What is Passive Optical LAN?

The pervasiveness of Internet of Things (IoT) with IP-based edge devices monitoring and controlling HVAC equipment, security devices, and the convergence of voice and data networks is putting a great strain on legacy copper-based Ethernet local area networks in terms of bandwidth.

This problem can be solved by installing networks using fibre optic cable.

This blog post will introduce you to Passive Optical LANs (POLs) also known as Passive Optic Networks (PONs)

Fibre optic networks use light to transmit data over a network. A Passive Optical LAN uses point-to-multipoint fibre cable runs to connect end-points in which unpowered optical splitters are used to enable one single-mode optical fibre cable to serve multiple endpoints.

It is a Layer-2 data link transport medium, consisting of only three major components: an Optical Line Terminal (OLT) at one central location connected by fibre optic cable to a number of Passive Optical Splitters (POS) which are then connected to Optical Network Terminals (ONTs) near the end user workstations as illustrated in the following diagram.

Passive Optical LAN Overview

All the POL optical splitters are passive (hence the name), meaning that no power is required for switching.

Rather, the splitters receive and then split light waves using wavelength division multiplexing (WDM) that “divides” the light source into several different colours, each of which is sent along its own separate fibre pathway to an end device.

Power is only needed at the head-end OLT and end point ONT within the network.

Network Components

A typical POL is comprised of three main components connected by one single mode fibre as shown in the following diagram from APOLAN.

In this part, we will examine each of these components in more detail.

Optical Network Terminal (ONT)

The ONT is located at the user end of the POL and is connected to end devices such as computers and IP telephones by standard copper patch cords. The ONT converts the electrical signals it receives from the end device into an optical signal which is then transmitted upstream.

Conversely, the ONT converts downstream optical signals into electrical signals for the end devices. Typically, the ONT is low-voltage device connected to a DC power supply module and can be mounted under a desk, on a wall, in a ceiling or in a rack.

Depending on the application, several ONT configurations are available ranging from 2 to 24 Ethernet ports, multiple analog voice ports, coaxial video ports and wireless support.

Passive Optical Splitter (POS)

As the name suggests, the function of a POS is to split the optical signals into different branches connected to the ONT’s. They require no power – hence the name “passive” – and are typically deployed in above-ceiling fibre zone boxes near end user work areas.

Each POS uses Wave Division Multiplexing (WDM) to split the optical signals over the single mode fibre. Communication is bidirectional with upstream signals from the ONTs being transmitted at 1310 nanometers (nm) and downstream signals being sent at 1490 nm.

Several different split ratios are available, typical ones being 1:8, 1:16, 1:32, 2:8, 2:16 and 2:32.

Optical Line Terminal (OLT)

As shown in the diagram, the OLT is installed in the datacenter or main distribution frame (MDF) and is connected to the core switch using traditional Ethernet components. It combines all optical signals from the splitters and converts them back to electrical signals for the core router.

A typical OLT is a chassis device housing modular cards and may support 8 to 72 fibre ports with each port connecting a fibre cable to the splitter. A typical port connects 32 ONTs. Whereas WDM is used to split the downstream optical signals, Time-Division-Multiple-Access (TDMA) is used for upstream traffic to the core switch.

The OLT provides redundant switching, control, and power capability and may also have a range of built-in functionalities such as integrated Ethernet bridging, VLAN capability, end-user authentication and security filtering.

Comparing Passive Optical LANs and Copper LANs

As discussed in the previous two parts, a Passive Optical LAN (POL) transmits data through a strand of single mode fibre from a head-end component called an Optical Line Terminal (OLT) through Passive Optical Splitters (POS’s) to end-user interfaces called Optical Network Terminals (ONT’s.)

By contrast, a traditional Ethernet LAN requires core switches in the Main Distribution Frame (MDF’s) connected to distribution switches located throughout the facility in Intermediate Distribution Frames (IDF’s) by copper cable.

The following diagram compares a traditional copper LAN on the left with a POL architecture shown on the right. Elements of both networks above and below the red lines are the same. In the POL architecture, the core and distribution (or edge) switches are replaced with OLTs, passive optical splitter(s) and ONTs.

Essentially the “traffic directing” function performed by active switching gear on the left has been replaced with passive optical splitters on the right. The Ethernet switches are typically installed in a telecommunications closet along with patch panels and require power and cooling.

Conversely, the splitters aren’t powered, don’t need cooling and are small enough to be installed in ceiling, thereby eliminating the need for a telecom closet.

The following photo from APOLAN illustrates how significant the space differential is between a copper LAN and a POL.

As shown on the left, the rack space required by the POL equipment is about one-sixth that required by the Ethernet equipment while providing connectivity to 8.5 times as many ports. The photo on the right compares one strand of single-mode fibre with 128 copper cables.

POL provides significant space-savings advantages versus traditional copper-based approaches as well as lower cost.

Passive Optical LAN Advantages

The previous sections have introduced Passive Optical LANs (POLs), examined the three main components of a POL and compared a POL to a traditional copper-based Ethernet LAN.

This last chapter will review the financial and technical advantages and benefits that a POL offers over traditional networks.

Cost Savings

Let’s begin with the most important advantage – cost savings. The Total Cost of Ownership (TCO) for a POL is typically 40% – 60% less than a comparable Ethernet installation.

This is illustrated in the following table which is based on a recent case study by Tellabs an APOLAN founding member.

All figures in Canadian dollars.

Ethernet LANPassive Optical LANSavingsSavings %
Total Number of Ports:17281704  
Active Equipment Costs:$756.10$293.80$462.3061.1%
Structured Cabling Costs:$919.80$235.70$684.1074.4%
Capital Expenditure:$1,675.90$529.50$1,146.4068.4%
Annual Support Costs:$54.10$7.10$47.0086.9%
Annual Power Costs:$7.60$3.20$4.4057.9%
Operating Expenses / Year:$61.70$10.30$51.4083.3%

These substantial cost savings alone are a compelling reason to deploy POLs.

Additional Benefits of POLs

However, there are even more benefits.

  • Because a POL is a fibre-based network, data transmission speeds are substantially higher than in a copper-based LAN, measured in Tbps as opposed to Gbps.
  • The length of cable runs is not limited by the 90m rule but can be up to 20km.
  • Fibre cable is immune to EMI and RFI.
  • Because fibre cable is virtually impossible to tap into, a POL offers increased security.
  • Substantial floor space savings are achieved because POLs don’t require telecom rooms or wiring closets.
  • Fibre cable has a much longer lifespan than copper, making it more future-proof.
  • Moves, adds and changes are easier because of the simpler, flatter architecture of a POL.
  • It is easier to get a LEED (Leadership in Energy and Environmental Design) certification because of the passive, non-energy consuming nature of a POL and the elimination of cooling requirements.

Simply put, a Passive Optical LAN gives a bigger bang for fewer bucks.

Give us a call!

At CORE we can help you design a POL. If you are considering upgrading your existing network, building a new one, or just want more information about Passive Optical LANs, please email ed@corecabling.ca or call 905.235.7955.

*This blog is based on several white papers which can be found in the Resources section of The Association for Passive Optical LAN.

For further reference, go to www.BICSI.org and find the presentation “Fundamentals of Passive Optical LAN” in the conference archives.

Finally, BrightTALK has several TIA/FOTC webinars pertaining to POLs (as well as many other topics) which can be downloaded from their website.

Written by:
Michael Lundgren, P.Eng. Engineering Manager, CORE Cabling Inc.