Attenuation in Fiber Communications Systems

I'm teaching a fiber optics communications course this semester and - like just about every communications course - we started out talking about attenuation.

Attenuation is just a fancy word for loss. In any communications system you've got a certain amount of signal strength going in and a certain amount of signal strength coming out. If there is no amplification in a system there is always going to be loss and the output signal will always be weaker than the input signal.

In fiber systems attenuation is caused by three things:

1. Absorption - Glass, whether it is fiber or the windows in your house, will always absorb a small amount of light going through it. The amount depends on the wavelength of light and what the glass is made of.
2. Scattering - Atoms in glass cause a certain amount of scattering of light and scattered light will not emerge at the output.
3. Leakage - Light will leak out of fiber, especially if the are a lot of bends in the fiber.

Fiber manufacturers typically provide specifications for all three of these, along with total attenuation per kilometer.

One of the primary goals in any communications system is to keep the attenuation to a minimum. Even so, there will always be a loss in signal intensity when comparing output power to input power. Calculating attenuation in a system is pretty simple. Attenuation is cumulative so basically you just add up the signal loss for each component in the system. Here's an example:

Question: A 50 km fiber run has been spec'd at 99% transmission per km. What percentage of light will emerge at the output?

Answer:

The fiber run is transmitting 99% per km so after the first km 99% of the input signal will be available, after the second km, 99% of what's left after the first km will be available, etc. So we can say:

60.5% of the original input signal strength will emerge at the output.

When Will Batteries Last A Week?

Like many of you, I'm constantly on the look-out for wall outlets. Meetings, airports, etc - if there is an outlet, I'm trying to find it. I've always fantasized about not having to carry around a charger. Wouldn't it be nice to get 40 hours out of a battery before having to recharge? It sure would! When will it happen? Lenovo has an interesting infogram out that take a look back and look forward when it comes to battery life technology.



Looks like we've got about ten years to go!

Memories of September 11, 2001

I first published this four years ago.

I was walking into the office when our technician told me a plane had hit one of the towers - he said "they think it was a small plane" and I did not think too much about it. 20 minutes or so later I was in a meeting and the same tech came in saying it was an airliner. We all left the meeting and turned on a small television in our lab. I also made sure I had a computer close by so I could watch email.......

At the time we were running a national listserv for a large group of community college faculty and administrators involved in a Working Connections grant with Microsoft and the American Association of Community Colleges (AACC). I've pulled out a few emails that came to the list. Here's one of the first from Mete at Borough of Manhattan Community College (BMCC):

11:50 AM, 9/11/01

It is chaos here, but everybody at BMCC is OK. We are closed for the day and the roads/subways to in/out of Manhattan are blocked. I am in Brooklyn (home) now and the sky is dark from smoke/ash/soot. I have a feeling it is going to take a long time to recover from this one.
Hope all is well with everyone around the country,

Mete

BMCC is on Chambers Street, next to ground zero and a college building was damaged from the attack. Mete was on the subway on his way in when the attack started and I believe he walked home to Brooklyn.

Here's a reply message from Lynn at the AACC in Washington, DC::

12:05AM, 9/11/01
We are ok here, but our office is closing so people can try to get home. The smoke from the Pentagon is visible from our 4th floor conference room. Most of the federal offices have now closed, a couple of subway stations near the Pentagon are closed, the streets are crowded with people driving and walking home from downtown offices, and cars with sirens go by every 5 minutes or so. Folks who live near Capitol Hill are sticking around the office until things calm down in that part of town.

Lynn

A flurry of emails went back and forth during the day from people all around the coutry. We were all worried, frustrated and upset about the attacks and our friends in New York, Washington and Pennsylvania. Here's a sample of the response from Paula at Richland College in Dallas:

11:50AM, 9/11/01
Thank you both for taking the time to provide us with an update of your safety. Our prayers are with you and all Americans during this tragedy. As in other states, thousands are donating blood. A major sports arena in Dallas has been setup as a blood donation facility. Churches are conducting special services. Please assist us to remain informed as to organizations/drives that are established that will provide direct support.

