Friday, July 10, 2009

Wireless Technology Part 2 - Mobile Wireless



First, for sake of clarification, what I mean by mobile wireless is this. Wireless devices mounted to cars, trucks and any other types of transportation used to transmit data, specifically video. Examples, video from an on board camera in a police car to the police station or video from a school to a mobile command vehicle.



I have seen a very large increase in interest for wireless download of video from police cars to the police station where a central video server is kept. On the surface this is a great use of wireless technology. In theory, an officer will pull into the police department’s parking lot with 8-12hours of fresh video and audio from his night shift. Once the car pulls in the on-bard video management system (VMS) links with the wireless network at the Police Department and automatically offloads the video to the server. Mean while the car is checked in, gassed up and prepped for the next shift. Once the day shift officer is ready, he jumps in the car and off he goes with a clean hard drive, ready for the next 12 hours of audio and video to be recorded. Great idea, great solution… right?



Let’s look at the math:
1 camera = 30FPS = 1800 FPM (frames per minute)

Continues recording, one 12 hr shift = 1,296,000 images per shift

Each image uses 5 kilobytes = 5000 bytes

5,000 bytes x 1,296,000 images = 6,480,000,000 bytes per shift

6,480,000,000 bytes = 6,480 Mb

6,480 Mb = 6.48 GB per car, per 12hr shift
- The mic will add 2Mb of data every minute. With two microphones in the car you will add 2,880Mb of audio = 2.88Gb

2.88 + 6.48 = 9.36Gb of audio and video per 12hr shift, per car

Note: transmission speed, or bandwidth, is measured in megabits per second, or Mbps, while our earlier calculation of file sizes were done using megabytes, or Mb.

1Mb = 8.3 Megabits

9,360Mb x 8.3 = 77,688 Megabits

Assuming perfect, test lab perfect environment (which is not reality) we can hope for 70Mbps.

70Mbps divided by 7 cars = 10Mps per car

Total time to upload 12hr shift = 2.2Hrs per car

The above numbers are only factored for a 7 car system. If your department has 30-40 cars, then the bandwidth is squeezed a lot more.

The short of this is, in reality this is not feasible.

The solution – there are many ways in which we can do this. First is lower the Frames per Second (FPS) of the cameras. In this application the customer wanted 30fps. One could cut the fps down to 7-3fps and save a lot. Second, have multiple receiving nodes on the building, allowing for multiple paths to the central server. This will eliminate the potential bottleneck and provide faster download time. Third option is to record video only on events, like when the lights are turned on, or when the officer hits a record button, this too would reduce the amount of video being transmitted.

Again, a properly designed system that fits the department’s needs could work. But under normal assumptions, wireless downloading of video from a vehicle is not too practical.

The other aspect of mobile wireless I want to address is one that no one, to my knowledge, has taken advantage of. With the increase in school shootings which has resulted in the increase of school security awareness there has been a lot of investment into schools security. But there is one small and very simple step that most schools are missing, cooperation with local law enforcement and emergency management. Yes, schools have basic cooperation in the form of liaison officers in the schools but what I am referring to here is another level.

Here it is… schools are putting in place advanced video surveillance and life safety systems. These systems offer better video quality and in some applications advanced video analytics. But these systems are still in the CCTV mind set. Let me explain, CCTV stands for Closed Circuit Television, which in its day was the way to go. But we live in the new IP world. Everything is on the network, eliminating the “closed circuit” part of video surveillance. But, with the implementation of IP and networkable video surveillance systems they are still being closed, not allowing for outside access. So… the answer is cooperation. By putting in place a node for a wireless mesh network in the school building and a node in a police car or mobile command vehicle, officers with the proper credentials will have the ability to access the video from within the school and be able to provide another level of support in the case of an actual event. This is a simple and a very low cost addition but one that is over looked way too often.

Yes there is a bit more to this than what I just laid out but the idea is there. By properly implementing wireless in mobile applications there are a lot of benefits. But in a lot of situations the system is either over sold or miss understood, resulting in less then desirable results. The key to this is to make sure that you are working with an integrator that understands wireless.

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