In an Emergency, Why Cellular Data Is Better Than Voice

By Craig Johnston on 12 Apr 2010 01:26 pm EDT

Cellular Network

Back on September 11th 2001, terrorists flew two fully fueled planes into the World Trade Center. The result was catastrophic and forced people to jump to their death before eventually both buildings collapsed. I was there that day to experience it first hand. Other than proving that humans have a lot more evolving to do, the chaos it caused made a lot more people than usual use their cell phones. Because too many people were trying to make calls, the cellular towers in the area of the World Trade Center became saturated and a large portion of the voice calls could not be placed successfully.

On that day I had a RIM 857 (a BlackBerry). Back in 2001, BlackBerry (RIM) had no phones and operated on dedicated data networks. The two that RIM devices used were the Mobitex network run by Bell South in the United States (which was later absorbed into AT&T Wireless), and the DataTAC network run by Motient in the United States. These networks were available in a few other countries including Canada and Australia although the RIM devices were only used in the US and Canada.

Because my BlackBerry operated on a completely isolated network, I was freely able to send and receive emails, while the cellular networks were completed jammed with voice calls. (In a related note, if you want to read my account - which is essentially the thread of emails I was sending and receiving on my BlackBerry in real-time that day - click here. Its kind of chilling to re-read it). Actually there were 2 reasons. The first was because of a separated network, but because the BlackBerry has always used a store and forward approach to data, even if there was congestion on that network, it would have simply kept retrying until that packet of data was sent. So on that day I was able to communicate with work colleagues, friends, and family, while others using regular cell phones were not.

The very next year, the first GSM BlackBerrys started shipping, the 5810 and 5820. Both had a phone, and sent data using the GSM cellular network, making use of a technology called GPRS (General Packet Radio Service). Many people worried that the huge benefit of using their BlackBerrys on 9/11 would be lost as the data part of BlackBerry moved to the same cellular network that carries voice calls. But as it turns out, it wasn't. Here is why.

Why does the data get through?

We recently saw that after the earthquake in Haiti, people were using BlackBerry Messenger to communicate, while others trying to make regular cell phone calls, could not get connected. Why would that be? Surely if a cell tower is not able to allow more calls, how can data get through? The answer lies in the way in which the cells handle voice and data.

Because radio spectrum is limited, cellular carriers had to come up with a way to share the smallest amount of radio spectrum with the largest number of simultaneous connections. There are 2 main ways to achieve this (there are others but these are just the most widely used).

Code Division Multiple Access (CDMA) takes a spectrum of bandwidth, and uses unique codes to identify each phone that connects. This means that all phones use the same frequency range (or bandwidth) at the same time, while the code they use when transmitting allows them to be distinguished (think all the people in a room talking at once, but in different languages).

Time Division Multiple Access (TDMA) is a method of sharing a spectrum of bandwidth by breaking it up into time slots. When a phone connects it is given a time slot. In a round robin fashion, each phone is then given a time to send a burst of data in its time slot (think all the people in a room taking turns to speak).

GSM, which is used by 75% of the world, uses TDMA mixed with another technology called FDMA (Frequency Division Multiple Access), while carriers like Verizon and Sprint in the US use CDMA. No matter what technology is used, the bandwidth is shared somehow.

When you make a voice call, your voice is converted to data. When your phone connects to a cell tower for a voice call, your phone needs to send data at a speed of around 13kbps for the outgoing audio, and receive data at that same speed for the incoming audio. This causes it (in the case of GSM) to take 2 time slots for the duration of the call. If the cell tower runs out of time slots, your call will not go through.

Regular data traffic however is different. This includes email, BlackBerry Messenger, web browsing, etc. The phone and cell tower handle data as the Internet does. It breaks it up into packets. When your phone needs to send an email, firstly the BlackBerry compresses it to half its size (so for example a 2K email [which by the way is a pretty long email] ends up at 1K before it is sent), but your phone then only needs 1 time slot for a very short period of time to actually send the email.

