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The Fiber Optic Update

Every year is another busy year for fiber optics. New technology, components, applications and usually a few surprises. On this page we've gathered some of the more important stories for the year, stories covering topics that FOA believes every tech needs to know. Many of these articles are from the FOA monthly newsletter, which you can subscribe to here.

We also recommend the FOA "Fiber FAQs" page with tech questions from customers originally printed in the FOA Newsletter. We had lots of interesting questions in 2018.

This page is part of a Fiber U Tech Update Course.



FOA Guide

Got questions? Try the FOA Guide and use the site search.



Technology



Cable Bend Radius

All fiber optic cables have specifications that must not be exceeded during installation to prevent irreparable damage to the cable. This includes pulling tension, minimum bend radius and crush loads. Installers must understand these specifications and know how to pull cables without damaging them.

The normal recommendation for fiber optic cable bend radius is the minimum bend radius under tension during pulling is 20 times the diameter of the cable. When not under tension, the minimum recommended long term bend radius is 10 times the cable diameter.

Note: Always check the cable specifications for cables you are installing as some cables such as the high fiber count cables have different bend radius specifications from regular cables!


Fiber Optic Cable Bend Radius
Under tension (top) and after pulling (bottom)

Bend radius example: A cable 13mm (0.5") diameter would have a minimum bend radius under tension of 20 X 13mm = 260mm (20 x 0.5" = 10") That means if you are pulling this cable over a pulley, that pulley should have a minimum radius of 260mm/10" or a diameter of 520mm/20" - don't get radius and diameter mixed up!

Why is it important? Not following bend radius guidelines can lead to cable damage. If the cable is damaged in installation, the manufacturer's warranty is voided. Here is what one manufacturer's warranty says: "This warranty does not apply to normal wear and tear or damage caused by negligence, lack
of maintenance, accident, abnormal operation, improper installation or service, unauthorized repair, fire, floods, and acts of God.
" And their specifications call our the minimum bend radius as "20 X OD-Installation, 10 X OD-In-Service."

And When An Installer Gets it Wrong

Cable pull

There are two problems here, one visible and one hidden. The visible one is the pulley mounted on the side of the truck used to change the direction of the cable to allow using the capstan mounted on the rear of the truck. The cable is being bent about 120 degrees over a pulley that appears to be about 120mm (5 inches) diameter. That's a radius of 60mm or 2.5 inches. That pulley looks like a stringing block uses for stringing ropes when pulling in power lines.

We believe the cable was a 864 fiber ribbon cable with a diameter of 24mm (0.92") with a minimum bend radius of 360mm or 14".  That means the pulley the cable is being pulled over is ~1/6th the size it should be - shown by the dotted red circle above.

The second problem is the angle of the cable coming out of the manhole. It is exiting a conduit and being pulled almost straight up out of the manhole. If there is no hardware in the manhole, the cable is being pulled over an edge exiting the conduit or the manhole, bending with a very, very small radius.

One can only speculate about the possible damage to a cable when treated like this. What comes to mind first is broken fibers, and that is a possibility. But bending this tightly can also damage the cable structure, including the fiberglass stiffeners, strength members and jacket. Compromising the integrity of the cable reduces its protection for the fibers. Even the fiber ribbons can be delaminated and fibers put under stress. A cable pulled under these circumstances can have damage along the entire length, not just a point where it was kinked.

What should have been done on this pull? The 120mm/5" pulley should have been replaced with one at least 6 times larger. The truck could have been further from the manhole (and maybe turned to be inline with the pull) so the angle of the cable exiting the conduit was less. Hardware should be attached to the conduit to provide a proper bend radius for the cable as it exited the conduit and the cable should have been protected if it contacted the edges of the manhole..

Bottom Line
  • All cables have specifications for minimum bend radius
  • Violating this spec may permanently damage the cable
  • Bend radius is generally 20X cable diameter under tension - 10X after installation



Optical Loss: Are You Positive It’s Positive?

A recent post on a company’s blog and article on the CI&M website discussed the topic of the polarity (meaning “+” or “-“ numbers) of measurements of optical loss, claiming loss was a positive number. The implication was that some people failed fourth grade math and did not understand positive and negative numbers. The claim is that insertion loss is always a positive number.

Is that right?

The asnwer is no - loss is a negative number, but instruments that only measure loss - OLTS and OTDRs - display loss as a positive number. 

Suppose we set up a test. Let's measure power out of a cable with a power meter and then attenuate the power by stressing the cable. What happens?

FOA created this short movie on the FOA Guide page explaining dB showing how a power meter shows loss when a cable is stressed to induce loss:

dB on a power meter

As the fiber is stressed, inducing loss, the power level goes from -20.0 dBm to --22.3 dBm.That's a more negative number. (-22.3dB) - (-20.0dB) = -2.3dB That's basically 4th grade math.

No question – loss means a more negative power reading in dB and a negative number in dB indicates loss.
If you want to calculate this yourself, FOA has a XLS spreadsheet you can download that will calculate the equations for optical power for you.

But if you are a manufacturer of fiber optic test instruments that offers optical power meters and sources to test loss, why would this confuse you?  Well, it seems they think when we talk about loss, we do not give it a "+ or -" sign, we just say loss, so they just display it as a number without sign,


The Detailed Explanation
In order to understand this you need to understand logarithms and that’s Algebra II*, way beyond fourth grade addition and subtraction. You see dB is defined as a logarithmic function calculated from the ratio of optical powers expressed as power in Watts, the standard measurement units for optical power, just like the output of light bulbs and LEDs:

dB equation

With logarithms, if the ratio of measured power to reference power is greater than 1, e.g. measured power is more than reference power, the log is positive. If the ratio of measured power to reference power is less than 1, e.g. measured power is less than reference power, the log is negative. If the ratio is 1, the log is 0.

Let’s try a graphic explanation of this equation. We measure absolute power in fiber optics referenced to 1 milliwatt power.  Substitute "1mw" for "reference power in the equation above and we get power in dBm.  Take a look below at this of dBm vs optical power in the range commonly used for fiber optics calculated with our equation above. Remember dBm means all power is referenced to 1 milliwatt optical power.

dB to watts
As the power in milliwatts goes up, the equivalent in dBm become more positive - 1 milliwatt is 0dBm and 10 milliwatts is +10dBm. Going the other way, 100microwatts = 0.1 milliwatt = -10dBm, more negative.


How about an example? Let’s say we decide to test a singlemode cable plant. We start with a laser source and launch cable which we measure our reference level for loss with a power meter to have an output of 0dBm. That’s 1 milliwatt of power, about the normal output of a fiber optic laser. After we attach the cable plant to test and a receive cable to our power meter, we measure 3dB loss.

What power did we measure? The power must be lower, of course, since we have loss, and 3dB is approximately a factor of 2, so the power the meter measured is 1mw divided by 2 = 1/2milliwatt or 0.5mw. Since our power meter is measuring in dBm, it will read minus 3dBm (-3 dBm), since lower optical power is always more negative. If it read +3dBm, the power measured would be 2mw and that would be a gain from our reference (0dBm) which we know is incorrect – passive cable plants are not fiber amplifiers.

Here is the graphical version of this loss test:

 dB loss or gain



Perhaps we should blame accounting.

Suppose you have a company that has $1million in sales and $900,000 in expenses. What’s the profit? It’s $1,000,000 - $900,000 = $100,000. That’s a profit, right?

But suppose your company has $1million in sales and $1,100,000 in expenses. What’s the bottom line? It’s $1,000,000 - $1,100,000 =  - $100,000. Wait a minute, that is a negative number – that’s not a profit, it’s a loss.

So in accounting, profits are positive numbers and losses are negative numbers when we do the math, but when we talk about loss, we don’t say we have a loss of “-$100,000,” we just have we have a loss of $100,000. Then we’ll put that number in parentheses when we publish our P&L like this ($100,000) and hope it doesn’t get noticed by investors, but you know it will.

Maybe the people from fiber optic test equipment companies went to business school instead of engineering or science!


Lost In Translation
Loss and gain in fiber optic measurements are similar. If you are using a separate source and power meter, loss will be a negative number and gain will be a positive number. But because of convention, we sometimes drop the signs when we report the values - we're supposed to know loss always means the optical power measurement in dB was negative and gain means the optical power measurement was positive. But maybe that’s not what the convention has evolved to.

Optical loss test sets (OLTS) aren’t designed to measure and display optical power, just loss. The actual power measured is lost in the algorithms used for calculating loss based on the “0dB” reference power and the measured loss. Long ago, most OLTS measured loss and displayed it as a negative number, but some companies who got into the fiber optic test equipment business from other test businesses arbitrarily decided to display loss as a positive number, and today most OLTS do show loss as a positive number. But when the instrument sees a gain, which it can do if improperly used, it therefore displays a negative number, which can be very confusing to a trained fiber tech who understands fiber optic power and loss measurements.

