Convergence Channel: How to Avert Traffic Gridlock on the Network

October 31, 2010 SSI Staff

Megapixel cameras, Internet streaming, file transfers … what do all these things have in common? They take up large amounts of bandwidth on a network. To put it simply, bandwidth is the size of the pipe. Bandwidth tells you how much data you can push down that wire/fiber/wireless link.

I want to stop here briefly to clarify something that still seems to cause confusion. Bandwidth and throughput are not the same thing. As I just mentioned, bandwidth is the amount of data that a pipe can pass. Throughput is the amount of data actually passing through that pipe. Another way to look at throughput is as utilized bandwidth; in other words, the amount of space the data is taking up.

Now that that’s out of the way, let’s take a look at what insufficient bandwidth and excess bandwidth utilization can do to a network, and some ways to mitigate the damage.

Accounting for Data Throughput

Just like a garden hose and water, every copper or fiber cable has the capacity to carry only so much information. With copper (Cat-6, coax) that information is in the form of electric impulses. Fiber, however, carries light, and while it seems like the amount of light you can send should be unlimited, that’s just not the case.

Data throughput, measured in bits per second (bps), is an extremely important number in network design. Knowing how much information is passing through each segment of your infrastructure is critical to maintaining network uptime.

Let’s look at a couple of reference points so we’re all on the same page. A commercially available T-1 wide area network (WAN) connection carries 1.5Mbps (megabits per second) up and down. Most local area network (LAN) ports are now at least 100Mbps, although there are still many 10Mbps out there. Plus, many of them today are now becoming 1000Mbps, or 1Gbps (gigabits per second). Backbone connections between high-end switches in larger networks are starting to use 10Gbps connections.

Dealing with these larger numbers, it’s hard to see how a few IP cameras, even megapixel cameras, can throw a network into turmoil. But as we’ll soon see, it’s not all about the numbers.

Doing the Calculations

It’s vital to understand just how much data is flowing past each and every point in your network before you can understand how to control it.

In a regular IT network, throughput would be measured at each port with an edge device on it (PC, printer, etc.), but it would be especially important at servers and storage devices as multiple other nodes could be accessing them at the same time, causing a backup.

In the security world, our equivalent point would be the NVR. It’s important to know, worst case, how much traffic will be hitting the port feeding the NVR on the switch.

Network throughput measurements should be taken at the camera input, at each backbone/trunk connection, at each server/NVR connection and at each viewing location. Only with all these points being measured can you truly get an accurate picture of what’s going on.

Managed switches will generally have some kind of way to monitor throughput, errors and dropped packets at every port. It is also possible to get network monitoring software that can retrieve that data from the switches and give you more detailed measurements.

Don’t Overlook Overhead

Remember one important rule in life: What you see is not necessarily what you get. With network numbers that rule applies too. Just because a port is rated for 100Mbps doesn’t actually mean you can cram that much data down it.

In networking, there’s a pesky little thing called overhead. Miscellaneous bits of data and other information that takes up space on the network and has nothing to do with the information you’re trying to send.

The actual amount of overhead on a network varies, but I have seen people estimate up to 40 percent of a link should be reserved for overhead, meaning you only have 60 percent of that link’s capacity available to you.

In other words, if you have a 100Mbps link it’s not a good idea to put more than 50-60MB of data on that link. Now, you are probably asking what devices produce that much information? I have seen some larger megapixel cameras pushing that high.

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