Thursday, October 30, 2014
Satellite Internet Access
Satellite Internet access is Internet access provided through satellites. The service can be provided to users world-wide through low Earth orbit (LEO) satellites. Geostationary satellites can offer higher data speeds, but their signals can not reach some polar regions of the world. Different types of satellite systems have a wide range of different features and technical limitations, which can greatly affect their usefulness and performance in specific applications.

Signal latency

Satellite Internet access via VSAT in Ghana

Latency 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 it is received at its destination. Compared to ground-based communication, all geostationary satellite communications experience high latency due to the signal having to travel 35,786 km (22,236 mi) to a satellite in geostationary orbitand back to Earth again. Even at the speed of light (about 300,000 km/s or 186,000 miles per second), this delay can be significant. If all other signaling delays could be eliminated, it still takes a radio signal about 250 milliseconds (ms), or about a quarter of a second, to travel to the satellite and back to the ground.[1] For an internet packet, that delay is doubled before a reply is received. That is the theoretical minimum. Factoring in other normal delays from network sources gives a typical one-way connection latency of 500–700 ms from the user to the ISP, or about 1,000–1,400 ms latency for the total round-trip time (RTT) back to the user. This is much more than most dial-up users experience at typically 150–200 ms total latency.[2]

[edit]Geostationary unsuitable for low-latency applications

Satellite Internet dish on the side of a house in rural New York State

A geostationary orbit (or geostationary Earth orbit/GEO) is a geosynchronous orbit directly above the Earth's equator (0° latitude), with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often given geostationary orbits, so that the satellite antennas that communicate with them do not have to move to track them, but can be pointed permanently at the position in the sky where they stay. Due to the constant 0° latitude and circularity of geostationary orbits, satellites in GEO differ in location by longitude only.

The internet latency mentioned above makes satellite Internet service problematic for applications requiring real-time user input, such as online games or remote surgery. This delay can also be irritating and debilitating with interactive applications, such as VoIPvideoconferencing, or other person-to-person communication. It will cause most general market applications (such as Skype) to behave unpredictably and fail, as these are not designed for the difficult compensation required for the high-latency connections. Some people find that the delays inserted into conversation over a high-latency connection make communication difficult and may lead to a feeling of mistrust or hesitation, even when both sides are aware of the lag.

Latency also impacts the initiation of secure Internet connections such as SSL which require the exchange of numerous pieces of data between web server and web client. Although these pieces of data are small, the multiple round trips involved in the handshake produce long delays compared to other forms of Internet connectivity, as documented by Stephen T. Cobb in a 2011 report published by the Rural Mobile and Broadband Alliance.[3]This annoyance extends to entering and editing data using some Software as a Service or SaaS applications as well as other forms of online work.

The functionality of live interactive access to a distant computer can be impaired by high latency. While these problems may be tolerable for basic email access and web browsing, the use of character-by-character command shell or virtual private networks (which typically involve several round trips using layered protocols) is almost impossible through geostationary connections. For this reason, the two largest satellite Internet providers in North America, HughesNet and WildBlue, do not recommend their services be used for VPNs. The HughesNet FAQ states: "Virtual Private Networks do not work well over satellite…HughesNet Technical Support does not provide help with…problems associated with VPN clients."[4]

[edit]Acceptable latencies, but lower speeds, of lower orbits

For geostationary satellites, there is no way to eliminate latency, but the problem can be somewhat mitigated in Internet communications with TCP acceleration features that shorten the round trip time (RTT) per packet by splitting the feedback loop between the sender and the receiver. Such acceleration features are usually present in recent technology developments embedded in new satellite Internet services.

Medium Earth orbit (MEO) and low Earth orbit (LEO) satellites do not have such great delays. The current LEO constellations of Globalstar and Iridium satellites have delays of less than 40 ms round trip, but their throughput is less than broadband at 64 kbit/s per channel. The Globalstar constellation orbits 1,420 km above the earth and Iridium orbits at 670 km altitude. The proposed O3b Networks MEO constellation scheduled for deployment in 2010 would orbit at 8,062 km, with RTT latency of approximately 125 ms. The proposed new network is also designed for much higher throughput with links well in excess of 1 Gbit/s (Gigabits per second). The planned COMMStellation™, scheduled for launch in 2015, will orbit the earth at 1,000 km with a latency of approximately 7 ms. This polar orbiting constellation of 78 microsatellites will provide global backhaul with throughput in excess of 1.2 Gbit/s.

[edit]Ultralight atmospheric aircraft as satellites

A proposed alternative to geostationary relay satellites is a special-purpose solar-powered ultralight aircraft, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20,000 meters. Onboard batteries would be charged during daylight hours by solar panels covering the wings, and would provide power to the plane during night. Ground-based satellite dishes would relay signals to and from the aircraft, resulting in a greatly reduced round-trip signal latency of only 0.25 milliseconds. The planes could potentially run for long periods without refueling. Several such schemes involving various types of aircraft have been proposed in the past.