OTDRs do the same thing. We looked at traces from a half-dozen OTDRs and all showed loss as a positive number and gain as a negative number. And yes, when you have a gainer in one direction, they show it as a negative number.

*So the problem is not simply fourth grade math, it also involves a bit of convention and tradition and marketing.


More Reading
This requires understanding logarithms that create the negative number of loss. That’s more like Algebra II or 7th grade math, and here is a good tutorial from Kahn Academy on that: https://www.khanacademy.org/math/algebra2/exponential-and-logarithmic-functions/introduction-to-logarithms/v/logarithms

And more basic, here is a tutorial on adding and subtracting negative numbers https://www.khanacademy.org/math/cc-seventh-grade-math

** If you want to calculate this yourself, FOA has a XLS spreadsheet you can download that will calculate the equations for optical power for you.

The FOA has an explanation of dB on our online Guide and a couple of graphics that illustrate what happens with loss.


Note: In IEC (and TIA documents adopted from IEC documents, the definition of attenuation in Sec. 3.1 is written to have attenuation calculated based on  Power(reference)/Power (after attenuation). This definition leads to attenuation being a positive number as it is normally displayed by an OLTS or OTDR. However if one uses  a fiber optic power meter calibrated in dBm, the result will be a negative number, since dBm is defined as Power(measured)/Power(1mw) (see FOTP-95, Sec. 6.2). If dBm were defined in this manner, power levels below 1mW would be positive numbers, not negative as they are now, and power levels above 1mW would be negative!
Bottom Line: Confusion
  • Loss in dB is a negative number
  • Instruments that measure loss do not display negative signs with loss
  • Gains are displayed with a negative sign




What's That Fiber?

Regular and BI MMF

A FOA Newsletter reader sent FOA these microscope photos of two MM (multimode) fibers, asking what was the difference with the one on the right. It is a bend-insensitive (BI) fiber and compared to the regular graded-index MM fiber you readily notice the index "trench" around the core that reflects light lost in stress bends right back into the core. You can read more about bend-insensitive fiber in the FOA Guide.




What does Bend-Insensitive Fiber Look Like?

While researching the answers to the question above, we talked to Phil Irwin at Panduit. He mentioned that you could see the structure of BI fiber and sent along this photo:
Bend-insensitive fiber photo    BI fiber structure
At the left, you can see the gray area surrounding the core, shown in the drawing in the right as the yellow depressed cladding region.

If you want to try to see it yourself, it's not easy. Phil tells us that OFS fiber is the easiest to see, Corning a bit more difficult. You need a good video microscope. You may need to vary the lighting and illuminate the core with low level light.


Today most multimode (MM) fibers are bend insensitive fibers. If you buy a MM cable or patchcord, it is probably made with bend-insensitive fibers. That's generally good because thee fibers are less sensitive to bending or stress losses which can cause attenuation in regular fibers. 


The Problem Comes When Testing
The problem comes when testing these fibers and using them as reference cables for testing. BI fibers have regions outside the core that reflect light lost in bends back into the core, reducing the effect of bending. However that mechanism causes these fibers to have more high-order modes than regular fiber which can affect testing.

The problem with testing is as follows:
1. Most standards say do not use BI MMF for reference test cables. This is because standards call for launch cable mode conditioning (mandrel wrap or encircled flux) that is ill-defined for BI MMF. Except one standard. FOTP-171B has one section saying no BI MMF (Sec. 3.6.1) but another (A.3.2) says to use the same type of fiber for reference cables as in the cables you are testing.
2. Manufacturers are mostly making BI MMF today. OFS is the one manufacturer that says they continue to make non-BI MMF.
3. As far as anyone knows, nobody marks on the jacket of cables whether the fiber is BI or non-BI. There seems to be consensus that is should be marked, but no standard exists to require it.
4. Some test equipment vendors provide non-BI MMF for test cords. Nobody else has been identified as a supplier.
 
Conclusion? Confusion!

Testing Multimode Fiber With BI Fibers: Real Test Data

FOA received a technical inquiry regarding insertion loss differences in test results when using OM2 and OM4 fiber for launch cables. This table shows losses measured with OM4 bend-insensitive (BI) fiber launch cables and OM2 non-bend-insensitive fiber launch cables.

BI fiber losses

Note the same cable under test (DUT) showed significantly lower loss when tested with an OM2 (non-BI) launch cable than with an OM4 (BI) launch cable (-the same test source was used.) The OM4 cable, based on the photo below, is BI fiber, had higher modal fill and gave much higher loss.


Bottom Line: Confusion!
  • Most MM fibers are BI fibers
  • Standards say don't use BI fibers for reference cables
  • Cables are not marked so the user doesn't know if its BI or not



Manhole/Handhole Size

Q: What you recommend when it comes to manjole/handhole sizing.  If they are being used for splicing, do you have a general formula of length of splice closure plus X factor more for cables in/out of closure and slack storage? 

manhole

A: FOA has been doing some research on underground construction to expand our section in the FOA Guide. We are looking at what people are specifying on some projects since we do not know of any industry standards.

There are links of some interesting/useful information below.

From our standpoint, the minimum size would be determined by the bend radius of the fiber optic cable (see article above), how much slack (service loops) would be stored (slack from how many cables - see photo below) size of the splice closures, and how many ducts and cables would be served. Generally you will have 20-30 feet in service loops to allow for splicing, Typical cable up to 1/2” needs a loop >20” but I don’t know how you would ever get a loop that small for that much cable, so you probably have minimum 2’ loops, at least 5 coils. Add a closure and you probably need a 2’X4’ handhole, at least 2’ deep, as a minimum - see the “good” photo below.

We were at Corning training last Spring on high fiber count cables and those cables require ~6’ X 4” min manholes just to fit the loops of cable. Handholes can be smaller, depending on the type of splice or drop, midspan access, etc.

The FOA Guide pages on OSP Construction created by Joe Botha for his course in South Africa talks about manholes and handholes on this page near then end.

This Jensen web page shows the number of different designs and sizes.

Detail from Central FL Expressway Design Standards offers several sizes: 4' X 4' X 4',  4' X 6.5' X 6.5', 4' X 6.5' X 6.5' and specifies a duct organizer.

Here is a Wisconsin DOT spec for a 4’ diameter manhole.

NYC Broadband General Network Specifications:  see page 24ff


Bottom Line:
  • Manholes need to be big enough for the cables they must contain
  • They usually aren't!


Blasting Near Fiber Optic Cables

Last month we ran a Q&A question about blasting near fiber optic cables. Bill Graham, FOA Board Member and long-time instructor in Canada, tells us that he suggests a more conservative approach. This is what he has taught in his classes:

Blasting and Fiber Optic Cables

Two situations:
1.)    Aerial on poles or towers
2.)    Buried in ground

Aerial:

Blasting near fiber optic cables
Blasting close to poles with fiber optic cables can depend on the soil between the blasting area and the pole.  For example:  In parts of Ontario there are large areas of solid granite. The pole is generally bracketed to the rock. (Some areas have started drilling the rock) there is a solid connection from the blast hole to the pole, up the pole onto the bracket and to the Fiber Optic cable, which they seem to forget is glass.

Blasting near fiber optic cables

We recommend the following:
1.)    Cover the aerial cable with Big O (4” perforated drain pipe slit along the length) as shown in the photo above. This protects only from flying rock.
2.)    Removing the cable from the pole clamp and hanging it from the bracket with a  Bungie strap.

Blasting near fiber optic cables

Blasting locations are carefully engineered… however, if the crew wants to get home early on Friday and they double up on the blasting the damage risk increases substantially. ( The red cups in the photo are blast holes.) Most companies are wary  enough not to guarantee “no glass damage”. This area is almost all solid granite.

Bottom Line:
  • For Cables Buried Underground
  • Regardless of Kinder Morgan’s recommendation, We suggest 5-6 meters separation is not adequate
  • Bleasting should be at least 12 to 15 meters away




The Perils Of 2-Cable Referencing

FOA received an inquiry about fluctuations in insertion loss testing. The installer was using a two cable reference method for setting a "0dB" reference where you attach one reference cable to the source, another to the meter and connect them to set the "0dB" reference. The 2-cable reference method is allowed by most insertion loss testing standards, along with the 1- and 3- cable reference methods, although each gives a different loss value.

reference 3 ays
3 different ways to set a 0dB reference for loss testing

When a 1-cable reference is used, one sets a reference value at the output of the launch cable and measures the total loss. With a 2-cable reference, a connection between the launch and receive reference cables is included in making the reference, so the loss value measured will be lower by the amount of that connection loss. The 3-cable reference includes two connection losses so the loss will be lower still.

two cable reference
The problem with the two cable reference is the uncertainty added by including the connection between the two reference cables when setting the
"0dB" reference.

