Author: Fiber-MART.COM

While fiber optic technology has been utilized for many years in the communications industry, consumers generally identify with the role that it plays in wired communications such as Cable TV, Fiber-To-The-Home, and the related networking equipment.  However, what most overlook or do not realize is the significant impact that deploying optical fibers has also had on something consumers use every day – mobile devices.  In order to achieve the high speed data levels that we have become accustomed to when using mobile devices, cell towers and their supporting networks had to be retrofitted with optical fiber cables.

The transition from copper to fiber first started when 3G mobile technology was first introduced, but when 4G LTE technology was deployed, the service providers’ equipment in almost every cell tower had to be upgraded.  The primary reason for this was to support the need for the higher frequencies and faster speeds that the existing 1 5/8 ” coax cables on most cell towers could not handle. Since the primary feed line to most cell towers had been upgraded already, connecting the cell systems in the towers with fiber was the next step.

So what positive changes occurred when transitioning to optical fiber in the cell tower?

First, engineers could now design systems with fiber that run solely off of DC power.  The result was that a very small (less than a ½” in diameter) 16-pair optical fiber cable and two small multi-strand DC cables could replace as many as 12 to 18, 1 5/8” coax cables which are sometimes called “hard lines”.  As you can see, this is a significant improvement.

Secondly, after the hard lines are taken off and replaced with optical fiber cables, both the weight and wind drag are drastically reduced on the cell tower.  The amount of weight and wind drag that is reduced when swapping coax for a fiber-based system is almost unbelievable.  Thousands of pounds of materials are removed and space on the tower is dramatically increased.  In addition to amount of material, a lot of time is saved in comparison to having to add 12 to 18 more hard lines to each system.

By upgrading to incorporate optical fiber cables into the infrastructure, today’s cell towers have realized significant improvements not only in mobile network performance, but also from an architectural standpoint.

In many cases, the primary focus of a fiber network simulation platform is on the electro-optical equipment at both ends of the fiber optic link.  Since the purpose of network simulators is to evaluate the DUT (Device Under Test) equipment, it only makes sense that this is where the attention will be. However, it is a crucial mistake to forget about both the stability and consistency of the test fiber because the DUT test results (performance, pass/ fail, etc) totally depend on knowing that the optical fiber characteristics are consistent and very reliable.

As a provider of test fiber, we often hear statements such as “if we can get light through, it’s good enough” or “new fiber is too expensive, so we’ll just buy a used spool on EBay” or “the shipping package our fiber spool arrived in provides enough protection”.  Unfortunately, the reasoning behind these comments has everything to do with saving a very small amount of money and nothing to do with the expectation of being able to provide reliable test results that can confidently be presented to a customer or included on a data sheet.

As many times as we hear the statements above, we also often hear “the fiber I am using is not providing consistent results”, “the second-hand fiber I purchased arrived damaged”, and “our fiber was broken accidentally while exposed on the bench.”  Fiber network simulation platforms are designed to resolve all of the potential issues arising from mishandling of fiber spools, including damaged connectors, pinched or bent fibers, and fiber movement on the spool due to improper storage.  By using these platforms, the risk of damage and breaks to fiber and connectors is greatly minimized, while protecting it from dust and debris.  In addition, connector interfaces (adapters) are secured, cleanable, and replaceable.  Lastly, only new fiber is used in these platforms to assure the quality of performance, so users do not have to worry about any issues with the fiber that might lead to poor performance and test results.

If you do not take the appropriate measures to build a reliable test platform, you run the risk of inconsistent or marginal results which can lead to more serious issues down the line.  A small investment into a quality network simulation platform is an important, yet simple way to ensure the accuracy and performance of your equipment.

There are so many cable options available the one might wonder where to start. This article will set you on the right path in the decision process. Let’s begin by focusing on single mode and multimode fiber optic cables.

