- A TechNote on Wireless and Mobility
- Steven Taylor
- Publisher and Editor-in-Chief, Webtorials
It's almost impossible to overestimate the impact that Telstar and its plethora of satellite cousins had over the past half-century. It's also time to realize that we may soon be bidding a fond farewell to the majority of satellite communications for corporate networks, with this technology possibly destined to become almost as archaic as fax machines.
The key to satellite communications, of course, is that the vast majority of the "birds" are in a geosynchronous orbit, which is a fancy way of saying that they orbit the earth once every 24 hours, thereby appearing to stay in the same location in the sky. This allows for aiming the dishes at a given point and also ensures that the service is available at all times. (The above-mentioned resource on the initial Telstar satellite points out that it had a relatively low orbit and, as such, it was available using special antennas for roughly 20 minutes every 2.5 hours.) As it turns out, the distance above earth for a geosynchronous orbit is about 24,500 miles, and that is one of the greatest disadvantages.
You see, radio waves have to travel to and from the satellite to get from one point to another. Since the speed of light is 186,000 miles per second, a back-of-the-envelope (speaking of vanishing breeds) calculation says that the requisite distance to and from the satellite is about 45,000 miles, resulting in a transit time of about a quarter-second in one direction. So for a round-trip "question and response," the means almost a half-second delay.
This was acceptable (but annoying) for voice conversations, and the minor inconvenience was certainly acceptable as compared with the great advantages that satellite communications brought for essentially "anywhere-to-anywhere" communications - provided that both endpoints could still "see" the satellite. This tolerance of delay versus convenience, by the way, is still being played out today in our putting up with the delay in cell phone calls.
Back in 1962, data communications barely existed. And for a while data did pretty well, especially as long as it moved as "data over voice" using traditional telephony at relatively low speeds. But that darn delay was a super pain in the rear, especially as data speeds sky-rocketed (pun intended).
Data protocols were designed assuming terrestrial delays from point to point (for the more robust protocols). In some cases, which tended to work poorly outside the LAN, each packet of data was sent, and a positive acknowledgment (ACK) was needed before the next chunk of information was sent. In more "sophisticated" protocols, "windows" were developed so that a certain number of packets could be sent while awaiting acknowledgment. For instance, if a "modulo eight" protocol were used, up to seven packets of data could be unacknowledged at one time.
To address this problem, "extended ARQ" protocols were developed especially for satellite communications, allowing for a large number of packets to be unacknowledged, such as might be the case with "modulo 128." This worked fine - unless an error required a LOT of packets to be resubmitted.
However, this need for guaranteed accurate data transmissions simply could not battle the speed of light. Consequently, as our oceans have increasingly been crisscrossed with fiber optic cable, the need for satellite communications for data has rapidly diminished. And voice circuits per se are almost gone. Instead, voice now rides over the same low-latency paths as data.
So what's the future for satellites?
Certainly, for those of us who grew up with the space race as a part of our great adventure, satellites will always have a soft spot in our hearts. However, if we think a "truck roll" is expensive, we don't even have a maintenance option for a "shuttle roll" these days.
Nevertheless, a few applications will continue to see satellite communications as being important over the next several years:
- Broadcast. From a broadcast perspective, delay is not a big issue. So long as transmissions are "simplex" (rather than "full duplex"), delays won't be really significant. (An exception to this might be when a hard-core sports fan likes to watch on TV and listen to their favorite commentator on the radio. It's not clear these days as to which will be ahead of which, but there is often as much a couple of seconds difference.)
- Really Remote Access. Of course, satellite communications will continue to be quite important for accessibility to places that aren't reachable by wired (glassed?) communications. In particular, the Iridium network of low-orbiting satellites provides service virtually anywhere in the world.
- Backup. While satellite communications is no longer a "first choice" in many cases, there is still an extremely reasonable case for using satellite services, such as those offered by Hughes Networks as a backup for terrestrial services. Satellite can be competitive for the SOHO (Small Office / Home Office) worker who happens to live where there are limited other options available.
Looking forward, we acknowledge that satellites have served us well over the years. So happy birthday to these great "birds," and may today's new technologies enjoy the same longevity.