Imagine that a voice call is a constant 13kbps on 2 time slots. Your data is tiny in comparison, and takes only 1 time slot for the briefest of time. BlackBerry Messenger is even less data.

So in a large congested area, where many people are taking up time slots for extended periods of time, your BlackBerry is able to quickly zap out that data in between people hanging up and making the next call, or using leftover time slots. It is still possible to have absolutely no time slots available if there are many voice calls and many data devices using a cell tower and that's where the second feature comes into play, the store and forward.

Even if time slots are hard to come by, your BlackBerry will keep retrying to send that data. Eventually it will get it out, even if it takes so long that you get the red X and have to do a resend.

So the moral of the story is, if you find yourself in an emergency situation, try and use data (email, BBM, PIN), instead of a voice call to communicate. Not only will it allow you to communicate, but it will allow more people to communicate at once.

Topics: BBM Editorial

Reader comments

In an Emergency, Why Cellular Data Is Better Than Voice


What about sending an SMS? During Obama's Inauguration carriers said that sending an SMS is better than making calls, since they were trying to keep network congestion to a minimum. But SMS uses 1X (for CDMA) right?

Sending SMS messages are similarly, if not more, reliable because they are sent on the same signal that establishes the connection between your phone and the towers (if you have a signal, you should be able to send SMS messages). Also, SMS messages are stored on carrier severs, so your message should eventually be delivered, even in times of high traffic. Google SMS limitations and advantages.

Very helpful to know. I live in the US on the edge of the US/Mexico border. Just about 30 miles from where the 7.2 quake hit this past Easter. This helps to explain why I was able to sent and receive data, but very few phone calls right after the quake and the remainder of the day.

This is what a blog should also be able to offer. This is probably the best infotainment article I've seen on CB and its based on an event 8.5 years old! Way to make it relevant

Good job Craig

Back in the Mid 90's I did a High School report of mobile phones and found that the word "Cellular" or "Cell" is technically NOT correct since the Cellular technology is no longer used. Digital technology is now used for mobile phones. Is that different in the Blackberry data world somehow or is the word cellular just used because it is just what know like Kleenex, Xerox and JetSki? Just curious.

There isn't anything different, I think you got it right when you said the word "cellular" is used because it's what we know, like Kleenex, or Xerox. You don't go out go buy a digital phone, you're buying a cell phone... just the term it has.

Mobile Phone, not Cellular Phone. No matter what the technology evolves into in the future, it will always be "mobile".

Oh, and it is FOOTBALL, not soccer!

I agree. Even when bluetoothes r bio-intergrated into our ears it will still b considered a "mobile" device. Thx again Craig.

I live in Bergen County, NJ and knew many people that died that day. Reading this gave me goose bumps. Thanks for posting.

I explain to people how blackberry actually works since everyone and their mother come in looking for I-Phones.

RELIABILITY= Blackberry's

I use the 9-11 story all the time and it is very true. I really appreciate the breakdown of how it all works in this write up.

Crackberry = Knowledge!

What is being described here is not BlackBerry specific. Your BlackBerry uses the cellular network the same was that an iPhone, Android phone, dumb phone, etc. BlackBerry of course does its compression which always helps.

And of course your BlackBerry has BBM so as long as the RIM network remains online, you're in business.

im glad this was posted....if i had to think about myself in an emergency i would've most def tried to call someone as opposed to using data

This just makes me hate the wireless companies even more. It's absolutely rediculous that they charge extra for text messaging (one of the reasons I love BBM so much) Back when questioned about it by congress, the wireless industry claimed increased texting popularity put a "strain" on their networks, requiring them to charge more to improve their networks to handle the "extra" traffic. As it turns out, with this article, it proves Voice transmission strains their networks, and adding more customers requires them to improve their networks. Charging for texting is profiteering and infuriates me!!!!!

