Convergence Channel: The Surveillance Storage Story
When designing video systems, we tend to focus on things such as network utilization or storage retention times, but what happens when I need to get video off of one of these devices? It is very inconvenient to pull a whole DVR out of service to take it to court.
Many people have no idea how much space digital video uses, or that the terms export and archive should not be used interchangeably. Let’s look at what’s involved with getting video off the chosen recording device.
Video Has a Voracious Appetite
How much space does recorded video take up? Take a recent example I was involved with: The application was observation of an interview room for test subjects. Each interview could last five to six hours. The two cameras recording the interview were set for full resolution (720 X 480) at 30 images per second. This rate generated a stream at about 2.2Mbps per camera.
At this rate, after converting megabits to megabytes, one camera alone would generate about 18.75MB per minute. This would give us 1.12GB per hour. At the end of the six-hour interview session, we would have more than 6.5GB of video on that camera. Multiply that by two, and you’ve now got 13GB of information. The customer was concerned after their efforts to export the video clip to a DVD kept failing! Of course, a single layer DVD only holds about 4GB.
Meet the Media
So, what are some of our options for exporting the video? Obviously a standard DVD is out. What about a dual-layer DVD? Well, unfortunately, even it only holds a little more than 8GB. That would have worked if we had only one camera in the application.
The newest optical media is Blu-Ray. With a single-layer capacity of 25GB and dual-layer storage of 50GB, it would definitely fit the bill. However, not a lot of devices in our industry take advantage of it yet. While it would be possible to use an external drive, that could be cumbersome.
Another technology gaining widespread acceptance is the USB flash drive, which currently holds up to 16GB. At less than $100, it is an amazing amount of storage for the price. Remember when storage was a dollar per megabyte? I still have a 300MB external hard drive (about 12 inches X 12 inches X 2 inches in size) that cost me $300. My, how times have changed! A flash drive is a convenient way to export small- to medium-sized video clips.
Exporting Vs. Archiving?
Notice how I used the term “export” in that last sentence. The interview room application I mentioned pushes the limits of the term “exporting.” Most clips in a security application last minutes, not hours (with some exceptions, of course). In some cases though, a customer may need to store a large amount of video (like all of it) for a long time.
Some markets such as corrections and healthcare are seeing more internal regulations for security video storage and retention times. It is not uncommon for these industries to request a year or more of storage. With most customers, we would explain that an incident can be exported and stored on removable media for as long as they need it, and anything else, where nothing happened, could be overwritten. Not so with these industries. They need to keep everything, for a long time.
Even in other industries, new government regulations such as Sarbanes-Oxley are making companies rethink data retention needs. Security video is being rapidly swept up in this new paradigm. When a customer needs to take all of the video off their recorder, and move it to a larger storage array, we call this archiving, as opposed to exporting. This is a time-, bandwidth- and storage-consuming process.
Storing Voluminous Video
There are two main ways to move large amounts of archival video: a direct small computer systems interface (SCSI) connection or via a network. For years SCSI was the standard, but as with all things swept up in the Convergence Wave, the network is taking over.
Most mainstream DVRs today allow archiving of video over the network. This can be done by mapping a large storage array or network-attached storage (NAS) device as a network drive on a DVR, or by providing an iSCSI (Internet SCSI) connection directly to a storage area network (SAN).
A NAS connects to a device using a file- and folder-based interface, much like the hard drives in your computer. A SAN makes a connection at a much lower level, sending the data in blocks directly across the network. A SAN tends to be used for much larger amounts of storage than a NAS, which is usually a single box.
Either way, if a network is used to archive video, care must be taken to calculate the bandwidth available and the time it will take to move the given amount of data. Going back to the numbers presented in our first example, each of those cameras would give us 26.88GB per day (assuming continuous recording in a security application). If there were 16 cameras at that location, you would collect a whopping 430GB per day! Now I know some of you out there see that as a drop in the bucket for some of the systems you work on. Just imagine a casino with 5,100 cameras.
So, if I had to move 430GB of raw video from one box to another per day, what size pipe would I need? A 1.5Mbps T1 connection won’t do it. Neither would a 10MB network connection. It would take hours on a 100MB network port, and be relatively manageable over a gigabit connection. As mentioned many times in this column, all these calculations need to be considered during the design phase of any decent-sized system, if success is to be achieved.
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