Unless you carefully inspect and clean the two connectors and check the loss of that connection before setting the
"0dB" reference, you add a large amount of uncertainty to measurements of loss. The best way to use a 2-cable reference is to set up the source and reference cable (with inspected and cleaned connectors), measure the output of the launch cable, attach the receive cable (with inspected and cleaned connectors) and measure the loss of the connection before setting the "0dB" reference. If the connection loss is not less than 0.5dB, you have connectors that should not be used for testing other cables. Find better reference cables.

The two cable reference is often used when the connectors on the cables or cable plant being tested are not compatible with the connectors on the test equipment, so you must use hybrid launch and receive cables. Then you can only reference the cable when connected to each other. In that case, you need the 2-cable reference but should expect lower loss and higher measurement uncertainty.

Experiments have shown that the uncertainty with a 1-cable reference is around +/-0.05dB while the 2-cable has an uncertainty of around +/-0.2 to 0.25dB caused by the mating connection between the two reference cables. Those experiments also showed the uncertainty of the 3-cable reference was not significantly larger than the 2-cable reference.

When possible, use a 1-cable reference. When you must use the 2- or 3-cable reference, inspect and clean all connectors carefully before making connections for the reference or test.

Bottom Line:
  • The value of loss you measure depends on how you set your "0dB" reference - more reference cables means less loss.
  • Connections between reference cables when setting a 0dB loss add uncertainty to measurements


Fiber Bend Sensitivity

Here is a OTDR trace from EXFO that FOA Master Instructor Eric Pearson uses in one of his books to illustrate bend sensitivity of fiber and different wavelengths. It shows the loss of a bend in a singlemode fiber at 3 wavelengths, 1310, 1550 and 1625nm. You can see the loss is greater at longer wavelengths, the reason that OTDR testing at longer wavelengths can be used on most fibers to find bending or stress losses. However, this will not be as useful in the future as bend-insensitive fibers are more widely used.

OTDR bend sensitivity



Historical Footnote 2: We found another interesting Corning ap note, AN3060, March 2014, on OTDR testing of SM fiber fusion splices.


What interested us was this graph, showing OTDR loss measurement differences depending on direction and mode field diameter MFD differences.

OTDR errors with MFD

This shows the direct relationship between MFD and OTDR errors - if you test a joint or splice going from smaller MFD to larger, you get a gain. Going from larger MFD to smaller, you get an exaggerated loss.

Note the magnitude of the loss difference for such small differences in MFD - up to 0.25dB!

Bottom Line:
  • Gainers are caused by differences in mode field diameter (MFD) in fiber splices or joints.
  • OTDRs can have big errors when tests are done in only one direction.




Multimode Loss With A Mandrel Wrap, Testing The Effect In Class


The fiber optic industry has always known about the effects of modal distribution and has created metrics to measure and standardize it for testing multimode fiber. The methods included MPD (mode power distribution), CPR (coupled power ratio) and the latest, EF (Encircled Flux.) 

In class, we decided to test connectors with and without a mandrel wrap mode conditioner to see if it made a difference.

mandrel wrap


After adding the mandrel wrap to the launch cable, we tested the LED test source using a HOML (higher order mode loss) test as described in the page on Encircled FluxWith the mandrel wrap, the power was reduced by ~0.6dB, so we left the mandrel on for our testing.

Adding the mandrel wrap certainly did make a difference. Connections tested single-ended withowithut the mandrel wrap  ~0.6dB loss were measured at ~0.2dB loss with the mandrel wrap - that's 0.4dB less. That's how much difference modal conditioning can make on a single connection.

Think about that the next time you are testing multimode fiber!


Bottom Line:

If you are testing multimode fiber, use a mandrel wrap mode conditioner.









Troubleshooting With A VFLFibers Damaged In Splice Trays

Is this a trend? Twice in one week, we have inquiries from readers with problems and both were traced to fibers cracked when inserted in splice trays. The photo below shows one of them illuminated with a VFL. This was the same issue we found in the first field trial of a VFL more than 30 years ago that led to its popularity in field troubleshooting.

Broken fiber found with visual fault locator
Photo courtesy Alan Kojima.

Bottom LIne:
  • VFLs are invaluable troubleshooting tools for finding cable faults
  • But only work close by - 3-4km range max


They've ALL Got It All Wrong - And They Confuse A Lot Of People - YOU CANNOT STRIP THE CLADDING OFF GLASS FIBER!!!

We recently got this email from a student with field experience taking a fiber optic class:""The instructors are telling us that we are stripping the cladding from the core when prepping to cleave MM and SM fiber.  I learned from Lenny Lightwave years ago, this is not correct. I do not want to embarrass them, but I don't want my fellow techs to look foolish when we graduate from this course."

I'll share with you our answer to this student in a moment, but first it seems important to understand where this misinformation comes from. We did an image search on the Internet for drawings of optical fiber. Here is what we found:

bad fiber drawings

EVERY fiber drawing we found on the Internet search with one exception (which we will show in a second) showed the same thing - the core of the fiber separate -sticking out of the cladding and the cladding sticking out of the primary buffer coating. Those drawings are not all from websites that you might expect some technical inaccuracies, several were from fiber or other fiber optic component manufacturers and one was from a company specializing in highly technical fiber research equipment.

The only drawing we found that does not show the core separate from the cladding was
- really! - on the FOA Guide page on optical fiber.

correct fiber drawing



No wonder everyone is confused. Practically every drawing shows the core and cladding being separate elements in an optical fiber.

So how did FOA help this student explain the facts to his instructors? We thought about talking about how fiber is manufactured by drawing fiber from a solid glass preform with the same index profile as the final fiber. But we figured a simpler way to explain the fiber core and cladding is one solid piece of glass was to look at a completed connector or a fusion splice.

We started with a video microscope view of the end of a connector being inspected for cleaning.

fiber view - core/clad

Here you can see the fiber in the ceramic ferrule. The hole of the connector is ~125 microns diameter (usually a micron or two bigger to allow the fiber to fit in the ferrule with some adhesive easily.) The illuminated core shows how the cladding traps light in the core but carries little or no light itself. This does not look like the cladding was stripped, does it?

Here is the same view with a singlemode fiber at higher magnification.

Fiber view - SM

And no connector ferrules have 50, 62.5 or 9 micron holes so that just the core would fit in the ferrule, do they?

What about stripping fiber for fusion splicing. Here is the view of fiber in an EasySplicer ready to splice.

Fusion splice - core/clad

What do you see in the EasySplicer screen? Isn't that the core in the middle and the cladding around it? In fact, isn't this a "cladding alignment" splicer?

We rest our case. If that's not sufficient to convince everyone that you do not strip the cladding when preparing fiber for termination or splicing, we're not sure what is.

Special Request: To everyone in the fiber optic industry who has a website with a drawing on it that shows the core of optical fiber separate from the cladding, can you please change the drawing or at the very least add a few words to tell readers that in glass optical fiber the core and cladding are all part of one strand of glass and when you strip fiber, you strip the primary buffer coating down to the 125 micron OD of the cladding?

Bottom Line:
  • Most diagrams of fiber construction are wrong - showing core and cladding as separate - but they are one solid peice of glass.
  • You cannot strip the cladding from glass fibers.


Are Manufacturers Beginning To Realize That It's Time To Go Singlemode?

Back 40 years ago when optical fiber technology was being developed by the scientists at Bell Labs in Murray Hill, NJ they were focused on future technologies. At the time, the installed fiber links were based on 850nm Fabry-Perot lasers (VCSEL technology was 20 years in the future) and multimode fiber (62.5/125 micron was their standard then, later replaced with higher bandwidth 50/125 fiber for the early long distance links.)

But they knew the future would be dominated by singlemode fiber. These scientists, many of whom had worked on the Bell Labs projects associated with millimeter wave transmission in the 1960s, knew that multimode fiber had the same problem as mm RF waveguides - noise and bandwidth limits caused by multimoding. To realize the potential of optical fiber, it was necessary to move transmission to singlemode fiber.

And that's what happened. Telcos went multimode in the mid-1980s. Multimode fiber was still used for premises cabling and short links because the transceivers were cheaper.

But that may be changing. Links speeds are getting higher so multimode is running our of bandwidth - but singlemode is still cruising along.  The millions of links in data centers and millions of subscribers connected on FTTH networks has caused the price of laser transceivers to drop to prices comparable to multimode versions with cheap VCSELs and now singlemode links are often cheaper since singlemode fiber is much cheaper than MM fiber.

Fiber techs who have worked with multimode in premises cabling often claim singlemode is much harder to install. Maybe that was true a decade ago, but the technology developed for outside plant and FTTH, especially multi-dwelling units and data centers has changed all that. Bend-insensitive fiber allows the cables to be made much smaller (microcables) and the same for ducts. Microducts allow fiber to the "blown in" more quickly. Splice on connectors (SOCs) and low cost fusion splicers solve the termination problem.

Bottom Line:
  • Singlemode is needed for today's high speed links
  • Singlemode is becoming cost effective for all applications
  • New technology makes singlemode easy to install
  • Most new networks are based on singlemode fiber (see below)


Maybe it's time. But don't hold your breath. Like Cat 5, multimode will not "go gentle into that good night..." Dylan Thomas.


