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It’s easy to be streaming a demo video in the middle of a teleconference with an important customer and NOT think about how this is actually happening. But if you are at all involved in structured cabling or network management, or share responsibility for these things from a facilities perspective, then you DO think about it. You know, for instance, that in recent years Ethernet speeds and bandwidth have increased by yet another order of magnitude. You also know higher speeds are the result of technological refinements that increase various interactions in the physical and data layers of a cable link over shorter periods of time in the same amount of space.

This complex interweave of materials, signal and math is the primary reason devices for testing LAN links have become so sophisticated. The sophistication comes in many forms—type and number of tests, speed, ergonomics, full color touch screen, document-based reporting—and causes devices to fall into a few basic categories. This is important because the cost associated with testing devices varies considerably with their purpose and their capabilities. This in turn suggests an overall strategy for cabling contractors to deploy multiple devices and helps both contractors and facilities managers avoid over- or under-spending in this area. More on that in a moment.

Cable and Network Testing

Let’s take a quick look at the breakdown of testing and devices. One of the simplest cable tests, called wire mapping, sends a signal down the link to see if anything is wrong, broken or missing.

A method called time domain reflectometry measures the speed of the signal and maps the channel topology, precisely locating any of a number of possible faults. Sometimes called OTDR or verifiers, these simple wire mapping devices are the equivalent of a pocket knife and should be hanging from every technician’s belt.

A somewhat more useful foundational cable testing device will be able to detect Power over Ethernet (PoE) and capture any networked phones, cameras or other low-voltage devices in the wire map.

The next step up in verifiers enables the technician to troubleshoot the network by identifying specific faults in both the physical and link layers in a channel. What does this mean? Simply that, in addition to broken or split wires, bad connections or other issues associated with the signal itself, the tester will perform a network discovery that tests the data protocols at each link to make sure all devices on the network are properly identifying one another.

Whether you are installing a new run or troubleshooting a faulty one, someone is going to need to test both the physical and link layers and prove that the network is operating to specifications. That takes us to a considerably higher level of testing, typically called qualification or transmission testing. But before we go there, let’s skip to the top for a moment.
Cable Systems and Network Certification

New builds and component manufacturer warranties typically require cabled systems to be certified. Certification means a pass/fail judgment of network performance based on a battery of tests performed, recorded and reported within a very specific procedural framework set forth by applicable TIA and ISO/IEC standards. This goes beyond the ability to determine whether your network does what you and your customer want it to do. You are proving, guaranteeing, certifying to the rest of the world that your network meets the same standards applied to every other network of that type or class. Moreover, it is not you that is doing the certifying, but rather your sophisticated, impartial, and, yes, expensive testing device. And there it is: the certifying tester itself is a product of global telecommunications standards. To be useful, it must be able to measure specifications well beyond those standards in order to render results within comfortable margins and provide headroom for the future. That’s why, for example, the WireXpert certifier from Softing was designed years ago with a frequency range up to 2,500 MHz—far in excess of the Category 8 cabling standard, which has only just now been approved.
Network Link Qualification

As a practical matter, certification only happens after everything has already been installed, fixed and tested. That leaves a lot of situations short of certification where the testing and reporting capabilities of a certifier could come in handy. That is the space occupied by qualifiers.

Softing’s qualifier, called NetXpert, leans toward the same lab-grade testing technology in a battery-powered handheld device that engineers developed for the WireXpert. Basically, the NetXpert qualifier does all the troubleshooting of cable/network testers with one very important addition: It can verify Gigabit Ethernet operation compliant with the IEEE 802.3ab standard. It does this through bit error rate test (BERT), a form of data transmission testing that sends 10 million bits in 10 seconds (1 Gbit/sec), counts the errors, then issues a PASS/FAIL. While distinct from certification, a BERT pass essentially proves the speed of a cable channel is up to standard.