Paula

This came from Chris at the Fashion Institute of Technology (FIT) in New York City a few days later:

10:50AM, 9/14/01
FIT is safe and sound and open for business but few classes are running normally. It is hard to describe the experience of walking south on Manhattan's avenues and seeing a column of smoke where the World Trade Towers should be. When the wind shifts, the smell of the fire comes to Chelsea with a light dusting of the cement that is ankle deep a few blocks away.

We are glad to hear that all are well at BMCC and in Washington.

Chris

Here's a followup from Mete that was also sent on September 14:

12:36AM, 9/14/01

The building that we (CIS) dept was suppose to move this Sept., (but did not because of delays) is quite damaged. They are using our main building as command/triage/morgue center. We will be closed until the end of next week. The cleanup is going very slowly and there is very limited access to downtown Manhattan.

The subways are not running and all the outer borough are choked with traffic with people bringing their cars and parking them as close to Manhattan as possible. The air quality is bad, there is possibility that some more buildings (including our own that was next to a collapsed building) may come down aggravating the situation.

There are a number people that I know, with families, that perished in the bombings (we were going go to a 10th bday party this weekend, but the mother is missing - what do we do now ??) from my daughter's school and our neighborhood. But they are defiant, and most of the businesses try to operate as usual with a backdrop of surrealism...

Thanks for all of your e-mails and good wishes. We appreciate it and find comfort in them.

Mete

Hundreds of emails went back and forth over the next few weeks on the listserv. I've saved them all.

eReaders Becoming Niche Market Players?

I've always been a fan of the Kindle and other eReader type devices, especially when compared to bulkier and heavier tablets that are difficult to view in direct light conditions. Lately though, I've been wondering how long eReaders would continue to exist as tablets become lighter and smaller with better displays. Well, I just read an interesting study from ABI Research titled eReaders and the Digital Publishing Market that takes a look at where the eReader market is going.

Here's some details from the ABI report:

• Eleven million eReaders are projected to be shipped globally in 2012, down from a peak volume in 2011 of 15 million devices.
• The growing popularity of media tablets along with declining US “baby boomer” population and lack of organized digital bookstores outside of the US and Western Europe will reduce the eReader opportunity over the next five years.
• Despite the average tablet selling for more  than $465 as a result of Apple’s dominant market position, tablets are expected to outsell eReaders 9 to 1 this year. 
• Over the next five years, annual eReader shipments are projected to drop by a compound annual growth rate (CAGR) of 6.1%. In contrast, global media tablet shipments are predicted to increase from approximately 102 million annual device shipments in 2012 to nearly 250 million in 2017.

All bad news for eReaders. Well, not so fast.... the report continues... However, eReaders maintain advantages over media tablets for reading purposes. Electronic paper (ePaper) displays are able to better replicate the print reading experience and are usable in direct sunlight conditions unlike LCD technologies. The eReader battery life of weeks between charging is significantly greater than the media tablet. And of course, eReaders are priced significantly less than entry-level tablets.

ABI senior mobile devices analyst Joshua Flood is quoted “Regardless of the tremendous historical eReader success, the market tides have already begun to turn. Nevertheless, the eReader market will not be totally cannibalized by media tablets. We believe there will always be a niche market for the dedicated reading device for voracious readers, business travelers, and educational segments, particularly ones that are low-priced.”

ABI senior practice director Jeff Orr continues “The decline of buying audiences for dedicated digital readers in the US is more rapid than the digital publishing ecosystems organizing for growth in Asia or Eastern Europe. Development of content digitalization systems and services in all world regions should continue without delay as the effort will be necessary for developing mobile app catalogs that provide easy search, discovery, and monetization.”

Be sure to check out the full ABI Research report.

Curiosity on Mars Bringing Back Memories

The Mars landing of Curiosity has brought back some memories when it comes to sensors and systems in space.....