See also the article below on fiber types in data centers.


New High Speed Networks

In the article below on faster Ethernet standards, we've highlighted the fiber types to emphasize the dominance of SM fiber. Note that 6 of the 7 use singlemode fiber and 4 of the 7 use WDM over 2 fiber links.

200/400G Ethernet Approved

Last December, the Ethernet committee approved a new standard, IEEE Std 802.3bs-2017: 200 Gb/s and 400 Gb/s Ethernet with seven variations.

200GBASE-DR4: 200 Gb/s transmission over four lanes (8 fibers total) of singlemode optical fiber cabling with reach up to at least 500 m

200GBASE-FR4: 200 Gb/s transmission over a 4 wavelength division multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode optical fiber cabling with reach up to at least 2 km

200GBASE-LR4: 200 Gb/s transmission over a 4 wavelength division multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode optical fiber cabling with reach up to at least 10 km

400GBASE-SR16: 400 Gb/s transmission over sixteen lanes (i.e. 32 fibers total) of multimode optical fiber cabling with reach up to at least 100 m

400GBASE-DR4: 400 Gb/s transmission over four lanes (i.e. 8 fibers total) of singlemode optical fiber cabling with reach up to at least 500 m

400GBASE-FR8: 400 Gb/s transmission over an 8 wavelength division multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode optical fiber cabling with reach up to at least 2 km

400GBASE-LR8: 400 Gb/s transmission over an 8 wavelength division multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode optical fiber cabling with reach up to at least 10 km

An MPO-16 plug and receptacle is required to support the 32-fiber 400GBASE-SR16 multimode application. The MPO-16 plug is designed with an offset key to prevent accidental mating with a standard MPO/MTP receptacle. All 2-fiber applications may be supported with a 2‑fiber LC singlemode interface and all 8-fiber applications may be supported with standard MPO/MTP receptacles.

This graphic shows how many "outlaw" Ethernet versions there are:


Ethernet speeds

100G SWDM4 is for VCSEL WDM on OM5 fiber.  100GBase-ZR is a tech marvel using special modulation and coherent technology like long haul telecom. As you can see from the list above, the 400G standards are now approved along with some 200G not listed here. Remember our quote (above) from Bob Metcalfe, co inventor of Ethernet: 
"The wonderful thing about standards is we have so many to choose from."



How "Fast" Is Fiber?

We've probably all heard the comment that fiber optics sends signals at the speed of light. But have you ever thought about what that speed really is? The speed of light most people think about is C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec. But in glass, the speed is reduced by about 1/3 caused by the material in the glass. The light is slowed down and the amount is defined as the index of refraction of the glass. V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec. So in glass, the "speed of light" is about 2/3 C, the speed of light in a vacuum.

One of the FOA instructors sent us this question:  "I work with at Washington Univ with an engineer who works for an electrical utility. He asked a question about the speed of signal transmission over fiber optics, single mode, at top of towers. They need signal to be sent in 18 millisecs for relays to function properly. Is there a problem over a distance of 150 miles?"

Electrical transmission lines

Let’s do a calculation:

C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec
V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec
 
150 miles / 127,000 miles/sec = 0.00118 seconds or ~1.2 milliseconds

Another way to look at it is 127,000 miles/sec X 0.018 seconds (18ms) = 2,286 miles

So the fiber transit time is not an issue. The electronics conversion times might be larger than that.

I used to explain to classes that light travels about this fast:

300,000 km / sec
300 km / millisecond
0.3km /microsecond or 300m / microsecond
0.3 m per nanosecond - so in a billionth of a second, light travels about 30cm or 12 inches

Since it travels slower by the ration of the index of refraction, 1.46, that becomes about 20cm or 8 inches per nanosecond.

That is useful to know since an OTDR pulse 10ns wide translates to about 200cm or 2 m pr 80 inches (6 feet and 8 inches), giving you an idea of the pulse width in distance in the fiber or an idea of the best resolution of the OTDR with that pulse width. 

Bottom Line
  • Fiber is "fast" because of its bandwidth capability
  • Light travels in fiber at the speed of light
  • But the speed of light in glass is only 2/3 as fast as the speed of light in air or a vacuum




What Does A FTTH ONT Look Like Today?

ONT

That's all there is to the ONT that goes into the home. The arrow points to the 1310 TX/1490 RX transceiver for SC-APC connectors.





Components

Here are several technologies that have continued growing in importance in the fiber optic marketplace  -  components that every tech needs to learn about and become familiar with their use.




Taking Care of Your Fiber Optic Tools

fiber optic toolkit

    Nothing is more frustrating that trying to accomplish a task and having problems with your tools. It doesn’t matter whether it is not being able to find a tool or finding a damaged tool put back in the toolbox without being repaired or replaced, it is a problem.

Have you ever noticed how careful automobile mechanics are with their tools? The right tools are absolutely necessary for their work and they know they must keep them in good condition and stored in the tool’s proper location when not being used. Tools are expensive - not just for mechanics but for fiber technicians too - so learn how to use them correctly and take care of them so they will work properly when you need them.

Never, ever, go out on a job unless you have inspected your tools and test equipment back in the office and verified your tool kit is complete, your test equipment is working properly and you have all the supplies and consumables you need.

A corollary of this is never take new gear into the field until you have tested it in the office and are familiar enough with its use that you will not have problems in the field caused by unfamiliarity with it. When I was in the fiber optic test equipment business, it amazed us how many help calls we got from customers who were at the job site and wanted to know how to use the equipment!

Let’s get more specific. Start with your tools. Clean off a table and open your fiber optic tool kit. Are all the tools there? Grab a notepad and list what you are missing. Even better create a list of tools you need and use it as a checklist so you don’t forget anything. In fact, I’ll include on the online version of this article at http://www.ecmag.com a list of recommended tools you can use as a checklist. And keep a copy of that list in your toolkit for reference.

Some of your tools are bulletproof, but some are delicate and/or wear out. Check the condition of your fiber optic strippers and scribes in particular. They both should be carefully cleaned and inspected. Use a magnifying glass or loupe to check the working areas. Then get some fiber and test them to make sure they work properly. I recommend you have spares of these two tools in your kit since they do wear out or can be damaged, so spares are warranted.

Most test equipment is battery powered, so having spare batteries and/or keeping the batteries charged is important. Check the condition of the batteries in each piece of gear, turn it on and make sure it works properly. If your gear has adapters for various fiber optic connectors, make certain that all those adapters are there and kept in marked plastic bags to identify and protect them. Find all your reference test cables and mating adapters.

Grab one of your reference test cables and use it to check the operation of your connector inspection microscope. At the same time, you will be checking the condition of the reference cable connector. Does it look nice and clean and free from scratches? Reference test cables wear out after hundreds of tests, even when you clean them regularly, so use the microscope to check the condition of every connector on every reference cable and set aside those which look questionable.

Next use your light source and power meter to test all those reference cables. Use a single-ended insertion loss test to determine if they are still in good condition, with a loss of well under 0.5 dB, and discard the bad ones or set them aside for re-termination. If you have a checklist, keep track of the loss and watch how the loss will increase as they are used more and more. If you have an OTDR and associated launch cables for it also, use the light source and power meter to check them too.

    Finally, check all your cleaning supplies. Make sure you have enough for the next job. If not, add that to your notepad list of things to order ASAP. Don’t wait until the next job comes up; order all the replacement tools and supplies you need now and be ready.

Bottom Line
  • Your work depends on your tools - take good care of them!


High Fiber Count Cables

We ran this first in the March 2019 FOA Newsletter with updates in May and June. We received some feedback and have been talking to people in the industry also. We even have samples  to photograph and have visited a manufacturer for demos and training. We thought we'd share some of what we've been told and see if others agree. Feel free to comment!


High Fiber Count Cables

FOA has recently gotten several inquires about the new high fiber count cables - 864, 1728, 3456 or even 6,912 fibers. Like this one from Prysmian with 1728 fibers:

Prysmian 1728 fibers

These cables use bend-insensitive fibers to allow high density of fibers without worrying about crushing loads affecting attenuation. Most also use fibers with 200 micron buffer coatings instead of 250
micron buffer coatings to allow even higher density. Many, or even most, use ribbons of fiber, either the conventional hard ribbons or the newer flexible ribbons, since, as we show below, the time to splice even a 1728 fiber cable is extremely long unless ribbon splicing is used.

High Fiber Count Cables may not be for everyone. Maybe only for a very few. A single cable that has as many fibers as 12-144 fiber cables (1728 fibers) in a cable with a diameter of only twice that of a conventional 144 fiber cable can present challenges.

