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VSATs

VSATs, or Very Small Aperture Terminals, are a breed of satellite system involving terrestrial dishes of very small diameter (aperture). Operating in the C–band and Ku–band, VSATs are digital and are designed primarily to support data communications on a point-to-multipoint basis for large private networks in applications such as retail inventory management and credit verification, and authorization. While some newer systems also support mesh networks and voice communications, they are unusual at this time. Bandwidth is in channel increments of 56/64 Kbps, generally up to an aggregate bandwidth of 1.544 Mbps. By far the largest concentration of users is in North America, claiming 68% of the market. According to Chuck Emmert of Telecom Applications Corp., Mobil Oil has an installed network of 5,000 sites and Shell Oil has a network of 3,000 sites [3-11].

Bandwidth

Satellites can support multiple transponders and, therefore, substantial bandwidth, with each transponder generally providing increments of [le]36 MHz. Each increment of bandwidth within a given frequency band is provided through a transponder. The amount of bandwidth, the number of frequency bands supported, the number of transponders, and the specific area of coverage all influence the size and power requirements of the satellite.

As in the case of other transmission systems, the higher frequency bands offer greater bandwidth, or capacity. C–band is the most limited, while Ka-band is the most attractive, in this sense, of the commercial satellite frequency bands. As a point of reference, Intelsat I could accommodate only 240 voice circuits, while Intelsat VI supported 120,000 voice circuits and 3 TV channels, with a total bandwidth of 3.46 GHz [3-12].

Error Performance

Satellite transmission is susceptible to environmental interference, particularly at frequencies above 20 GHz. Sunspots and other types of electromagnetic interference particularly impact satellite and microwave transmission. Additionally, some satellite frequency bands (e.g., C–band) compete with terrestrial microwave (see Tables 3.2 and 3.4), again illustrating the requirement for careful frequency management. As a result of these several factors, satellite transmission often requires rather extensive error detection and correction capabilities [3-13].

Distance

Satellite, generally speaking, is not considered to be distance-limited, as the signal largely travels through the vacuum of space. Further each signal travels approximately 22,300 miles in each direction, whether one is communicating across the street or across the country, and assuming that only a single satellite hop is required. However, additional power is required to serve areas which are far removed from the equator (e.g., New Zealand and South Africa), as the signals must travel through substantial atmosphere and as they are more likely to be reflected by the earth’s magnetic field at such a severe angle.

Propagation Delay and Response Time

GEOs, by virtue of their high orbital altitude, impose rather significant propagation delay on the signal and, therefore, doubly affect response time (see Figure 3.8). Given the fact that the radio signals must travel approximately 22,300 miles up to the satellite and the same distance on the return leg, the resulting delay is about 270 milliseconds (.27 seconds). Considering the amount of time required for processing on board the satellite, as well as at the earth stations, the total delay for a one-way transmission is about 320 milliseconds. Therefore, the delay between signal origination (transmission) and response is about 640 milliseconds (.64 seconds), assuming that the response is immediate and that only a single satellite hop is required. Hence, highly interactive voice, data, and video applications are not effectively supported via two-way satellite communications.

Security

As is the case with all microwave and other radio systems, satellite transmission is inherently not secure. Satellite transmission is especially vulnerable to interception, as the signal is broadcast over the entire area of the footprint. Therefore, the unauthorized user must know only the satellite and associated frequency range being employed. Security must be imposed through encryption (scrambling) of the signal.

Cost

The acquisition, deployment, and rearrangement costs of the space segment of satellite systems can be quite high (approximately US$200 million). However, the satellite can be shared by a large number of users, with each user perhaps connecting a large number of sites. As a result, satellite networks often compare very favorably with cabled systems or microwave systems for many point-to-multipoint applications. Cost elements include leasing capacity from a satellite provider (e.g., GE Americom, Hughes Communications Inc., and AT&T Skynet), as well as the acquisition cost of the terrestrial antennae. It should be noted that the acquisition cost of receive-only dishes is relatively low, while transmit/receive dishes are considerably more expensive. As is the case with microwave, satellite transmission is not affected by backhoe fade, as are cabled systems.

Regulation

The space segment of satellite communications is more or less carefully regulated by national, regional, and international bodies. Additionally, local zoning ordinances and health and safety regulations may affect the placement of terrestrial antennae. While satellite network technology is widely available in North America and other countries, it is not widely available to end users, worldwide. Many countries in Asia and Europe have not adopted open sky policies, in support of the incumbent carriers, commonly known as PTTs (Post, Telegraph and Telephone agencies) or TOs (Telecommunications Organizations).

Applications

Satellite applications are many, and increasing rapidly as the traditional voice and data services have been augmented with more exotic applications such as GPS and ATMS. Traditional international voice and data services have been supplanted, to a considerable extent, by submarine fiber optic cable systems.

Traditional, and still viable, applications include international voice and data, remote voice and data (e.g., island nations, isolated areas and sparsely populated areas), television and radio broadcast, maritime navigation, videoconferencing, inventory management & control (VSATs), disaster recovery and paging. More recent and emerging applications include air navigation, Global Positioning Systems (GPS), mobile voice & data (LEOs), Advanced Traffic Management Systems (ATMS), Direct Broadcast Satellite (DBS) TV, Integrated Digital Services Network (ISDN), interactive Television, and interactive multimedia.


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