Multimode

Multimode optical fiber cable has a larger diametrical core that permits multiple modes of light to pass through at a given time. This characteristic allows the number of light reflections created as the light passes through the core to increase, creating the ability for more data to pass through at any given time. The attenuation rate and high dispersion of this fiber reduce the signal quality over long distances. Multimode optical fiber is commonly used short distances, audio/video applications, and Local Area Networks (LANs). From core to cladding, the diameter ratio is 50 microns to 125 microns and 62.5 microns to 125 microns.

Single Mode

Single mode optical fiber cable a small diametrical core that allows one mode of light to pass through at a given time. Because of this, the light reflections created as light passes through the core decreases, reducing attenuation and enabling the ability for the light signal to travel further. Single mode optical fiber would be ideal for long distances that require more bandwidth such as telecommunications companies, cable television providers, and colleges and universities. From core to cladding, the diameter ratio is 9 microns to 125 microns.

Depending on your application will determine with fiber would necessary. We provided a quick optical fiber guide for you to use as a guide to learn more about the types of fiber and the comparison between each manufacturer.

Since 2001, fiber-mart.com has been an established manufacturer and innovator of professional optical fiber platforms for fiber network simulation, latency / optical time delay, training, and demonstration applications. Our customer base includes many of the world’s most recognized communications service providers, equipment manufacturers, data centers, web service providers, financial institutions, research institutions, and government agencies.

Shielded cables are not all the same

These are the six shielded network cable construction types that you will encounter:

F/UTP

This construction type is more commonly referred to as FTP. FTP only has foil shielding around the bundle of the 4 pairs. The individual pairs are NOT wrapped, this is the most common type of shielded cable and is perfectly acceptable for the majority of applications. This is the construction type used in our Cat5e and Cat6A shielded patch cables.

S/UTP

The difference between this type and the foil type described above is the shielding material. With S/UTP the shielding material is metal braid rather that foil, the individual pairs are not wrapped. Metal braid is far less flexible than foil shielding and therefore this cable type will be more rigid than its foil counterpart.

SF/UTP

This cable type is a combination of the two types above. Both foil and braid are used as the outer shield, individual pairs are not shielded. As you can imagine this shielding type is very effective.

The above two types are designed to deal with interference from outside the cable, but the individual pairs can interfere with each other. This is not usually a consideration in the home and office, but for some commercial, data center and industrial applications it may be desired or even necessary.

S/FTP

This cable type has a single metal braid shield around the four wire pairs, and each one of those pairs is individually wrapped in metal foil. As described above this limits the amount of crosstalk between the pairs. This is the construction type used in our Cat7 shielded patch cables.

F/FTP

The overall shielding material is foil, with each individual pair wrapped in foil. This cable type is commonly used for 10GBaseT applications.

U/FTP

There is no overall shielding material utilized, the individual pairs are foil wrapped only.

In summary

The main point we want you to take from this article is that there are different types of shielded cable, but more importantly the term “STP” is fairly misleading. The natural assumption is that if UTP stands for Unshielded Twisted Pair, then STP stands for Shielded Twisted Pair. But as you have seen in this article there are many types of shielding that have very specific applications.

Most applications only require F/UTP (commonly FTP) this is the type that we sell the most of. Our Cat 7 patch cables are S/FTP, this is for 10GBaseT transmission rates as well as protection from external interference sources.

Today, I am going to discuss what happens at the other end of a fiber link — detectors. Optical detectors, as the name implied, can detect the amount of light received. Our very own eyes are a pair of detectors as they can receive light information with the retina and transmit that light data to our brain. In the visible light spectrum, our eyes are great detectors to inspect fiber break or light leakage. However, most fiber works in the invisible wavelength spectrum where human eyes won’t be able to see. That is the where the optical detectors come in.

Photoelectric effect

It is impossible to explain how optical detectors work without mentioning the photoelectric effect.

