Ask Lemmy

Broadcast versus on demand.

Cable sends the sane data to everyone at the same time. So it is something like, read from the hard drive once. Send it out once. Everywhere it goes, it is just the same thing replicated to each and every reciever, no changes, just copy and paste.

Streaming is different. Every piece of information sent is basically unique, you need to send each piece of information perfectly, you need to read from the hard drive thousands of times, as everyone is watching something different, you need to send unique information to the right location perfectly and in order and at the right time. If it goes wrong, you get buffering.

Cable and Broadcast, no buffer ing, but no choice.

Streaming, choice but with buffering

Broadcasting and the Internet work in fundamentally different ways. With Broadcasting a transmitter sends radio waves out into the world (and beyond). It does not care how many receivers there are; there could be millions or there could be none—literally broadcasting into the void. There is a bit of a disconnect between the transmitter and the receiver. The transmitter doesn’t need to know anything about the receiver; its transmission is ultimately independent of the receiver. The receiver can tune in or not, much like a boat raising its sails to catch a passing wind. One receiver generally will not impact another, just as many boats can sail on the same wind.

This is a really efficient way to get a lot of identical data to many people at once. Especially when we switched to digital television this became easily apparent. ATSC 1.0, the standard the US switched to about 15 years ago, was able to carry about 19.3 mbps of data over a normal TV channel. Because the system was designed in the 1990s this is MPEG-2 video (the same as used on DVDs), but it still works pretty well for 1080i or 720p. In fact as encoders improved we could usually fit two HD streams in there at 6-8 mbps that looked pretty decent and still have room for one or two more SD streams.

At the same time we were able to pretty significantly reduce the power of the transmitters. I think the last station I worked at was something like 125 kilowatts out of the transmitter in the analog days but with the switch to digital we were at 28 or 40 kilowatts (it’s been about a decade since I left television engineering). In the analog days a huge percentage of the power usage was to keep an adequate picture at the very fringes of our broadcast license, which effectively meant an increasingly crummy picture was pushed well beyond our license area (this was factored into how the system was designed). With digital, you either get enough of a signal to produce the picture or you don’t; there’s not really an in-between (other than a picture that keeps freezing up). This means a weaker signal far away from the transmitter that would produce a marginal signal in the analog days can produce a picture that looks just as good as it does much closer to the transmitter with a stronger signal.

All of this means that with just 19.3 mbps of data coming from the transmitter, potentially millions of people can see the same video in real-time. Satellite is basically the same thing except instead of an antenna on top of a tower that’s 3,000 feet tall and can cover an area maybe 150-200 miles in diameter, the antenna is placed 22,000 miles high and can cover an entire continent. Cable works pretty similar except instead of transmitting through the air, the coaxial cable carries the entire spectrum protected from outside interference. It pushes all the signals out of its “transmitter” (called the head end) down a cable and then splits that cable and amplifies the signal (and then splits and amplifies again and again and again) as needed until it reaches all the customers. There can be some complications with digital cable, but that’s the basic concept.

In contrast, the Internet very much designed for one-to-one communication. This works fine for everyday communication, but if you have something where a lot of people want to see the same thing, each of those people have to make their own connection to the server. Even if the video stream is only 5 mbps, if 100 different people want that same stream at the same time, you now need 500 mbps of bandwidth to handle all those connections. You also need a computer that can handle all those connections simultaneously. If you have thousands, hundreds of thousands, millions of people trying to stream the same video at once you can see how much of a problem this becomes. It’s one thing if the video is already recorded, like a movie, you can just distribute it to many servers in advance. But if it’s a live event that’s ultimately coming from one source you have to set up multiple servers to connect to the source and then forward that, perhaps to other servers that will forward to other servers that forward to other servers until you have enough servers and bandwidth for the end customers to connect to. If you have a million people trying to watch your 5 mbps video one might think you need 5 million mbps of bandwidth, but actually you need even more to connect all your servers back to the source, plus many servers. This is a hugely intensive usage of resources. Streaming companies will try to setup in advance for the number of viewers they expect, but if they guess too low they’ll have to scramble to increase capacity. I suspect this is more challenging for companies like Netflix that rarely do live video as opposed to companies that do it every day like YouTube or Twitch.

This isn’t even getting into complexities like TCP vs UDP for the protocol. At the end of the day, the way the Internet is designed each client needs to be sent their own personal stream of data. It just can’t compete with the efficiency of everybody sharing the same stream of data that comes from broadcasting. In that sense, for big, shared experiences, it’s kind of a shame that broadcasting is dwindling away. How many people do you know who still can get a TV signal from an antenna or cable/satellite?

Short version: cable is more optimized for sending everyone the same content at the same time. (And all users connected to cable get all channels all the time, even if they're only watching one or two at the time.) Internet is made for each user getting what they ask for when they ask for it.

Either technology can be used for either use case, but they were originally built for different purposes and so are optimized differently.

Just like a subway train would make a pretty crappy private one-person vehicle for commuting to work and the grocery store. As would a fleet of cars be crappy for public mass transit.

The answer goes deep into networking technology. I try to explain a few major points:

Streaming is built on IP networking, which in turn can be built on different cable technologies. IP transmits every piece of data reliably, but asynchronously - the sender cannot know how long it will take. This is good for things like the www: everybody gets his individual request fulfilled sooner or later.

"Cable" is built directly on one cable technology. It transmits all things synchronously: you know exactly how long the transmission takes, and the amount of data is the same at all times. This is good for one long movie without any disturbances, but it does not give much flexibility when many different users have many different needs.

Cable infrastructure is built different, with multicast streams. All the channels are broadcast on their network at all times to relays all the way up to your neighbourhood, if not your cable box. It's got dedicated, guaranteed bandwidth. It can't get overloaded.

With streaming, each user is one connection using one stream worth of bandwidth, so it doesn't scale too well to millions of viewers. Technically there's multicast stuff but it only works locally, and I'm sure all those cable companies that are also ISPs aren't all that interested in making it work either. So for now we have thousands of identical streams crossing the country at the same time hogging bandwidth and competing with everything else using bandwidth.