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XipLink Successfully Demos LTE Using Satellite Wireless link optimization specialist XipLink has completed a test with a significant Asian cellular service provider, delivering accelerated voice, data and video traffic exceeding 90 megabits per second (Mbps) to a remote LTE base station over a satellite connection... 5 Things We Learned at Global MilSatCom Today Global MilSatCom is one of the main events for military and communications executives and officials... Cobham Satcom Debuts Lightweight Sailor VSAT Antenna Systems Cobham Satcom has revealed two new 60cm Ka-band VSAT antenna systems for maritime customers... Eutelsat Inks new Broadcast and IoT Deals Eutelsat Communications has signed up a number of customers in Africa and in Europe for broadcast services, and also tightened it’s collaboration with Internet of Things (IoT) pioneer Sigfox... Avanti, Global Invacom Demo Encrypted Satellite TV Over Ka-band IP Networks Avanti Communications and Global Invacom, in partnership with Microsoft and the European Space Agency’s (ESA) Advanced Research in Telecommunications Systems (ARTES) program, are demonstrating satellite broadcasting using standard IP-enabled networks such as Ethernet or Wi-Fi... Iridium GO! Gains Five Third Party Apps Iridium has made five mobile applications from third party developers available for Iridium GO!, the company’s global smartphone access device... Industry Coalition Launched to Extend Role of Satellite Broadcasting Six leading satellite operators and manufacturers announced the formation of the SAT>IP Alliance at the 2015 NAB Show in Las Vegas, Nev... New Market Opportunities for Satellite Start to Emerge From Global Connectivity Boom The “connected car” and “smart cities” are two potentially new and exciting markets the satellite industry could play in over the coming years, particularly the former as the potential for “always-on” connectivity could be a big draw for satellite... Swiss Space Systems Opens Croatia Subsidiary, Plans Spaceport Swiss Space Systems (S3) has inaugurated a new subsidiary in Croatia, where the company hopes to build a spaceport as well as a facility for manufacturing the upper stage of the SOAR launch vehicle... SpaceX Satellite Project Boosted by $1 Billion Google, Fidelity Investment Google and Fidelity have confirmed an investment of approximately $1 billion in SpaceX, powering the second large-scale Low Earth Orbit (LEO) satellite communications constellation... GX Launches Getting Dicey after Proton Puts Telecom Satellite in Wrong Orbit Failed Proton Launch - Opps - Wrong Orbit: After last Tuesday's failed Proton launch Inmarsat's GX launches look riskier than ever...more Explore some of the locations with our communication systems.more dr.sc. Draško Marin, dipl.ing. «Elektroničke komunikacije-regulativa, sustavi, norme» Following the launch of the first satellite, Sputnik 1, by the Soviet Union in October, 1957, the US successfully launched the Explorer 1 satellite in 1958. The first commercial communications satellite was Telstar 1, built by Bell Labs and launched in July, 1962. The idea of a geostationary satellite — one that could orbit the Earth above the equator and remain fixed by following the Earth’s rotation — was first proposed by science fiction writer Arthur C. Clarke in 1945. The first satellite to successfully reach geostationary orbit was Syncom3, built by Hughes Aircraft for NASA and launched Aug. 19, 1963. Succeeding generations of communications satellites featuring larger capacities and improved performance characteristics were adopted for use in television delivery, military applications and telecommunications purposes. Following the invention of the Internet and the World Wide Web, geostationary satellites attracted interest as a potential means of providing Internet access. A significant enabler of satellite-delivered Internet has been the opening up of the Ka band for satellites. In December, 1993, Hughes Aircraft Co. filed with the Federal Communications Commission for a license to launch the first Ka-band satellite, Spaceway. In 1995, the FCC issued a call for more Ka-band satellite applications, attracting applications from 15 companies. Among those were EchoStar, Lockheed Martin, GE-Americom, Motorola and KaStar Satellite, which later became WildBlue. Among prominent aspirants in the early-stage satellite Internet sector was Teledesic, an ambitious and ultimately failed project funded in part by Microsoft that ended up costing more than $9 billion. Teledesic's idea was to create a broadband satellite constellation of hundreds of low-orbiting satellites in the Ka-band frequency, providing inexpensive Internet access with download speeds of up to 720 Mbit/s. The project was abandoned in 2003. Teledesic's failure, coupled with the bankruptcy filings of the satellite communications providers Iridium Communications Inc. and Globalstar, dampened marketplace enthusiasm for satellite Internet development. It wasn’t until September 2003 when the first Internet-ready satellite for consumers was launched by Eutelsat. In 2004 with the launch of Anik F2, the first high throughput satellite, a class of next-generation satellites providing improved capacity and bandwidth became operational. More recently, high throughput satellites