Tip

Use satellite links for VoIP

Conventional wisdom says you must use "landlines" for VoIP. By landlines, people refer to a transmission facility that is terrestrial, such as fiber-optic cables buried in the ground, or laid across the floor of the ocean. This is opposed to satellite links, which consist of signals shot into orbit and bounced off telecommunications satellites, back to the Earth.

The justification for this school of thought is fairly simple. Most of these satellites are between 35,000 and 40,000 km from the surface. (Specifically, 35,767 km above the equator, to maintain them in geosynchronous orbit.) Assuming the voice packets would travel up and back, which is a pretty safe assumption, and return via the same path, which may or may not be the case, we're looking realistically at having our packet traverse 76,000 km one-way, or 152,000 km round-trip. Our packet will likely be traveling at a little shy of the speed of light, which is 300,000 km/s, but we'll use that number to make the math easy. This leaves us with something around 1/4 of a second best-case, one-way delay and 1/2 second best-case round-trip delay. This is where the rule of thumb comes from for adding 1/2 second of delay to the calculation for delay when a satellite link is involved. (It is precisely 479 ms at the equator.) And it is why TCP timers are often adjusted for these links for regular data traffic.

Relatively, the Earth is only about 24,000 miles around, so even if you ran a fiber-optic cable

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from one side to the other, the 12,000 miles, while not insignificant, still leaves plenty of room in your delay budget.

But for VoIP, obviously, the satellite delays are far beyond the ITU G.114 recommendation of 150 ms maximum one-way delay. Worse, we're already close to twice the recommended maximum before we even add delay for the codec or packetization or queuing or the receiver's jitter buffer, etc.

Thus, satellite links for VoIP are often dismissed out of hand. So some might be surprised to learn that there are many satellite companies offering VoIP services. There are a number of reasons for this. One is that while the satellites mentioned above are "geo-synchronous," there are other satellites in medium- or low-Earth orbits. In fact, these are networks of satellites that do not stay in the same position, but communicate with each other to achieve a service similar to that you can get from geosynchronous satellites. The advantage of these systems is that they are much closer to the ground (as low as a few hundred miles) so transmissions through them have much smaller delays.

Various other methods have been used to minimize delay for VoIP services, and the VSAT systems are apparently growing rapidly in popularity.

If you're interested in some of these providers, a short list can be found here.


Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


This was first published in October 2002

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