VSATs

With fiber taking over their traditional role in intercontinental links, satellites must compete by reaching customers directly. The innovation that made this possible was the directional antenna, first implemented in 1975 by a satellite called Satcom. It enabled satellites to focus their beam on one particular area of Earth’s surface, usually North America or Western Europe. Newer satellites launched from 2000 onward incorporate even narrower beams, which can switch rapidly between hotspots, such as individual towns.

With a well-focused beam, dishes can have a diameter of one meter or less, a small fraction of the earlier systems. These are known as VSATs (very small aperture terminals). All VSATs use geostationary satellites, so just how small they can be depends on the latitude as well as on how close they are to the center of the beam's target.

Dish size also depends on the frequency used. Higher frequencies have shorter waves, permitting a smaller dish. Most commercial satellites broadcast at substantially higher frequencies than mobile systems, usually in the K part of the microwave spectrum, from 10.9 to 36 GHz. For satellite purposes, this is subdivided into the Ka and the Ku bands. The a and u stand for above and under, so the Ka-band is the higher of the two.

Higher frequencies also permit higher bandwidth, though increased interference can negate this. Shorter waves are more easily blocked by particles of dust, rain, and even mist, as shown in Figure 12.3. Like terrestrial fixed wireless equipment, a good vendor or operator will ensure that the equipment has enough spare capacity to cope with adverse weather, so that in most conditions performance should actually be better than stated. The exception is a blizzard—no satellite receiver will work if the dish has filled with snow.

The VSAT Industry

Geostationary satellites are operated by one of a very few specialized corporations or international organizations. The largest and first was Intelsat, the International Telecommunications Satellite Organization, created by various governments and companies in 1964. Cold-war politics prompted several rivals to be set up, including the maritime-focused Inmarsatand the European Union's Eutelsat.

Despite the apparent competition between them, these groups all have the same owners, namely the major telecom companies in each country. The exception is the U.S., where the government was in the middle of an antitrust battle with its major operator, AT&T, at the time the satellite industry began. Rather than hand it even more power, the government created and then privatized a satellite-only company, Comsat Corporation.

Some of the companies that design and launch satellites also operate them. Hughes Network Systems, for example, runs several satellites and plans many more. Boeing, more famous for aircraft, also has major investments in several satellite firms. The reason is that the satellite industry has become one of its biggest customers. Airlines each buy only a few jumbo jets, but satellite operators have a near-limitless demand for rockets.

These groups rarely deal with customers directly. Instead, they lease capacity to satellite service providers, in units called transponders.These originally represented physical devices, each of which would send and receive at a slightly different frequency, but have become shorthand for blocks of bandwidth.

The satellite industry is obviously more international than other types of wireless communications, and so it is particularly important that regulators cooperate internationally. Rather than national governments, nearly all regulation is handled by the ITU. It allocates satellite frequencies on a worldwide basis and also governs positions in orbit, a role that has caused some controversy.

Because of interference problems, only a limited number of satellites can use the same spectrum within a given part of the sky. To keep their signals separate, geostationary comsats need to be separated by at least two degrees, meaning there is room for a maximum of only 180. The ITU originally allocated orbital slots on a first-come, first-served basis, a process which upset the countries underneath the equator. They demanded that orbital slots over their nations should be given to them, not mainly to foreign countries.

National jurisdiction doesn't normally extend into space, a precedent set in the early days of the space race, when no country dared challenge the superpowers' right to overfly them at much lower orbits. Nevertheless, the ITU decided that geostationary orbits are a special case and allocated the equatorial nations slots of their own. Few could actually afford to develop space programs of their own, so the slots went unused until the ITU began to demand that everyone with a slot either launch a satellite or relinquish it.

The slots most in demand are above very poor parts of Africa and South America, as they can cover the lucrative European and North American markets. The obvious solution is for satellite operators to pay these countries a share of their revenue in rent, probably through a market mechanism similar to spectrum auctions, though so far arguments between regulators have prevented such a system being set up.

Traditional VSAT Architecture

A VSAT system is usually configured in a star or mesh topology around a central master Earth station known as a hub. This has a giant dish up to 10 meters in diameter, together with routing equipment and a fast terrestrial connection, usually over fiber. For a corporate system, this is connected to the company's headquarters; for an ISP, it's connected to the Internet backbone.

