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Maintenance & Testing

TSB 140: An update

August 1, 2003  

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The soon-to-be ratified document promises to clarify optical fiber testing and ensure overall network integrity and performance.

There are three basic laws that dominate networking: Networks never go slower, never get smaller and never stay the same. Users and system designers are constantly planning for higher speeds, more traffic and capacity as well as flexibility and reconfiguration for manageability. This entails among other complications, keeping pace with the need for the correct support and test procedures to ensure that networks meet the specified system requirements.

Today’s fiber-based LANs are all about speed and capacity. The need for bandwidth continues to be driven by information rich, delay sensitive applications, which demand ever increasing network speeds.

This is a far cry from the early days of networking, where the term simply meant a few computers sharing a printer, common files or an Internet connection. These devices were interconnected with horizontal structured copper datacom cabling.

The highly complex local area networks (LANs) of today consist of an office full of computers sharing multiple common resources. The complexity grows as multiple LANs are networked into campus, metropolitan and wide area networks.

As a result, the backbone cables that link together LANs must possess an order of magnitude greater bandwidth than the LANs they support. And the escalation continues, with the newest drivers being Gigabit Ethernet, Ethernet in the first mile and even discussions of 40 GbE on the horizon.

LAN networks differ from their long-haul wide area cousins. Multimode fiber dominates in premise networks where link lengths are relatively short with few or no splices. Optical loss events are closely spaced, leading to a large number of terminated links to test. It is not unusual for the number of links to exceed 1,000, especially when fiber is installed in the horizontal to desktops.

In a recent study conducted by BICSI, 10,000 surveys were sent out to designers, project managers, installers and end users to gauge the concern for evolving gigabit and multi-gigabit applications and the need to service them.

The results clearly proved that there is a major concern about supporting gigabit applications. Of those who responded, 82 per cent stated that there is an industry need to clarify optical fiber testing.

Evolution, not revolution

Traditional premise design includes an optical fiber backbone and UTP Category 5e or 6 to the desktop. However, technology and applications are pushing the bandwidth closer to the desktop to put more fiber in the networks.

Fiber standards have gone through a considerable number of permutations, including a newly approved standard for 10-gigabit Ethernet to support these applications in commercial building cabling networks. Because this fiber also supports available applications such as FDDI (fiber distributed data interface) and Gigabit Ethernet, the new standard is a natural extension of previously recognized fiber standards for these areas.

In the continuing drive to standardize proper test procedures, a new document called the TIA/EIA TSB 140 is in approval stages. Once approved, this document will describe field-testing of length, optical loss and polarity in optical fiber cabling using an optical loss test set (OLTS) optical time domain reflectometer (OTDR) and a visual light source, such as a visual fault locator (VFL).

The purpose of the TSB 140 standard is to clarify optical fiber testing for premises to ensure overall network integrity and performance. (Currently, TSB 140 is on its third ballot although approval is expected sometime this quarter.)

TSB 140 will not replace ANSI /TIA/EIA-526-7 (for singlemode) or ANSI/TIA/EIA-526-14A (for multimode) standards, which were first written almost 20 years ago. Rather it will serve to enhance these testing procedures by enhancing and defining the test methods needed for fiber installed in buildings and premises networks.

Three test methods

TIA/EIA-526-14A specifies three test methods using an OLTS. Method A tests the multimode cable used in outside plant but does not take into consideration all the connections within the link. Method B tests the cable in buildings and the connections at each end. Method C provides the user with channel testing, which is not realistic, as patch cords are usually not left at the end of the fiber cable, either in the TR or at the outlet. The results obtained will vary depending on the test method used.

A choice of two Tiers of testing is given, depending on the installation and requirements of the end user. TSB140 describes the functional use of the different tools — OLTS, a visual light source and OTDR – and testing methods for a different array of connectors, fiber sources and test jumpers, as well as proper documentation to ensure certification of the cable plant. Although the various test procedures can be contained in separate instruments, the preferred tool is one that combines all features in one instrument.

Tier 1 of TSB140 (which uses Method B from TIA/EIA 526-14A for multimode) accommodates loss and length testing with an OLTS and polarity verification by a VFL to most closely simulate the system.

Tier 2 testing includes tests described for Tier 1 plus the addition of an OTDR trace. Tier 2 testing involves testing the installed cabling for irregularities and assuring uniformity of cable attenuation and connector insertion loss. It provides a higher level of testing to provide quantitative measurements of the installed condition and overall performance of the entire cabling system and its components.

A visual light source, such as a VFL verifies polarity and can detect bends or breaks in the cable. Basically, a VFL is a high-powered infrared laser flashlight that streams light into one end of the fiber. In doing so the VFL identifies continuity, as well as connectors in the patch panels or outlets, and fibers during splicing.

An OLTS incorporates two components: A light source and an optical power meter. OLTS offerings on the market can perform tests from simple loss measurements to automating referencing and testing of the cable and recording the results.

The basic OLTS is a meter and a source. Using this, a user would have to manually calculate the reference power from the power resulting from a cable measurement and then compare the results to standards. The second type of OLTS automatically makes the calculation with reference saving capabilities although results are not saved within the instrument.

A third type of test tool, an advanced OLTS – also known as a CTS (certifying test set) – can measure with the reference power reading from the power reading of the cable measurement and calculate the loss, then saving the results to compare standards-compliance and make reporting quick and simple.

A CTS comprises a main and remote unit – one for each end of the link under test. Each unit houses a power meter and a dual wavelength source. It incorporates loss testing, length measurement, pass/fail analysis and data logging in a single pair of instruments.

An OTDR is an advanced diagnostic tool for optical fiber and is used to characterize the power loss by sending short pulses of light from one end of a fiber. Light reflected back from the fiber travels back to the OTDR where its optical power and arrival time are recorded and graphically displayed. The OTDR has the capability to measure the length of the fiber and determine the power loss between any two points along the fiber.

The OTDR enables measurement of elements along a fiber link, including the fiber segment length, attenuation uniformity and rate, connector location and loss and other power loss events such as a sharp bend. Users can visually locate reflective events (connections, fiber breaks) and non-reflective events (splices, tight bends) by evaluating the graphical “trace”. In Tier 2 of the TSB140, an OTDR trace provides a more thorough analysis of the link under test.

An OLTS and an OLTR are complementary tools for testing and certifying o
ptical fiber premises networks and capabilities. Combining the best capabilities of both testers is crucial for designers and installers and ultimately, the end user.

The use of fiber cable in the LAN will continue to grow as networks expand and demand additional bandwidth. At the same time, test standards are evolving to keep up with technology changes which means LAN cable installers are facing a whole new level of required certification that demands more advanced tools.

The soon-to-be ratified TSB140 will change the way system designers specify testing procedures. It is only with the deployment of more advanced test tools, that users will be able to certify that their fiber is properly installed and terminated to meet specified network reliability for today, as well as provide the scalability for tomorrow.

Brad Masterson is Canadian Product Manager for Fluke Networks. Involved in the field of networking and network testing since 1995, he is a Certified Engineering Technologist registered with OACETT and is a member of BICSI