  • First of all, the cost - it's high. You do not want to waste cable at this price. Engineering the cable length precisely will save lots of money.And it's worse for higher fiber counts.
  • Likewise, making mistakes when preparing the cable for termination can be expensive.
  • The cable may require special preparation procedures to separate fibers for termination. Most use new methods of identifying cables and bundles.
  • Besides skill, the tech working with high fiber count cables needs lots of patience.
  • Splicing multiple cables at a joint can get complicated keeping all fibers straight.
  • These cables will generally use 200 micron buffered fiber and often a flexible ribbon instead of a typical rigid ribbon structure to reduce fiber sizes. This may complicate splicing as the methodology to splice the flexible fibers and splice 200 micron fibers to regular 250 micron fibers is a work in progress.
  • Splicing 200 to 250 micron fibers may be easier with the flexible ribbon designs which make it easier to spread fibers to the same spacing.
  • We've heard the splicing time for flexible ribbons is about 50-100% longer than that of conventional rigid ribbons. So if you use that table below, you may need to increase your ribbon splicing estimates when working with flexible ribbons.

We've been looking for directions on how to deal with high fiber count cables. Several contractors tell us ribbon splicing is the way to go, and most of these cables now use a version of the new ribbon types that are flexible. We've  put together this table from some articles on splicing ribbons:


June 2019

High Fiber Count Cables

We've had a continuing feature on high fiber count cables in the FOA Newsletter and we now have some interesting photos to show you. Corning generously sent FOA some samples of 1728 and 3456 "RocketRibbonTM" cable. We took some photos and must admit that these cables are fascinating updates on the traditional fiber optic cables.

high fiber count cable


Here are Corning RocketRibbon 1728 fiber (bottom) and 3456 fiber (top) cables. To get an idea of these cables size, look at this photo:

cable

The 3456 fiber cable is 32mm diameter, 1.3 inches. The 1728 fiber cable is 25mm, 1 inch diameter.

These are cables made from conventional "hard" ribbons, not the "flexible" ribbons used on some cable designs. As a result of using hard ribbons, the fibers are arranged in regular patterns to get high density.

cable

These are the tubes of ribbons from these cables. Each of those tubes of ribbons has the equivalent of 24 ribbons of 12 fibers each (actually 8 X 12 fibers and 8 by 24 fibers stacked up) for 288 fibers total. The 1728 fiber cable has 6 tubes and a center foam spacer, with 144 ribbons total. The 3456 fiber version has 12 tubes and no spacers, 288 fiber ribbons total.

What amazes us is the density of fibers.

cable

We calculated the "fiber density" of this 3456 fiber cable based on 200 micron buffered fibers and determined that 54% of the cable is fiber. Compare that to a typical 144 fiber loose tube cable, which is about 14% fiber or a 144 fiber microcable which is about 36% fiber.

Looking at the end of this cable reminded us of nothing so much as this PR photo from AT&T from their intro of fiber in 1976:

cables

Not the fiber, the dense cable of copper pairs!

Of course the cable is much lighter than copper but much heaver than you are used to with fiber - it weighs 752 kg/km or about 1/2 pound per foot. And it's stiff. Very stiff. The minimum bend radius is 15 times the cable diameter or 480mm (~19 inches), about a meter or yard in diameter.

As we noted in the photo above, Ian Gordon Fudge of FIBERDK taught some data center techs how to handle a 1728 fiber Sumitomo cable with a slotted core. Ian sent FOA this photo to illustrate the number of fibers in the cable he was using for training. Impressive!

Fiber DK

Here is the slotted core that separates the flexible fiber ribbons
in the Sumitomo cable:

slotted core

More on high fiber count cables and our continuing coverage



ribbon splicing

High fiber count cables are all ribbon cables, some with hard ribbons and some with flexible ribbons, All require ribbon splicing because of the construction and the time it would take to terminate them. This is a table of estimated termination times. Is that realistic? We've heard the flexible ribbons may take
50-100% longer than conventional ribbons due to the need to carefully arrange and handle fibers.



High Fiber Count Cables - Continued Updates - Installation

Continuing our ongoing research on high fiber count cables, last month we were invited to visit Corning's OSP test and training facility to experience the processes of installing these cables for ourselves. We had the opportunity to handle some of these cables ourselves and see how experienced techs worked with this cable.

Once you get a chance to handle this cable and see how big, stiff and heavy it really is, you understand that it's quite different from any fiber optic cable you have worked with, with the possible exception of some hefty 144/288 fiber loose tube cable that's armored and double jacketed. With a bend radius of 15X the diameter of the cable, the minimum bend radius of a 1728 fiber cable is 15" (375mm) and that's a 30" (750mm - 3/4 of a meter) diameter. Just the reel it's shipped on is outsized - it should have a ~750mm (30 inch) core and will be probably ~1.8m (6 feet ) in overall diameter. 3300 feet (1km) of this cable will weigh 550-750kg (1200-1700 pounds.) and the reel will weigh another ~300-400kg (700-900 pounds). Will that fit on your loading dock? Can you handle a ton of cable? (Metric or English)

I tried bending one of the 1728 fiber cables and (with the manufacturer’s OK) tried to break it. The 1728 fiber cable I was bending took an enormous amount of muscle to bend, and when I got down to about an 8 inch radius, it broke, with a sound like a tree limb of similar diameter cracking. In the field, that would have been an expensive incident.

The stiffness of these cables affects the choice of other components and hardware. You will not fit service loops into a typical handhole, you need a large vault like the one shown in the photos taken at Corning. You will also need close to 100 feet (30m) of cable for a service loop. You may need to figure 8 the cable on an intermediate pull and that will require lots of space and a crew to lift the cable to flip it over.

This 1728 fiber cable is stiff, does not easily twist and only bends in one direction because there are stiff strength members on opposite sides of the cable. Placing it into a manhole or vault and fitting service loops into it is not easy. In this case, it helped to have two people and one was the trainer. You need to have a "feel" for the cable - how it bends and twists - to make it fit. The limits of bend radius, stiffness and unidirectional bending makes it necessary to work carefully with the cable to fit loops into the vault. Sometimes it's necessary to pull a loop out and try in a different way to get it to fit. But it can be done as you see at the right.

cable handling
 
Pulling the cable out of conduit in the vault without damaging it also requires care. You can see in the back the orange duct coming into this vault. When pulling the cable, it's important to not kink the cable while pulling it out of a duct. A length of stiff duct can be attached to the incoming duct to limit bend radius. Capstans, sheeves and radius cable sheaves need to be chosen to fit the required cable bend radius. A a radius cable sheave with small rollers can damage the cable under tension and are bot a good choice unless the rollers are used with a piece of conduit to just set the bend radius.

Corning also showed us a new feature of their RocketRibbon Cables. A high fiber count cable has a lot of fibers, even a lot of ribbons, so identifying ribbons can be a problem. In addition to printing data on each ribbon, Corning now tints the ribbons with color codes to simplify identification. Great idea.

tinted ribbons

Here's links to some of the information we've been reading and watching online:
Corning sticks with solid ribbons in high density cables.
Corning ribbon splice closure for 1728 fibers.
Directions from Corning on ultra high-density cabinets
Designing a high fiber count cable with flexible ribbons - SEI.
Fujikura (Japan) Highest density Optical Fiber Cable.
OFS Presentation on 200micron buffer, bend insensitive, high fiber count cables.
Ribbonizing 250 micron loose tube fibers for splicing, AFL Fujikura. (video).  (Written instructions too.)
Splicing AFL "SpiderWeb ribbon cable.
Ribbonizing 250 micron loose tube fibers for splicing, Sumitomo. (video).  
Procedures for ribbonizing and de-ribbonizing fibers.  (Telonix)
Some things you need to know about splicing 200micron buffered fibers.

Some things you need to know about the new ribbon cables (Prysmian)

These current links to news may disappear over time.

Bottom Line
  • High fiber count cables allow extremely high fiber counts in small cable sizes, perfect for dense applications in data centers and metro areas
  • With so many fibers, ribbon splicing is the only sensible way to splice them
  • Ensure you splicing machines can handle 200micron buffer fibers
  • Because bend radius limits are so high, they require special consideration for installation and storage - BIG manholes for example




New Connectors

We're seeing some interesting new connectors being introduced. 3M announced a multifiber array connector using expanded beam technology and several new ideas of making a duplex connector smaller.

3M Expanded Beam Connector
3M Expanded Beam connector 3M

Details are sketchy but from the video on the 3M website, the connection is made by a small plastic fixture that is shown by the arrow in the top photo. The plastic seems to turn the beam 90 degrees so the connection is made when two pieces overlap., in the direction of the arrow in the lower photo. The connectors are hermaphroditic - that is two identical connectors can mate. There are models for singlemode and multimode fibers and you can stack the connection modules to handle up to 144 fibers. We understand this was not part of the 3M fiber optic product line recently acquired by Corning. 3M Expanded Beam Connector.  

For more information on expanded beam connectors, see the FOA Newsletter for October 2018 that discusses the R&M QXB, another multifiber expanded beam connector announced last Fall. 


