IP 101 Tutorial – Multicasting

DO YOU KNOW the difference between IP broadcast and multicast? When is it appropriate to use multicasting for network video transmission? This month’s column is a quick tutorial on the basic methods of IP data transmission: unicast, broadcast and multicast.

Unicast
Unicast is the most common form of IP data transmission. It is point-to-point communication between a sender and receiver device, each identified by an IP address. No other computers or devices on the network will receive the unicast message. Unicast messages can be routed and forwarded through a network until they arrive at the receiving address.
Most IP cameras used for surveillance implement unicast video streams using real-time transport protocol (RTP).

Broadcast
A broadcast message is sent from a single sender to all devices on the network. Many security devices such as IP cameras and access control readers will broadcast small “discovery” messages that allow receivers such as NVRs and video management servers to automatically find the device. 
Too many broadcast messages will clutter a network and slow down all of the network devices since broadcast messages are visible to any device on a network.
Most network routers are programmed to block broadcast messages. This means that broadcast messages, such as the IP camera discovery messages, cannot move across the Internet and are usually contained within a subnet in a private local area network (LAN).
Broadcasting is not a practical transmission method for moving high data rate messages such as video.

Multicast
Multicast messages are sent from a single sender to a subset of the receiving devices on the network. Receiving devices must join a multicast group in order to receive the messages. Devices that are not members of the multicast group will not see the multicast message from the sender. In video surveillance streaming applications, multicast is commonly used when there are many devices that need to view the same video stream.

With multicasting, network traffic is reduced since the video stream is sent only one time versus unicast which requires a copy of the message for each receiver.
Multicasting is a function of the sending device and the network switches. Not all network switches support the multicast setup protocol, Internet Group Management Protocol (IGMP).
RTP can be implemented with either multicast or unicast messaging.

Requirements
Most video surveillance applications do not have requirements for many users to simultaneously view video streams. Such applications are typically limited to casinos, airports or other large infrastructure systems. Unicast RTP streaming works just fine for most surveillance applications that are primarily implemented for video recording or a small number of viewers.

Tom Galvin of NetVideo Consulting is a network video specialist. His company Web site is www.netvideoconsulting.com.

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