Satellite Internet access
Internet via satellite is a satellite- based broadband Internet access, use of which by the use of a geostationary satellite in the entire area into which the satellite is possible, for example Europe via Eutelsat and SES Astra.
- 3.1 Very high latency
- 3.2 vs. transparency. transfer rate
- 3.3 Slow Start
Variants
Basically, two types of satellite link must be distinguished.
2-way satellite link
In this form of the compound of the return channel ( upstream and downstream ) are prepared via a satellite.
In the downstream stand depending on the provider transfer rates between 64 kbit / s and 150 Mbit / s willing to put in separately negotiated individual cases, different transmission rates. The transmission rate of the upstream, depending on the target group ( the consumer and professional users) between 64 kbit / s and 20 Mbit / s Technically, however, much higher transmission rates are possible. So the Belgian satellite technology company Newtec announced in mid-June 2012 that it speeds of over 500 Mbit / s reached by a 72 MHz transponder on a Eutelsat satellite.
1-way satellite link
More than ten years ago the first asymmetric compound used in which only the downstream was passed through a satellite, but the upstream ran over terrestrial links. This technical solution was used until a few years ago in Europe by a few tens of thousands of users. Downstream, the user transmission rates were at that time depending on the provider prepared between 256 kbit / s and 36 000 kbit / s. The reception of the satellite signals can also be performed using commercially available DVB -S card, provided that the modulation support and a compatible software is available.
The data transmission rate of the return channel was based on the technology used. Most modem connections over POTS or ISDN connections have been used, which provides set up to 128 kbit / s upstream rate. For mobile applications, GPRS, UMTS or GSM connection for the return channel could be used.
For connections with terrestrial return channel of this was used in parallel to the satellite link for data transmission in the downstream direction with some providers. When this channel is utilized in the downstream, the flow further packets over the more expensive for the provider satellite transmission path. By implication, this means that if the terrestrial line is not at full capacity - for example, due to a slow Internet server - take all the dates terrestrial. With providers who were not using this technique, the actual download running exclusively via satellite.
Benefits
The advantage of satellite links is the combination of the relatively large bandwidth and its wide availability, which is largely location-independent.
Advantage of the pure satellite connection is that they are data or telephone lines here is completely independent of terrestrial available and no fixed location is required. It thus stands for ships and aircraft. The user must only be in the broadcast range of a suitable satellite or move it.
The advantage of a connection with terrestrial return channel to a two-way solution was earlier in the lower cost for the satellite transmission rate, the lower signal propagation delays (latency), and the cheaper devices. If for the return channel, a mobile system is used, the user can also be mobile.
Disadvantages
Very high latency
Alone by the large distances and the speed of light by ~ 300,000 km / s resulting for instance with geostationary satellite from the earth station to the satellite and back to the service user signal propagation times of at least 239 ms. In bi-directional communication via the satellite must be overcome twice for the way of inquiries and the way back a reply that route. Together with other factors delay resulting latency between 500-700 ms, which is far worse than, say, the usual dial-up modem latencies of "only" 150-200 ms. DSL latencies are on the order of 20 ms. Internet services using the satellites for the round trip, therefore not suitable for video telephony and games or other applications where high latencies are associated with strong restrictions.
Transparency vs. transfer rate
The requirements for effective use possible of the theoretically available data transmission rates and the greatest possible transparency application can usually not be met simultaneously. The most effective data transmission rates of use offers the one-way service, the client / server version. She heads before the connection and behind the satellite connection through a proxy server (client and remote proxy ). In order to remain application- transparent, while one-way service usually a variant tunnel ( VPN / PPTP) is offered.
Slow Start
The Transmission Control Protocol ( TCP) is not designed for signal propagation times, as they occur in satellite links. The propagation delay is about 240 ms for the path from the Earth to the satellite and back. The resulting by these terms Round Trip Time (RTT) means that a TCP connection according to the so-called slow start, the transfer rate does not substantially increase, and thus, for example, to a compound with 2 Mbit / s theoretical data transmission rate, under unfavorable conditions, only few 10 kbit / s could be transmitted. This problem can be circumvented by the use of special proxy server.
Provider
IpStar satellite modem