SENKO CS and SN
In the FOA Newsletter for January 2018, we featured the SENKO CS connector, a miniature duplex connector using two 1.25mm ferrules, but much smaller than a duplex LC. The CS is sell on its way to becoming standardized with a FOCIS (fiber optic connector intermateabliity standard), but on the SENKO web page, there is another new connector, the SN, that makes the SC look huge! The big difference is the vertical format that allows stacking connectors very close. That can allow transceivers to have more channels, a big plus for data centers. Here is more information on the SENKO CS and SN connectors.

SENKO CS and SN
SENKO

CS-SN
Comparison of SENKO CS (L) and SN (R) connectors with duplex LC.

US Conec MXC and MDC Connectors
The R&M and 3M expanded beam multifiber connectors reminded us that US Conec introduced the
MXC connector over 5 years ago, using similar technology for up to 64 fibers per connector. The MXC is on the US Conec website, but seems to be aimed at board level connections, not far off its original purpose as a connector for silicon photonic circuits. But when we checked the US Conec website, there was a connector name we dis not recognize, the MDC. The MDC (below) is a vertical format duplex connector using 1.25mm ferrules that looks similar to the SENKO SN above. Here is information on the US Conec MDC duplex connector.

US Conec MDC connectorUS Conec

Its All About The Data Center
Just like the high fiber count cables discussed above, the CS, SN and MDC connectors are aimed at high density cabling and transceivers for data centers. All three are specified for the new QSFP-DD pluggable transceiver multi-source agreement.


Bottom Line:
  • Like everything else, connectors keep getting smaller
  • Too early to determine if they will be accepted in the marketplace and can compete with LCs



Micros: Microcables, Microducts and Microtrenching

Corning MiniXtend cable

144 fiber Corning MiniXtend cable is smaller than a pencil

MIcrocables, microducts and microtrenching - three technologies that have more in common than the prefix "micro" are gaining in acceptance along with blown cable, the obvious method of installation using them. Smaller is always better when it comes to crowded ducts, especially in cities where duct congestion is a problem in practically every city we have contact with.


Bottom Line:

  • Like everything else, cables keep getting smaller
  • Work well with microducts and microtrenching
  • Installers need to become familiar with "blown cable" technology
  • They are already accepted in the marketplace


Fiber Ducts

With the demand for more fiber for smart cities services like small cells and smart traffic signals, not to mention a ton of other smart cities services, installing more cables in current ducts - without digging up streets - is a major interest. Sometimes it's possible to install microducts in current ducts with a cable and blow in a new microcable. Sometimes it's worth it to pull an older cable out and install a new microduct that will accommodate 6 cables, making room for future expansion. The makers of the fabric ducts, Maxcell, can even show you how to remove the ducts in conduit without disturbing the current cables and pull in fabric ducts to install more cable.

Maxcell
Comparison of MaxCell ducts to rigid plastic duct

samples of microducts

Microducts are small ducts for blowing in cable. In the size of a traditional fiber duct, you can get 6 microducts for 6 288 fiber cables.


Microducts And Microtrenching

Nearly invisible microtrenching
Nearly invisible microtrenching

If you have to trench, microtrenching is probably the best choice for cities and suburbs. Rather than digging wide trenches or using directional boring (remember the story about the contractor in Nashville, TN using boring to install fiber who punctured 7 water mains in 6 months?), microtrenching is cheaper, faster and much less disruptive.

All of this implies that contractors are willing to invest in new machinery and training, sometimes an optimistic assumption. Microtrenching machines and cable blowing machines are available for rent, but personnel must be trained in the design of networks using these technologies and operating the actual machinery in the field. That's still a considerable investment.


Bottom Line:
  • Cables and ducts are getting smaller allowing more and more fibers in the same space
  • Microtrenching allows "construction without disruption"




Splice-On Connectors

SOC
Terminating with SC SOC in EasySplicer

Termination has been seeing greater acceptance of the SOC - splice-on connector - using fusion splicers. It's popularity started in data centers for singlemode fiber where the number of connections is very large so the cost of a fusion splicer is readily amortized and the speed of making connections is the real cost advantage. The performance of SOCs is much better than prepolished/splice (mechanical splice) connectors simply because of the superiority of a fusion splice and the cost of the SOCs are much less since they do not have the complex mechanical splice in the connector.

We have used SOCs in training and the techs take to them readily. In classes you can combine splicing and termination in one session. The cost of fusion splicers has been dropping to near the cost of a prepolished/splice (mechanical splice) connector kit so the financial decision to use SOCs is easier to make.


Bottom Line:
  • Splice-On Connectors (SOCs) are easy to install, low loss and low cost
  • Less hardware than pigtail splicing
  • Premises or OSP
 


Fiber For Wireless


WiFi, DAS or Small Cell?

Nothing provides perspective better than looking at something as an outsider. Especially an outsider who's just trying to understand something instead of an insider trying to perform successfully as an insider. That's how we feel about wireless communications. If you say "wireless" to an IT or LAN person, they think WiFi. But to a telecom person they think cellular. FOA's involvement is based on trying to understand the infrastructure to support wireless, OSP or premises, WiFi or cellular, tower site or small cell.

We're basically outsiders on the technology looking at the infrastructure to support them. Recently we've been trying to understand the technologies, markets and applications for both to better include the two technologies in our training and certification programs.

The initial question we had dealt with distinguishing DAS (distributed antenna systems for cellular) and small cells (also cellular). In most ways they seem to be very similar, except perhaps DAS is indoors and small cells outdoors.

We've started to interview insiders in both technologies to try to understand how they work and why we should have both. Right off, we found that there appears to be a general lack of technical understanding about the other from almost everybody we talk to who works with one of them. And we're talking real basics - what frequencies are used, protocols, coverage, bandwidth, etc. etc. etc. Even the jargon is different, but that's not unexpected. So we've tried to consolidate information on the three different premises wireless technologies appropriate for general usage. Over time we expect to refine this comparison with more data and user feedback. (got any? send it to us)

Based on the current evaluation, WiFi is essential to premises spaces and because of the ubiquity of WiFi, it is inexpensive. However, WiFi connections for cellular mobile devices appears to have not yet been refined sufficiently to provide reliable coverage for cellular voice, but data is good and video, maybe. Given the cost structure of data plans, using cellular for video can be very expensive but WiFi is preferable since it is only limited by bandwidth.

The choice between small cell and DAS in premises spaces is simple - small cells are generally single carrier connections and that is too limiting for most users. DAS is similar technology but has the advantage of offering multiple service providers. If better cellular service is desired indoors and WiFi connections for cellular calls is unreliable, a DAS is the best solution.

Small cells appear to be a good solution for better cellular service outdoors in metropolitan areas but the capital costs for building systems is quite high - Deloitte, you might remember from an earlier FOA Newsletter, forecast a cost of over $200 billion. It makes one wonder if the carriers can make that investment while simultaneously investing in 5G.

Premises Wireless
WiFi DAS (Cellular)
Small Cell (Cellular)
Connects to: PCs, tablets, phones, many other devices Phones, tablets, some other devices Phones, tablets, some other devices
Usage Free, sponsored Paid Paid
Origin Private, LAN Public, telco Public, telco
Frequency Ranges Unlicensed
2.5GHz (802.11n, 14 - 40MHz channels, 3 max non-overlapping)
5GHz (802.11ac or 802.11n, 24 -  80 MHz channels, 23 max non-overlapping)(more bandwidth, less range)
Licensed
3G:  850, 1700, 1900, 2100 MHz
4G/LTE:  600, 700, 850, 1700, 1900, 2100, 2300, 2500 MHz
CBRS (Citizens band Radio Service, shared, unlicensed): 3600 MHz, 20MHz channels,
5G: Eur: 24-27GHz, US: 37-48GHz, 71-74GHz
Licensed
3G:  850, 1700, 1900, 2100 MHz
4G/LTE:  600, 700, 850, 1700, 1900, 2100, 2300, 2500 MHz
CBRS (Citizens band Radio Service, shared, unlicensed): 3600 MHz,
20MHz channels,
5G: Eur: 24-27GHz, US: 37-48GHz, 71-74GHz
Connects to: Internet Multiple telco carriers Single telco carrier
Mobility Log in to each new private system required, limited handoffs between WiFi systems or WiFi and cellular
Seamless handoffs Seamless handoffs subject to coverage
BYOD (bring your own device) OK OK Depends on service provider device connects to
Optimized for Data 3G: voice
4G/LTE/5G: data
3G: voice
4G/LTE/5G: data
Data: Max data rate: 802.11n: ~35-300Mb/s
802.11ac: ~400Mb/s - 7 Gb/s (MIMO)
4G/LTE: ~100Mb/s
5G: ~Gb/s (proposed)
4G/LTE: ~100Mb/s
5G: ~Gb/s (proposed)
Voice VoIP: good
Cellular on WiFi: not optimal, depends on device/carrier/implementation
Good with proper coverage Good with proper coverage
Video Good 4G/LTE: marginal, expensive
5G: Good (proposed), cost?
4G/LTE: marginal, expensive
5G: Good (proposed), cost?
Cabling (typical)
Fiber backbone to Cat 5, POE
Fiber, sometimes Cat 5
Fiber, sometimes Cat 5
Summary
Best for data on PCs, tablets, smartphones, good for VoIP systems, marginal on cellular devices
Best for cellular devices since can cover all service providers, not optimal for high throughput data (today, future 5G ?)
Good for cellular devices but can cover only one service provider, not optimal for high throughput data (today, future 5G ?)