The hub is connected via satellite to any number of client installations, which can be based over a wide region and may span many countries. The remote client sites use VSAT dishes, which are much smaller and can even be portable. A portable VSAT doesn't provide the full convenience of genuine mobile communications, but it allows all the equipment for emergency communications to be stored inside something no larger than a briefcase.

Such a setup is useful for any application that needs to connect a large number of users in a private network, especially those in remote areas. Common applications include file transfer, credit card authentication, telemetry, and distance learning. Even in heavily populated areas well-served by terrestrial communications, VSATs are still popular. For example, around 20 percent of lottery terminals in the UK use satellite technology because it is faster and cheaper than using BT's network.

The disadvantage of a traditional VSAT system is that all data has to go through the hub. This doesn't matter if most people are communicating with landlines, but it does make a difference when two VSAT terminals need to connect with each other. As shown in Figure 12.4, data has to make two trips to the satellite and back, doubling latency. This is fine for something like email, but leads to very noticeable delays with real-time interactive applications such as voice.

Switching in the Sky

The next generation of VSAT systems transfers the hub's intelligence to the satellite itself, meaning that customers can be connected directly. The most well known example of this is Hughes's Spaceway Constellation, due for completion in 2004. By eliminating the ground-based hub, it frees up more bandwidth for individual VSAT users and cuts latency in half. Hughes plans to reduce latency yet further by giving Spaceway a MEO component, though this will be further in the future.

Another innovation, under development by Spaceway and its competitors Loral and Gilat, is the switchable directional antenna. This is very highly focused, perhaps even onto a particular user, and moves depending on where it is needed. The switching is handled electronically rather than by physically rotating the satellite, allowing capacity to be deployed very efficiently.

Systems based on this new technology are touting spectacular data rates. For example, a company called V-Star claims that it will be able to deliver capacity OC-12 (622 Mbps). Such speeds are normally achievable only by fiber or laser links, and even then over distances far less than the thousands of miles between Earth and the GEO orbit.

One-Way Data Systems

The growth of the Internet created a huge demand for high-speed data services, often among people who could not afford a full VSAT system. Canny satellite operators realized that they didn't really need one. In Web activity, most traffic flows from a Web server to a user, with only a small amount going the other way. If this small amount could be reduced to zero, customers could access the Internet over the same type of cheap dish that many already use to watch TV.

The first system to take advantage of this idea was DirecPC, released by Hughes in 1996. It didn't supply a complete Internet connection, just a downlink. Customers needed to have some other type of connection, usually an ordinary phone line and modem. Mouse clicks and URLs were sent via the phone line, with Web pages coming back by satellite.

DirecPC was very successful, leading to plenty of copies from rival operators. They have proven most popular in the U.S., for the same reason as ordinary ISPs—unmetered local telephone calls. Europeans are less eager because they have to pay for the uplink phone call as well as for the downlink satellite. Many companies also have "enterprise" versions of DirecPC-style systems. These do have a satellite uplink, but differ from traditional VSATs in that they are highly asymmetric. A typical system might transmit at 256 kbps and receive at 4 Mbps.

For Web surfing, caching can reduce latency to near-zero. A cache is simply a hard disk that stores Web pages closer to the user to improve performance and reduce bandwidth consumption. Every Web browser has its own small cache within a PC, usually a directory called "Temporary Internet Files," for users of MS Internet Explorer. Many companies, universities, and ISPs have special appliances or even entire server farms dedicated to caching. If everyone in a building is accessing the same site, a lot of capacity is saved by fetching the pages only once and storing them locally.

A satellite cache uses the same principle, with a relatively inexpensive hard disk sitting between the user's PC and satellite dish. The satellite automatically broadcasts popular Web sites to everyone so that they will already be stored on this hard disk when the user wants to access them. Back at the main hub, the satellite operator checks these sites for changes, broadcasting them to every user's cache to ensure that content is up-to-date.

Super VSATs

Some companies are planning huge geostationary satellites, equipped with transmitters so powerful that they can be accessed via a nondirectional terminal no larger than a mobile phone. Inmarsat's "M" (Mobile) series can already be used through a portable antenna weighing only 3 kg, about the size of a laptop computer. Aces (Asia Cellular Satellite), a consortium including Lockheed Martin and Ericsson, is taking this further with terminals that they say will be no larger than most GSM phones.