23-07-2014, 02:03 PM
Satellite Internet Access
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Introduction
Although Internet is considered to be a global network, nowadays there are many places, large areas, rural and isolated locations and even entire nations without the possibility to access to the network or, in best cases, to use broadband connections such as fiber optic networks and ADSL modem since terrestrial networks are relatively expensive to build and maintain. In short, there are many places where IP communications seemed impossible.
Several methods was proposed and studied over the years to solve this problem but the majority of them didn’t have success due to the vast set of constraints and problems that make this situations difficult to resolve. However solutions based on satellite technology had a moderate success even if it presents many limitations. As a matter of fact in this context satellite technology could plays a leading role because of its intrinsic multicast and broadcast capabilities, mobility aspects and global reach
Satellites
A communications satellite is basically a microwave repeater station revolving around the earth in a specified orbit. Satellites are meant for sending, transmitting, receiving and processing of electromagnetic signals (radio waves) of frequency larger than 1 Ghz. A ground station transmit the signal on a given frequency to the satellite which operates a frequency translation of the received signal and finally retransmits it to the ground stations. Transmission of signals is performed via transponders. A transponder which is usually capable of providing 40-155 Mbps bandwidth consists of a transceiver and antenna tuned for a specific frequency. Most satellites have multiple transponders (even over 70) which can be used simultaneously not only to gain higher bandwidth on demand but also to regenerate the signal (to its original shape and power level). A typical satellite has 24 transponders: 12 to regenerate the consecutive frequency blocks consisting of the
segments assigned for the vertical signal polarization and 12 for the frequency blocks comprising the segments assigned for the horizontal signal polarization. Satellites are powered by solar cells and a backup battery and use propulsion jets to adjust the orbit when needed. Majority of
Positioning
Satellite communication is a line-of-sight one-way or two-way radio frequency transmission system that comprises a transmitting station, a satellite system that acts as a signal regeneration node and one or more receiving stations. Satellites orbit the earth at different altitudes and are thus categorized as LEO, MEO and GEO according to their altitude.
Operating Frequencies
The transmission channel of a satellite system is a radio channel using a direct-wave approach, operating at specific radio frequency bands within the overall electromagnetic spectrum. The frequency of operation is the Super High Frequency (SHF) range that is from 3 to 30 Ghz. A satellite link further is a radio link between a transmitting earth station and a receiving earth station through a communication satellite and consists of one uplink and one downlink.
According to the application of a given system, satellites use different frequencies for uplink (from ground station to satellite) and downlink (from satellite to ground station) and these bands are controlled by the International Radio Frequencies Board and by the Federal Communications Commission. In addition to this also the channel and subchannel bandwidth are dictated from applications requirements. More specifically the most common frequency bands are C-band, X-band, Ku-band, Ka-band and V-band
Subscriber Side Ground Terminal
Each satellite subscriber has an outdoor unit which is a small (65-240 cm in diameter) low-cost dish antenna with transmit and receive components placed at the focal point of the antenna. The antenna must have a direct Line-of-Sight (LOS)
to the satellite since the signal is not able to penetrate any obstacles. Larger dish usually gives better transmission power and wider receiver coverage.
The indoor unit is a satellite modem which serves as an interface between the ODU and customer equipment and controls satellite transmission. The demodulation and transmission optimization is often handled by a custom made software. The satellite modem can be connected to a PC using an USB port or DVB/MPEG-2-card depending on selected technology. Once the PC is able to receive the packets, they can be rerouted using standard Ethernet card or wireless communication techniques such as WLAN.
Latency
The most evident problem with the satellite communication system without doubt is the latency that is the delay between requesting data and the receipt of a response, or in the case of one-way communication, between the actual moment of a signal’s broadcast and the time received at its destination. This problem is closely correlated with the satellite orbital position. As a matter of fact, compared to ground-based communication, all geostationary satellite communications experience high latency due to the signal having to travel 35.768 km to a satellite and back to Earth again. More specifically, the major consequence is that signals experience a propagation delay no less than 119 ms on an uplink and no less than 240 ms for an uplink and downlink or a one-way end-to-end transmission path. A two-way interactive session with a typical communications protocol, such as TCP, will experience this roundabout delay twice (no less than 400 ms) because the information is making two round trips to the satellite and back. One-way or broadcast applications easily deal with this issue, as the delay is not noticeable to the video viewer or the receive data user.
Merits of the Satellite Communication Systems
. Merits of the Satellite Communication Systems
Such as any other technology, satellite systems have several clear advantages. First of all they offer a wide geographic coverage including interconnection of remote terrestrial networks and currently there are several satellite operators who offer global coverage without any further investments in expensive terrestrial networks which is good for Internet Service Providers. Above all satellite systems run not only to solve the digital divide question, allowing Internet access in all isolated or rural areas, but also to play a leading role in allowing communications over all zones struck down by natural disaster. The second important point is that satellite connectivity offers single-hop transmission which avoids the packet loss
TCP Exploiting
As we saw, using TCP over satellite links brings a lot of issues.
Usually, for accessing internet over satellite we base over GEO satellites, and this signifies a lot of delay over our signals, because of its long feedback loop. TCP has poor performances in this scenario. So we could think to LEO as a good alternative to GEO, but this brings other difficulty in using IP protocol. LEO satellites doesn't seem fixed to the observer as GEOs but the period of its revolution over the earth is of about 90 minutes. So this mean that our base station (the observer) should change frequently its antenna position but above all the satellite whom it's linked. This would take a lot of problems with traffic routing due to this frequently handover, and also due to the high fluctuation of the terrestrial-satellite propagation delay.
TCP is very prone to delays over the channel: the first problem is that TCP transmitter will get its ACK only after a long delay but, before the reception, he could think that its packet weren't received. It could be compelled to keep a large number of packets sent but not yet acknowledged and, thus, to require expensive buffers (this is mostly related to the large delay*bandwidth product typical of GEO satellite systems).