In multicast transmission, there is no direct connection between the source and the destinations. The connection to the video stream of the IP camera is done by joining a multicast group, which in simple terms means actually connecting to the multicast IP address of the video stream. So the IP camera only sends a single copy of the video stream to its designated IP address and the destination simply connects to the stream available over the network with no additional overhead on the source. In other words, the destinations share the same video stream. In figure 2 below, the same three destinations requesting the video stream have the same impact on the network as a single destination requesting the stream in unicast and there is no more than 4 Mbps of data travelling on each segment of the network. Even with 200 destinations requesting that video stream, the same amount of data would be travelling on the network.
It is evident at this point that using multicast transmissions in an IP video surveillance application can save a lot of bandwidth, especially in large scale deployments where the number of destinations can grow very quickly.
Making Multicast Transmission Worth Your While
Multicast is a relatively simple method to improve bandwidth management. However, it must be supported by the following three components:
- Multicast-enabled routers and switches
- IP video sources supporting multicast transmission
- Back-end video system supporting multicast management
In network terms, a switch supporting multicast can also be referred to as a switch supporting IGMP snooping or IGMP snooping and querying. IGMP snooping is a functionality that is usually handled by a layer 2 switch. IGMP querying can be managed by the layer 2 switch or by a multicast-enabled router. This equipment is the key component for building a multicast-capable network. A switch supporting IGMP snooping will ensure that the multicast traffic is only sent to the destinations that have requested the multicast stream and that it is filtered from the rest. By simply sending messages such as “IGMP join” or “IGMP leave” on the network, destinations will or will not receive a multicast stream. To ensure that IGMP snooping is working correctly, at least one IGMP querier must be running on the network. The IGMP querier will ensure proper transmissions through the multicast connections.
Without these components, the multicast traffic will not be handled properly on the network causing unwanted traffic. A non multicast-enabled switch will in fact interpret the multicast packet as an unknown packet and transmit the packet of data to all destinations. Typically, IT departments are involved in providing a multicast-capable network and in ensuring that it has been properly configured.
In terms of IP video sources, most of the IP cameras and encoders on the market have support for multicast. An exception to that is some MJPEG IP video sources, as these types of sources essentially only stream in unicast. This limitation is not due to the compression format (MJPEG), but mainly occurs because MJPEG compression is usually coupled with HTTP for transmitting the stream, resulting in unicast streaming only. However, as RTSP is taking over the control of the stream, this will all soon change, allowing for MJPEG IP video sources to stream in multicast.
Now, having both a multicast network and a video unit streaming in multicast is still not entirely sufficient to effectively manage the multicast traffic over the network. The VMS that is being employed also needs to properly manage that multicast traffic. The main objective of this software is to provide the viewing client application and the recording server with the ability to join the multicast group over the network as described above. Of course, this is merely the minimum requirement for a VMS when it comes to facilitating the transmission of multicast video streams.
Taking Multicast to New Heights: Intelligent Video Management Software
In addition to this, the VMS must be much more intelligent to provide good control of the multicast traffic, and therefore should be able to conduct all the following: