Overview
Multicast is a method of network communication where data is sent from one device to multiple specific devices simultaneously.
This differs from:
- Unicast, which is one-to-one communication between devices.
- Broadcast, which is one-to-all communication across the network.
Multicast requires active management by network switches to ensure that data is delivered only to devices that have explicitly requested it. This selective distribution conserves bandwidth and prevents unnecessary network congestion.
How Multicast Works
Receiving devices signal their interest in a multicast stream using the Internet Group Management Protocol (IGMP).
- When a device wants to receive a multicast stream, it sends an IGMP Join message.
- When it no longer needs the stream, it sends an IGMP Leave message.
The source device sends each multicast stream to the network only once, regardless of how many devices are subscribed. The switch then replicates the data only for those ports connected to subscribed devices, preventing unwanted traffic from reaching other parts of the network.
IGMP Snooping
IGMP Snooping is a switch feature that listens to IGMP traffic and manages how multicast streams are distributed.
With IGMP Snooping enabled, the switch:
- Detects Join and Leave messages from receiving devices.
- Forwards multicast traffic only to ports with subscribed devices.
- Blocks multicast traffic on ports without subscriptions.
This intelligent filtering reduces overal network load and prevents devices from being overwhelmed with unnecessary data.
IGMP Querier (Multicast Router)
The IGMP Querier, also known as a multicast router, coordinates multicast group membership across the network.
It:
- Maintains a record of which devices are subscribed to each multicast group.
- Shares this information with other switches to ensure accurate and complete memebership data.
Only one device in the network, typically the core switch, is configured as the IGMP Querier. All other switches forward IGMP Join and Leave messages to this device for processing.
Multicast in Just Add Power Systems
In a Just Add Power Video Distribution system:
-
Encoders (Source Devices)
Each Encoder is assigned a unique Channel, which corresponds to a Multicast IP Address identifying that stream on the network. -
Decoders (Display Devices)
When a command is sent to a Decoder to switch Channels, it triggers the necessary IGMP Join and Leave requests. This allows the Decoder to begin displaying the desired video stream. -
Switch Management
The network switch handles the Join and Leave messages, ensuring multicast data is forwarded only to the ports connected to active Decoders. This separation prevents standard data devices from receiving video traffic. -
Default Settings
- Multicast IP Range: 239.92.0.0 - 239.92.100.0
- IGMP Version: IGMPv2
