Saturday, March 19, 2005

When The Speed of Light Is Too Slow

If there is anything fast, it's light. Nothing is faster if you believe Einstein. The speed of light is so fast compared with everything else we experience that we tend to think of it as instantaneous. But it's not. There is even a small but imperceptible delay between the time you flick the light switch to ON and the time you see a room full of light.

Imperceptible is the key. Newton's laws of motion work just fine at the speeds that everyday things move. Us, cars, jet aircraft, even speeding bullets are too slow for the speed of light to have any effect. But communications is not in the material realm. We engage light to transmit information through fiber optic cables. Light's cousin, radio waves, are in the same electromagnetic family and subject to the same speed limitations. Believe it or not, that Einstein speed limit can cause us real grief.

Let's say you are in New York and want to talk to someone in Tokyo. Your voice will travel 6,760 miles. The speed of light is about 186,000 miles per second. That's 186 miles per millisecond. The fastest any signal could get between New York and Tokyo is about 36 milliseconds. Most of the way, the phone call will be carried by laser light in fiber optic cable. Light travels much slower in glass than it does in a vacuum or air. It's about 65% as fast or 120,900 miles per second. So the delay between when you speak and the person at the other end hears you is at least 56 milliseconds.

That 56 milliseconds of transport delay is called latency. In a real phone call the latency would be 1.5 to 3 times as long because the fiber optic cable isn't stretched between the two cities. It follows a longer path around the country and under the ocean. There are also electronic regenerators along the way that add their own latencies. That 56 milliseconds may well be least 84 to 168 milliseconds. To call half way around the world, about 12,500 miles, the delay just due to the speed of light would be 67 milliseconds, with a likely total latency of 101 to 202 milliseconds. At 250 milliseconds, the delay starts to become annoying.

Where does 250 milliseconds make a difference? Say you want to play a real time action game hosted on a server on the other site of the Earth. The round trip from mouse movement or keyboard entry to screen update is twice that or 500 milliseconds which is also half a second. Can you notice a half second lag?

Here's another example. Say you want to host a concert with musicians located in various countries around the world. You connect the various studios through digital fiber optic cables and combine all the vocals and instruments on your mixing board. A singer half way around the world will hear the mixed audio in their headphones a quarter of a second later than you do and you'll receive their voice a quarter of a second after they start singing. Does a half second make a difference in this case? You bet it will. No two performers will hear the same mix. Everybody will be slightly off in their timing.

Well at least you can make a phone call, right? Sure, as long as you avoid sending it through a satellite in geosynchronous orbit. Those birds are out there 22,300 miles away. The path is almost totally a vacuum, so the radio waves will travel or propagate at 186,000 miles per second. The satellite will hear you 120 milliseconds after you start speaking and will take another 120 milliseconds to send it to the party you called. That's 240 milliseconds best case. If every data packet was acknowledged before it was accepted, the delay would double to 480 milliseconds at a bare minimum. For voice, it's probably better just to shoot those packets up there and hope they come back intact. Even so, other latencies due to landline transport and routers that add perhaps 10 milliseconds each will push the total delay to the point where you have to treat satellite VoIP calls like walkie-talkies rather than full duplex telephones. Ping times, a measure of the round trip latency, have been reported over a second and about 850 milliseconds on average for two way satellites in geosynchronous orbit.

Want to call someone on Mars? It's a mere 35 million miles away so plan on your call taking 3 minutes to get there and 3 minutes to come back. Forget music on hold, you'll need background music while waiting for responses. The moon is a lot closer. When we establish a base there, you can call your friends 239,000 miles away. They'll hear you in about a second and a quarter. It's a mere two and half seconds round trip at the speed of light.

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