Satellite data communications: The space race is on
Here's a rundown of the types of satellite services and how they compare in their capabilities
Satellite data services will change how we connect to the office and to the Internet. The word telecommuting will take on a whole new meaning when you can actually work from any location of the world. In his continuing series on wireless communications, Rawn Shah looks at what the new satellite service providers are aiming for in the next century. (2,100 words)
The vendors see it as a new race. They were there when cellular services took off. They were there when Internet services hit it big. Now they're jockeying for position in the space race. The goal? To lead the pack in providing the next generation of digital voice, data, and video services.
With tens of billions of dollars invested in starting up these services, vendors are going to need a lot of customers and time before they'll even begin to turn a profit. Still, individuals and companies like Craig McCaw, Bill Gates, Motorola, Hughes, PanAmSat, Inmarsat, etc., are all throwing hundreds of millions of dollars at such projects. And, with the average cost of a phone call running somewhere between 30 and 50 cents per minute, satellite services can almost compete with their terrestrial counterparts.
Global data access
Grouped together as mobile satellite services (MSSs), part of the reason for the race is that the vendors already see this as the new "Internet-in-the-sky" as allegorized by Teledesic. Among the various satellite providers, the aggregate data throughput will be in the terabits per second (Tbps). Of course, this speed is to be shared between 100 and 250 million subscribers worldwide, giving a global average of about 2 to 10 kilobits per second (Kbps) per subscriber.
Such low average data rates aren't the biggest concern. Most users won't be connecting at the same time, so the practical data rates end up being 2 to 10 times higher. More importantly, it's either cost-prohibitive or impossible to attempt even a basic telephone line over the majority of the land surface area of the globe. The quality of service in rural and remote areas is also very low, and often suffers at the hands of nature. Satellite service provides almost fiber-like clarity of service in connections to all open-air locations.
Although there are some MSSs that will provide absolute global coverage, most of them describe themselves as global but actually only serve the major continental areas. For example, although the MCHI Ellipso project has South Africa as an investor, it does not actually provide service in the southern-most part of Africa (See Table 1.1). Most services cover the continental U.S. and much of Europe, except near the polar regions. Other services such as Constellation ECCO are intended only for the countries along the tropics (23 degrees north and south).
Table 1.1 Well-known satellite service
provider coverage and launch dates
Latin America, Africa, Mid|
East, South Asia, Australia,
(btwn 23 deg N and 23 deg S)
All N. Hemisphere, Africa,|
Australia, S. America, S. Asia
(btwn 80 deg N and 55 deg S)
Most Europe, Russia, and N.|
America, all S. America, New Zealand
(btwn 70 deg N and 70 deg S)
Most Europe, Russia, and N. America,|
all S. America, Africa, S. Asia,
Australia, New Zealand
(btwn 70 deg N and 70 deg S)
"Most major continents,|
except Asiatic Russia"
In its smallest format, satellite-based pagers, two-way data service is compact enough to fit in your pocket. Hand-held satellite phones are more practical because they are not much bigger than the smallest of current analog cellular phones. Despite the fact that you can use these anytime, anywhere (except maybe on airplane take-off and landing) these end-user services are fairly slow when it comes to data transmission.
Most mobile end-user data services range only between 2.4 and 9.6 Kbps -- the speed of analog modems about 5 to 10 years ago. Stationary terminals fare significantly better, potentially hundreds of megabits per second -- but at high cost, of course. (See Table 1.2.)
|Table 1.2 Satellite service particulars|
|Project||Satellite Type||Number of Satellites||Frequency Bands||Signaling Protocol||Voice (Kbps)||Data|
|Celestri||LEO/GEO||63 + 9 GEOs||Ka||TDMA||N/A||64 Kbps to 155 Mbps|
|Ellipso||LEO/MEO||14 + 3 spares||N/A||CDMA||N/A||9.6 Kbps|
|Globalstar||LEO||48 + 8 spares||L, S, C||CDMA, FDMA||
|ICO||MEO||10||C||TDMA, FDMA||4.8||38.4 Kbps|
|IRIDIUM||LEO||66||L, Ka||TDMA, FDMA||
|Orbcomm||MEO||28 + 8 spares||N/A||N/A||None||2.4 Kbps|
|GEO||4 + 1 spare||N/A||N/A||2.4||64 Kbps to 144 Kbps|
60 Mbps down,|
2 Mbps up
|Spaceway||GEO||9||Ku, V||N/A||None||16 Kbps to 6 Mbps|
|16||16 Kbps to 2 Mbps|
Floating in space
There are three kinds of earth orbit satellites. LEO (low earth orbit) satellites are the closest to us, flying several hundred to a thousand miles overhead. At these distances, the satellites need to be moving to service the entire coverage area. Each satellite can usually only cover a surface area of 2000 to 5000 miles. Medium earth orbit (MEO) covers even more surface area, but unless you have enough satellites, you still need to be moving to keep the coverage constant. Finally, geostationary (GEO) satellites are usually so high up that they can pretty much hover over a fixed map location. In truth, they are really rotating around the axis together with the earth's rotation.