What We Learned From Visiting A Wireless Conference

FOA  attended the WIA's Connect(X) conference in Charlotte, NC. This was the first wireless show we'd attended in over a year and the topics of conversation were similar to last year - 5G topped the list. We attended several tech sessions and our takeaway from one was the answer to an attendees question to a speaker: "When can we expect a standard for 5G." The answer was revealing: "5G is not a standard, 5G is a goal."

If you search the web for cellular standards, you will probably end up at a Wikipedia page called "Comparison of Mobile Phone Standards." It's an interesting history of the development of cellular systems. Nothing on that page refers to 5G, but there is a page on 5G that starts off saying "This article is about proposed next generation telecommunication standard. 5th-Generation Wireless Systems (abbreviated 5G) is a marketing term."

Generally we don't recommend using Wikipedia for technical information because it is too often edited for commercial bias, (that's why we created the FOA Guide,)  but in this case the candor is refreshing.

Zinwave  Ericsson DAS

As we toured the trade show exhibits, we did see something new, this "Standalone Small Cell" from Zinwave. What's notable, is that like a similar device we saw last year from Ericsson that saw at the IWCE wireless meeting and we reported on in the June 2017 FOA newsletter, it looks similar to a WiFi wireless access point including Gigabit Ethernet interfaces to standard Category-rated copper cabling. DAS, it seems is migrating to operating off Cat 6/Cat 6A in a structured cabling system. Since most offices need both cellular (small cell or DAS) and WiFi, this makes sense.

When we tried to find a link to this Zinwave device on the company website and could not find it, we found something even more interesting on a page called "Cellular As A Service": Unfortunately, carriers are no longer spending on in-building commercial cellular coverage in the way they used to. That means building owners—whether they are in commercial real estate, healthcare, hospitality, or the enterprise—are now having to find and fund the solution themselves, and it’s not easy. It’s difficult to budget for the kind of capital outlay needed to deploy an in-building connectivity solution.

This may indicate a movement to make indoor cellular more accessible using small cells replacing DAS. We've been told that DAS is a declining market because most of the large public areas like sports arenas and convention centers have been done. Enterprise DAS has not been as big but if small cells on LANs, similar to WiFi, becomes cost effective - and at least one person told us it would be - then we are looking at a change in enterprise networks.

Wireless At FOA: This addition of cellular wireless to WiFi and of course the usual fiber or copper Ethernet connectivity expected of a corporate network is something we've seen before, and it's the reason FOA expanded it's course offerings and certifications to include a general "Fiber For Wireless" programs. We now offer a free
"Fiber For Wireless" program on Fiber U, a curriculum for our schools to teach, and of course a page on the FOA Guide.

Recommended Reading: "Revealing Underlying Wi-Fi Problems with Ultrafast Broadband" by Adtran, a provider of equipment for networks.

Recommended Watching: A YouTube video on how the Mexican city San Miguel Allende installed a small cell/DAS system in a historic city. La ciudad de San MIguel de Allende se pone a la vanguardia en telecomunicaciones, al contar con un sistema de antenas distribuido conectado por fibra óptica. (In Spanish with English subtitles)


Bottom Line
  • DAS and WiFi will probably coexist indoors since they have different goals
  • Small cells are outdoors, being installed for 4G/LTE with expected upgrades in the future to 5G





Nanotrenching Failure In Louisville, KY

Google FIber tried a new way to install cable in Louisville, KY, that turned out to be a very expensive failure. Nanotrenching is what some call very shallow trenching for installing fiber optic cable - see the photo below - and filling with rubber cement. It did not work.

Google Fiber ending service in Louisville

Chris Otts, WDRB Louisville,
Feb 7, 2019

LOUISVILLE, Ky. (WDRB) – Google Fiber is leaving Louisville only about a year after it began offering its superfast Internet service to a few neighborhoods, citing problems with the method it used to build the network through shallow trenches in city streets.

The shut off will happen April 15, said Google Fiber, a unit of Silicon Valley tech giant Alphabet, in a blog post Thursday.

Google Fiber has served about a dozen cities, and Louisville is the first it has abandoned."

Shortly after Google announced Louisville as a possible location in 2015, the Metro Council passed a utility pole ordinance at Google’s behest, then spent $382,328 on outside lawyers to defend the ordinance in lawsuits from AT&T and the cable company now called Spectrum.

Mayor Greg Fischer said in early 2016 that Louisville’s landing Google Fiber was “huge signal to the world.”

Louisville’s public works department allowed Google Fiber to try a new approach to running fiber – cutting shallow trenches into the pavement of city streets to bury cables.

It led to a lot of problems, including sealant that popped out of the trenches and snaked over the roadways.


trx

Louisville street,
Copyright 2019 WDRB Media. Reproduced with permission.

It feels like you are using us for a science-fair experiment,” Greg Winn, an architect who lives on Boulevard Napolean, told Google Fiber representatives during a Belknap Neighborhood Association meeting last year. “…Our streets look awful.”

Google Fiber would go on to fill in the trenches with asphalt, what company executives said was like filling a 60-mile long pothole.

Google Fiber never ended up using the utility pole law -- a policy called One Touch Make Ready -- that Louisville passed at its behest, as the company only buried its wires instead of attaching them to poles.

A public relations representative for Google Fiber said no one was available for an interview.

In written responses, the spokesman said Google Fiber initially chose not to use the utility pole access because of "uncertainty" about whether the ordinance would hold up. Now that it has cut trenches in the streets, the company has no desire to start over.

Even using (One Touch Make Ready), we’d need to start from scratch, and that’s just not feasible as a business decision," the spokesman said.


FOA: Be sure to watch the video from WDRB.

Copyright 2019 WDRB Media. Reproduced with permission.



Bottom Line:
  • Before you try some new idea, ask some experienced installers what they think


FCC Adopts One Touch Make Ready (OTMR) Rules For Utility Poles

On August 3, The US Federal Communications Communications Commission adopted a new rule that allows "one-touch make-ready" (OTMR) for the attachment of new aerial cables to utility poles. From the FCC explanation of the rule, "the new attacher (sic) may opt to perform all work to prepare a pole for a new attachment. OTMR should accelerate broadband deployment and reduce costs by allowing the party with the strongest incentive to prepare the pole to efficiently perform the work itself."

You may remember that FOA has reported on the "Pole Wars" for several years. Battles over making poles available and/or ready for additional cable installation has been slowing broadband installations for years and now threatens upgrading cellular service to small cells and 5G in many areas.

Is OTMR A Good Idea?

OTMR has the potential to speed deployment of new communications networks if handled properly. However, one hopes the installers doing OTMR know what they are doing. We've heard so many horror stories about botched installations, cut fiber and power cables, punctured water mains and gas lines done by inept contractors that we just hope this doesn't cause even more trouble.

For example, here are 2 poles in the LA area where small cells are being installed. Can just any contractor handle OTMR on these poles?

Pole wars  figure 8 on a pole

Bottom Line
  • OTMR may be problematic if contractors doing installation are not competent

Data Centers

Data Center Connections - 40G Is Obsolete Already


40G

Bottom Line
  • Data centers have moved on - 100G is now standard, 200/400G + is starting to become accepted



Which Fiber For Data Centers?

From The Leviton Blog:

The Market Has Spoken: OM4 (MMF), OS2 (SMF) Leave No Place for Unproven OM5 (MMF)

Typically, industry standards and associations set the stage for the next-generation of cabling and infrastructure that support network communications. But there are instances when the market decides to take a different route. This is currently the case with the recently standardized OM5 fiber. Even though TIA developed a standard for OM5 (TIA-492AAAE), this new fiber type very likely won’t see wide industry adoption because there is no current or planned application that requires it.

Due to new transceiver launches, coupled with customer perception of their needs and network requirements, the market is ignoring the new, unproven OM5 cable and sticking with proven solutions like OM4 and single-mode fiber.

This trend is supported by a recent Leviton poll that found a significant jump in OS2 single-mode, compared to surveys from previous years.

poll

Some of the follow-up comments from the Leviton survey included responses about OM5:

“I do not believe that OM5 offers a real advantage, it’s mainly a marketing ploy by manufacturers.” — IT manager at a global financial company

“OM5 isn’t needed. There is no real place for it between OM4 and OS2.” — communications consultant

Thanks to CI&M for bringing this to our attention.
 

