Wi-Fi VoIP At 80 mph 142
fredo123 writes "Almost faster than a speeding bullet.
As reported in Muniwireless minutes ago, RoamAD and WI-VOD have tested mobile VOIP over Wi-Fi at over 130 Km/h over an 8km stretch of Interstate highway somewhere near the Mexican border. Gee... I wonder what this is for?" No need to guess: according to the MuniWireless link, "the network is for public safety personnel (police, fire, ambulance and border patrol) first, with various community agencies, schools, business and local residents being added as the deployment expands beyond its targeted coverage areas."
Security? (Score:4, Interesting)
-nB
How Fast? (Score:5, Interesting)
Public safety? (Score:5, Interesting)
I don't know what the rage over VoIP is -- the telephone system has worked for many, many years. We're just opening ourselves up for another avenue of attack. Can anyone say terrorists with WiFi blockers?
eh (Score:2, Interesting)
Folks are doign it at 30k feet too (Score:4, Interesting)
Three letters Q.o.S (Score:3, Interesting)
In cell phone network _maybe_ something like this is possible, but it would not be that easy to adjust in real time, I'd guess...
A friend of mine told me that when he was stuck in really bad traffic on I5 (he used to commute LA to San Diego) his cellphone was almost useless exactly because everyone else was also trying to call home...
Paul B.
Cops and Cell Phones (Score:5, Interesting)
In response to your statement about giving cops cell phones instead of a WiFi VoIP based solution. I'd like to add my opinion to the argument. Everyone ready?! It is time once again for my Bullshit Theory of the Day . (Patent Pending, of course.)
Let's review what the police already use to their job. Every officer where I live, be they local, county, or state, has a laptop in their car. Their radio system is trunked and the laptop receives information from the station as well as offering multiple channels for voice communication to dispatch. At this time the technology, though having been in use for a while, is still somewhat proprietary and thus is expensive. A station must buy the trunking hardware to digitize, and mux traffic, then transmit that into the ether where it is picked up by everyone.
Let's review that last word. Everyone. Where I live it is illegal to have a police scanner in a moving vehicle. (Technically, during transport ie you just bought it, the scanner must be in the trunk.) There really isn't anything to keep normal people, as well as criminals, from listening to communications. At best, the consumer scanners don't have the proper computer communication from headquarters and most sometimes can't follow a full conversation. (The trunks switch every mic key release, and the "computer" channels change every couple of days.) But you typically can hear what you need to in order to know where your friendly neighbor Officer Mitchell is doing his job.
Also, pushing information like that through the ether can be hit or miss in rural communities. You have to remember, that the curvature of the Earth dictates distance for RF travel. Typically 70 miles before you hit the ground itself, unless you get the signal on a high tower. However, the trunk receiver on the cars can't be equally as high (and I'm starting to wondering if satellites are not getting involved. The trunk receivers now look like XM antennas) anyway, I digress. This means, technically, that unless you are bouncing the signal to orbit and back you can not talk to a field agent that is over 70-ish miles from home base.
Enter tomorrows technology today. Setting up WiFi that allows vehicle transmission to push VoIP so that as long as you have an internet link, you can communicate with dispatch. This will not be limited to voice. The laptops the officers use to get information about plates and criminals will also switch to this WiFi based system, and for the Law Enforcement Pointy Haired Bosses, here comes the best part. PGP type encryption for PTP tunnel building so that the information between agent and base is "secure". Technically, it would take someone long enough to get the encrypt key, even if it's measured in minutes, to keep from knowing exactly when and where officer movement is occuring real time.
The funny thing is that I used to do tech support for Motorola, and they have a wireless networking technology that is pretty cool. We also did tech for their international customers, and had this one crazy chick from China continuously calling. Had to be two or three times a week, for about four months. Asking all kinds of technical and really out there questions about the system, and why the system didn't work. We puzzled through it and finally got an interpreter involved and found out she had these things on *trains* Apparently Asian WiFi has already been doing this moving hand off for a while now, at least experimentally. The Chinese chick couldn't understand that this product was like ethernet cabling, without the cable. Had to be aimed and left. So the control center kept losing, and then regaining, contact to trains on board systems. So people want this to work, for a variety of reasons.