Fig 1: LEO, MEO, and GEO orbit around the earth
|Table 1.3 Three types of satellites|
|Feature||Low Earth Orbit||Medium Earth Orbit||Geostationary Orbit|
|Orbit distance||300 to 1,000 miles||
- 10,000 miles
(the Clarke Orbit)
|Communications Latency||20 to 40 ms||50 to 150 ms||250 ms|
However, the further up you go, the longer it takes for a signal to reach its destination. It takes 20 to 40 milliseconds for a signal to go from a terrestrial location up to the satellite and then bounce down to a terrestrial receiver elsewhere -- and this rate is dependent on both locations being serviced by the same satellite. As you reach geostationary orbits, it takes half a second for any communication to happen. In human terms that's not slow, but to a computer this latency of data transmission takes forever. By comparison, the average T-1 connection has a latency of 15 to 30 milliseconds and a 28.8-Kbps modem has something like a 100 to 160 millisecond latency.
Many of these satellite systems are placed in a network surrounding the earth. They can communicate with each other using inter-satellite links (ISLs), usually phased-array radio or lasers aimed at each other. These lasers transmit data at speeds much higher than ground-to-satellite links, primarily because there are practically no obstructions -- physical, frequency spectrum usage, etc. -- or hazards to electronics and biological life out in space. Most of these satellites have a lifespan of 5 to 15 years before being considered for replacement. In truth, many of these satellites may remain in orbit for decades before crashing back down to earth, making the space around our planet more crowded every year.
The network protocol side poses other problems with satellite communications. LEOs. for example, are constantly on the move, so users are most likely to move frequently from one LEO to the next. Depending upon its distance from the ground and the number of satellites in the MSS network, each user connection has to be constantly recalculated in order to determine whether or not it needs to be passed on. The effect of this on a TCP/IP network is that the packets encounter a constantly changing delay factor known as jitter. Voice applications survive well enough, given their significantly low data rates, but time-sensitive applications, such as video conferencing, can get erratic if the jitter isn't minimized. Some vendors approach this by buffering data at each satellite and passing it in a smoother fashion across the inter-satellite links, but this can affect other types of bursty traffic, such as Web access and remote-user sessions.
The actual network protocol and signaling systems used by the MSS networks will vary. Some vendors are going for ATM connections or even straight TCP/IP while others are building proprietary protocols for communications. The aggregate or total capacities of each of these networks vary from 1 gigabit per second (Gbps) to hundreds of gigabits per second. The larger networks have ISLs running at 2 to 10 Gbps. This is pretty fast, considering that the fastest terrestrial lines hit a technological wall at 2.4 Gbps on OC-48 or 10 Gbps on OC-192 fiber links.
Don't be fooled: Behind the calls for provision of services in rural areas and in areas currently not served by terrestrial means, the real motivation for satellite-based phone and data communications is money. Most vendors, recalling the trend in analog cellular communications, picture themselves delivering services to high-end clients and customers who absolutely have to have a reliable connection from anywhere in the world.
Security now becomes a global concern. At the raw protocol level it takes quite a bit of effort to intercept digital voice and data sessions. Most vendors are implementing a form of the time division multiple access (TDMA) or code division multiple access (CDMA) systems we discussed in April's Connectivity column. CDMA is inherently more secure but also more costly per user. At the network protocol level, expect to be using some form of security for data, such as the IPSec virtual private networking system.
The pricing for data services has not been set, but most MSS providers are placing estimates of voice pricing at 15 to 30 cents per minute wholesale and 50 to 75 cents per minute retail. The larger broadband-based systems that can deliver several megabit-per-second data rates will probably have much higher charges. The MSS networks will be closer to the structure of current telephone service providers. Every minute will be counted and charged for accordingly. There are only a few major vendors creating these "floating backbones," which may be the first truly accountable pay-services to hit the Net.
In spite of the multibillion dollar plans involved, analysts predict that there will be a shakeout among the competitors until two or three take the lion's share of the service market. MSSs focusing on specific markets such as the ECCO project for Brazil and the tropics may fare better. It's still too early to tell which ones will lead the pack, but the bets are on Teledesic, Iridium, and Globalstar. Let's just hope they have a decent call support center.
About the author
Rawn Shah is chief analyst for Razor Research Group covering WAN and MAN networking technology and network-centric computing. He has expertise in a wide range of technologies including ATM, DSL, PC-to-Unix connectivity, PC network programming, Unix software development, and systems integration. He helped found NC World magazine in December 1996, and has led the charge to the deployment of network-centric computing in the corporate world. Reach Rawn at email@example.com.
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