Satellite Internet access
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Satellite Internet services are used in locations where terrestrial Internet access is not available and in locations which move frequently. Internet access via satellite is available worldwide, including vessels at sea. There are three types of satellite Internet service:
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[edit] One-way multicast
One-way multicast satellite Internet systems are used for IP multicast-based data, audio and video distribution. In the U.S., an 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 pages 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 a TV or radio content which offers little user interface.
[edit] 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 FTP server to queue data for broadcast.
[edit] 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 teleport must provide access control, priority queueing, sending, and encapsulating of the data.
[edit] 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 are sent via satellite at a speed near that of broadband Internet access. In the U.S., a FCC license is only required for the uplink station, no license is required for the users.
[edit] System hardware components
The transmitting station is called the teleport (also called "head end", "uplink facility", or "hub"). It has two components:
- Internet connection: The ISP's routers connect to proxy servers which can enforce Quality of Service (QoS) bandwidth limits and guarantees for user traffic. These are then connected to a DVB encapsulator which is then connected to a DVB-S modem. The RF signal from the DVB-S modem is connected to an up converter which is connected via feedline, to the outdoor unit.
- Satellite uplink: The Block Up Converter (BUC) and Low Noise Block converter (LNB, optional) which may use a wave guide to connect to the OMT (optional) which is bolted to the feedhorn which is connected by metal supports to the satellite dish and mount.
At the remote location (Earth station) the setup consists of:
- Outdoor unit
- Indoor unit
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 transfers.
The most cost effective way though to use 1-way internet via satellite is to use GPRS.[citation needed] By utilizing a 9600bps (0,9kb/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 24Mbits. 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 53dbW. Using a 33cm wide satellite dish, a notebook and a normal GPRS equipped GSM phone, users can get broadband everywhere.
[edit] 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 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 Mbit/s,4 Mbit/s,16 Mbit/s packages are generally offered.
[edit] Theory of operation
Remote sites use the proxy server at the 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).
[edit] 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 detectors 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 TDMA or SCPC.
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.
INMARSAT offers a satellite internet service called BGAN that does not have to be precisely aligned but the speeds are lower than the dish-based systems and bandwidth costs are much higher. Thuraya offers a similar service but this is not as fast as BGAN
See the standard DVB-RCS.
[edit] Reducing satellite latency
The only real alternative 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. The lower orbits would also subject the satellites to a slight drag effect from the upper atmosphere, requiring a means to boost the orbits back up as the satellites gradually slow down.
A theoretical alternative to satellites that is being explored is the use of ultra-light solar-powered airplane (see the Helios Prototype) or an airship (see Stratellite) that could fly in a continuous a circling path perhaps 70,000 feet (20 km) high. These would act as flying satellites, providing high-speed service to customers below the aircraft. Since the roundtrip signal distance would only be 30 miles, 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.
[edit] See also
- Back-channel and return channel
- HughesNet (formerly DIRECWAY)
- IP over DVB
- Satellite dish
- SkyFX
- StarBand
- VSAT
- WildBlue
[edit] External links
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