The most well-known and traditional method of video distribution is broadcast. Multicast is a one-to-many transmission method over IP and Unicast is a one-to-one connection for transmissions over the internet.
Mark Horchler*
Today we address a question we are frequently asked when we talk about live video streaming and distribution: what do the terms unicast, multicast, and broadcast mean? This manual will give you a basic overview of the differences between the three most common ways to distribute live videos.
A very simple way to remember the difference is one-on-one, one-to-many, and one-to-all. Read on for a more detailed explanation of each.
What is Unicast?
Unicast is a one-to-one connection that uses TCP (Transmission Control Protocol) to deliver transmissions over the Internet. One user requests information from a server, website, or other user, and the other party sends it after establishing a unique connection. Unicast is great for on-demand content, look at whatever, whenever you want content. OTT or Over the Top services are unicast-based for live video streaming to smartphones, tablets, SmartTVs and other internet-connected devices.
When a live sporting event is delivered through an OTT application, a source server is used to stream live content to thousands of individual viewers around the world. Each person watching the live stream has a unique connection between their display device and the source server in unicast mode. Because unicast transmission takes place over the Internet, network paths and bandwidth availability can vary greatly from transmission to transmission.
Since a single-origin server can struggle to deliver thousands or even millions of unique unicast streams, content delivery networks (CDNs) are used to distribute the streams over the Internet and share the load.
What is multicast?
Multicast is a one-to-many transmission method over IP similar to traditional streaming. Multicast uses UDP (User Datagram Protocol) to "transmit" a stream over a closed IP network, such as a LAN (Local Area Network) or an IP service provider's own network. Multicast live TV streaming is commonly referred to as IPTV, while OTT is unicast over the Internet.
In a multicast IP network, the sender of the content only needs to deliver a single stream, and nodes along the network will replicate that flow across the network, such as in a relay race. Using multicast to distribute video to hundreds or thousands of users is the preferred video delivery option for businesses and organizations, as it avoids flooding the network with duplicate streams. Multicast is also used to transport SMPTE ST 2110 metadata, audio, and video streams within a transmission facility or OB production network.
What is broadcast?
Finally, the most well-known and traditional method of video distribution is broadcast. The transmission sends a signal to everyone at once from a single source, and anyone who has a receiver within range on a network can view the content. Predominantly used by TV stations and cable TV channels for live content distribution, the biggest advantage of the broadcast approach is that it can reach large-scale audiences. Think of one for all.
A single video source is transmitted over a specific antenna frequency directly to the home or satellite and fiber links to satellite and cable television services. Digital television antennas, satellite dishes, and set-top boxes receive and decode the transmission signal for viewing on a TV. However, the streaming approach is not suitable for watching on mobile devices.
Which one is suitable for streaming video?
While the one-to-one approach is still widely used to deliver live content through traditional linear TV services, broadcasters are also increasingly adding unicast OTT services. While enterprise customers will want to share live content, such as manual meetings or IPTV channels over multicast on their internal network, they may also want to include unicast streaming to share videos with mobile workers.
Each of these approaches has its uses, and no matter the content or organization, there is a growing need for a one-to-one and one-to-many mix of streaming based on application, network, and audience.
*Mark Horchler is an experienced technologist and marketer who has worked in Europe, North America and the APAC region with leading transmission providers, including ATEME, Anevia, Elemental and now at Haivision, as director of product and solutions marketing. He has written and presented on topics of live production workflows, OTT and 4K TV.
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