Optical fiber serves as the primary medium for transmitting data in today's high-speed communications networks and latency, one of the most critical performance measurements, is the time it takes for a light signal to travel from one point to another. While there are numerous factors that contribute to this value like installed devices when looking at overall network latency, the time delay incurred during light transmission across the fiber itself, also known as fiber latency, is a significant component in the equation.
While the physical length of a fiber is a large driving factor in the latency calculation, it's important to know there are additional and important fiber-specific considerations that will affect or change the final latency value. In this article, we will briefly review all of these key considerations which will help ensure greater accuracy the next time fiber latency calculations are being made.
The physical length of an optical fiber is usually top of mind when thinking about a calculated fiber delay value and for good reason since it is one of, if not the most, significant factors. Looking at only the fiber itself (ie, no other network factors involved), the longer the physical fiber path that an optical signal must travel between two points of a matching fiber type, the longer the time it will take to arrive at its destination. As an example, a signal transmitted across a 100km distance of standard G.652D optical fiber will incur a longer fiber delay than a signal transmitted across a 50km distance of that same fiber. As a result, the physical distance of an optical fiber will always be an important consideration in the latency equation.
Technical Note: Recent advances in how optical fibers are manufactured and constructed have led to improvements in latency performance, resulting in some specialty fibers having a distinct performance advantage over traditional solid glass core fibers. This is briefly covered later in this article in the "Optical Fiber Design" section.
Another key consideration when seeking to accurately calculate fiber latency is the type (or types) of fiber being used or installed in the network. As the old saying states, not all fibers are created equal, and there are multiple factors within this category to review that contribute to differences in latency performance.
Index of Refraction (IOR) / Refractive Index
Generally defined as the ratio of the speed of light in a vacuum to the speed of light in another medium, IOR is a foundational element of light transmission including optical fiber transmission. Every optical fiber has a specific IOR at a specific transmission wavelength and this is the most important fiber-specific characteristic in the fiber latency calculation since IOR is directly correlated to the speed of light. A fiber with a higher IOR will transmit light slower than a fiber with a lower IOR. Due to design differences between different fiber types, along with materials and production process differences between manufacturers, virtually every type of fiber will have its own unique IOR specifications, which can be found on the manufacturer's data sheet for the respective fiber.
The correct and known IOR value should always be used when seeking to accurately calculate optical fiber latency, since different fiber types will deliver a different delay value. It's also important to take these differences into consideration when looking at the overall network. As an example, if one brand of G.652D single mode fiber is used for 25% of a link and a comparable but different brand of G.652D fiber is used for the remaining 75%, the resulting fiber latency value will be different than if the same fiber brand was used for 100% of the network link. (Note: The difference may be minimal overall if the IOR difference is small, but it is still a difference and that may or may not be important depending on the goals of the network operator).
Transmission Signal Wavelength
In the previous section, it was noted that every fiber has a defined IOR specification for specific transmission wavelengths. Therefore, a fiber's IOR at the 1550nm wavelength will be different from its IOR value at the 1310nm wavelength. Because IOR is the key characteristic in the fiber latency calculation, transmitting at a different wavelength and thus a different IOR will result in a difference in the delay measurement. As a result, one must always consider and use the correct transmission wavelength when calculating fiber latency for a specific application.
When a manufacturer publishes the IOR for their fiber on a datasheet, they also include the wavelength for that IOR value. In most instances, the IOR specification will be provided for the 1310nm and 1550nm wavelengths at a minimum, while many now also include the IOR at 1610nm.
Published Group Index of Refraction for Corning® SMF-28® Ultra SMF
Optical Fiber Design
When discussing fiber distance earlier in this article, it was briefly noted that new fiber design approaches have led to significant improvements in latency performance. Therefore, it's important to highlight that some types of specialty fibers, specifically hollow core optical fibers, allow for a drastic reduction in signal delay. This is because in a hollow core fiber, light signals are traveling through the air instead of a solid glass core. Because air has a lower refractive index than glass, light travels at a faster speed. Companies like Lumenisity in the UK are pioneering the development and production of hollow core fibers such as their CoreSmart® fiber which offers as much as a 30% reduction in latency versus traditional glass core fibers.
One of the lesser-known and discussed variables that impacts fiber latency is temperature, so in latency-driven systems where every fraction of a second is important, environmental temperature is another key consideration. Temperature change impacts the refractive index of fiber, so if extreme temperature shifts are expected in a network fiber span, this should be accounted for when seeking the most accurate latency calculations.
A practical example of this arose a few years ago when a fiber infrastructure provider supporting the financial trading industry was acquiring optical time delays from M2 Optics for accurately synchronizing signal timing at the ends of a link. Based on the geographic location of this particular link and the environments in which they were deployed across the span, it was not uncommon for portions of the link to experience temperatures below 0° in the winter season and greater than 80° temperatures in the summertime. To optimize latency and signal timing calculations they inquired as to how temperature affects IOR. When checking with the manufacturer of the fiber used in their cable, every change in 1° temperature resulted in a change of 1 in the 4th decimal point of the IOR (ex: 1.4680 to 1.4681).
While not a significant change during mild temperature swings, the difference is much greater during instances of extremes. To combat the impacts of environmental elements, cable manufacturers and network operators make an effort to insulate and protect cables from the elements as much as possible. Some cabling technologies like Phase Stabilized Optical Cable available from Linden Photonics are designed for harsh environments and utilize a special jacketing material that helps minimize temperature variance and impacts on signal delay.
TESTING CAPABILITIES AND LIMITATIONS
Another important consideration when calculating fiber latency involves the testing capabilities and limitations when evaluating optical fibers in the field. Latency and timing-based systems rely on accurate fiber span measurements which are then incorporated into the respective delay calculations. Understanding the accuracy and capabilities of the test devices being used, along with the expertise of the user conducting the tests, is essential.
Test Device Capabilities
When validating an optical fiber length, an OTDR device is often used to test and certify fiber continuity and length. There are many types of OTDRs that are designed for different applications and use cases while offering a wide range of performance features and capabilities. As a result, differences arise in terms of fiber length accuracy measurements between different devices from a manufacturer, as well as devices across multiple manufacturers. As no two manufacturers build a device in the same manner, there will naturally be minor or major differences in measurements across devices throughout the marketplace. For a given fiber length, some may measure to within a few meters, others within a few feet, and some new highly-specialized devices are capable of measuring to within millimeters up to a certain length. Therefore, selecting the most applicable device for the task at hand is extremely important when seeking to validate a length as accurately as possible for a latency-driven application.
Device User Knowledge and Expertise
It has been said that a device is only as good as the knowledge level of the person using it, while effectively using an OTDR and specialized fiber length measurement devices is an "art form". In networks where timing is critical and testing/validating accuracy is essential, not only is using the correct device necessary but also having someone with a high level of expertise perform the tests to ensure the most accurate measurements are being achieved.
THINGS TO REMEMBER
To summarize, calculating fiber latency to a high degree of accuracy is a very challenging endeavor due to the numerous variables and factors that are involved from a fiber perspective, along with limitations and tolerance differences of test devices, and the expertise level of the parties involved.
However, taking the following key considerations into account highlighted in this article will help to achieve a greater level of measurement accuracy:
- The fiber length
- The fiber type; including critical attention given to the IOR and Transmission Wavelength parameters
- The environmental temperature and accounting for possible variances
- Test device selection; ensuring the appropriate device is selected for the application
- Device user knowledge; ensuring individuals involved are well-trained
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