My first phone on Celluar One had free texting, it just took so long to type out a message since no one had a querty keyboard and T9 programming wasn't out yet.

Compared to now the system for sms was rudimentary. SmS wasn't as popular n standard as it is now. Building this existing system needed funding. Who better than the ppl using the service to fund the amioleration of the system structure. I am a frugal person but I can understand things just don't run on the barter system. If the funds aren't there, they aren't there.

N speaking of funds I just wanna mention fees our cell provider charge us can be challenge if enough of us feels as though those fees are unreasonable.

I remember 1 year at after a very large sporting event at the rosebowl, the networks got jammed and the only way I could get any info out was to text people. Thanks for the great blog!

Reading this article just blew my mind away. I had no idea this is how all BlackBerry models work and it just makes me even prouder to be a BlackBerry owner. In the end its not how many apps your phone has but how it performs and manages your data efficiently. Good Job!! and thank you for the lesson! i will share this with all my friends in hopes to make them see how smart and efficient the BlackBerry device is!!

i knew that bbm and pins can easily sneak by durnig heavy loads but really didnt know exactly why. great info
thanks CB.

any idea about sms? (one time slot? two? kb?)

i'm assuming an adroid or iphone same for sending an email as bb, only bbm even less kb right?

Good article but don't you think that is a bit of an INSULTING comment to 911 families to say that because of people jumping...humans have more evolving to do.
I don't think it was meant to come off like that but if I wrote what you wrote, I would be happy if someone informed me how it sounded.

Or maybe i'm over reacting. what do you guys think?

I don't think it was about the jumpers ... it's the fact that humans were responsible for such a catastrophe in the first place.

Those statements were in two different sentences. He says that the CHAOS proved human beings have more evolving to do... and hell as far as I'm concerned just the fact that someone planned these attacks also proves we have a long way to go. I don't think he was referring ONLY to people jumping out of windows... I think we can all understand why some people did that.

The result was catastrophic and **forced people to jump to their death** before eventually both buildings collapsed. I was there that day to experience it first hand. **Other than proving that humans have a lot more evolving to do**, the chaos it caused made a lot more people than usual use their cell phones.

It didn't come off sounding like that at all. It is in fact two separate sentences and two separate thoughts.

lol at people still thinking it was a bunch of terrorists with box cutters, high-jacking planes and crashing them into two buildings. It is true that humans have more evolving to due, apparently we are still spoon-fed information and have not developed the common sense in order to figure things out. Hint: insurance policy/get rid of expensive building...i'll leave it at that

I didn't misconstrue what the intended statement meant. I guess its true. You can never expect 10 out of 10 to totally understand. Do you guys think as intelligent lifeforms we will ever evolve into a peaceful species? Like in the Star Trek Universe? And speaking of Star Trek (yes I'm a trekkie, TNG thk you) they were a military entity yet they had abolished war and terrorism. Things that make you say "hmmmm" Pat!

I can say with confidence that the evolution part was meant for the assholes who crashed the planes into the building moron.

I worked at RIM back then. In the aftermath, the news articles about the BlackBerry availability during the crisis were circulated within the company (note: in those days, as was any BB news article; there were relatively few compared to the years that followed!)

While we were certainly proud that BBs proved helpful and were able to bring comfort to some, it was made very clear that we should not go around trumpeting that. Obviously would be in poor taste to take advantage of the situation.

I was just explaining to a friend of mine the same thing over the weekend but not as scientifically thorough as your article Sir.I will forward the second section to him so he can see that I wasn't full of crap. I do have one question does the Android have something similar built in as BBM ? Im due for renewal and haven't made up my mind between a Storm 2 or an Android.

I went to the Storm2 after a year of using the iphone. I love the Storm2.
You won't regret getting the Storm2... except maybe when Storm3 arrives this fall :)

It was the same on Saturday here in Poland,after the president's plane crashed. For a few hours I was unable to place a phone call(my blackberry bold displayed "congestion" and that was it), but I could send messages on bbm without any problems. Also I could send and receive emails in real-time.

