How Nighttime Video May Crash Your NetworkAuthor: Ethan Ace, Published on May 02, 2012
Nighttime video may crash your network. During the day, under ideal conditions, bandwidth remains relatively stable. However, when the lights go down, bandwidth changes can become severe, creating problems like lost video, degraded quality and network instability. In this note, we look at the causes and negative effects of these low-light issues, and give our recommendations for overcoming them.
Most surveillance networks are designed using average numbers (typically from daytime video) or manufacturer camera calculators. Both of these methods are, if not best case, optimistic, and do not account for nighttime spikes. In other video industries, such as broadcast and A/V, constant bitrate encoding is generally used, to provide more stable, predictable bandwidth consumption. In surveillance, this is not the case, as our poll shows most prefer VBR to CBR, due to the storage savings it provides. However, it creates the potential for spikes and variations which CBR does not.
The Causes of Nighttime Bandwidth Issues
VBR encoding, along with noise created when automatic gain controls turn on, are the typical causes of this issue. Under normal circumstances, video is relatively clear and noise-free. As light levels lower, however, gain controls activate to digitally increase the brightness of the image. This digital correction introduces noise, however, which results in increased video bandwidth.
The following video illustrates surging bandwidth when using an Axis Q1604:
The increase from ~100 kBps (800 kbps) to over 2800 (over 22 Mbps) illustrates just how potentially dangerous surges are.
A nearly 28x increase in bandwidth, as shown above, may easily overload the network. For example, assuming five cameras and a server are connected to a 10/100 switch, bandwidth surges at this level would quickly overwhelm the server's connection to the network, as it totals over 100 Mbps. This could result in a dropped connection, lost video, and other unexpected issues.
The issue is even more critical when using limited-bandwidth connections, such as wireless, 3G/4G, or cable/DSL. Even a moderate surge in bandwidth on a single camera may exceed available bandwidth, dropping the connection.
This is also dangerous when sharing the network with other services. If designers are accounting for 4 Mbps bandwidth on all cameras sharing a LAN with voice and data applications, these spikes may result in interruption to other services, potentially overwhelming switch interconnects and backbone connections.
Surges may greatly reduce expected storage durations, as well. If storage calculations are based on cameras using average bandwidth, even moderate surges could cut days or even weeks off of retention periods. This is especially dangerous if the organization must meet state or federal regulations, such as the gaming industry, or in critical use systems, such as municipal or government security.
There are two ways to account for these nighttime bandwidth spikes: setting bitrate caps on VBR streams, or using constant bitrate streams. We cover both of these in detail in our VBR vs. CBR streaming report:
- Use VBR with a cap: The preferred way to handle these spikes is to set a maximum bitrate when using VBR streaming. This allows bandwidth to remain low most of the day, but increase to reasonable levels in low-light situations.
- Use CBR: The second, though less preferred method for dealing with variations in bandwidth is to use constant bit rate streaming. This allows designers to account for variations, as bandwidth should generally not exceed the set rate. However, the CBR bitrate must be set high enough that degradation does not occur during bandwidth surges. This results in higher overall bandwidth and storage usage, where VBR would be lower during other times.
Using gigabit switches may also alleviate minor or moderate network issues, given their higher capacity. However, in larger systems, using 24- or 48-port switches, even gigabit links may not provide enough headroom.
Finally, gain management should be carefully considered. Most often, installers leave cameras set to default gain settings, which typically results in very aggressive levels, and resulting increased noise and bandwidth. Gain should be set to the lowest possible levels which still produce quality images, to avoid this. Users may see our automatic gain control test results for more information on setting gain, as well as comparisons of differing gain levels.
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