The Line on Casting

How Multicast Groups Work

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When the camera first joins a multicast network, it tells the switch it’s connected and that it wants to establish a multicast group. The switch advertises this group to the network. Those join messages we just talked about are sent up the network to these source switches where the multicast groups reside. The client is now a member of that multicast group and the video is sent directly to it.

All devices on a network have an IP address, but in this case a device can have a couple of them. Multicast addresses are only used for multicast traffic and are in the range of 224.0.0.1 to 239.255.255.255. Addresses in this range should never be used to address devices on a network. When you set up a device to support multicasting, you would assign one of these addresses to each multicast stream the camera would be initiating.

This would be a good time to mention a couple of cautions. Multicasting is primarily used in a LAN situation. Most multicast traffic, without special handling by service providers, will not be able to travel across the Internet.

Also, all switches and routers on the network must support the multicast standard. If not, a couple of things can happen. In some cases the traffic will be flooded out all ports (a broadcast storm) or it will be blocked entirely. Either way it will be a problem. Support for multicast may be as simple as a programming command or it may require a software upgrade or even additional licensing for advanced functions. 

Variations Of Multicasting

So multicast is a very efficient way to send large amounts of video, as we’ve seen. Due to its complexity, however, it generally only applies to the larger systems, but not always.

Some products, such as Pelco’s Endura network video system, utilize both unicast and multicast to build an efficient system. A unicast, point-to-point stream is sent from each IP camera or encoder to the system NVR. This ensures the traffic gets where it is going. And as this isn’t a live stream, error checking can be used to replicate any bad packets, so no frames are dropped.

The live video in Endura, however, is multicast using the UDP protocol, so multiple workstations and other viewing devices don’t put an undue load on the network.

Lack of Info Jeopardizes Network

Setting up a multicast network isn’t for the faint of heart or the network amateur. It requires an advanced knowledge of switch/router programming, network architecture and several protocols, especially routing protocols. Misconfigured multicast programming on a network can do more than just stop your video; it can cause broadcast storms and crash a network in minutes, if not seconds.

It is, however, a very useful tool for reducing the amount of traffic on a network for large video installations. Different video syste
m manufacturers will implement multicast in different ways, but a basic grasp of multicast concepts will go a long way to helping you communicate with the ever-skeptical IT guy.

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