What is "latency"?

June 30, 2012 10:10 AM

In regard to satellite internet service, can you explain “latency” to me? I’m currently a Hughesnet customer and get a deplorable 1.5 mbps. A new satellite will go online later this summer, and though download speeds will supposedly greatly increase, I’m told there will still be a latency issue. I’ve poked around online, but the explanations don’t really address the practical issues, like, can I play realtime games online and finally log on to secure sites without them timing out?
posted by jackypaper to technology (7 answers total)

Latency, in your terms, is how long it takes a message to transmit from your computer to another computer on the internet.

Because you're transmitting to a satellite in geosynchronous orbit, that means the signal needs to make a round-trip to space and back, 57600 miles up. Even at the speed of light, that still means around half a second to send a message to someone and another half second if they respond.

While that's perfectly usable for email or logging into a webpage, it makes gaming really teeth-grating.
posted by JoeZydeco at 10:19 AM on June 30, 2012 [3 favorites]

Latency is the time it takes to get a response back after you send out a request to the internet. Latency is an insurmountable problem with satellite internet: the satellites are 22,236 miles up and it takes a quarter second for signals to cross that distance and back due to the speed of light. So you send a request to a server, it goes up to the satellite and back, taking a quarter second, then to the server. The response back to you then has to go back up to the satellite and back again, adding another quarter second. So anytime you're playing a realtime game online, you're between a quarter and a half second behind everybody else. As far as secure sites timing out, I'm not sure why that'd happen because of latency. Sites will timeout on the order of 30 seconds or so, not fractions of a second.
posted by zsazsa at 10:23 AM on June 30, 2012 [2 favorites]

Whoops yeah, I goofed. Zsazsa's altitude is correct.
posted by JoeZydeco at 10:26 AM on June 30, 2012

To elaborate on what everyone said, when you're dealing with online games, it's not so much the bandwidth (your download speed) as the latency (how long the data takes to get from point to point).

Like if you're playing a first person shooter, the game is already downloaded and sitting on your system, the only thing your computer is doing is telling the server "Okay, I'm over here now shooting my gun" and the server goes "Okay, hey, everyone, he's over here now and he's shooting his gun."

It's not a lot of data in the bandwidth sense, what matters is how fast the "Okay, I'm over here now" gets communicated. This used to be (and may still be on same games) called your Ping and having a low one, generally speaking, is good. If we're playing Battlefield and my ping is 46 milliseconds and yours is 300 milliseconds, I may see you and shoot you before the message that "Hey, Ghostride is shooting at you" gets through to your client because of the time difference between us.
posted by Ghostride The Whip at 10:27 AM on June 30, 2012

Let's say that you run a grocery store.

You have an arrangement with a farm to buy milk. That farm can deliver up to 40 gallons of milk to your store per day, and they have a two day turnaround on any request for delivery. Some weeks you may want to order 20 gallons of milk, and other weeks you might want to order 30 gallons of milk, and they'll accomodate you, but whatever amount you order, it always comes two days after you place the order.

Similarly, since their farm's capacity doesn't allow them to ship more than 40 gallons per day to any given customer, if you wanted 60 gallons, you would have to place a 40 gallon order and then a 20 gallon order, and you would receive 40 gallons in two days and then 20 more gallons after that.

Bandwidth is the amount of milk that the farm can deliver each day. Latency is the time it takes the farm to get the milk to you.

Satellite Internet, which sends each request for data up to space and back, is a bit like ordering milk from California in Maine. You can certainly do it, and you can actually get enough milk if you stagger your orders correctly, but you'll always have to wait for the milk to make its way across the country.
posted by eschatfische at 10:33 AM on June 30, 2012 [8 favorites]

Real world example: have you ever watched the news and they have someone on satellite from the other side of the world? That delay thing that happens is latency.

Another real world example: when they opened up the Croton Aquaduct, it took (if I remember the number correctly) 22 hours for the water to get to New York City. So the time from when they guy in New York said "open the floodgates" to the time when water came out the tap is the round-trip latency. The time it takes for the water to head down the pipe + whatever time it took for his message to get to the headwaters.

Some computer games will have latency. You push the button, some amount of time passes, then your guy jumps. In steady state play, you can usually adjust to this and just hit the button early and get the guy to jump. But latency becomes troublesome when the lead-time for your reaction becomes longer than the time you have to get out of the way. If the zombie is going to eat you in 1 second, but it takes 2 seconds to move out of the way, you die.
posted by gjc at 11:19 AM on June 30, 2012

In re: gaming, the relavent number is your Ping, which the time it takes a packet to make the round trip from the server to you and back, measured in milliseconds. Great pings are <1>
Your pings will be utterly terrible-- over 500, perhaps closer to 700. Online gaming with combat interactions with other players is a complete no-go. That is because your signal has to travel via radio to satellites 22,300 miles above the equator-- not above you, mind, so the signal goes further), down to an earth station somewhere, to the game server, and then the same path back to you. Lightspeed is 186,000 miles/second, and let's say your satellite round-trip alone is just 70,000 miles, you can see how that's over a third of a second, to say nothing of processing delays and internet latency from earth station to the gameserver, which itself is a fixed quantity and could be as much as 200-400 ms by itself. Your best pings will be to gameservers close to the earth station, which appears to be... El Segundo in LA-- actually that's not so bad, they'll be lots of high-speed infrastructure network there.

In that case, games might be marginally playable. Good luck.
posted by Sunburnt at 3:20 PM on June 30, 2012