Page 24 - Security Today, February 2017

ted, but instead simply prevents any amount of data over the con- figured limit to be streamed.
The feature allows administrators to accurately forecast the amount of data that could be consumed at a given time. This is most relevant when users of the VMS system may call up video remotely. Remote viewing would cause the video to be streamed over an internet connection, which may be shared by other net- work applications. Limited bandwidth availability may trigger a need to apportion available bandwidth in order to guarantee throughput for other uses.
One problem with bandwidth throttling for this use case is any user of the VMS platform would be affected by the limit that was set, not just remote users. If video were displayed on a separate client computer on the same LAN where sufficient bandwidth is available, the performance of local client viewing would be lim- ited by this feature. To avoid local client performance issues, the configured bandwidth limit may need to be set too high to ef- fectively prevent the demand for remote video viewing from con- suming too much bandwidth over the internet connection.
When the configured limit is reached by a client, often some number of live camera feeds will freeze. If a relevant security event is occurring, this effect can cause monitoring professionals to miss the action, preventing a real-time response.
In practice, the feature is of limited benefit and other com- monly available network services, such as Quality of Service (QoS), can provide the same functionality.
Multi Streaming
Multi streaming is much more advanced than bandwidth throt- tling. Multi Streaming works by pulling multiple streams of video from an IP camera simultaneously, and delivering the most ap- propriate stream of video, based on client viewing requirements.
Most modern IP cameras support delivering multiple versions of the same video feed simultaneously. Using a simple example, one stream can be delivered to the VMS at full resolution, 1080P perhaps (1920x1080), and a second stream can be delivered at a lower resolution, say VGA (640x480). Typically, the highest reso- lution stream is recorded to the VMS, so video evidence is always available at full quality.
If the client workstation is displaying the video feed in a small area of the screen, it may be unnecessary to transmit the full reso- lution. In a case where VGA resolution is a higher resolution than the target display area but also lower resolution than the full-size video stream, the VGA stream would be transmitted. Because the VGA stream is still higher resolution than the target display area, video will be displayed at full quality. No additional benefit would be gained from sending the 1080P stream. In this case, the benefit is the reduced bandwidth of transmitting the VGA stream as compared to 1080P.
Although a VMS platform with Multi Streaming capability is likely to be much more optimized than one without, there are some limitations to its application:
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For there to be any benefit, the system must use Multi Stream capable IP cameras. Most cameras support multiple, simulta- neous streams of video. However, there are often performance limitations of the camera that may require sacrifices with reso- lution or frame rate. If pulling many streams simultaneously, each stream may need to be set at a lower frame rate than desired because of CPU limitations on the camera.
Although Multi Streaming saves bandwidth between the VMS server and the live video display client, it consumes more bandwidth between the camera and the VMS server. Each ad- ditional stream from the camera to the VMS takes bandwidth on top of what would have been consumed by pulling only the stream used for recording.
When using digital zoom on a camera, the VMS will stream the highest resolution video feed from the camera to provide a greater number of pixels and therefore more detail. When digital PTZ is in use there are no bandwidth saving benefits from Multi Streaming.
Multi Streaming does not send the exact resolution needed by the client. The limited granularity can add up to substantially more data transmitted than what is necessary which translates to more bandwidth consumption. Using the previous example, if a client is displaying 16 cameras simultaneously on a 1080P resolution screen, the client only needs a total resolution of about 2 megapixels, or about 0.125 megapixels per camera feed. If each feed is being transmitted as VGA resolution the VMS would be sending a total of about 5 megapixels of video which is vastly greater than the client’s display requirements.
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Multicasting
Using multicasting, data can be sent to multiple recipients simul- taneously while using a minimum amount of bandwidth. With a multicast capable system, the sender will transmit data to the network, and each recipient will ‘subscribe’ to the data stream. Instead of the sender transmitting a unique stream of data to each recipient, the network replicates the data for each recipient.
Multicast works best when there is a single source of data and many recipients. An example of where this model is best used would be transmission of a live video broadcast, like a sporting event. Because many recipients want the same data all at the same time, Multicast conserves a lot of bandwidth by sending only a single copy of the data to each network segment where there are multiple recipients.
In a video surveillance system, there are many sources of data (cameras) and usually very few recipients (users). Think of a sys- tem with 100 cameras and a single video monitoring room.
VMS BANDWIDTH