I can't even begin to tell you how often I look and listen to what is going on without thinking to myself, "My God, we're in a badly ghostwritten William Shatner novel." ... or any other post apocalyptic work that envisions the future of the world with computers in our head. Ever hear of Masamune Shirow? I'm starting to think that dude is dead on about what's coming in the next 50 years.
Well after signal strengh abates (Score:3, Interesting)
If everybody had a nice high-gain antenna on their roofs this would seem practical, but the little linksys dipoles aren't meant for and don't cut it for MAN'ing.
Raleigh Fading (Score:5, Interesting)
The big problem with mobile radio sysems (particularly in urban environments) is Raleigh Fading, otherwise known as "picket fencing" noise. What happens is that one receives the radio signal via multiple paths, reflected from buildings in the "urban jungle". Sometimes these signals interfere constructively, and sometimes destructively. When driving, in an urban environment, one tends to move from areas of constructive to destructive interferance and back again, on a surprisingly regular basis. The effect is called "Raleigh Fading", after the statistical distribution of constructive and destructive zones. On an analog voice radio channel, it sounds like someone running a stick past a picket fence, hence "picket fencing noise". Of course, in environments with less opportunities for radio signal reflections, the effect is less predictible, but it still happens.
Naturally, transmitting and receiving a checksummed packet while driving through one of the areas of destructive interferance is, well, a challenge. If the non-acknowlegement retransmission rate, and speed are just so, you'll never get a packet through.
There are two ways of dealing with this: spacial diversity antennas (multiple antennas separated at carefully computed distances so that one is always in an area of constructive interferance when the other is in an area of destructive interferance), and interleaved error correcting codes. The spacial diversity antennas work well at the higher VHF and greater frequencies, because the distance between individual antennas isn't all that great. However, at frequencies of around 150 Mhz and lower, the required distance between individual antennas is too great to allow for automobile mounting. So, one uses interleaved error correcting codes (generally Reed Solomon), and hopes that one travels between zones of constructive and destructive interferance "fast enough". Yes, there is a mimumum driving speed related to data rate, carrier frequency, and error correcting code and interleave chosen, below which the system would not work. One generally picks an error correcting code so that the minimum speed is low enough that it would be practical to stop in an area of constructive interferance.
As I recall, at least one rural police force in Quebec, Canada was outfitted with the equipment we produced. Needless to say, the fade rate was not a problem when "Enos" (well, Jean-Guy in the Quebecois version of "Dukes of Hazzard") was in in "hot pursuit".
No, we did not interface the modem to the cruise control to ensure the vehicle was moving "fast enough", though it was damn tempting...
Of course, at modern data rates and carrier frequencies, spacial diversity antennas are a far better choice to combat this problem (and why wireless data network interfaces usually have two antennas).
Re:Public safety? (Score:3, Interesting)
This I can answer:
The office I work at has 3 locations ( soon to be four ) in wildly different area codes. We are getting larger, so we want to make our very own call center in one of the offices ( dr's office ), so the other three can simply focus on the patient.
Using traditional methods, this would require all sorts of funky telco stuff, and a few 1800 numbers. Not even going to mention the shit from a phone supplier.
However, using voip ( asterisk specifically ), I was able to build this for them with about a 30g budget. That includes the 4th office. And tons of redunancy. All calls from the different locations get routed ( over our vpn ) to a central office which then handles them efficiently. And it's completely transparent to the patient.
A similar system from a phone companie ( *cough*avaya*cough* ) would run you will into the 100s of thousands. And it wouldn't be an open standard system. And it wouldn't have most of the features that this system does.
Put simply; voip is a way to package and control a phone call. One I have the call, I can do whatever I want with it. The possibilities are staggering.
Are they smoking crack? (Score:3, Interesting)
The network described is best effort service with no built-in QoS features. Yes, you can set the qos bit, but can't users do that same with custom voip software aswell?
I'd perfer my emergency calls routed through circuit switched network, since there's actually chance for them to get through in it.
And what's with this reinventing the wheel again?
TETRA is already existing standard for public safety communications, it still works at speeds of 200km/h, circuit switched, encrypted secure transfer medium by default, nationwide user groups, integraded ptt in devices etc etc.