For example, watching a paid soccer online broadcast – if a lot of people watch it then the quality will drop and it will start buffering and lagging. But that never happens with a soccer TV broadcast no matter how many people watch it. Why is that?
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TV is based on broadcast technology. You cast the single signal broadly to anyone with the capability to receive it. So the transmitter only sends the single signal no matter how many people watch it. Internet is inherently unicast. The server cast the signal to one single client. If two people are watching the same channel then the server will send out two signals, one to each client. That means that when many people watch the servers need a lot of bandwidth.
There are ways of solving this. Internet was developed with multicast functionality. This is a feature where anyone can subscribe to a signal and the routers would make sure to split the signal to every subscriber. But this was hard to implement so it was not initially working. It is just fairly recently that ISPs have started implementing it due to the issues with TV broadcasts over unicast. However it is still very limited and usually does not work between ISPs. Adding to this most people today use video on demand extensively, if only because they want to pause the live transmission for a bit or that they arrived a few minutes late for the opening. Multicast (and broadcast) only work when everyone is watching the exact same thing so when people are in different parts of the video then it have to revert back to unicast anyway. So the benefits of multicast is not easy to gain.
There are content delivery networks who implement something similar. They send servers to the ISPs to install fairly close to the clients. The client would automatically find the closest of these servers and get the signal from this server. When multiple people watch the same broadcast from the same server then only one set of the signal will be sent to it saving some upstream bandwidth. In addition these servers usually caches the data for some time which allows video on demand clients to retrieve the data from the local server. This is still not a perfect solution but it is fairly good.
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