Fiber Optic cabling is hands down the preferred transmission medium for high bandwidth systems such as core, metro and access networks. As the demand for bandwidth continues to increase, so does the foot print and reach of these networks. As these networks expand and upgrade, the physical condition of the fiber plant must be evaluated to ensure the system can handle new configurations. Fiber Characterization is the preferred group of measurements used by network engineers to fine tune their approach to network design and turn up.
In today’s market, networks are often operating in a reactive fashion to keep up with user demand. As video streaming and high bandwidth applications become more available for end users, the demand for large volumes of data increases. Industry professionals indicate that bandwidth demand grows at an incredible rate annually. Some estimates suggest that bandwidth demand grows by as much as 50% each year as cell networks deliver streaming video to more smartphone and tablet users than ever before.
For those unfamiliar with Fiber Characterization, it is a group of 5 tests performed to assess the fiber plant’s physical health. Tests performed include:
Optical Return Loss – The amount of light being reflected back to the source.
Insertion Loss – The total end to end attenuation value of the span of fiber.
OTDR – Measure, map and classify events in the span of fiber.
Chromatic Dispersion (CD) – Measuring pulse spread over distance, caused by wavelengths traveling at different velocities.
Polarization Mode Dispersion (PMD) – Measuring physical signal warp, caused by small imperfections in the fiber core, also influenced by heat, vibrations and wind.
While many end user systems will not require such in depth testing solutions, the increase in metro network activity with 10 gig and 40 gig links will certainly require this style of testing. The most common reasons for a company to perform Fiber Characterization include dark fiber deals, network acquisitions, upgrades and expansions. Let’s take a minute to examine a few scenarios where a project manager, company supervisor or end user would want to have fiber characterization performed.
Dark fiber or unused fiber is in high demand today as building high speed long distance networks has become very expensive. Instead, a company may choose to rent or lease fiber from a dark fiber provider. This option creates a quick solution to the need for long distance fiber optic links. A dark fiber deal should definitely have fiber characterization performed on the link before it is finally commissioned. Since these types of links will deliver high speed transmissions for businesses, it is important for the dark fiber provider and end user to be sure the link can handle the transmission. Many times these types of links are running DWDM or 10 Gbps speeds over long distances with critical information such as credit card transactions, government agencies, cell networks and municipalities. A link of this type may even have a Service Level Agreement in place which will specify network downtime limitations. Any interruption of this type of service could create huge financial losses for both fiber provider and end user.
Network acquisitions are another prime example of situations which require fiber characterization. Suppose ABC Telecom, a metro network and dark fiber provider was being purchased by XYZ Telecom in order to increase XYZ Telecom network’s reach into new markets. XYZ Telecom would want to characterize the fiber optic links that are being purchased before the sale. XYZ would want to have a very accurate analysis of the links that are to be purchased as active equipment or other delivery system components may be upgraded shortly after the sale. Any problematic spans of fiber would be identified prior to upgrade installations, which allows technicians time to perform corrective actions to that link.
Active equipment or network element upgrades are also common scenarios where fiber characterization is necessary. Consider the following: a company has been running long distance 2.5 Gbps links for the past 8 years and now it is time to upgrade the system to 10 Gbps. The operational thresholds for 10 Gbps are significantly more stringent than 2.5 Gbps. Chromatic Dispersion and Polarization Mode Dispersion are two of the most critical factors when upgrading to 10 Gbps. Chromatic dispersion allowable thresholds for 2.5 Gbps links are 12,000 – 16,000 ps/nm while 10 Gbps links allowable thresholds are 800-1000 ps/nm. Polarization Mode Dispersion thresholds for 2.5 Gbps links is 40 ps/nm while 10 Gbps is 10 ps/nm. These are significantly different threshold values which will have a major impact on a systems performance.
As the core and metro markets begin to upgrade to 40 and 100 Gbps transmissions, the networks are changing. The high speed transmission systems now incorporate Forward Error Correction, which allows systems to recover from errors more easily which actually loosens thresholds for certain parameters like PMD. This does not mean that fiber characterization is not required for these links. As 100 Gbps systems are coming online, the distances they cover are increasing which means changes in dispersion compensation plans may be required. Fiber Characterization testing is going to be critical in order for engineers to have recent data in order to make decisions for compensation and amplifier schemes.
Network expansions are the final common scenario which requires fiber characterization testing. When building high speed links it is not uncommon for different singlemode fibers to be spliced together which means a variety of fiber in the system. Fiber Characterization testing can help indicate which is the dominant fiber in the system. This information will be very important if amplifiers and dispersion compensation plans will be used in the system.
Polarization Mode Dispersion is perhaps the most important test performed on network expansion projects. As old and new fiber are used together in a system, the opportunity for PMD to be an issue is greatly increased. Since PMD is random and span specific we must be aware that any fiber, no matter how long or short, can have PMD issues. Remember that PMD is caused by microscopic variances in the fiber core during manufacturing. Combine that with the fact that PMD can be influenced by other factors such as heat, wind and vibrations; PMD can quickly become an issue on any span of fiber.
To answer the question “When should I characterize fiber?” In the most simple fashion, anytime you have a link with speeds 10 Gbps and faster. This rule should apply for links which are new builds, leased, upgraded or expanded. Knowing that a span of fiber can handle a signal is probably more important than anything. After all, the cost of fiber characterization is minimal compared to the cost of systems taking errors, extra truck rolls and service level agreement penalties for network downtime. Feel free to give us a call to discuss any projects which may require fiber characterization.