Multicast vs Unicast vs Broadcast: Which One for Video Distribution

The three models in one table

Model	Delivered to	Replication happens at	Role in video distribution
Unicast	One receiver per connection	The sender (one copy per viewer)	VOD, interactive sessions, cross-segment delivery
Broadcast	Every host in the subnet, wanted or not	Every host receives a copy	Effectively unused for video
Multicast	Only receivers that joined the group	Forks in the distribution tree	One source to many screens on a LAN

The real difference: where packets are duplicated

• Unicast: the sender builds a separate point-to-point connection with each receiver. 100 viewers means 100 copies from the sender. Sender bandwidth and CPU become the bottleneck.
• Broadcast: packets go to every host in the IP subnet whether they want them or not. Because routers do not forward it, its scope is tiny — local subnet only — and it disturbs every device on that subnet, so video distribution avoids it.
• Multicast: the sender transmits one copy. Multicast routers build a tree using multicast routing protocols and replicate only at the furthest possible fork, raising efficiency and reducing backbone congestion.

Why multicast saves bandwidth, in real numbers

Take one 8 Mbps 1080p stream going to 60 screens:

Model	Sender uplink	Backbone load	Verdict
Unicast	8 Mbps × 60 = 480 Mbps	480 Mbps	Saturates gigabit; the encoder cannot sustain it
Broadcast	8 Mbps	8 Mbps, but every device must process it	Disrupts other traffic; unusable
Multicast	8 Mbps	About 8 Mbps, growing only after forks	Adding screens barely loads the sender

That is why multicast is the standard answer for one source to many screens on a LAN: transit PIS, campus signage, showroom video walls and surveillance walls all follow this model.

Multicast addresses and IGMP basics

• Address range: IPv4 Class D, 224.0.0.0 – 239.255.255.255. The 224.0.0.0–224.0.0.255 block is reserved for local network control (224.0.0.1 is all hosts) and must not carry business traffic; 239.x.x.x is the administratively scoped range, suitable for planning inside an organisation.
• IGMP: receivers use IGMP to tell the local router they want to join a group. The encoder and decoder must agree on multicast IP and port.
• Practice: allocate addresses from 239.x.x.x with a written plan, and never let two different streams share one multicast address.

The switch feature you must enable: IGMP Snooping

The fix is IGMP Snooping: the switch listens to IGMP traffic and forwards each multicast stream only to ports that actually joined the group.

• Always enable: IGMP Snooping on Layer-2 switches.
• As needed: an IGMP Querier. With no Layer-3 device querying, one switch must take the role, otherwise group memberships age out and the stream stops.
• Verify: check port counters — ports that never joined the group should carry no multicast traffic.

Typical multicast faults and how to isolate them

Symptom	Likely cause	Action
No picture on any receiver	Multicast address or port mismatch, or multicast output disabled on the encoder	Test with VLC at udp://@address:port on a receiver
Picture appears then dies after minutes	No IGMP Querier; group membership aged out	Enable a querier in the network
Whole network slows, including non-receivers	IGMP Snooping off; multicast is flooding	Enable IGMP Snooping
Nothing arrives across subnets	No multicast routing (PIM) on the router	Multicast does not cross Layer 3 by default — add multicast routing or a unicast relay
Some screens fine, others stutter	Insufficient bandwidth or loss on one link	Inspect per-port traffic and error counters

When not to use multicast

• Across the public internet: multicast is not forwarded publicly; use unicast plus CDN, or SRT.
• Only two or three receivers: unicast is simpler; the switch configuration is not worth it.
• Each viewer needs independent control: on-demand playback is inherently unicast.
• You do not control the network: if the switches are not yours and IGMP Snooping will not be enabled, multicast puts the whole LAN at risk.

In practice the common pattern is multicast inside the LAN plus unicast relays for the parts that must cross subnets. Draw the topology first, then decide which leg uses multicast.