such as ViaSat's ViaSat-1 satellite in 2011 and HughesNet’s Jupiter in 2012 have achieved further improvements, elevating downstream data rates from 1-3 Mbit/s up to 12-15Mbit/s and beyond. Internet access services tied to these satellites are targeted largely to rural residents as an alternative to Internet service via dial-up, ADSL or classic FSSes. ViaSat-1, the highest capacity communications satellite in the world, was launched Oct. 19, 2011 from Baikonur, Kazakhstan. With 140 Gbit/s total throughput capacity, the new satellite services the Exede Internet service, with download and upload speeds much faster than anything previously offered in the satellite industry. The EchoStar XVII satellite with JUPITER High-Throughput Technology, built by Space Systems/Loral, was launched July 5, 2012 by Arianespace and was placed in its permanent geosynchronous orbital slot of 107.1° West longitude. The satellite services the HughesNet Gen4 satellite Internet service. Employing a multi-spot beam, bent-pipe architecture, this Ka-band satellite has over 100 Gbit/s of throughput capacity. 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. 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. Satellite Internet access is Internet access provided through communications satellites. Modern satellite Internet service is typically provided to users through geostationary satellites that can offer high data speeds, with newer satellites achieving downstream data speeds up to 15 Mbps Satellite Internet generally relies on three primary components: a satellite in geostationary orbit (sometimes referred to as a geosynchronous Earth orbit, or GEO), a number of ground stations known as gateways that relay Internet data to and from the satellite via radio waves (microwave), and a VSAT (very-small-aperture terminal) dish antenna with a transceiver, located at the subscriber's premises. Other components of a satellite Internet system include a modem at the user end which links the user's network with the transceiver, and a centralized network operations center (NOC) for monitoring the entire system. Working in concert with a broadband gateway, the satellite operates a Star network topology where all network communication passes through the network's hub processor, which is at the center of the star. With this configuration, the number of remote VSATs that can be connected to the hub is virtually limitless. At the center of the new broadband satellite networks are a new generation of high-powered GEO satellites positioned 35,786 kilometres (22,236 mi) above the equator, operating in Ka-band (18.3–30 GHz) mode.[5] These new purpose-built satellites are designed and optimized for broadband applications, employing many narrow spot beams,[6] which target a much smaller area than the broad beams used by earlier communication satellites. This spot beam technology allows satellites to reuse assigned bandwidth multiple times, enabling them to achieve much higher capacity than conventional broad beam satellites. The spot beams also increase performance and consequential capacity by focusing more power and increased receiver sensitivity into concentrated areas. Spot beams are designated as one of two types: subscriber spot beams, which transmit to and from the subscriber-side terminal, and gateway spot beams, which transmit to/from a service provider ground station. In conjunction with the satellite's spot-beam technology, a bent-pipe architecture is employed in the network in which the satellite functions as a bridge in space, connecting two communication points on the ground. The term “bent-pipe” is used to describe the shape of the data path between sending and receiving antennas, with the satellite positioned at the point of the bend. Simply put, the satellite's role in this network arrangement is to relay signals from the end user’s terminal to the ISP’s gateways, and back again. The satellite receives, amplifies, and redirects signals carried on a specific radio frequency through a signal path called a transponder. The satellite has its own set of antennas to receive communication signals from Earth and to transmit signals to their target location. These antennas and transponders are part of the satellite’s “payload”, which is designed to receive and transmit signals to and from various places on Earth. What enables this transmission and reception in the payload transponders is a repeater subsystem (RF (radio frequency) equipment) used to change frequencies, filter, separate, amplify and group signals before routing them to their destination address on Earth. The satellite’s high-gain receiving antenna passes the transmitted data to the transponder which filters, translates and amplifies them, then redirects them to the transmitting antenna on board. The signal is then routed to a specific ground location through a channel known as a carrier. Beside the payload, the other main component of a communications satellite is called the bus, which comprises all equipment required to move the satellite into position, supply power, regulate equipment temperatures, provide health and tracking information, and perform numerous other operational tasks. Along with dramatic advances in satellite technology over the past decade, ground equipment has similarly evolved, benefiting from higher