Standards-based Design and Testing for PON

Patch Cable


With more customers adopting PON technology, integrators, installers and end users are finding it beneficial to hone their design and test skills of the optical fiber cabling infrastructure that supports PONs.

Passive optical network (PON) technology has been optimized for enterprise environments, changing the way we think about architecting the LAN. Now that PON technology has become an affordable LAN alternative to traditional copper-based networks, large enterprise and government customers alike are adopting it due to the many immediate and long-term benefits.

The point-to-multipoint network architecture of a PON provides a fiber-to-the-desktop (FTTD) solution, using unpowered (hence passive) optical splitters to enable a single optical fiber to serve multiple end points with voice, data and video services. This accommodates a tremendous amount of Internet protocol (IP) traffic over a lightweight, all-optical fiber passive infrastructure, eliminating distribution and workgroup switches, cables and wiring closets typically found in a traditional network.

Enterprises now taking advantage of these benefits include government agencies, large office buildings, universities and hotels. Energy savings (i.e., lower carbon footprint), simplified IT management, greater bandwidth, higher reliability, improved security and lower capital and operating expenses are some of the benefits that can be achieved through PON deployments.

With more customers adopting PON technology, integrators, installers and end users are finding it beneficial to hone their design and test skills of the optical fiber cabling infrastructure that supports PONs. To that end, the following discussion of industry standards, best practices and emerging hardware technologies is intended to help those seeking more knowledge.

Industry Standards

The Telecommunications Industry Association (TIA®) establishes and maintains standards for the premises wiring industry, which cover a variety of aspects relating to the design, planning, implementation and testing of a generic cabling system. The following standards are applicable to PONs in a commercial building/campus environment for the North American market:

  • ANSI/TIA-568-C.0, Generic Telecommunications Cabling for Customer Premises
  • ANSI/TIA-568-C.3, Optical Fiber Cabling Components Standard
  • ANSI/TIA-569-C, Telecommunications Pathways and Spaces
  • ANSI/TIA-606-B, Administration Standard for Telecommunications Infrastructure
  • ANSI-J-STD-607-B, Generic Telecommunications Bonding and Grounding (Earthing) for Customer Premises
  • ANSI/TIA-758-B, Customer Owned Outside Plant Telecommunications Infrastructure Standard
TIA-568-C.0 establishes the overall premises infra-structure for PON optical fiber cabling and provides detailed requirements for cabling installation and field testing. It calls for:

  • Singlemode fiber for the backbone and horizontal cabling subsystems
  • Generic structured cabling to be installed in a hierarchical star layout
  • Optical splitters in various distributor spaces. This allows flexibility of placing splitters in a telecommunications room (TR) or distributor enclosures in zone areas
  • Two or more optical fibers to each work area
While the standard recommends two or more optical fibers, only one is needed to deploy an operational PON—two or more optical fibers is recommended to prepare for future growth and applications.

In August 2012, the TIA published Addendum 2 General Updates to the 568-C.0 standard, which adds support for PONs by providing distance and attenuation guidelines with respect to PON singlemode optical fiber applications for the LAN.

The addendum represents a significant development for the enablement of PON technology in the LAN by providing industry-recognized generic cabling guidelines to support the application. PONs can now be designed and supported in accordance with the industry standards intended to ensure the longevity of the infrastructure lifecycle, as well as the performance of the application system bandwidth over the distances specified. The addendum also recognizes optical splitters, which are the key element to the design of a PON. The optical splitter is now included in the definition and testing of the channel. The PON channel is the end-to-end transmission path between application-specific electronics, including any splitters and patch cords. Furthermore, the addendum clarifies that the splitter is not considered a component of the optical fiber link segment, which is the path between two points not including splitters or equipment cords. This provides clarification to the integrator/installer on how the passive infrastructure should be tested for compliance to industry standards. Compliance testing may thereby qualify for warranty, according to some manufacturers.

TIA-568-C.3 prescribes component-level testing and performance requirements for optical fiber connectivity devices. It sets maximum allowed attenuation levels for singlemode optical fiber and connectors, as well as acceptable return loss levels for connectors. It also establishes acceptable pull strength and maximum bend radius levels for the inside optical fiber plant. In addition to TIA standards, BICSI will cover best practices for PONs in Chapter 5 of the BICSI Telecommunications Distribution Methods Manual (TDMM), 13th edition, which is targeted for release in 2014.

Infrastructure Fundamentals

Optical Fiber Cabling
PON designs call for singlemode optical fiber. Multimode optical fiber cannot support the bandwidth, reach and multiple signal wavelengths enabled by PON applications in the way that singlemode can. PONs therefore do not require separate transmit and receive optical fibers, as would be the case for multimode optical fiber.

