbps.  APON allowed internet service providers to supply multiple customers from a single router and use unpowered splitters to send data to end users. 

BPON – The ITU standard APON was improved upon and developed into Broadband PON (BPON) in 2007.  BPON has an upstream transmission rate of up to 622 Mb/s and downstream transmission from between 155 Mbps and 622 Mbps. 

GPON  – The early 2000s saw the emergence of the next ITU standard G.984 – Gigabit-capable PON (GPON) – which uses asynchronous transmission (ATM) and enables the speed of the data communication to be set for each user.  GPON increased the speed of downstream data to 2.5Gbps and upstream data to 1.25Gbps.  The GPON standard also specifies protocols for error correction, encryption (AES), line control (OMCI) and password or serial number authentication. In 2014 the standard was extended to include wavelength division multiplexing (WDM) which enabled the transmission of multiple services (video, data and voice) over the same fiber.

XG-PON / 10G PON – 10G-PON (also known as XG-PON) is the next generation standard G.987 developed by ITU in 2010.  Asymmetric 10G-PON (XG-PON1) enables downstream speeds of 10Gbps and 2.5Gbps upstream.  10G-PON uses wavelength division multiplexing, but different wavelengths from the G-PON standard, so that it is possible to upgrade GPON subscribers to 10G-PON while other GPON users continue to receive services.

NG-PON2 / TWDM-PON – the NG-PON2 standard developed by ITU in 2015 provides an architecture that uses Time Wavelength Division Multiplexing (TWDM) with 4 or more wavelengths per fiber which can each deliver symmetrical bit rates of 2.5Gbps or 10 Gbps.

XGS-PON – The XGS-PON standard introduced in 2016 now provides synchronous transmission of up to 10 Gbps both downstream and upstream. XGS-PON is not, as it would seem, an evolution of XG-PON, but is an evolution of the NG-PON2 standard.  XGS-PON uses different wavelengths for transmission from the original GPON standard, allowing for simultaneous transmission of GPON, XGS-PON and NG-PON2.

Higher speed passive optical networking standards continue to be developed by ITU including 25G-PON and 50G capable PON.

IEEE PON Standards

EPON – In 2004, IEEE published an alternative standard to the ITU standard called EPON, which based communication on the two-way Ethernet protocol.    Ethernet Passive Optical Network (EPON) uses packets for synchronous communication (rather than ATM – Asynchronous Transfer Mode used in GPON), and provides a bandwidth of up to 1 Gbps. 

GEPON  – was the next standard issued by the IEEE, which enabled speeds of 10G.

10G-EPON – Ratified by IEEE in 2009. The 10G-EPON standard provides either symmetric transmission at  10Gbps downstream and upstream, or asymmetric transmission with 10G downstream and 1G upstream.

Image Source: Researchgate

How a Passive Optical Networking works and key components

OLTs and ONTs / ONUs

Two key components in a passive optical network are the optical line terminal (OLT) and the optical network terminal (ONT) which is sometimes also referred to as an optical network unit (ONU).  These devices, along with passive optical splitters take care of the downstream and upstream traffic.

OLT – The OLT is a device used at the central office of the network provider which converts electrical signals from the service provider’s equipment into fiber optic signals to be sent by the passive optical network. The OLT takes care of multiplexing and sends data over a single fiber optic cable to a passive optical beam splitter, which then sends the fiber optic signals to multiple ONT / ONU.

ONT / ONU – The optical network terminal/unit at the receiving end in the house or building filters out the fiber optical signals which are meant for them.  The ONU also takes care of transmitting signals back upstream to the OLT. 

Passive optical network splitters – The passive optical network splitter enables fiber optical signals received from the OLT to be sent to a larger number of individual end-customers using the same fiber optical medium, but without interfering with transmissions to other users.

Wavelength Division Multiplexing (WDM), Time Division Multiplexing (TDM) and Time and Wavelength Division Multiplexing (TWDM) – PON Networks use either Time Division Multiplexing or Wavelength Division Multiplexing to transmit on designated bandwidths over fiber to each of the end-customer connections. 

PON and GPON both take advantage of WDM using one wavelength for downstream traffic and one for upstream traffic using a single fiber.

In contrast to PON / GPON which use ATM (Asynchronous Transmission Mode), EPON uses Ethernet packets to transmit over a single optical fiber using time division multiplexing (TDM).  TDM transmits signals at different times, with TDM being used for downstream traffic and time-division multiple access (TDMA) for upstream traffic.

As mentioned earlier, NG-PON2 is the next-generation passive optical networking standard and makes use of Time and Wavelength Division Multiplexing (TWDM) to deliver symmetrical bit rates of 2.5Gbps or 10 Gbps over 4 or more wavelengths per fiber.

Key benefits of PON networks

For network service providers, PON offers some key advantages:

• The ability to offer fiber to a large number of households and premises, each with different bandwidth requirements.
• Ensuring robust network services, not only voice, video and data to consumers, but also ensuring a mission-critical services for the large number of employees who are now working more and more from home.
• Low latency for the growing number of applications which businesses rely on, such as data analytics, machine automation, robotics and real-time decision making.
• Different PON standards are able to share the same fibers over segregated wavelengths, enabling networks to be easily upgraded to different standards and bandwidths.
• Energy efficiency, easy to install and expand.

Future of Passive Optical Networking

While PON networks have predominantly been used in FTTH and FTTB applications, it is now being seen in the implementation of Fiber for Everything, powering 5G, smart cities and industry 4.0 which require high performing fiber optical infrastructure, as well as in the expansion of business and residential broadband services.  GPON, 10-GPON and XGS-PON are the ideal fit for FTTH and FTTB while 25G PON can meet the needs of large enterprises and large scale applications which demand higher and higher bitrates.  With the ability to cater for different bitrates over different wavelengths, passive optical networking is incredibly flexible and is paving the way for 50G PON (standard launched in 2021) and 100G PON applications of the future.