Multicast vs unicast: why operator IPTV can scale differently from streaming apps

When a million people watch the same live match on a streaming app, the service sends a million separate streams. When a million customers of a telecom IPTV service watch the same channel, the operator may send that channel across much of its network only once. The difference is unicast versus multicast delivery, and it shapes how operator TV behaves in ways most viewers never see. This explainer covers multicast vs unicast from a viewer's point of view.

Unicast: one stream per viewer

Unicast is a one-to-one connection between a server and a single device. Web browsing, downloads and almost all internet streaming (HLS and DASH) use it.

• Pros: works across the public internet, works with any device, and suits on-demand viewing where everyone watches something different.
• Cons: traffic grows with the number of viewers. Services rely on CDNs, large networks of caching servers placed close to viewers, to absorb that load. The formats they use are covered in HLS vs MPEG-DASH.

A worked example of the scaling difference

Imagine a live final watched by 100,000 homes in one city, each receiving a stream of about 8 Mbps.

• Unicast from a distant server: 100,000 × 8 Mbps = 800 Gbps of traffic leaving that server. CDNs exist to bring servers closer to viewers and spread this load, but the total still grows with every viewer.
• Multicast across an operator's network: each network link carries the channel once if anyone downstream is watching. A core link carries 8 Mbps for that channel whether 10 or 100,000 homes are tuned in.

That's why multicast was so attractive to operators building TV services on their own networks, and why it's being explored again for very large streaming events.

Multicast: one stream, many viewers

Multicast sends a single stream to a group address. Network routers copy the packets only where the network branches toward viewers who have asked to join. A channel being watched in 10,000 homes behind the same router is still carried only once on the links upstream of that router.

• Pros: very efficient for live channels with many simultaneous viewers, and predictable quality on networks engineered for it.
• Cons: multicast generally doesn't work across the public internet. It needs a network where every router is configured for it, which in practice means an operator's own managed network.

That is why multicast is closely linked to operator IPTV, the TV services sold by broadband providers, rather than to streaming apps. For how the full service fits together, see what is IPTV.

Multicast addresses

Multicast uses a reserved range of IP addresses: 224.0.0.0 to 239.255.255.255 in IPv4. Each channel in an operator's network is typically assigned a group address in this range, and devices "tune" a channel by joining its group. IPv6 has its own multicast address range. Addresses in this range are only meaningful within networks configured to route them. You can't reach an operator's multicast channels from elsewhere on the internet.

How channel changes work: IGMP

In a multicast IPTV network, a channel change is essentially a request to switch groups. The set-top box uses the Internet Group Management Protocol (IGMP, version 3 in RFC 3376) to say "I'm leaving channel A's group and joining channel B's group." The nearest multicast-capable router starts sending channel B and stops sending A if no one else nearby is watching it.

Operators work hard on channel-change time. Video can only start decoding from certain frames (keyframes), so a naive join can leave a viewer waiting for the next one. Many systems send a short unicast burst from a nearby server to get the picture on screen quickly, then hand over to multicast.

Source-specific multicast (SSM, RFC 4607) is commonly used in these networks. A receiver asks for a group from one specific source, which simplifies routing and improves security.

What operators do to make it reliable

Multicast IPTV networks usually add:

• Separate VLANs for TV traffic, keeping it apart from internet traffic in the home gateway
• Quality of service settings, giving TV packets priority on shared links
• Forward error correction or retransmission, to recover from occasional packet loss without visible glitches
• Fast channel change servers, which send a brief unicast burst so the picture appears quickly

These features are invisible to viewers but explain why operator IPTV can feel as responsive as cable.

Home network considerations

If you use an operator's IPTV service, multicast traffic also flows through your home network, which can cause problems:

• IGMP snooping on switches and routers makes sure multicast reaches only the ports that asked for it. Without it, a cheap switch may flood every connected device with video traffic and slow down the network.
• Wi-Fi handles multicast poorly (see Wi-Fi vs Ethernet for streaming). Many access points send multicast at low data rates. That's one reason operators often recommend wired connections or their own Wi-Fi extenders for set-top boxes.
• Replacing the operator's router can break IPTV if the new router doesn't support the operator's VLAN and multicast settings. Check the operator's documentation first.

Troubleshooting multicast IPTV at home

Where multicast and streaming meet

The lines are blurring. Some operators deliver their channels as HLS or DASH from CDN caches inside their own network, which is unicast but close to the viewer. Standards bodies have also worked on multicast ABR, which carries adaptive-bitrate segments by multicast to a gateway in the home and then serves them to devices by unicast. This combines multicast's efficiency with the device compatibility of standard streaming formats.

Will streaming move to multicast?

Large live events push streaming services and CDNs to their limits, which renews interest in multicast-style delivery. Approaches include multicast ABR inside operator networks, broadcast features in mobile networks, and partnerships between streaming services and internet providers to place caches and multicast-capable equipment deeper in networks. For viewers, the goal is the same: big live events that start quickly and don't buffer, however many people are watching. Our analysis of how TV delivery is converging looks at the bigger picture.

Questions people ask

Can I receive multicast IPTV over the internet at home?

Not from an operator other than your own. Multicast traffic is confined to networks configured for it. Your broadband provider's IPTV service can use multicast because it runs on that provider's network.

Why does my operator insist on its own router?

Operator IPTV often relies on router settings such as dedicated VLANs, IGMP proxying and quality-of-service rules. The operator's router comes preconfigured. Replacing it can break TV service unless the new router is set up identically.

Is multicast more secure?

Multicast itself isn't a security feature. Operator networks combine it with access controls and encryption, such as conditional access or DRM, so only subscribers can view channels.

Does multicast reduce picture quality?

No. Multicast changes how many copies of a stream travel over the network, not the stream's quality. Operators can actually afford higher bitrates for popular channels because each is sent once per link.

Why do some IPTV apps on my TV use more data than the operator's box?

Apps typically use unicast over the internet, so each stream is delivered separately to your home. The box may receive the same channel by multicast inside the operator's network. On your home connection, a single stream uses similar capacity either way. The difference appears in the operator's network, not your home.

Glossary

Key takeaways

• Unicast sends a separate stream to each viewer; it powers almost all internet streaming.
• Multicast sends one stream per network branch, which makes operator IPTV efficient for live channels.
• Channel changes in multicast IPTV are IGMP group switches, often sped up with a brief unicast burst.
• At home, IGMP snooping and wired connections keep multicast IPTV running smoothly.
• Interest in multicast-style delivery is growing for huge live streaming events.

Multicast vs unicast: summary

Sources and further reading

• IETF RFC 3376: Internet Group Management Protocol, Version 3
• IETF RFC 4607: Source-Specific Multicast for IP
• Wikipedia: IP multicast