Passive Optical Splitters
TIA-568-C.0 requires generic structured cabling to be installed in a hierarchical star configuration. The star layout can accommodate passive optical splitters located between the main equipment Ethernet aggregation chassis or optical line terminal (OLT) and workgroup terminals, also known as optical network terminals (ONTs). The splitters contain no electronics and use no power. Signal attenuation is the same in both directions. The splitters branch the signal on a single optical fiber from the OLT or head-end chassis onto multiple optical fibers, typically up to 32. Those optical fibers then connect to ONTs located in or near work areas. The ONT provides the end user with gigabit Ethernet and other service ports. ONTs can be deployed up to 20 kilometers (km [12 miles (m)]) away from the main OLT equipment. Passive optical splitters are the key enabling technology for passive optical signal distribution and a significant part of the PON optical fiber plant investment. Therefore, network designers are wise to choose products of high quality and reliability from a trusted manufacturer that offers technical service support and an extended warranty.

Optical Fiber Connectors
Subscriber connector-angled physical contact (SC-APC) optical fiber connectors are typically required for PON applications. These are allowable connector types according to ANSI/TIA 568-C standards. Designers can choose between preterminated patch cords and long optical fiber cable assemblies, or field-terminated connectors that enable customizable network connectivity. use clever but inexpensive non-powered plastic tools to enable low-cost, quick and easy onsite optical fiber connector terminations. They perform at specifications on par with fuse-on splices without the need for pricey fusion splicing equipment and extensive training.

MPO Trunks and Modules
Multifiber push-on (MPO) trunk cable assemblies are typically used for backbone cabling in PON networks. When routing multiple optical fibers between TRs or out to the zone distribution area, preterminated trunk cables are a good option. The trunks can be routed from the equipment room (ER) to preconnectorized multifiber modules with SC-APC breakout ports in the TR or zone enclosure for quick and easy installation.

Figure 1



























Figure 1. The two most common PON configurations. Source: TIA Fiber Optics Technology Consortium

Typical Configurations

With regard to physical topologies and the layout of the cabling, a primary consideration for PON design is where to place the passive optical splitters. Because the splitters require no power to operate and are modular, they can be placed practically anywhere in the network. Note that the splitter is an aggregation device—the closer it is located to the end users, the less optical fiber cabling required. Other factors to consider regarding splitter location and configuration include ease of access to accomplish moves, adds and changes (MACs) or testing, and whether a full cross-connect solution is desired (discussed later in this article). As shown in Figure 1, the two general types of layouts commonly deployed for passive optical LAN solutions (POLS) include:

Configuration A–
The most common design, this configuration locates splitters in the TR and:

  • Follows traditional hierarchical star physical cabling LAN design, so is familiar to most end users and integrators/installers
  • Utilizes TRs and allocated floor space that may already exist in the building
  • Allows for an interconnect or a full cross-connect at the TR, where there is typically abundant room for equipment
  • Allows easy access for IT personnel for any required maintenance away from end users
Configuration B–
This is an alternative common design where the splitters are installed in a zone area enclosure out in the horizontal floor area. From there, shorter patch cords of singlemode optical fiber are used to connect the ONTs located in or near the work areas. Configuration B:

  • Follows fiber-to-the-enclosure (FTTE) design as per the standards, but is not as common in the industry as hierarchical star configuration
  • Utilizes enclosures located in a raised floor, on a wall or above a drop ceiling in zone areas on each floor. The enclosures may be an added expense for existing buildings but a lower expense than TRs for new buildings
  • Allows an interconnect or a full cross-connect at the enclosure location (space required depends on the number of ONTs supported from each zone enclosure)
  • Allows easy access for IT personnel for any required maintenance, but the work will take place closer to end user areas

Design Considerations

Designing a PON and installing the required optical fiber plant can present some unique considerations, choices and challenges. Following is a discussion of some of the common issues related to deploying PON technology in a LAN environment.

Optical Link Attenuation Budget
In an optical fiber transmission system, attenuation (e.g. loss) is the light loss or decrease of signal power. It is the primary limiting factor for system performance. Therefore, attenuation testing must be performed following optical fiber cable installation to ensure the system meets the original design intent and that the application can be supported within the criteria specified. The optical link budget allowance is used during the design phase to plan for the expected performance of the end-to-end optical fiber system to ensure that the receiver and transmitter application equipment can reliably operate. The optical link budget is calculated loss expectancy based on the end-to-end components incorporated within the link or channel design.

It is important to remember that according to the standards, the channel is the end-to-end transmission path between two points at which application-specific equipment is connected. For PONs, this includes the various constituent links, connectors, splitters and optical fiber patch cords between the OLT optical port and the corresponding ONT optical port. Therefore, the channel attenuation includes the sum of the attenuation of all the components in the path. Calculating the optical budget allowance per the standards involves the following steps:

Step 1—
Calculate optical fiber loss at the 1490 nanometer (nm) wavelength (1550 nm for optional video overlay applications).
• 0.5dB/km for outside plant
• 1.0dB/km for inside plant

Step 2—
Calculate loss of connectors. • 0.75 dB max/connector pair

Step 3—
Calculate loss of any splices. • 0.3 dB per splice

Step 4—
Calculate the splitter(s) loss.

Step 5—
Include the loss of the connector at the end of the channel (and optical fiber patch cords).

Step 6—
Sum all losses.