This phenomenon was first observed by German scientist Heinrich Hertz who only published his observations. It was Albert Einstein who later studied this effect and quantified the discrete light energy as photons in one of his famous papers that won him a Nobel Prize in 1921. Vacuum photodiodes and photomultipliers take advantage of this technology and can convert the light signal back to electric signals. One critical parameter for characterizing detector is responsivity. It is the ratio of output electric current to the optical input power, with the unit A/W.

In the end, we will compare the responsivity of different detectors and choose wisely based on each application.

Vacuum Photodiode and Photomultiplier

A vacuum photodiode (or phototube) is mainly comprised of a cathode and an anode. When the cathode detects photons, electrons are emitted according to the photoelectric effect, and current will go through the circuit since electrons are attracted to the anode. The following sketch shows how vacuum photodiode works2.

The limitation of a vacuum tube is that it is physically too big and operates in a wavelength range lower than what fiber communication requires. Another issue is that it also involves much voltage to power it. The typical responsivity of a vacuum photodiode is in the magnitude of mA/W.

Photomultiplier, on the other hand, works more efficiently because of its built-in gain mechanism. In addition to the anode and cathode, it also has a series “dynodes” for accelerating the electrons. The following illustration shows the simplified circuitry of a photomultiplier3.

Just like in vacuum tube, electrons are radiated after photons got absorbed by the cathode. However, the emitted electrons are attracted by intermediate dynodes which have very high voltage. What is so good about dynodes is that there can be more than one electron gets emitted when only one electron is attracted to it. This is called secondary emission caused by high kinetic energy electrons possess. Each electron now becomes more than one electron after hitting each dynode, causing a series of multiplying which eventually leads to electric signal amplification.

The gain at each dynode is about 5, so if there are 3 dynodes in the tube, the total increase will 125 (5x5x5). In reality, there are typically 5 to 10 dynodes in each photomultiplier, so the actual gain is in the magnitude of millions. Photomultiplier tubes are high-speed but also consume hundreds of voltage to power each dynode. It is heavy and big, almost the size of a hand grenade4. Unfortunately, photomultipliers are not suitable for the fiber optic communications.

Learning to operate an OTDR properly is a critical skill for field technicians managing and servicing fiber optic networks.  The OTDR is used frequently to determine length and loss characteristics, including testing optical fibers for faults and related issues that can negatively affect network performance.

There are a number of excellent OTDR devices available in the market, from small portable units designed for field use, to sophisticated laboratory-grade devices that can provide a wide range of features intended for advanced users.  Regardless of the type of OTDR itself, the one complimentary training item that is always required is a length of test fiber.  (Note:  An OTDR launch fiber may also be required for overcoming the “dead zone” of a fiber-under-test, which is discussed here).  More importantly, the length of fiber should include issues or “events” to be identified by the device, simulating real-world factors in the classroom.

While most training facilities use unsecured fiber spools on the desktop, this is generally not a best practice, as the delicate fiber is frequently at risk of damage during handling.  Since the fiber will be used by many students over time, it is a benefit to both the instructors and the students to utilize a more professional setup designed to protect the fiber and provide consistent results.

By utilizing a Fiber Lab solution from fiber-mart.com, a number of benefits can be realized that are not available when using a traditional, unsecured fiber spool.  For convenience, we have outlined a number of key benefits below.

Key Benefits of Using a Fiber Lab for OTDR Training:

Easy-to-handle, professionally-designed enclosures protect fiber from accidental damage while providing consistent results

Any types or custom lengths are available, enabling a wide array of real-world scenarios, especially when combining multiple spools

The fiber can include in-line events representing field splices, connectors and patch panels, or other factors that affect performance.

A Fiber Lab offers both a training facility and it’s students the opportunity to bring the field network right into the classroom for practical, hands-on experience.  For the instructors, it is a great value to have a useful tool that can be used for teaching demonstrations and tests, while ensuring that students have learned the OTDR skills necessary to be successful in the field.