Does it still have the ability to display profile pics and what ur lissening too as well? Just curious as to the capabilities of bbm. I'm loving bbm more n more the more I use it.

Just a little fyi, the direct connect network of Nextel was getting through also while the phone side was jammed. I was at the Pentagon that day.

N when I saw the Towers get hit I really thought it was a MOViE! Then I saw the Pentagon was hit n called my family who worked tht day their. They was evacuated n walking down Route 66. Very scary. I was all for the war then. It happened on American soil. And that's a MAJOR No-No. We as Americans have a superior sense of national pride. We feel if any 1 wrong us it only will be US. No 1 else will be tolerated.

Yeah my ROTC instructor retired after 20 or so years from the pentagon and he was teaching a class when I saw the footage. I felt the urge to break him the bad news and it was like he saw a ghost. You are correct also by saying it changed your outlook on war.
As bad as it sounds I think every American generation needs a catostrophic event to really gauge how precious a human life really is. Especially if it is an innocent life.

Another reason for me to justify staying with RIM and upgrading my 8130 pearl to the bold 9650.

Interesting to read an article on here that doesn't have people trolling the comments for a change.

One slight correction, I don't know when the 5810 was released in the states, but the 5820 was certainly launched by o2 in the uk (when they were still known as bt cellnet) towards the end of 2001 and might have even been out by september 11th although I think they arrived shortly after that.

The term cellular refers to the fact that the device communicates with local towers which have a limited range or "cell" that they cover. When you move out of the range of that cell, you call is "handed off" to another cell.

This is in contrast to something like terrestrial radio/TV or satellite broadcasts, which typically are broadcast by a single antenna to the whole coverage area. (with some exceptions these days where you have a "repeater" to fill in poor-coverage areas)

This article is by far the best (not that others were bad) in terms of information.
Simple language yet effective!
Hats off to Craig!
Well done mate.
Keep up the good work "The Crack-Team"

Man reading your 9/11 story gave me the chills. I used to work for RBC at 1 Liberty Plaza in 2005. Looking at ground Zero everyday from my office window was sad and depressing. My uncle used to work in Tower 2 and my cousin worked at a Starbucks at the WTC. Its crazy to think all this destruction went on where i worked. In reality I or any of my family members could have been killed that day. Especially my uncle who lost many friends. He constantly worked from the WTC for most of his years in New York. Only missing 9/11 by a few months because he took another job offer. With another banking firm in lower Manhattan. Goose Bumps,. i know that area very well

This is absolutely crazy to see, but I am glad everything went ok for you! If having a blackberry can keep in contact where everyone else will have issues I am so happy to have one incase of an emergency.

I genuinely enjoyed reading this. Even though I was 10 at the time, only remembering watching the footage in that afternoon (I'm in Europe) still brings chills to my spine.

Truly another reason to stick to a BB than the other latest models of the gadget phones. A boon to mankind by RIM

Wow that brings back so many emotions from that horrible day. Thank you very much for sharing, and educating me at the same time.

Never forget 9/11!

Thanks for the info. I was in an earthquake a while back and found I was unable to call or text but emails were getting through just fine. I always tell people if there is an emergency, I'll email my location. They can call for help.

I was with Ladder Co. 3 in the North Tower. I remember not being able to get a hold of my wife for hours. I had a Nokia 7500 series back then.

Yeah, from Jersey had a few friends that worked in the area luckily one of them got a call out to another friend outside of the area and she contacted the rest of us. Of course for me the problem would have been almost no one I knew at the time had 24/7 email, but good to know for now. Thanks for the post.

Hi I read your emails. You should forward these emails to RIM. And RIM should be proud to have those. They should give this a wider publicity. It will help lot of people worldwide.

Thanks for the great tech info Craig!! Always learn something new at

Thanks for sharing your experiences and the emails as well...

