Lasers perform cable testing - Technology Information

Communications News, May, 2001 by Hugo Draye

With higher speeds and performance requirements, more accurate testing methods are needed.

Certifying an installed fiber-optic link to support Gigabit Ethernet presents special measurement challenges that can be overcome by carefully selecting the right tools and methods. Traditional light-emitting diode (LED) testers cannot accurately measure losses at gigabit speeds. As a result, the industry is moving toward testers that use laser sources to test fiber connections, providing loss measurements that accurately characterize the headroom of the Gigabit Ethernet application over fiber.

Gigabit Ethernet is expected to rapidly proliferate within corporate networks because of its outstanding performance, low cost and relatively easy implementation. According to one study, Gigabit Ethernet switches cost less than $2 per Mbps, a fraction of the cost of fiber-distributed data interface and asynchronous transfer mode, and less than half the unit cost of 100 Mbps ethernet. Switches on the market today perform at full wire speed of 1.0 billion bits per second with 64-byte frames.

The fact Gigabit Ethernet network topology follows the traditional rules of ethernet helps leverage the installed base on existing ethernet equipment. Network managers are able to utilize the skill sets and training of their existing network administration staff and use familiar network analysis and management tools. Deployment and administration are also simplified by the fact there is no need to add other protocols or technologies.

Yet, implementation of Gigabit Ethernet is far from a simple process. Selecting and certifying cabling capable of meeting the demands of 1,000 Mbps transmission is one of the most complex issues. The IEEE 802.3z Gigabit Ethernet standard includes five physical-layer specifications, three for fiber-optic media and two for shielded copper media.

THE ADVANTAGES OF GIG E

The majority of larger installations are moving to fiber-optic cabling because it offers advantages in Gigabit Ethernet applications. For example, research demonstrates that the longer link lengths possible with fiber-optic cabling, combined with the recent reductions in component costs, have reduced the expence of fiber local area networks (LANs) to 15% to 22% below copper in typical applications.

The type of fiber-optic link used has an impact on the testing process. With single-mode fiber, the radius of the fiber core is on the order of five microns and 10 microns, close to the wavelength, so that only one single light angle or mode of light can pass through the fiber. The fact the light is limited to a single transmission path means that an optical signal can travel for long distances with relatively low losses.

Multimode fiber, on the other hand, has a much larger internal radius. This means light propagates along multiple paths, each of which has a slightly different length, and thus causes distortions that limit the distance over which the integrity of the light can be maintained. Because it is less expensive than single-mode fiber, multimode fiber is the predominant type used in LAN.

One of the hazards of Gigabit Ethernet is that traditional testing and certification methods will not necessarily hold up at gigabit speeds. The loss budgets--the allowable loss through the fiber cable plant--are much tighter with Gigabit Ethernet than with 10 Mbps or 100 Mbps. In the past, most fiber cable used in LANs could pass certification with loss budgets as high as 5 dB or 10 dB. In both 1000Base-SX and 1000Base-LX, the actual link loss budget is 7.5 dB, but this needs to account for the effects of both attenuation and dispersion.

With dispersion factored out, loss budgets for Gigabit Ethernet are as low as 2.35 dB. With a typical loss of 0.5 dB to 0.75 dB per connector pair, many installations can be close to the limit, even if they are done perfectly. They therefore require highly accurate testing methods to avoid having them fail certification, even when they are within the acceptable range.

LED TESTING PROBLEMS

Many network engineers may still be using LED sources when measuring losses in cabling installations intended for use with Gigabit Ethernet. At 10 Mbps or 100 Mbps, LED sources almost always provide perfectly acceptable results, which helps to explain why much of the existing test equipment base still uses LED sources. Yet, there are some significant problems with LED testers when they are used on cabling intended to run gigabit networks.

An LED transmits a wide and diffuse array of light energy that fills a multimode fiber and has far higher modes than any laser. These higher-order modes are more susceptible to bending loss. Also, inevitable misalignment in the fiber-to-fiber connection will prevent the receiving fiber from capturing all of the light energy. The LED measurement will consequently exhibit greater loss, due to both connector loss and bending loss, than will be seen in the actual laser transmission.

Lasers, however, launch light in more powerful and concentrated beams that have fewer higher-order modes than LED beams--so they are less susceptible to bending loss. In addition, a concentrated beam of laser light is less sensitive to misalignment between fibers. Consequently, the laser test will generally exhibit lower and more accurate loss measurements, particularly on multimode fiber.