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Subject to technical reliability, and attenuation of the microwave signals (typically around 2 cm wavelength) by intense rain and hail, geostationary satellites provide carriers with permanent high-capacity communication links for digital and analogue voice and video signals. Due to the limited directionality of ground-station antennae, there are problems with having two satellites using the same frequencies within a few degrees of each other. Consequently there is intense and sometimes bitter jostling for position over populated parts of the globe.
While it is possible for mobile apparatus with compact and non-directional (and therefore very low gain) antennae to communicate with geostationary satellites, the long 36,000 km distance to the satellite makes this difficult. Mobile use of satellite communications are increasingly based on Low Earth Orbit (LEO) satellites.
Carrier Earth-Stations
Telecommunications links with geostationary satellites fall into two categories. Firstly the traditional, large dish (for instance 3 to 15 meters) carrier-operated earth station, and secondly the much smaller dish VSAT (Very Small Aperture Terminal) earth stations. VSAT dishes may be half to two meters in diameter, and may be used for mobile purposes - although they need to be carefully aligned to point towards the satellite.
The large earth stations use large and therefore highly efficient and directional antennae. They typically transmit and receive multiple frequencies at once. The large earth stations are few in number, and are operated by the carrier who owns the satellite, rather than by customers who purchase satellite communication services from the carrier.
VSAT - Very Small Aperture Terminal Earth-Stations
Recent developments have lead to increased performance and lower costs in the VSAT field. This means that customer sites may have a relatively small dish, with low-maintenance, remotely managed electronics, which provides them with a complete two-way link, via the satellite, to a carrier earth station. With expenditures in the tens of thousands of dollars per site, VSAT terminals are an attractive proposition for companies wishing to link many remote sites into a Virtual Private Network. Generally all the sites receive a common, relatively large bandwidth (perhaps a few Megabits per second) data stream, and have a limited upstream capacity. Each VSAT transmitter shares a frequency with other transmitters, so they take it in turns to transmit data according to demand.
Satellites cannot match the enormous bandwidth of optic-optic cables using WDM but they can enable many remote sites to be linked without the need for laying cable, or worrying about the cable being damaged by earthworks or fishing operations.
Latency and Geosynchronous Satellites
A second potential disadvantage of geostationary satellites compared to LEO satellites, fiber and terrestrial microwave links is the time delay - or latency - of the 60,000 km round trip between one earth-station and the other. Since light and radio waves travel through air and vacuum at 300,000 km per second, this is a 200 msec (millisecond) delay. In a VSAT-based VPN example, with a telephone conversation between people at two remote sites, the delays add up. For instance one person speaks and their speech signal reaches the main ground-station 200 msec later, where it is sent up to the satellite again and down to the second site where it arrives 400 msec after the person spoke. The response of the second person is similarly delayed by the time it reaches the first. Since telephones typically echo back part of the signal they receive, this can lead to echoes every 800 msec. These echoes can be eliminated with echo-cancellers - but this introduces annoying problems with cutting off the speech of one person when the other person's speech arrives there.
Many data applications are insensitive to such delays - but some, such Telnet or terminal access to a remote server would be more difficult with such delays. In addition, TCP - the layer of the Internet protocols which ensures reliable message delivery for all higher level protocols, such as HTTP is affected by these delays, but such effects should be minimal when the data rate of the messages is below 100 kb/s or so.
In contrast, a fiber link halfway around the Earth would be just over 20,000 km, and with the lower speed of light in the fiber, the one-way delay time would be around 100 msec.
Microwave Links
PDH and SDH Microwave Links
Microwave links have long been used by carriers in areas where the costs of laying cable were prohibitive and as a backup for such cables. Initially these links carried multiplexed analogue voice and video signals but most carrier microwave links today carry digital data using the PDH or perhaps SDH standards described above. Microwave towers can be used over several hundred kilometers of ocean, or for 50 to 100 km hops over land, depending on the terrain and the elevation of the dishes.
Functionally similar to the lower capacity PDH/SDH fiber systems, but potentially affected by intense rain, these microwave links of carriers are part of their backbone and generally of little direct interest to the carrier's customers. Low-Cost, Shorter Range Microwave Links
In the late 1990s major advances were made in smaller microwave systems, which are suitable for deployment, by large and small organizations. Many of these use 'spread-spectrum' modulation of the microwave signals, which enables reception in the presence of multipath propagation, which would upset other modulation schemes. While these links are limited to line-of-sight or near line-of-sight and so to ten or twenty km or so, they generally have a number of characteristics which make them very attractive alternatives to purchasing high-cost telecommunications services from carriers:
- Data rates of 0.5 to 2 and sometimes to 10 Mb/s
- Transmit powers so low that they may not need to be licensed.
- Complete point-to-point 2 Mb/s LAN links costing less than US$10,000
- Direct interface to LANs, for instance some units behave as a remotely managed router and have an Ethernet connector, enabling a pair of them to be used to link two LANs
- Integration of LAN, telephony and other data streams
- Encryption to ensure security against interception
- Antennae sizes ranging from 30 cm to 1 meter
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