What is Latency?


latency <communications>

1. The time it takes for a packet to cross a network connection, from sender to receiver.

2. The period of time that a frame is held by a network device before it is forwarded.

Two of the most important parameters of a communications channel are its latency, which should be low, and its bandwidth, which should be high. Latency is particularly important for a synchronous protocol where each packet must be acknowledged before the next can be transmitted.

(2000-02-27) Source:
The Free On-line Dictionary of Computing, 1993-2001 Denis Howe


Satellite Internet Access Q & A


Reprinted from dslreports.com


 Q: What are the average ping times for satellite?

 A: Latency for satellite connections is very high due to the distance the satellites are from earth. One-way systems typically get pings in the 400-500ms range with 450ms about average and two-way systems in the 690-1150ms range, with 850ms about average.


 Q: Can I use Instant Messaging with Satellite?

 A: Yes. We aren't aware of any popular instant messaging services that don't work with satellite connections. That being said, there are a few caveats. None of the instant messenger file transfers seem to work with Hughes based satellite systems. Also, users running WIN98SE with MSN Instant Messenger have reported that after about 15 minutes of idle time, the MSN Instant Messenger shows that you are signed-on, but actually, you aren't. Other users will see you as "offline" and if you attempt to send a message you will have to sign on again.


 Q: Does Internet Connection Sharing (ICS) work with Satellite?

 A: Yes! This is a very easy and inexpensive way to share your satellite connection on your local area network. Make sure you read the tweak section of this FAQ for information on getting ICS client computers up to speed. Without tweaks, your ICS client computers will be very slow compared to the host. With a little tweaking you can have the same, fast speed on all your computers.


 Q: How do games work with Satellite Broadband?

 A: For the most part, dismally. Any interactive, "shooting" type game will not work well with the high latency associated with satellite connections. Most gamers will be better off sticking with dial-up connections if they have no other alternatives.


 Q: Can I use FTP with Satellite?

 A: Yes and no. As far as using your FTP client to download and upload to a server, you should have no problem. Note that most files will download in passive (pasv) mode.


You cannot, however, run a successful FTP server on a DirecPC/Direcway based system, including Earthlink and Pegasus Express. Since with these systems, you have no public IP address, there is no way to set up a server. 


 Q: Why is the latency so high?

 A: The way a satellite stays in orbit, without using any type of engine or rocket to correct its orbit, is to gain a balance between gravity and centrifugal force. The closer to earth, the greater the gravity pulling the bird toward earth. The faster it moves the greater the force pulling it away from the earth. It turns out that there is a spot, about 22,300 miles above the equator, where the speed an object must travel to gain equilibrium against gravity is exactly the same as the speed the earth is turning. By placing a satellite in this spot, its relative position above the earth stays constant. Satellites used for broadband internet and TV must stay in one place in the sky so that you can point your dish in one spot and get the signal. The only other alternative would be to have a constellation of satellites in a lower orbit, that would rise and fall like the sun and stars. There would need to be enough of these flying that there was always one overhead, which requires a lot of satellites, a lot of infrastructure, and therefore a lot of money.


In the case of a two-way satellite system, when you request something by clicking on a link, or any other way, that message travels 44,600 miles just to get to the NOC. The stuff coming back to you must travel the reverse route, so the round trip is 89,200 miles. The speed of light is 186,000 miles per second in a VACUUM, slower through the atmosphere. But even if you assumed 186,000 mps then the total time taken in space travel is about 480ms. Given the atmosphere problem, it is actually more like 500ms. Add to that the terrestrial internet latency, which should be about 100ms. Also you can add delays through transponders, gateways, proxies, etc.


 Q: Can I tweak something to reduce latency?

 A: No.


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