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ONE-WAY MULTICAST One-way multicast satellite Internet systems are used for Internet Protocol (IP) Multicast -based data, audio and video distribution. In the U.S. , a Federal Communications Commission (FCC) license is required only for the uplink station and no license is required for users. Note that most Internet protocols will not work correctly over one-way access, since they require a return channel. However, Internet content such as Web Page s can still be distributed over a one-way system by "pushing" them out to local storage at end user sites, though full interactivity is not possible. This is much like TV or radio content which offers little user interface. System hardware components Similar to one-way terrestrial return, satellite Internet access may include interfaces to the Public Switched Telephone Network for Squawk Box applications. An Internet connection is not required, but many applications include an File Transfer Protocol (FTP) server to queue data for broadcast. System software components Most one-way multicast applications require custom programming at the remote sites. The software at the remote site must filter, store, present a selection interface to and display the data. The software at the transmitting station must provide access control, priority queueing, sending, and encapsulating of the data. ONE-WAY WITH TERRESTRIAL RETURN One-way terrestrial return satellite Internet systems are used with traditional Dial-up Access to the Internet, with outbound data traveling through a telephone Modem , but downloads sent via satellite at a speed near that of Broadband Internet Access . In the U.S., an FCC license is required for the uplink station only; no license is required for the users. System hardware components The transmitting station (also called "teleport", "head end", "uplink facility", or "hub") has two components:
At the remote location (Earth station) the setup consists of:
Depending on the providers terms of contract, one cost effective way to use 1-way satellite internet is to use General Packet Radio Service (GPRS) for the back-channel. By utilizing a 9600 Bit/s connection that is offered in standard GPRS, the upload volume is very low and since this service is not per-time charged, users are able to surf and download in Broadband speeds. There are companies offering speed up to 24 Mbit/s. Another view of using GPRS as return would be the mobility when the service is provided by a satellite that transmits in the field of 50 to 53 DBW . Using a 33 cm wide satellite dish, a notebook and a normal GPRS equipped GSM Phone , users can get broadband everywhere. System software components Remote sites require a minimum of programming to provide authentication and set proxy server settings. Filtering is usually provided by the DVB card driver. Often, non-standard IP Stacks are used to address the Latency and asymmetry problems of the satellite connection. Data sent over the satellite link is generally also encrypted, as otherwise it would be accessible to anyone with a satellite receiver. Many IP-over-satellite implementations use paired proxy servers at both endpoints so that clients and servers do not need to accept the latency inherent in a satellite connection. For similar reasons, there exist special Virtual Private Network (VPN) implementations designed for use over satellite links because standard VPN software cannot handle the long packet travel times. Upload speeds are limited by the user's dial-up modem, and latency is high, as it is for any satellite based Internet. Download speeds can be very fast compared to dial-up:1 Mbits,4 Mbits,16 Mbits packages are generally offered. Theory of operation Remote sites use the Proxy Server at the Earth Station (teleport), which is configured to route all outbound traffic to the QoS server, which makes sure no user exceeds their allotted bandwidth or monthly traffic limits. Traffic is then sent to the Encapsulator , which puts the IP packets inside of DVB packets. The DVB packets are then sent to the DVB modem and then to the transmitter (BUC). TWO-WAY Two-way satellite Internet service sends data from remote sites via satellite to a hub, which then sends the data to the Internet. The satellite dish at each location must be precisely positioned to avoid interference with other satellites. The Oscillators in some Radar Detector s can cause interference with these systems. Also, each location must use power management to adjust the amount of transmit power to compensate for things like Rain Fade . There are several types of two way satellite Internet services, such as Time Division Multiple Access (TDMA) or Single Channel Per Carrier (SCPC). Each remote location is also equipped with a telephone modem; the connections for this are as with a conventional dial-up ISP. Two way satellite systems may sometimes use the modem channel in both directions for data where latency is more important than bandwidth, reserving the satellite channel for download data where bandwidth is more important than latency, such as for File Transfer s. Uplink speeds rarely exceed one megabit per second and latency can be up to one second. Satellite phone services such as Iridium also provide data services at the comparatively slow speed of 2400 bit/s. International Mobile Satellite Organization (INMARSAT) offers three bidirectional satellite internet services called Broadband Global Area Network (BGAN), Regional BGAN and MPDS, neither of which have to be precisely aligned but the speeds are lower than the dish-based systems and bandwidth costs are much higher. BGAN has the highest data rate. Thuraya offers a similar service but this is not as fast as BGAN. In 2006 the European Commission sponsored the UNIC project which aims at developing an end-to-end scientific test bed for the distribution of new broadband interactive TV-centric services delivered over low-cost two-way satellite to actual end-users in the home. The UNIC architecture employs DVB-S2 standard for downlink and DVB-RCS standard for uplink. REDUCING SATELLITE LATENCY One solution is to use satellites in much Lower Orbit very close to the Earth, to shorten the travel distance. Such orbital paths are no longer geostationary, and so would require a large number of satellites in orbit so that at least one is visible in the sky at all times. Communication dishes could no longer be fixed, and would either need some way to track the satellites as they move across the sky, or to work in an Omnidirectional manner without causing interference for anything else. A theoretical alternative to satellites that is being explored is the use of ultra-light Solar Power ed airplane (see the NASA Pathfinder ) that could fly in a continuous, circling path perhaps 70,000 feet (20 km) high or an airship (see Stratellite ). These would act as flying satellites, providing high-speed service to customers below the aircraft. Since the roundtrip signal distance would only be , the latency caused by the Speed Of Light is an almost insignificant 0.1 ms under the craft, and 2 ms at the edge of the covered area, at a 300 km (200 miles) distance. Such service via aircraft is still in the experimental stages as of 2006. Another practical method which can lower latency is to configure the satellite with a robust computer and cache. Much of the slowdown associated with satellite Internet is that for each request, many roundtrips must be completed before any useful data can be received by the requester.TCP is bound by the low latency of a three-way handshake. See Transmission Control Protocol . A well-maintained cache located in space would alleviate many of the full round-trips. Caching would assist in speeds as well; When cached data is requested, it takes less than half of the normal time to receive the response, speeding up the time normally wasted on mere latency. Of course, this method requires some forethought by the people who design the satellite - it is impractical to retrofit older satellites with new equipment such as this. REFERENCES SEE ALSO
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