How much data can you send per update?

Unity Engine
1

How much data can you send per update? Or rather, per FixedUpdate, so ideally 30 times a second.

I am looking at a game where I may be needing to send 40,000 byte value types a second. Is this too much? I was planning only to do multiplayer over bluetooth, on iOS devices, first. For bluetooth is this still too much?

Realistically how many byte values can I send per FixedUpdate?

2

So basically 40 Kilobytes a second? Or am I misinterpreting what you mean?

40 Kilobytes a second should be fine, but I wonder why you need to update these values so frequently?

3

Not 40 kilobytes second, 40 kilobytes an update, with ideally 15 updates a second. So 600 kilobytes a second. But I want to do multiplayer with 4 players, so the host device will have to be sending that 600 kilobytes out to 3 other devices simultaneously. Which is 1800 kilobytes a second.

I am looking at the bluetooth spec which says the application throughput is 2.1 megaBITS per second, or rather 275251 bytes, 275 kilobytes.

So I am WAAAAY over aren’t I?

The reason I am needing to send so much data is because the game has in it a gameplay mechanic that rests on a liquid simulation. The liquid is simulated in a 200x200 two dimensional grid. Which is 40,000 cells in the grid, each cells state can be sent as a byte, which is where 40,000 bytes come from. It is ideal for the sake of the gameplay that the state of the liquid simulation remains the same on all devices. My initial thought was, just have the host device simulate the liquid, and send it to all child devices. But as I’ve said above, thats nuts isn’t it?

Can someone give me some realistic limits of data throughput for bluetooth on ipad 2 or for online network play that I could work within to figure out how to do this? I am very unfamiliar with networking programming beyond just syncing a few player states and gun fires, which didn’t have the issue of having TOO much data. So if anyone has done tests, or knows anything about this, what is a realistic limit of how much data can be sent each update? Over bluetooth and wifi/internet?

Also, does anyone know if Unity’s built-in networking system can go over bluetooth on an iOS device? Or do I need to rely all on Prime31’s bluetooth plugin to do the bluetooth networking?

4

That’s an unreasonable amount of data, yes. Could the liquid simulation be made to be deterministic?

5

It already is deterministic…

But even with that I’m having trouble comprehending how to make this networked, I have sort of the general jist of it on the tip of my brain, but can’t quite wrap my head around it. So if you’ll excuse as I think out loud for a moment, and I’d appreciate it if anyone could correct or guide my thinking.

See each players input changes the liquid, the players essentially slosh around this goo. So the simulations input variable could be changing up to 30 times a second.

I am thinking to do this determinstically, what would have to happen is each players inputs would have to be synced. So each player inputs their input, sends it to the server, the server distributes all the input to each player, each player would wait to receive the updated inputs, and upon recieval, each player would update one cycle of simulation, all producing the same outcome. correct?

However this is where things get tricky for me and I can’t wrap my head around it. The simulation is running on a second thread. The simulation may actually update anywhere between 10 and 30 fps, but the actual game Update loop is on the main thread and is going 60 fps, interpolating the movements of liquid between each full accurate update of the simulation.

So I am thinking, the simulation thread would have to be put on pause, wait to recieve the next iteration of inputs from the server, while interpolating from the last simulation, then upon recieval of the inputs, unpause the simulation thread, let it run for one cycle, then repause it, wait for next iteration of inputs. But that seems like there would be such a delay… and how would I prevent it from over-interpolating further than it should? Like some of the liquid may be interpolated farther than another players liquid, and by the time the next cycle of the simulation thread occurs the liquid could all be out of sync from the interpolation. I would still have to verify each cell of the simulation grid against each client…

Unless I do something like keep track of the average time between network updates, and have it interpolate only up to that averaged amount of time, so then the interpolation would be controlled, and the same on all devices…

I’m confusing the hell out of myself here…

Am I thinking about this right?

After some more thought I am thinking potentially the best way to do this is to just have each player fully simulate the liquid at full speed individually, based on the stream of inputs from other players. Then as data and latency permits, it will gradually go through and verify the synchronization of the simulation. So it won’t sent all 40,000 cells every update. It may send only 2,000 cells an update, and then the next update send the next 2,000 cells, then next update, the next 2,000 cells. And if there is an out of sync cell disocvered, it will correct it. So the simulation may be ‘slightly’ off, but the mis-sync wouldn’t last more than a second typically.

6

The trick is to seed it, so a seed would generate the desired result, so you only need to send an integer per frame.

7

I’m still having trouble seeing how to get this to work given all the parameters I have to deal with… could you read what I posted above you, I imagine it’s kind of hard to follow what I wrote, but see if you can follow it somewhat and see what kind of thing I am trying to decipher. Then offer any tips on how to get that in there.

I’m struggling with understanding how to deal with the interpolation between updates…

I can follow how I can set up the simulation to be deterministic, and thus only need to send one integer, or a couple integers per update. But the simulation is going to have to interpolated between network updates and how am I going to avoid having to verify that the interpolation remains the same on each device?

8

Basically, don’t send the state, but the commands used to generate it. So the protocol would be somthing like this
Input(Player1, 101000100010001011000)
Input(Player2, 101000101010001011000)
Input(Player3, 101001100010001011000)
Input(Player4, 101000110010001011000)
Simulate()
Interpolate(5)
Interpolate(5)
Simulate()

9

Thank you that makes sense.

One question that, if I end up sending long int of 11000422110011444 which each numerical representing a different state in a grid, what is the most efficient, and ideal way, to iterate over each digit of that int to extract the individual digit?