passive optical network (PON)

What is a passive optical network (PON)?

A passive optical network (PON) is a system commonly used by telecommunications network providers that brings fiber optic cabling and signals all or most of the way to the end user. Depending on where the PON terminates, the system can be described as fiber to the curb, fiber to the building or fiber to the home.

How does a passive optical network work?

A PON system consists of an optical line terminal (OLT) at the communication company's central office and several optical network units (ONUs) near end users. Typically, up to 32 ONUs can be connected to a single OLT. The word passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is being transmitted across the network. This is in contrast to active optical networks, which require electrically powered switching hardware to pass cells or frames across the fiber cabling.

This image shows how redundant OLT hardware commonly sits in the telecommunications service provider's central office. From there, fiber cabling is distributed up to 20 kilometers from the central office and is split into multiple ONUs using a passive optical splitter, which terminates the fiber connection close to the customer's demarcation point. The ONU will then deliver a network handoff using copper or fiber Ethernet cables, making it easy for customers to connect to their existing local area networks (LANs).

What are the different types of passive optical networks?

All PON systems have essentially the same theoretical capacity at the optical level. The limits on upstream and downstream bandwidth are set by the electrical overlay, which is the protocol used to allocate the capacity and manage the connection. The first PON systems that achieved significant commercial deployment had an electrical layer built on asynchronous transfer mode (ATM) or cell switching protocols and were called APON. These are still being used today, although the term broadband PON, or BPON, is now applied. APON or BPON systems typically have a downstream capacity of 155 megabits per second or 622 Mbps, with the latter being the most common. Upstream transmission is in the form of cell bursts at 155 Mbps.

Multiple users of a PON could be allocated portions of this bandwidth through the application of optical splitters and wavelength division multiplexing techniques. A PON could also serve as a network trunk uplink between a larger system, such as a community antenna television system, and a neighborhood, building or home Ethernet network over coaxial cable.

The successor to ATM-based PONs are PONs that operate using Ethernet technologies. Gigabit PON (GPON), for example, offers a variety of speed options ranging from 622 Mbps symmetrical -- the same upstream and downstream capacity -- to asymmetric 2.5 gigabits per second download and 1.25 Gbps upload capacities. GPON is a hybrid system that uses ATM for voice transport and Ethernet for data transport. GPON is widely deployed in fiber-to-the-home networks.

A more recent Ethernet-based PON (EPON) is 10G-PON. This Ethernet-only technology delivers 10 Gbps download speeds with 2.5 Gbps upstream. Other EPON technologies, such as the International Telecommunication Union-Telecommunication Standardization Sector Next-Generation PON2 standard, are in the works and are expected to achieve up to 80 Gbps speeds in the future.

What are the benefits and limitations of PONs?

Benefits of PONs include the following:

• They are cheaper to deploy compared to most alternative broadband delivery technologies.
• They do not require electrically powered midspan devices to operate.
• They use existing fiber optic
• Their upgrade paths are plentiful, and throughput rates are keeping up with alternative technologies.
• They are considered a secure broadband technology.
• They can be transported over relatively long distances, such as 20 kilometers, on a central office loop.

The potential drawbacks of passive optical networks include the following:

• They require an extensive fiber deployment.
• Larger networks can become less efficient due to the amount of management traffic overhead being transported from the central office to individual customer ONUs.
• Unlike active powered network technologies, PONs must adhere to strict transport distance limitations.

Bandwidth is a vital factor in designing and maintaining a LAN. Learn the difference between bandwidth and throughput, as well as how to calculate network bandwidth requirements, to create a properly performing network.