Bottom Line
  • OM5 may not become accepted because of lack of equipment manufacturer support and the almost total move to SM fiber in data centers




Fiber Optic Testing


Test Sources For Multimode Fiber Testing

One of our schools recently asked up for recommendations on test sources for multimode fiber, wondering if the sources should be a LED or laser. Multimode test sources are always LEDs and these sources should be always used with a mode conditioner, usually a mandrel wrap. See here. This is how all standards for testing multimode fiber require test sources.

Years ago, as systems got faster and LEDs were too slow at speeds above a few hundred Mb/s. Fortunately 850nm VCSELs were invented to provide the solution for faster transmitters. But VCSELs were not good for test sources. They had variable mode fill and modal noise, so testers continued using LEDs for test sources, but with mode conditioners like the mandrel wrap that filtered out higher order modes to simulate the mode fill of an ideal VCSEL

The bigger issue with MM fiber is whether to test at both 850 and 1300nm. In the past, we did both because there were systems that used 1300nm LEDs or Fabry-Perot lasers for sources because the fiber attenuation was lower at 1300nm than 850nm. As network speeds increased to 1Gb/s and above, bandwidth became the limiting factor for distance, not attenuation.  VCSELs only worked at 850nm and all systems in MM basically have been switched to 850nm VCSELs.

We also used to test at both wavelengths because if a fiber was stressed, the bending losses were higher at 1300nm, so you could determine if a fiber had problems with stress. But since MM fiber has all gone to bend-insensitive fiber, that no longer works and the need or reason to test at 1300nm went away. It has not been purged from all standards yet however.

To complicate things, standards say that you should not use bend-insensitive fiber for test cables (launch or receiver reference cables) because they modify modal distribution, but it’s a moot point - practically all MM fiber is bend-insensitive so you have no choice but to use it. And most links will have BI to BI connections that should be tested. But we checked with some technical contacts and there are no specifications for BI fiber mandrels as mode conditioners.

Best solution - 850 LED with a mode conditioner on non-BI fiber (if you can find it - see above).


Bottom Line

  • Multimode fiber needs testing with a 850nm LED source


Power Budgets and Loss Budgets

Not only was this topic a long discussion with our new instructors but it's a common question asked of the FOA - we received two inquiries on loss budgets in the last month alone. The confusion starts with the difference between a power budget and a loss budget, so we'll start there. and we'll include the points where we were stopped to explain things.


What's The Difference Between Power Budget And Loss Budget?
  • A power budget is the amount of loss the link electronics can tolerate - transmitter to receiver. You use this to compare to the cable plant link loss budget when designing a cable plant to ensure the link will work on the cable plant design.
  • The link loss budget is the estimated loss of the fiber optic cable plant including the loss of the fiber, splices and connections. You compare that to the power budget to ensure the link will work on the cable plant being designed, then again after installation to compare to test results.


Consider this diagram:
Fiber optic loss budgets
 
At the top of the diagram above is a fiber optic link with a transmitter connected to a cable plant with a patchcord. The cable plant has 1 intermediate connection and 1 splice plus, of course, "connectors" on each end which become "connections" when the transmitter and receiver patchcords are connected. At the receiver end, a patchcord connects the cable plant to the receiver.

Definition: Connection: A connector is the hardware attached to the end of a fiber which allows it to be connected to another fiber or a transmitter or receiver. When two connectors are mated to join two fibers, usually requiring a mating adapter, it is called a connection. Connections have losses - connectors do not.

Below the drawing of the fiber optic link is a graph of the power in the link over the length of the link.  The vertical scale (Y) is optical power at the distance from the transmitter shown in the horizontal (X) scale. As optical signal from the transmitter travels down the fiber, the fiber attenuation and losses in connections and splice reduces the power as shown in the green graph of the power.

Comment: That looks like an OTDR trace. Of course. The OTDR sends a test pulse down the fiber and backscatter allows the OTDR to convert that into a snapshot of what happens to a pulse going down the fiber. The power in the test pulse is diminished by the attenuation of the fiber and the loss in connectors and splices. In our drawing, we don't see reflectance peaks but that additional loss is included in the loss of the connector.

Power Budget: On the left side of the graph, we show the power coupled from the transmitter into its patchcord, measured at point #1 and the attenuated signal at the end of the patchcord connected to the receiver shown at point #2. We also show the receiver sensitivity, the minimum power required for the transmitter and receiver to send error-free data. The difference between the transmitter output and the receiver sensitivity is the Power Budget. Expressed in dB, the power budget is the amount of loss the link can tolerate and still work properly -
to send error-free data.

Link Loss: The difference between the
transmitter output (point #1) and the receiver power at its input (point #2) is the actual loss of the cable plant experienced by the fiber optic data link.

Comment: That sounds like what was called "insertion loss" with a test source and power meter. Exactly! Replace "transmitter" with test source, "receiver" with power meter and "patchcords" with reference test cables and you have the diagram for insertion loss testing which we do on every cable.

The loss of the cable plant is what we estimate when we calculate a "Link Loss Budget" for the cable plant, adding up losses due to fiber attenuation, splice losses and connector losses. And sometimes we add splitters or other passive devices.

Margin: The margin of a link is the difference between the Power Budget and the Loss of the cable plant.




Determining The Power Budget  For A Link

Question: How is the power budget determined? Well, you test the link under operating conditions and insert loss while watching the data transmission quality. The test setup is like this:

Measuring fiber optic link power budget
Connect the transmitter and receiver with patchcords to a variable attenuator. Increase attenuation until you see the link has a high bit-error rate (BER  for digital links) or poor signal-to-noise ratio (SNR for analog links). By measuring the output of the transmitter patchcord (point #1) and the output of the receiver patchcord (point #2), you can determine the maximum loss of the link  and the maximum power the receiver can
tolerate.


From this test you can generate a graph that looks like this:
fiber optic BER
A receiver must have enough power to have a low BER (or high SNR, the inverse of BER) but not so much it overloads and signal distortion affects transmission. We show it as a function of receiver power here but knowing transmitter output, this curve can be translated to loss - you need low enough loss in the cable plant to have good transmission but with low loss the receiver may overload, so you add an attenuator at the receiver to get the loss up to an acceptable level.

You must realize that not all transmitters have the same power output nor do receivers have the same sensitivity, so you test several (often many) to get an idea of the variability of the devices. Depending on the point of view of the manufacturer, you generally error on the conservative side so that your likelihood of providing a customer with a pair of devices that do not work is low. It's easier that way.





Installation - Cleaning

Bad Advice

Our inbox recently had a message with this thought:

"It is time for spring cleaning, and we don't mean just at home. Contaminated fiber end faces remain the number one cause of fiber related problems and test failures. With more stringent loss budgets, higher data speeds and new multifiber connectors, proactively inspecting and cleaning will help you ensure network uptime, performance, and reliability. Despite "everyone" knowing this, fiber contamination and cleaning generates a lot of failed test results."

Well, experience tells us that "proactively inspecting and cleaning" can generate a lot of damage to operating fiber optic networks.

Inspection and cleaning should be done whenever a fiber optic connection is opened or made, of course. But the act of opening the connection exposes it to airborne dirt and the possibility of damage if the tech is not experienced in proper cleaning. Fiber optic connections are well sealed and if they are clean when connected, they will not get dirty sitting there. Fiber optic connections do not accumulate unseen dirt like under your bed or sofa, requiring periodic cleaning, as implied in this email.

Clean 'em, inspect 'em to ensure proper cleaning, connect 'em and LEAVE THEM ALONE!!!

And, duh, remember to put dust caps on connectors AND receptacles on patch panels when no connections are made

Was this perhaps another early April Fools' joke...like this one we ran several years ago about the wrong way to clean connectors:


Connector cleaning - NOT!


Why You Clean Connectors Before You Make Connections

Brian Teague of Microcare/Sticklers send us this series of photos showing what happens when you make connections with dirty connectors. It speak for itself!

Dirt on fiber optic connectors




How To Backfill A Trench For Underground Construction

backfill a trench

Here's the answer to a question we've gotten. Where did we find the answer? In the new FOA Guide section on OSP Construction developed using Joe Botha's OSP Construction Guide which is published by the FOA. Joe's book covers underground and aerial installation from a construction point of view, covering material after the FOA's design material and before you get into the FOA's information on splicing, termination and testing.

DO NOT FORGET THE MARKER TAPE! It makes the cable easy to locate and hopefully prevent a dig-up.

The 2019 update of the FOA Reference Guide To Outside Plant Fiber Optics contains this and lots of other new material on OSP construction.





Safety On The Job

bucket truck job  

Safety is the most important part of any job. Installers need to understand the safety issues to be safe. An excellent guide to analyzing job hazards is from OSHA, the US Occupational Safety and Health Administration. Here is a link to their guide for job hazard analysis.

Why We Warn You To Be Careful About Fiber Shards

Fiber in Finger

Photo courtesy  Brian Brandstetter,  Mississauga Training Consultants
1-844-440-0047
www.fiberoptictraining.com







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