In this way, LAN links are qualified as part of the installation, troubleshooting and repair procedure. You could think of a qualification pass as a sort of precertification that means whatever you are testing is working up to standards and you can move on. At less than a third of the cost of certifiers, it makes sense for every cabling crew to pack a network qualifier. On the other hand, at about twice the cost of a cable verifier it’s overkill to have everyone using them for routine wire mapping and troubleshooting.
Deploying Testing Devices

One way to understand the hierarchy among testing devices is to consider their deployment. Every cabling contractor location should have at least one certifier under a service contract plus one backup. Every crew needs to have a qualifier available to test the fix. Every technician should be carrying a cable verifier, whether they are part of a cabling contractor crew or a facilities management team.

Typically, cabling contractors will install, repair LANs for organizations even where there is an extensive facilities maintenance presence. But considering the high-performance nature of 10G+ Ethernet, it makes sense for in-house staff to be equipped to perform at least some of the initial wire mapping and troubleshooting activities. Where appropriate, cabling contractors should consider putting foundational tools like cable and network verifiers in their customers’ hands as part of a job or service agreement. It’s a good way to help customers retain control, save money and experience less network problem time. It also makes the contractor’s time and resources on service calls more efficient while giving their customers one less reason to do business with a competitor.

In addition to the WireXpert CAT-8 certifier and the NetXpert qualifier, Softing now offers the CableMaster line of cable and network verifiers to the North American market. (The CableMaster 800 is pictured above.) Don’t overspend and under deploy. Make sure you’re using the right testing device for the right job and that everyone on your team—including your customers—has what they need, when they need it.

If cost is the issue, there are some clues to make a choice: fusion is expensive equipment and cheap splices, while mechanical is cheap equipment and expensive splices. So if

you make a lot of splices (like thousands in an big telco or CATV network) use fusion splices. If you need just a few, use mechanical splices.

Fusion splices give very low back reflections and are preferred for singlemode high speed digital or CATV networks. However, they don’t work too well on multimode splices, so

mechanical splices are preferred for MM, unless it is an underwater or aerial application, where the greater reliability of the fusion splice is preferred.

Splicing is only needed if the cable runs are too long for one straight pull or you need to mix a number of different types of cables (like bringing a 48 fibre cable in and splicing it to six 8 fibre cables – could you have used a breakout cable instead?) And of course, we use splices for restoration, after the number one problem of outside plant cables, a dig-up and cut of a buried cable, usually referred to as “backhoe fade” for obvious reasons!

Splices are “permanent” connections between two fibres. There are two types of splices, fusion and mechanical, and the choice is usually based on cost or location. Most splicing is on long haul outside plant SM cables, not multimode LANs, so if you do outside plant SM jobs, you will want to learn how to fusion splice. If you do mostly MM LANs, you may never see a splice.

Fusion splices are made by “welding” the two fibres together usually by an electric arc. Obviously, you don’t do that in an explosive atmosphere (at least not more than once!), so fusion splicing is usually done above ground in a truck or trailer set up for the purpose. Good fusion splicers cost $15,000 to $40,000, but the splices only cost a few dollars each. Today’s singlemode fusion splicers are automated and you have a hard time making a bad splice. The biggest application is singlemode fibres in outside plant installations.

Mechanical splices are alignment gadgets that hold the ends of two fibres together with some index matching gel or glue between them. There are a number of types of mechanical splices, like little glass tubes or V-shaped metal clamps. The tools to make mechanical splices are cheap, but the splices themselves are expensive. Many mechanical splices are used for restoration, but they can work well with both singlemode and multimode fibre, with practice.

Visual tracing

Continuity checking makes certain the fibres are not broken and to trace a path of a fibre from one end to another through many connections. Use a visible light “fibre optic tracer” or “pocket visual fault locator“. It looks like a flashlight or a pen-like instrument with a light bulb or LED source that mates to a fibre optic connector. Attach a cable to test to the visual tracer and look at the other end to see the light transmitted through the core of the fibre. If there is no light at the end, go back to intermediate connections to find the bad section of the cable.