It was 1980. I was a year out of college with an undergrad degree in microbiology and working in a hospital lab trying to figure out what I wanted to do for the rest of my life. At one time I had thought I wanted to go to medical school but the more time I spent in a hospital working in a clinical setting the more I realized I was not cut out for that kind of work. I loved the biology but I was struggling with the procedures - the work was 100% protocol - the same procedures over and over again with zero room for mistakes. So many of the people I worked with were so good at it - they thrived on it.  Me - I had always loved tinkering, taking things apart and trying to put them back together. Trying new things that did not always work was what I liked. Trying this instead of that. What if I did this? What would happen?

But.... when you are dealing with people's lives you can't do that kind of stuff. I have so much respect and envy for people who do this kind of work but I knew it ultimately was not for me. I loved the science but knew I had to change directions with respect to my career. Fortunately, something came along (as it often does in life) that helped me with my dilema.

In the lab one day we started testing a machine called the VITEK for McDonnell Douglas and NASA. VITEK was a fully automated microbial identification and susceptibility system that had been developed in the 1960's for use in space. The system is based on microbial growth in thin plastic identification cards that have small wells with sugars, enzymes, etc. After the cards have been inoculated with sample they are incubated at 37 degrees C and photometrically scanned every hour for either color change or turbidity. Different bacteria ferment different sugars, produce different byproducts, grow or don't grow based on nutrients or conditions, etc. Basically how they grow or do not grow is how they are identified.

McDonnell Douglas was looking to move the product from outer space into the hospital lab and we had engineers coming in and out of the lab weekly tweaking the system. We were running tests in parallel - we'd run a sample using traditional microbiological ID methods and run the same sample through the VITEK, comparing identification results. There was about a three week period when the engineers were in lots - the system was not working properly - something was screwed up in the incubation cycle and they were having a hard time figuring it out.

One of the engineers I had got to know came in with a large stack of green-bar  paper with thousands of lines of code one day. I had never seen actual code before - it was so logical and simple and organized. Really cool! I was able to look at it and quickly pick out where the error was - the incubation line was not properly placed in the incubation cycle loop. I showed the engineer what code lines needed to be swapped around and remember him scratching his head and shrugging his shoulders. Still, he made the mod and....... it worked! It was so trivial but it worked!! The engineers were looking too deeply into the code - this was sort of like a TV not working because it was not plugged in type of thing. That simple. I remember the guy telling me I should have been an engineer. I also remember him telling me the first time someone writes a program it never works and you always had to go back and troubleshoot it. I was intrigued - engineering - could it be for a tinkerer like me who likes to learn from my mistakes?

Well - long story short - my sister and two brothers had degrees in electrical engineering. They had been bugging me for a while to check out engineering. They talked me into going to see Dr Jim Masi (the same Jim Masi who was our first Director at www.ictcenter.org) at Western New England University and we were able to work out an agreement where I could pick up the undergrad courses I was missing and then move into an MS Electrical Engineering program. It took a while and most of the time was incredibly hard but I was able to complete the degree.

As an engineer I could now tinker and try new (sometimes off the wall) things and make mistakes and learn from those mistakes. And that microbiology background has never gone to waste - I know it has helped me think and approach problems a little differently than traditionally trained engineers. All good!

The Future of Wireless: Spectrum and Cell Technologies

John Garvey from Garvey Communications dropped me an email last week and asked if I could write up a post about small cell wireless technologies. He had read a piece in Business Week titled Want to See the Future of Mobile Coverage? Go to a Baseball Game that describes how Verizon Wireless is using 4G LTE COWs (Cells On Wheels) and COLTs (Cells on Light Trucks) at sporting events. Both of these technologies provide portable bandwidth where and when it is needed. If you've ever been in a packed stadium and tried to make a call, upload a picture to Facebook, or post a tweet you've likely had problems getting a connection.