[edit]Rain fade

A foldable Bigpond satellite Internet dish

Satellite communications are affected by moisture and various forms of precipitation (such as rain or snow) in the signal path between end users or ground stations and the satellite being utilized. This interference with the signal is known as rain fade. The effects are less pronounced on the lower frequency 'L' and 'C' bands, but can become quite severe on the higher frequency 'Ku' and 'Ka' band. For satellite Internet services in tropical areas with heavy rain, use of the C band (4/6 GHz) with a circular polarisation satellite is popular. Satellite communications on the Ka band (19/29 GHz) can use special techniques such as large rain marginsadaptive uplink power control andreduced bit rates during precipitation.

Rain margins are the extra communication link requirements needed to account for signal degradations due to moisture and precipitation, and are of acute importance on all systems operating at frequencies over 10 GHz.[5]

The amount of time during which service is lost can be reduced by increasing the size of the satellite communication dish so as to gather more of the satellite signal on the downlink and also to provide a stronger signal on the uplink.

Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost.

Large commercial dishes of 3.7 m to 13 m diameter are used to achieve large rain margins and also to reduce the cost per bit by requiring far less power from the satellite.

Modern download DVB-S2 carriers, with RCS feedback, are intended to allow the modulation method to be dynamically altered, in response to rain problems at a receive site. This allows the bit rates to be increased substantially during normal clear sky conditions, thus reducing overall costs per bit.

[edit]Line of sight

Fresnel zone. D is the distance between the transmitter and the receiver, r is the radius of the Fresnel zone.

Typically a completely clear line of sight between the dish and the satellite is required for the system to work. In addition to the signal being susceptible to absorption and scattering by moisture, the signal is similarly impacted by the presence of trees and other vegetation in the path of the signal. As the radio frequency decreases, to below 900 MHz, penetration through vegetation increases, but most satellite communications operate above 2 GHz making them sensitive to even minor obstructions such as tree foliage. A dish installation in the winter must factor in plant foliage growth that will appear in the spring and summer.

[edit]Fresnel zone

The radio signal width between any two antennas is not perfectly straight and uniform, as if it were a beam of light. Instead as the signal propagates away from the transmitting antenna, it widens towards the centerpoint between the two antennas and then narrows again as it approaches the receiving antenna. This is known as the Fresnel zone (named for physicist Augustin-Jean Fresnel), and limits the usefulness of satellite dish antennas in locations where there is extremely limited open sky for signal reception. The signal path through space must be clear not only for direct line of sight, but also for the expanding Fresnel zone, which may be several meters[citation needed] larger in diameter than the ground-based satellite dish.

[edit]Two-way satellite-only communication

The back panel of a satellite modem, with coaxial connections for both incoming and outgoing signals, and an Ethernet port for connection

Two-way satellite Internet service involves both sending and receiving data from the remote VSAT site via satellite to a hub teleport, which then relays data via the terrestrial Internet. The satellite dish at each location must be precisely pointed to avoid interference with other satellites. The two way satellite market can be divided into those systems that support professional applications, such as banking, retail etc. and those built to provide home or small business users with access. The key difference between these systems can be seen in their ability to support advanced quality of service controls. While systems for professionals such as those from iDirect will allow the operator to define and meet strict service level agreements - those used for consumer access provide a 'best effort' service level.

Some providers oblige the customer to pay for a member of the provider's staff to install the system and correctly align the dish—although the European ASTRA2Connect system encourages user-installation and provides detailed instructions for this. Many customers in the Middle East and Africa are also encouraged to do self installs. At each VSAT site the uplink frequency, bit rate and power must be accurately set, under control of the service provider hub.

There are several types of two way satellite Internet services, including time division multiple access (TDMA) and single channel per carrier(SCPC). Two-way systems can be simple VSAT terminals with a 60–100 cm dish and output power of only a few watts intended for consumers and small business or larger systems which provide more bandwidth. Such systems are frequently marketed as "satellite broadband" and can cost two to three times as much per month as land-based systems such as ADSL. The modems required for this service are often proprietary, but some are compatible with several different providers. They are also expensive, costing in the range of US$600 to $2000.

The two-way "iLNB" used on theASTRA2Connect.

The two-way "iLNB" used on the ASTRA2Connect terminal dish has a transmitter and single-polarity receive LNB, both operating in the Ku band. Pricing for Astra2Connect modems range from €299 to €350. These types of system are generally unsuitable for use on moving vehicles, although some dishes may be fitted to an automatic pan and tilt mechanism to continuously re-align the dish—but these are more expensive. The technology for ASTRA2Connect was delivered by a Belgian company called Newtec.

The Toowaysatellite modem

[edit]Bandwidth

Satellite internet customers range from individual home users with one PC to large remote business sites with several hundred PCs.

Home users tend to use shared satellite capacity to reduce the cost, while still allowing high peak bit rates when congestion is absent. There are usually restrictive time-based bandwidth allowances so that each user gets their fair share, according to their payment. When a user exceeds their allowance, the company may slow down their access, deprioritise their traffic or charge for the excess bandwidth used. For consumer satellite internet, the allowance can typically range from 200 MB per day to 17,000 MB per month.[6][7] A shared download carrier may have a bit rate of 1 to 40 Mbit/s and be shared by up to 100 to 4,000 end users.

