I recently was able to sit in on a presentation that was given by the Arlington County PD on their response to the attack on the Pentagon. The whole thing was very interesting, but something I wanted to mention/ask about here is about cell phones.

It has been well documented that cell phones in their standard mode were near useless on 9/11 in both NY and the DC/MD/VA area. ACPD uses Nextels, and although the actual 'phone' part didn't work, the two-way radio feature did. Also, privately owned phones that didn't work in their normal mode sent and received text messages just fine.

I thought this was very interesting and I was hoping someone who knows more about these things could tell me why one mode would work while the other wouldn't.


Thanks in advance,

Chris.

Also for your info AOL two-way instant messenger pagers worked fine on 9/11.


I was told since nextel two way is on totally different network than cellurar it takes longer to get it knocked out. In NYC also WTC also had antenas and repeaters for cellphones (Verizon for sure) and when towers went down signal was lost.

My regular nextel didn't work during blackout but two way feature did.

If you ever learned anything about the TCP/IP protocol, you will have heard of 'connectionless' vs 'connection oriented' protocols. A connectionless protocol is like a note tied to a balloon and released: it's just thrown out there and you assume it will reach someone, but there's no subsystem between sender and receiver to make sure contact is established. A land-line call is representative of connection-oriented communications: both parties can confirm that the other guy is there, and if one party disappears, the loss becomes evident to the other.

A regular cell call is connection-oriented, and while you're on that call, the cellular system 'real estate' is dedicated to you and only you. Since the resources assigned temporarily to you can't be shared by other users while your call is happening, the whole system can saturate.

A text message, and to a lesser extent a PTT conversation, is sent by means of a best-effort or connectionless method. Your text message is released into the system and may or may not come out the other end - the system doesn't do extensive checks to make sure both ends are engaged. Your text message may also wait in line while system resources are freed up. This can't be done with standard voice calls, unless the cellular carrier wants to sacrifice quality of service.

Furthermore, like any computer, the cellular system requires much less horsepower to send the seven characters "I AM SAFE" than it does to send a recording of a human voice saying those words. That's why the text of Moby Dick can fit on a floppy disk, but the book-on-tape would fill several CDs.

Blackberries worked on 9/11 in NYC as long as you weren't stuck underground on the subway. (All of the phone lines to NYC were jammed practically all day, so it didn't matter whether you were trying to use your mobile or a landline phone.) The explanation I was given -- similar to dogplasma's explanation -- was that the e-mails were broken up into packets and so didn't get tied up the way "regular" phone calls were.

So, I bought some RIMM stock the minute the stock markets opened again, and made a nice bit of money, but it turns out I sold that stock too #$%* soon!

A few things are going on here.
First of all, the data vs.voice bit is correct, it takes much less "overhead" to move a bit of text than it does to make a voice connection.
As far as the PTT (Push-to-Talk) feature of Nextel, this is in many ways similar to a text message, in that the Nextels actually digitize the voice and then can store-and-forward it when the network can slip it into the next open slot.
We use a similar system in the fire department, it's called the "Astro" system, and sometimes, you call county and hear your own voice a few seconds later (an eternity by computer standards) as the network was too busy to accept my voice packet, but it was stored and forwarded as network bandwidth cleared.
Also, Nextel tends to build their own infrastructure, with more exclusive antenna sites and so forth, so unlike Verizon or Cingular, which typically lease tower space from a 3rd party, and share physical infrastructure, Nextel has (in many places) it's own towers and antennas.

I used to have Nextel, and it's the phone of choice for emergency responders, but their coverage in this area is poor, and so I was forced to drop Nextel and go to Verizon. But I miss the "bee-beep" of the walkie talkie.