Passive Optical Lan

For decades, the Local Area Network (LAN) has been a cornerstone for enterprise IT architectures. With cascaded aggregation and switching devices tied together by copper cabling, these sophisticated networks have endured a stream of never-ending changes – from best-effort access to always-available; from connecting computers to connecting Wi-Fi access points, cameras, and IP devices; and from malicious viruses to a range of security threats that include data theft and espionage. However, the bandwidth limitations of the copper cabling often require a disruptive rip-and-replace approach to keep up with evolving technological requirements. The need to avoid the bandwidth limitations of copper category cables led to development of a new, fiber optic-based architecture called Passive Optical LAN (POL).

POL is a derivative of the Passive Optical Networks (PONs) used in the successful Fiber-to-the-Home architectures that are deployed by Telecommunications Service Providers. The PON network is tailored for indoor use by shrinking the optical-to-electrical end device, called an Optical Network Termination (ONT), to the size of an AC gang box for easy installation. Other ONT adaptations include the ability to power PoE devices and the addition of advanced Ethernet protocols required for connectivity in a modern enterprise workplace. In all other regards, the POL architecture is consistent with its telecom cousin.

POL uses a common Optical Line Termination (OLT) to connect fiber cables to ONTs at the end points of the LAN network. To minimize cabling costs, passive optical splitters divide the fibers into multiple independent paths (e.g., 1x8, 1x16, 1x32) for connection to the end devices. The ONTs in turn connect to phones, computers, video monitors, Wi-Fi access points, cameras, smart building end-points, etc.

See Figure 1 for a block diagram of a typical POL network.

The POLs change the connection paths, but not the end-points of the LAN, so the same network connections and services are maintained. Desktop computers, phones, Wi-Fi access points, security surveillance, and video conferencing are left intact, but connected via ultra-fast, high bandwidth fiber cables. The elimination of the copper-based LANs enables the removal of costly, bandwidth-constrained equipment in the middle of the network, improves security by reducing the number of network points of vulnerability (e.g. equipment access and human management), lowers overall expenses by shrinking the cost and weight of cable, and eliminates the need to replace the cabling when bandwidth demand changes. When end devices change or more bandwidth is needed, the only upgrades required are at the OLT and ONT; in other words, the fiber cabling should never need upgrading.

Power – Achilles Heel or Deployment Enabler?

The one concern with POL is how to power the many ONTs. The conventional method of supplying power locally from an AC outlet is expensive and cumbersome. The cost of providing an AC outlet can be very expensive, sometimes costing as much as $1500. Even when an existing AC outlet is available, accessibility to the public can result in outages if accidentally unplugged by an unsuspecting user. Moreover, when battery backup is required, a bulky UPS is needed at each protected ONT. This not only increases the capital cost, but also creates a recurring operational expense to maintain the batteries. Clearly, for POL to become the standard solution for LAN architectures, a better power solution was required.

The local power issues led to the development of a new solution known as Remote Line Power (RLP). With RLP, the power for the ONT is not provided by a local AC outlet, but from a power supply located in a central site potentially up to a few hundred feet away. Power is delivered over conventional copper cables which are hardwired to the ONT. The cables can be installed behind the wall, solving the problem of accidental disconnection.

There are three fundamental electrical requirements for an RLP network. First, it must deliver enough power to energize the ONTs. A typical 4-port ONT supplying PoE to downstream devices consumes about 60-70 Watts. Second, the voltage must be at least -50Vdc at the ONT to deliver PoE+ power to downstream devices. Third, the power network must comply with the requirements of the National Electrical Code™.

Article 725 of the NEC defines a special type of remote power circuits known as Class 2 circuits. To ensure safety and fire prevention, these circuits are limited to a maximum power of 100W and maximum voltage of 60Vdc. In addition, they must achieve the 100W limit even when the primary protection circuit does not operate correctly. Compliance with Class 2 provides tremendous installation benefits because they can be deployed over conventional copper cables without requiring conduit or certification by licensed electricians.

How Does It Work

The source of power for an RLP network is a -48Vdc rectifier and a battery plant. Often, this is the same equipment used to power the OLT and the network equipment. To maximize reach, the -48V plant connects to a special DC-DC converter that elevates the voltage to a constant -57Vdc output, which in turn connects to the cables.

What makes the DC-DC converters special is the built-in active current limiting feature that limits the power to a maximum of 100W per circuit. The -57Vdc voltage level complies with NEC voltage requirement, is greater than the -50Vdc requirement for PoE+ at the ONT, and is sufficient to overcome the voltage drop in the copper cables that enables the circuits to reach ONTs at the edge of the network. For example, an 18AWG cable can power ONTs up to 300-400 feet from the power source.

The RLP network is depicted in Figure 2.

The design of the RLP architecture varies by site. The overall layout of the facility, the availability of riser space, the availability of space in the Intermediate Distribution Facilities (IDFs), and the number and location of the ONTs all factor into the design. There are two primary architectures for deploying Remote Line Powered POL networks. In the Distributed RLP Architecture, AC power is supplied in the riser to the power equipment located on select floors in the facility. In the Centralized RLP Architecture, the -48Vdc power and battery equipment is consolidated in a central location, which is often the first floor or in the basement. Cables carry DC power to the DC-DC converters placed on the floors. In both cases, copper cable is deployed alongside the fiber to each ONT.

Conclusion

POL is a future-proofed fiber-based LAN solution for the continual increase in bandwidth demand in a building and across a campus. When bandwidth demands increase in the future, for example from 1 gigabit to 10 gigabit speeds, the only items to replace are the electronics at the end points (OLT and ONT). The fiber infrastructure remains intact. Likewise, the copper cabling used in Remote Line Power networks is a one-and-done investment. The RLP method lowers the overall cost of installation, improves reliability by offering consolidated battery backup, and ensures compliance with the NEC. The combination of fiber and copper provides the ultimate solution for LAN deployments.