The uplink direction for shared user customers is normally time division multiple access (TDMA), which involves transmitting occasional short packet bursts in between other users (similar to how a cellular phone shares a cell tower)

Each remote location may also be 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.

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.

Normal VSAT dishes (1.2–2.4 m diameter) are widely used for VoIP phone services. A voice call is sent by means of packets via the satellite and internet. Using coding and compression techniques the bit rate needed per call is only 10.8 kbit/s each way.

[edit]Portable satellite Internet

[edit]Portable satellite modem

These usually come in the shape of a self-contained flat rectangular box that needs to be pointed in the general direction of the satellite—unlike VSAT the alignment need not be very precise and the modems have built in signal strength meters to help the user align the device properly. The modems have commonly used connectors such as Ethernet or Universal serial bus. Some also have an integrated Bluetooth transceiver and double as a satellite phone. The modems also tend to have their own batteries so they can be connected to a laptop without draining its battery. The most common such system is INMARSAT's BGAN—these terminals are about the size of a briefcase and have near-symmetric connection speeds of around 350–500 kbit/s. Smaller modems exist like those offered by Thuraya but only connect at 444 kbit/s in a limited coverage area.

Using such a modem is extremely expensive—bandwidth costs between $5 and $7 per megabyte. The modems themselves are also expensive, usually costing between $1,000 and $5,000.[8]

[edit]Internet via satellite phone

For many years[when?] satellite phones have been able to connect to the internet. Bandwidth varies from about 2400 bit/s for Iridium network satellites and ACeS based phones to 15 kbit/s upstream and 60 kbit/s downstream for Thuraya handsets. Globalstar also provides internet access at 9600 bit/s—like Iridium and ACeS a dial-up connection is required and is billed per minute, however both Globalstar and Iridium are planning to launch new satellites offering always-on data services at higher rates. With Thuraya phones the 9,600 bit/s dial-up connection is also possible, the 60 kbit/s service is always-on and the user is billed for data transferred (about $5 per megabyte). The phones can be connected to a laptop or other computer using a USB or RS-232 interface. Due to the low bandwidths involved it is extremely slow to browse the web with such a connection, but useful for sending email, Secure Shelldata and using other low-bandwidth protocols. Since satellite phones tend to have omnidirectional antennas no alignment is required as long as there is a line of sight between the phone and the satellite.

[edit]One-way receive, with terrestrial transmit

One-way terrestrial return satellite Internet systems are used with conventional dial-up Internet access, with outbound (upstream) data traveling through a telephone modem, butdownstream data sent via satellite at a higher rate. In the U.S., an FCC license is required for the uplink station only; no license is required for the users.

Another type of 1-way satellite Internet system uses General Packet Radio Service (GPRS) for the back-channel.[9] Using standard GPRS or Enhanced Data Rates for GSM Evolution(EDGE), if the upload volume is very low and since this service is not per-time charged, but charged by volume uploaded, costs are reduced for higher effective rates. GPRS as return improves mobility when the service is provided by a satellite that transmits in the field of 50–53 dBW. Using a 33 cm wide satellite dish, a notebook and a normal GPRS equipped GSM phone, users can get mobile satellite broadband.

[edit]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:

[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 certain communications between clients and servers [1] 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 (minimum of 240 ms one-way, resulting in a minimum round-trip time of almost 500 ms). Download speeds can be very fast compared to dial-up.

[edit]Theory of operation

Remote sites use proxy server or(and) Virtual private network servers 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)

[edit]One-way multicast, receive only

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 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 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 a File Transfer Protocol (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 transmitting station must provide access control, priority queuing, sending, and encapsulating of the data.

[edit]Efficiency increases

[edit]Reducing satellite latency

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.[10] Special IP stacks and proxies can also reduce latency through lessening the number of roundtrips, or simplifying and reducing the length of protocol headers. These types of technologies are generally referred to as TCP accelerationHTTP pre-fetching and DNS caching.

[edit]Elimination of advertising

While also effective for terrestrial communications, the use of ad-blocking software such as Adblock for Firefox is exceptionally beneficial for satellite Internet, as most Internet advertising websites use cache busting in order to render the browser and ISP's cache useless, by displaying advertisements (for the purpose of maximizing the number of ad views seen by theaffiliate marketing company's server).[citation needed]

[edit]Satellites launched

SkyTerra-1 was launched in mid-November 2010 and will provide service across North America while Hylas-1 was launched at the end of November 2010 and will target Europe.[11]

On December 26, 2010, Eutelsat's KA-SAT was successfully launched by an ILS Proton Breeze M vehicle at the Baïkonour Cosmodrome Kazakhstan. The last satellite was due in service in mid 2011. It covers the European continent with 80 spot beams—focused signals that cover an area a few hundred kilometers across Europe and the Mediterranean. Spot beams allow for frequencies to be effectively reused in multiple regions without interference. The result is increased capacity. Each of the spot beams will have an overall capacity of 900 Mbit/s and the entire satellite will have a capacity of 70 Gbit/s.[12][11]