Unicast, broadcast, and multicast

A Quick Primer on Broadcast, Unicast, and Multicast

Transporting video has never been easier thanks to the multitude of networking technologies on the market. It wasn’t that long ago that our televisions only had a handful of channels, or that we would dial into a conference call for a company all-hands meeting. Here’s an overview of the different video distribution technologies that exist, and how they work. This primer will give you a better idea of the differences between the more popular IP/broadcast distribution methods.

Broadcast

Let’s start with the most well known, broadcast. Broadcasting sends out a signal to everyone, and whoever has the receiver, or is on the network, can receive the signal to view the content. When we are thinking of radio or television broadcasts, powerful antennae send out a signal over a specific frequency. Receivers, either the one in your car or the dish on your roof, receives and decodes the signal. But, as we’ve all experienced when we miss one of our favorite shows, broadcasting is a passive experience where the viewer is unable to pause or rewind the content (DVR’s notwithstanding). What’s on, is on. Unlike Netflix, you cannot choose to watch your favorite rerun of Friends when you want. The biggest advantage of broadcast is it lets you reach nearly an unlimited audience.

Unicast

A contrast to broadcast’s one-to-all transmission of data, we have unicast, which is a one-to-one connection. The internet, at its core, is a unicast network. A user requests information from a source, website, other users, and the other party will send it. Unicast is great for on-demand, watch whatever, whenever you want content.

Imagine a household with five members. If two of those people decide to watch Friends on Netflix, they would both be making requests to Netflix’s server which would serve up two separate streams, even if it’s the exact same episode.

The main downside with unicast is bandwidth requirements. Let’s go back to our above example. If all five members of said household began watching 1080p video from Netflix, it’s more than likely that the connection will begin to suffer and so will image quality.

Multicast

Finally, we have multicast. Multicast is the more recent, IP answer to broadcast. Multicast is a one-to-many delivery, like broadcast, but it differs in a few ways. In a multicast network, not everyone receives the stream, only people who have requested it. The sender only needs to send out a single stream, and the nodes along the network will replicate that stream across a very large audience (think thousands of concurrent users). It’s more like a relay race.

In contrast, if you used unicast to stream to thousands of users on the same network, you’d risk flooding the network. Using multicast for distributing video to hundreds or thousands of LAN connected users is essential, and is a preferred video delivery option throughout most large enterprise campuses, organizations, government offices, and educational facilities.

Wrap up

Of course, there are other routing schemes which exist even the above mentioned carry more complexity and variety. Essentially, it comes down to a one-to-one or a one-to-many distribution of information.

One-to-one is great for single users, for example, a household, or a new employee watching training videos. Where people are watching varied content, at unknown times, as long as it’s only a few people at once (or you have a lot of bandwidth).

One-to-many is when you need to reach a large audience, say, people watching the Olympics, or an all hands meeting at your organization. One-to-many distribution works at scale but the viewer’s ability to control the stream aren’t as granular as a unicast stream.

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