Better to Switch Than Fight
Let’s say I have a 32-camera IP system. Let’s assume those cameras are running at their best quality settings, full 4CIF resolution, and 30 images per second. Let’s also assume that at those settings the camera is putting out two streams, a live stream at 2.2Mbps, and a recording stream at the same rate, for a total of about 4.4Mbps. Just to make it more realistic, let’s add some control data in there and make it an even 5Mbps from each camera.
So for an individual camera connection, is that 5Mbps going to make a 100Mbps connection breathe hard? Not at all, but what happens at the port where the NVR is connected to the switch?
Now, only one of the two streams coming from the camera is going to be sent directly to the NVR, so our math is simple, 2.2 X 32 = 70.4. Now if that NVR is plugged into a 100Mbps port, am I going to have to worry? That 70.4 is less than 100, right? Ah, here’s one of the danger spots.
A fact that’s known to most seasoned network guys but not very obvious to the rest of us is that just because a port says it’s rated for 100Mbps doesn’t actually mean you can push that much through it. Now, the numbers vary on this depending on who you talk to, but most network guys will say that a port shouldn’t be utilized beyond 40-60 percent of its capacity. Did you catch that? You can only fill 40-60 percent of that pipe!
So for a 100Mbps port, you can maybe put 40-60Mbps through it before you start seeing collisions and other errors. There are lots of factors for this, such as network overhead. Network overhead is additional data in each packet that network devices can see and use, but you don’t see as useful information (the payload) like a frame of video.
Going back to our example, 32 of those cameras sending a 2.2Mbps recording stream to our NVR gives us 70.4Mbps of throughput. I would worry about putting this on a 100Mbps port. But there is another consideration here, and that is playback. Recording is not the only function of a recording device. That device has to also send the video back out again on demand.
If I pull even a few of those cameras up for playback at the same time, I can easily push the 100Mb mark! So what is my solution? I would have to get a switch with at least one or two gigabit ports for my NVRs or trunk connections.
But again, as important as bandwidth is in the equation, it is not the only factor.
Principles of Packet Forwarding
Remember above when I mentioned that a switch’s primary function is forwarding packets from one device to another? How it does that forwarding is also a factor in how appropriate a switch is in an IP camera application. Other functions of a switch include making sure that the packet gets to the correct device, and that it makes it there intact.
Switches forward packets in two basic ways: store and forward and cut-through. Store and forward does just what it sounds like. The switch buffer (memory) will store a packet until the entire packet is received, make sure it is in one piece, and then forward it on. This is used where strong reliability and data integrity is needed, but it does one thing that makes it inappropriate for use with video. That is add latency, or delay. When you are streaming live, real-time video, you want as little latency as possible. Latency in a video stream causes packet loss and packets to arrive out of order, which makes for a very unusable picture.
Cut-through switching, however, only holds the packet long enough to get the source and destination addresses, and then fires it out as fast as it can. This is what we want for sending video across the network. Whole books have been written on just packet forwar
forwarding. It is a serious subject for switch manufacturers because, for them, it’s all about
the speed.
Saving More for Another Day
We’ve covered two main considerations when picking switches for IP video applications. There are more than these, however. If your video system of choice relies on multicasting, then every switch in the network has to understand that language, or it will be a catastrophe. You need to understand the specific requirements of the product you are installing in order to know what hardware to use.
Can you just walk in to your local computer mega-mart and pull a switch off the shelf? For small systems, probably so; but for larger, more involved IP video systems you will be starting out with problems. And no one wants to do that. Pick carefully.
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