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Advanced CBR for Better Image Quality with Limited Bandwidth

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Introduction

If you’re in charge of managing the security system for a facility, then you’ve undoubtedly had your fair share of “Picasso moments.” That is, picaso_momentone or more security camera frames that should show well-defined images have been mysteriously transformed into something the Master himself would’ve been proud of.

But what causes these episodes of ill-defined imagery? Suffice it to say that more than one system operator has been fooled into thinking that a CBR (constant bit rate) configuration provides consistently good image streams in limited bandwidth environments. The reality is more complex. When an IP camera tries to force too many packets over the network, extreme packet
loss can occur, and it’s more apt to occur when there is a flurry of activity.

The problem is this: If your system experiences too many such “Picasso moments,” the day may come when you cannot provide authorities with the images they need to determine who broke into your facility. In this paper we explain what causes video images to become corrupted, and describe the solution Moxa provides to avoid the problem.

The CBR Myth

Since surveillance camera images are streamed over a network, it certainly makes sense to configure the cameras to use CBR (constant bit rate), which requires setting the maximum bit rate (5 Mbps, for example).However, don’t make the mistake of assuming that CBR can always guarantee good image transmission in a limited bandwidth environment.

CBRWith CBR, video streams are downsized to match whatever maximum bit rate was configured by the operator. When there is minimal activity, such as when the camera continuously transmits essentially the same image, the sequence of images at the receiving end will be of high quality. But when there is a lot of
motion, as would be the case when one or more intruders enter the scene, the quality of the images suffers.

Although one could modify the CBR configuration to a higher maximum bit rate, doing so would put a real strain on the available bandwidth. The question then becomes: Can we augment the CBR methodology in such a way that packet loss is minimized, without compromising the network’s ability to provide reliable operation for all network devices and users?

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