And lucky for me, all my family members (except one of my brother, who will be getting the 9700 soon) and most of my colleges have blackberries :)

I just finished reading this book.... the story of the NFL players and two friends lost in the Gulf on a fishing trip. In recounting the story, the lone survivor says they tried repeatedly to dial 911 from an iphone and another unspecified phone, withoutluck. Would using email or BBM have worked 70 miles offshore? I doubt it, but just curious.

Very informative article. I worked for Bank of America during 9/11 and because of our Blackberry we were able to locate an employee whose tardiness saved his life!

The article helped me understand the true value of BB in digital world. Wow, it is a lot more powerful than I give credit for. Nice job.

I understand and agree with your comments. Disasters that don't touch the grid or the physical integrity of cell/radio towers are suited to the situation you mention. However in an earthquake or worse, power may be knocked out. If the power is not restored before the backup power (battery) for the cell towers dies then all are SOL. This is one of the reasons why landlines are still useful, providing everyone doesn't saturate the circuits...BTW I'm from CA and have been through these kinds of situations (earthquakes). You might want to look into what happened to cell towers during Katrina and how long service was out...

thank goodness for devices like these that can come in handy in such troubling times. this article has def served as an eye opener. im glad you were alright in that mess. i had family that worked in one of the buildings but didnt go to work that day. laziness......Thank God. But yeah BlackBerry for life!!

Thanks for the great info. I'll keep it in mind in the future. I text mostly anyway. Not many of my family have BB. They have yet to see the light. Also your e-mails on 9-11 was a riveting read. Sent chills up my spine.

thanks for all the cool info. I noticed the differences between a call and text years ago when I was on a POS flip phone lol when I only had one bar of signal I could still send a text to my Husband. How ever if I tried to call him with that same one bar it would cut out. Evidently my head has the ability to block all signal from my phone back then! ;-) lolol I got my first black berry curve a month ago and I am still trying to learn all the cool stuff on it. I will never go back to a flip phone again! lolol Doing some searches for themes and apps I found this site. Its a very cool and helpful site! I will be referring my friends to this site as they get Black Berrys. :-)

Wow I never knew how the actual cell technologies worked. Now I can see why gsm has been known for dropped calls more often then CDMA. Time slots is a weird thing to think about when hundreds of people are talking all at once. Lol

great writeup man. This one goes down as one of the most informative articles I've read in a long time.

Well its not the time slot model that causes dropped calls, its the hand-off between cell towers that can cause dropped calls. The CDMA networks have better hand-off technology than GSM.

The time slots work very well actually :-)

I always respect your opinion and thoughts on podcasts because this is the kind of stuff you talk about.

What do you know about SMS handling, it's size, and CDMA versus TDMA? Perhaps an explanation of circuit switched and packet switched? Is this how messages less than 160 and more than 160 characters are differentiated? Why does CDMA not allow multiple sequential SMS and how does a 3rd party program circumvent this? SMS data coding?

..reminds me when Katrina hit Louisiana, except that when almost all the networks (Sprint, Vzn, TMo, and ATT) were down, Nextel's iDen network was the only one working!


Excellent article! A few months ago there was a mayor earthquake in Chile, and the only way i was able to get in touch with friends and relatives there, was using BBerry Messenger.

best regards,

Those emails on 11 Septmeber are simply chilling and I could feel the fear and worry.

It was notable that you didnt appear as worried for yourself as those outside of the attack zone were.

I had always wondered why I could send and receive data and sms even when i had such a poor signal that voice was not possible.

Thanks for the great article.

I work for a carrier here in Canada and sometimes, I do get those clients who are very technically versed and this gives me a good insight and understanding how it all works. Thanks so much and I look forward to reading more.

What a great article My Dad and Brother were big ham radio buffs when I was growing up(way before anybody had even conceived the thought of cell phones. It was the only way of communicating, other than telegraph, in an emergency. This was back when phone numbers were 4 digits long.
Thanks for this useful information. Now if only we never have to use it.