A good example of how it can save time and money is testing fibre on a reel before you pull it to make sure it hasn’t been damaged during shipment. Look for visible signs of damage (like cracked or broken reels, kinks in the cable, etc). For testing, visual tracers help also identify the next fibre to be tested for loss with the test kit. When connecting cables at patch panels, use the visual tracer to make sure each connection is the right two fibres! To make certain the proper fibres are connected to the transmitter and receiver, use the visual tracer in place of the transmitter and your eye instead of the receiver (remember that fibre optic links work in the infrared so you can’t see anything anyway).
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OTDR manufacturers have give the users a realistic overview of what jobs they will see and what tools they will need. It is very important to understand when you need an OTDR and when it is not appropriate.

If you are installing an outside plant network such as a long distance network or a long campus LAN with splices between cables, you will want an OTDR to check if the fibres and splices are good. The OTDR can see the splice after it is made and confirm it’s performance. It can also find stress problems in the cables caused by improper handling during installation. If you are doing restoration after a cable cut, the OTDR will help find the location of cut and help confirm the quality of temporary and permanent splices to restore operation. On singlemode fibres where connector reflections are a concern, the OTDR will pinpoint bad connectors easily.

OTDRs should not be used to measure cable plant loss. That is the job of the source and power meter, which duplicates the actual fibre optic link, as we described in the first part of this article and is documented by every standard ever written for cable plant loss. The loss measured will not correlate between the two methods; the OTDR cannot show the actual cable plant loss that the system will see.

The limited distance resolution of the OTDR makes it very hard to use in a LAN or building environment, where cables are usually only a few hundred feet long. The OTDR has a great deal of difficulty resolving features in the short cables of a LAN and is more often than not simply confusing to the user.

And one OTDR manufacturer once told a class of students that they could justify the cost of an OTDR simply by using it to test the length of the fibre on a reel when they get it to make sure they got what they paid for. The class laughed at the instructor and pointed out the cable manufacturers mark length on the cable jacket and a £2 calculator would do as well!

Since OTDRs are very expensive and have only specific uses, the decision to buy one must be made carefully. For that reason, most instrument rental companies will rent one for a few days or weeks when you need them. However, if you are not familiar with their operation or cannot understand the results of OTDR tests, it would be much better to hire a specialist to do the testing for you.

Unlike sources and power meters, which measure the loss of the fibre optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fibre optic transmission link, so the measurement correlates well with actual system loss. The OTDR, however, uses unique phenomena of fibre to imply loss.
The biggest factor in optical fibre loss is scattering. It is like billiard balls bouncing off each other, but occurs on an atomic level between photons (particles of light) and atoms or molecules. If you have ever noticed the beam of a flashlight shining through foggy or smokey air, you have seen scattering. Scattering is very sensitive to the colour of the light, so as the wavelength of the light gets longer, toward the red end of the spectrum, the scattering gets less. Very much less in fact, by a factor of the wavelength to the fourth power – that’s squared-squared. Double the wavelength and you cut the scattering by sixteen times!

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While there has been some “buzz” about Optical Time Domain Reflectometer(OTDR)testing for premises cabling since the publication of TSB-140, Additional Guidelines for Field-Testing Length, Loss and Polarity of Optical Fiber Cabling Systems, many agree that the test is usually not necessary for the relatively short links found in a customer-owned network, and may merely add unnecessary expense and complexity.

When testing an optical premises network, the key measurement criterion is insertion loss, or attenuation. This is effectively measured by using a power meter and light source. If the attenuation is within the limits of the allotted power budget, the system will work. OTDRs measure Raleigh backscatter, not absolute loss.

Supporters of OTDR testing in the premise environment believe that testing with an OTDR can help identify microbends that could potentially cause a problem; as well as help document the system for future verification. However, visual inspection is usually the best way to locate breaks or bends. While there may be some minor advantages in performing OTDR testing on premise cabling, the added cost usually outweighs the benefits.