Figure 2





Figure 2. Example GPON channel link budget






The sum should fall within levels specified in Table 9 of the ANSI/TIA-568-C.0-2 standard. For gigabit passive optical network (GPON Class B+ (ITU-T G.984) compliant PON systems (as most are), the acceptable attenuation loss is a minimum of 13 dB and a maximum of 28 dB at a 20 km distance. PON equipment customers should confirm this with their particular active electronics vendor. An example of a channel link budget is shown in Figure 2.

Some manufacturers of optical fiber components offer lower loss products that out-perform the standard maximum allowable attenuation specifications. In this case, the designer can use the published specifications for the manufacturers’ components to calculate the expected optical budget attenuation for the system. In either case, the attenuation measurement results for the link or channel should always be less than the designed optical budget attenuation allowance. Also, it is recommended to plan for about a 3 dB of margin after testing the installed cabling plant to account for changes in the cabling design over time (e.g., equipment and cable degradation, dirty connectors and minor changes to the configuration).

Interconnect Versus Cross-connect
In both the aforementioned Configurations A and B, optional cross-connects are allowed between the OLT and the vertical riser backbone, and also between the splitters and the horizontal links to the ONTs.

Figure 3





Figure 3. Example of an interconnect and cross-connect configuration








Figure 3 illustrates examples of a splitter interconnect solution versus a splitter with full cross-connect solution.

The advantage of the cross-connect solutions is added flexibility to easily reconfigure the physical connections of the links, if necessary. It also eases testing the permanent links without having to unplug and plug in the links from the preconnectorized splitter ports. Only unplugging the cross-connect optical fiber patch cord is necessary to test at the fixed connection/coupler port of the permanent horizontal link.

The cross-connect solution simply adds one more connector at each location where it is implemented (and a max 0.75 dB for a connector pair in the channel attenuation calculations). Note that an added adapter plate (with the couplers) and additional patch cords are required to implement a cross-connect field.



Horizontal Installation Challenges
Most PON designs involve installing the horizontal optical fiber cabling above the ceiling or below raised flooring, if present. However, these methods are not always possible or desired in certain situations such as:

  • Retrofits, historical buildings or older buildings with asbestos
  • Hard-lid ceilings with difficult-to-penetrate construction
  • Buildings where access above the ceiling is disruptive (e.g., hotels and hospitals)
  • Congested conduits or no access to above-the-ceiling or in-floor spaces
  • High labor rate regions
For these situations, a below-the-ceiling deployment method may be more practical. Commonly used methods include plastic square latch molding, extra carpentry to cover the cable behind crown molding and stapling the optical fiber to the wall. The first two methods require a labor-intensive, two-step installation process of hanging the raceway and then pulling the optical fiber through it, while the third method runs the risk of damaging the optical fiber. The result of all three methods is often visually unappealing.

An easier, more aesthetically pleasing solution to the problem involves the use of an optical fiber pathway that consists of hollow polyvinyl chloride (PVC) ducts that are preloaded with bend-insensitive optical fiber and backed with a high-bond adhesive with a removable liner. The reel of optical fiber duct is applied on walls just below the ceiling with a rolling cart and a pole-top installation tool. Hence, the duct and optical fiber are installed simultaneously in just one pass around the hallway or room perimeter. Once installed, the pathways can be painted to match a unique wall color if desired. Thus, the pathway provides a high-performing, craft-friendly, aesthetically pleasing alternative for horizontal drops.

PON Splitting Methods
PON designers can choose between centralized splitting or cascaded splitting. Centralized splitting typically entails 1:16 and 1:32 split ratio counts. The more centralized, the higher the port aggregation, making it is easy to test and troubleshoot in one location and improving utilization of splitter output ports. Cascaded (also referred to as distributed or two-stage) splitting is possible in 1:4 and 1:8 or 1:2 and 1:16 split combinations. This method can enable PON port economization for sparser zones. It can also reduce cost in terms of horizontal optical fiber. In addition, optical fiber enclosures are much smaller due to lower port count.

Field Testing

Field certification testing of installed indoor singlemode optical fiber premise cabling per TIA-568-C.0 requires Tier 1 testing. This consists of visual inspection and an optical attenuation/loss test. The visual inspection should verify installed length, as well as minimum end-face scratches/debris and the polarity of any multifiber links. A power meter light source (PMLS) test set is used to measure end-to-end signal loss of the link. If attenuation is within the limits of the optical budget, the link passes for commissioning.

According to TIA standards, Tier 2 testing is optional for the inside cabling plant, and as such, it is not typically required by manufacturers to warranty the inside optical fiber cabling. However, if Tier 2 testing is performed (perhaps at the request of the customer), it involves using an optical time domain reflectometer (OTDR). The OTDR measures the length of the optical fiber, estimates the loss between any two points along the link and shows reflective events along the path. OTDR test results therefore enable a good baseline view of the installed optical fiber plant. Per TIA standards, OTDR testing is recommended for testing the outside plant and/or for troubleshooting. The OTDR testing provides further details on uniformity of cable attenuation, individual connector/splice losses and a historical database for future reference of individual points of network degradation.

Conclusion

Designing, deploying and maintaining PONs may require a new set of skills for many ITS professionals, including both designers and installers. Understanding industry standards and best practices related to optical fiber networking for the premises will help these individuals better serve customers interested in implementing PON technology.