Unicasting vs. Multicasting

Multicasting video over a LAN: Use the right switch.

In video extension applications where you want to distribute HD video across a network, you need to understand how it works and what kind of networking equipment to use with your extenders.

Unicasting vs. multicasting, and why an unmanaged Layer 2 switch isn’t sufficient.

Unicasting is sending data from one network device to another (point to point); in a typical unicast network, unmanaged Layer 2 switches easily support these types of communications. But multicasting is transmitting data from one network device to multiple users. When multicasting with unmanaged Layer 2 switches, all attached devices receive the packets, whether they want them or not. Because a multicast header does NOT have a destination IP address, an unmanaged network switch (a Layer 2 switch without supported capabilities) will not know what to do with it. So the switch sends the packet out to every network port on all attached devices. When the client or network interface card (NIC) receives the packet, it analyzes it and discards it if not wanted.

The solution: a Layer 2 switch with IGMPv2 or IGMPv3 and packet forwarding.

Multicasting with Layer 2 IGMP-capable switches is much more efficient than with unmanaged Layer 2 switches because it identifies the multicast packet and sends it only to the intended receivers. 

An unmanaged switch sends the multicast packets to every device and, if there are many sources, the network will slow down because of all the traffic. And, without IGMPv2 or IGMPv3 snooping support, the switch can handle only a few devices sending multicasting packets. Layer 2 switches with IGMP support, however, “know” who wants to receive the multicast packet and who doesn’t. When a receiving device wants to tap into a multicasting stream, it responds to the multicast broadcast with an IGMP report, the equivalent of saying,

“I want to connect to this stream.” The report is only sent in the first cycle, initializing the connection between the stream and receiving device. If the device was previously connected to the stream, it sends a grafting request for removing the temporary block on the unicast routing table. The switch can then send the multicast packets to newly connected members of the multicast group. Then, when a device no longer wants to receive the multicast packets, it sends a pruning request to the IGMP-supported switch, which temporarily removes the device from the multicast group and stream.

Therefore, for multicasting, use routers or Layer 2 switches that support the IGMP protocol. Without this support, your network devices will be receiving so many multicasting packets, they will not be able to communicate with other devices using different protocols, such as FTP. Plus, a feature-rich, IGMP-supported Layer 2 switch gives you the bandwidth control needed to send video from multiple sources over a LAN.

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