The more I thought the more I realized a good discussion will take a little more than a single post so I'm going to write a series on new antenna technologies over the next few weeks. Today let's discuss the way cellular is done now and why it's not going to work in the future.

Everyone know what a cell tower looks like and most would describe them as big, tall and ugly with lots of antennas hanging on them. If you look on the ground at the base of the towers you'll typically see a few "huts" that contain (among other things) large amplifiers that provide signal up to the antennas at the top of the towers. Connectivity to the towers (referred to as "backhaul") is provided by fiber. Here's a simplified drawing of a big tower setup.

The towers are fed by fiber and optical signals transmitted on the fiber get converted back and forth to wireless signals, giving us connectivity on our mobile devices. Seems pretty simple and if we need more capacity all we need to do is run more fiber to the towers, right? Well..... not really. Providers can bury as much fiber as they want and convert it to wireless frequencies but that's not going to solve the problem. The bottleneck today is spectrum and (especially) in congested areas this bottleneck is causing the dropped calls and slow wireless data access we've all experienced.

So, spectrum is a problem - what exactly is it? Each provider - AT&T, Verizon Wireless, etc - have licenses from the Federal Communication Commission (FCC) to use specific frequencies for information transfer. It is important each providers spectrum be allocated in a way that it is kept separate for interference free communications so different providers cannot typically share licensed spectrum. When I think about spectrum I think about tuning the radio in my car - each station is licensed by the FCC and has it's own "channel", broadcasting at a specific frequency.

Now - just like the numbers on your car radio tuner - there is only a finite amount of total spectrum available. This finite availability, along with not being organized in the most efficient way, limits things like coverage, connection speed and quality of service. The spectrum issue is really coming to a head now with high bandwidth LTE services being rolled out by all providers - higher speeds, more bandwidth requirements and more people using their phones for voice, video and data services.

What's being done by the wireless providers to get around this spectrum bottleneck? There's been a mad scramble to purchase as much spectrum as possible but..... utilizing a finite amount of spectrum the way it is currently being used is not going to solve the problem long term. They'll still end up running out. What's being done?

There are a number of solutions. We're also seeing providers building WiFi networks (WiFi uses unlicensed spectrum) to offload some of the licensed spectrum traffic. There's a bunch of other technologies including small cells (the COWS and COLTS John was asking about), spectrum re-farming and cell-splitting that look promising. I'll take a look at some of these in my next few posts.

Wireless Options: Virgin, Verizon, Sprint and CDMA

Virgin Mobile has announced they will soon be selling a pay-as-you-go iPhone4 and iPhone 4S, offering contract free service starting at $20 per month, moving all the way up to $50 per month for unlimited talk and data.

What's interesting about this is Virgin uses something called CDMA for channel access - that's the same technology used by Sprint and Verizon Wireless. And get this - Virgin is considered a virtual mobile network operator, purchasing network capacity from Sprint. This means when you use a Virgin phone, you're using the Sprint wireless network. Sprint service is just as good as AT&T or Verizon in my opinion. The latter two just do a lot more advertising to make us thing they are better.

So, let's think about this, Virgin uses the same access technology as Sprint and Verizon and Virgin's a lot cheaper. You're also not locked into a 2 year contract. Hmmmmm......switching sounds like a no brainer. There is a catch though - Virgin is charging $650 for a iPhone 4S - full price. It's still cheaper though over the long run. Let's compare Sprint's $80 per month unlimited data service (two year contract) to Virgin's $50 per month service (pay as you go).

I have not included Verizon Wireless in the table because Verizon is eliminating the unlimited data plans. If you've recently bought a Verizon iPhone 4S though you could switch by unlocking (not recommending you do this but many have or are considering) it and switching over to the Virgin network. To break your Verizon contract you would have to pay around $600 which sounds like a lot but it may be worth crunching some numbers to compare total cost.

If you've got an AT&T iPhone you're out of luck - it uses a different technology for channel access called GSM which is not CDMA compatible.