What Is a PON? Breaking Down the Passive Optical Network Architecture

A passive optical network, or PON, is designed to allow a single fiber from a service provider the ability to maintain an efficient broadband connection for multiple end users. These end users are typically individual clients using PONs in a commercial environment. The PON architecture is becoming increasingly popular for its efficiency and cost-effectiveness over copper networks. IT pros working on computer networks will likely need to understand the PON design and its components to implement and/or troubleshoot this type of network. Passive optical LAN (POL) — made possible by various types of PONs — has numerous FTTx (fiber to the x) applications that are changing the landscape of communications technology and IT infrastructures alike.

PONs and FTTx Applications

PONs are unique, as they can deliver high broadband connections using optical fiber for the last mile of telecommunications. FTTx is a generic term describing the way optical fiber provides broadband to a variety of devices in that last mile.

Common FTTx configurations include the following:

  • FTTH (fiber to the home): Optical fiber terminates inside an individual home or business. FTTP (fiber to the premises) may be used to describe optical cabling terminated at these places.
  • FTTB (fiber to the building): Similar to FTTH, FTTB optic cabling stops at a building’s electrical room.
  • FTTC (fiber to the curb): Cabling terminates within 300 yards of a subscriber's premises.
  • FTTN (fiber to the neighborhood): Optical cabling ends a few miles from a users' premises.

PONs are the next step in networking that can provide end users with the demanding bandwidth they desire, right to their doorstep (or other destination). IT professionals should make a point of studying and understanding optical networks and developing critical information technology skills with a great range of applications and industry demand.

What Is a Passive Optical Network (PON)?

What distinguishes a PON from other network structures is also what makes it highly dependable: a PON uses no electrically powered equipment in its path.

  1. A PON starts with the optical line terminal (OLT) at a service provider's central office.
  2. A single fiber is then run through a passive optical splitter to provide a broadband connection for up to 32 optical network units (ONUs) or optical network terminals (ONTs).

What makes a passive optical network passive is that it uses unpowered optical splitters — which is not a function of an active optical network.

Passive Optical Network vs. Active Optical Network

The two popular optical systems that make FTTx broadband connections possible both do the same job; however, they differ in how this job is done. PONs and active optical networks (AONs) separate data and route it to their respective users, but one difference creates pros and cons for each system.

Active Optical Network

AONs rely on electrically powered routers or switch aggregators in regulating distribution and signals for subscribers. In an AON, the fiber runs directly to the customer’s house.

While customers can choose hardware that fits their data transmission needs and scale it as they see fit, AONs are less reliable because they require power (at least one switch aggregator or router per 48 users).

Passive Optical Network

With PONs, power is only required at the source and receiving ends of the signal. Data is separated and collected by unpowered optical splitters along the line. It should be noted that PONs share fiber strands only for portions of the network.

Due to fewer moving and electrical components, PONs have lower building and maintenance costs than AONs. But two potential drawbacks are that subscribers must be close to the central source of data and there may be latency issues, as PONs are not dedicated to a single subscriber.

What Is a Passive Optical LAN (POL)?

POLs offer triple-play services — providing television, telephone and internet access through optical signals for up to 32 users — and is a step up from other local area networks. Split signals can be sent downstream to all devices, while encryption can be used to prevent eavesdropping. Bidirectional signals can be combined into one fiber while going upstream onto a single mode fiber. In many ways, passive optical LANS can improve the design and deployment of local area networks (LAN).

Different Types of PONs

Depending on the protocol, standards, upstream/downstream bitrates and other system specifications, a PON may be further defined. There are several types of PONs — as explained below — and, depending on the end user’s specific needs, an IT professional may choose one over the other as a solution.

  • APON: Also known as Asynchronous Transfer Mode passive optical network (APON), or simply ATM PON (as it contains an electrical layer built on ATM), APON is the original PON system that was used for commercial deployment.
  • BPON: Broadband PON (BPON) is the improved successor of APON and maintains the ATM structure. BPON has a transmission rate of up to 622 Mb/s, with downstream capabilities of 155 Mb/s to 622 Mb/s. BPON can be converted to EPON or GPON over time, as ATM bandwidth is not ideal for video.
  • EPON: Ethernet PON (EPON) uses ethernet packets rather than the ATM cells used in APON and BPON. EPON is popular for its 1 Gb/s bandwidth for modern networks.
  • GPON: Gigabit PON (GPON) provides a very high bandwidth of 2.5 Gb/s. GPON uses IP and ATM or GPON encapsulation method (GEM) for encoding.

There are several other types of PON structures that may better suit customer needs. In many cases, this selection will come down to which FTTx configurations are used in the last mile of service.

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