levels of integration and increasing processing power, expanding both capacity and performance boundaries. The Gateway—or Gateway Earth Station (its full name)—is also referred to as a ground station, teleport or hub. The term is sometimes used to describe just the antenna dish portion, or it can refer to the complete system with all associated components. In short, the gateway receives radio wave signals from the satellite on the last leg of the return or upstream payload, carrying the request originating from the end-user’s site. The satellite modem at the gateway location demodulates the incoming signal from the outdoor antenna into IP packets and sends the packets to the local network. Access server/gateways manage traffic transported to/from the Internet. Once the initial request has been processed by the gateway’s servers, sent to and returned from the Internet, the requested information is sent back as a forward or downstream payload to the end-user via the satellite, which directs the signal to the subscriber terminal. Each Gateway provides the connection to the Internet backbone for the gateway spot-beam(s) it serves. The system of gateways comprising the satellite ground system provides all network services for satellite and corresponding terrestrial connectivity. Each gateway provides a multiservice access network for subscriber terminal connections to the Internet. In the continental United States, because it is north of the equator, all gateway and subscriber dish antenna must have an unobstructed view of the southern sky. Because of the satellite’s geostationary orbit, the gateway antenna can stay pointed at a fixed position. At the far end of the outdoor unit is a small (2–3-foot diameter), reflective dish-type radio antenna constructed from and coated with a variety of materials. As indicated earlier, like the antenna used by the gateway, the VSAT antenna must also have an unobstructed view of the southern sky to allow for proper line-of-sight (L-O-S) to the satellite. There are four characteristic settings used to ensure that the antenna is configured correctly at the satellite, which are: azimuth, elevation, polarization, and skew. The combination of these settings gives the outdoor unit a L-O-S to the chosen satellite and makes data transmission possible. These parameters are generally set at the time the equipment is installed, along with a beam assignment (Ka-band only); these steps must all be taken prior to the actual activation of service. Transmit and receive components are mounted at the focal point of the antenna which receives/sends data from/to the satellite. The main parts are: Feed – This assembly is part of the VSAT receive and transmit chain, which consists of several components with different functions, including the feed horn at the front of the unit, which resembles a funnel and has the task of focusing the satellite microwave signals across the surface of the dish reflector. The feed horn both receives signals reflected off the dish’s surface and transmits outbound signals back to the satellite. Block upconverter (BUC) – This unit sits behind the feed horn and may be part of the same unit, but a larger (higher wattage) BUC could be a separate piece attached to the base of the antenna. Its job is to convert the signal from the modem to a higher frequency and amplify it before it is reflected off the dish and towards the satellite. Low-noise block downconverter (LNB) – This is the receiving element of the terminal. The LNB’s job is to amplify the received satellite radio signal bouncing off the dish and filter out the noise, which is any signal not carrying valid information. The LNB passes the amplified, filtered signal to the satellite modem at the user’s location. The satellite modem serves as an interface between the outdoor unit and customer-provided equipment (i.e. PC, router) and controls satellite transmission and reception. From the sending device (computer, router, etc.) it receives an input bitstream and converts or modulates it into radio waves, reversing that order for incoming transmissions, which is called demodulation. It provides two types of connectivity: Coaxial cable (COAX) connectivity to the satellite antenna. The cable carrying electromagnetic satellite signals between the modem and the antenna generally is limited to be no more than 150 feet in length. Ethernet connectivity to the computer, carrying the customer’s data packets to and from the Internet content servers. Satellite modems employ either the DOCSIS (Data Over Cable Service Interface Specification) or WiMAX (World Interoperability for Microwave Access) telecommunication standard to communicate with the assigned gateway. 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. The amount of latency depends on the distance travelled and the speed of light. Light including wireless radiation would take about 0.12 seconds to reach a geostationary satellite (at 36,000 km above the equator), so nearly 1/4 second for a round trip. Latency is the main difference between a standard terrestrial based network and a geostationary satellite network. The round trip latency of a geostationary satellite communication's network is almost 20 times that of a terrestrial based network. 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 (i.e. "circular orbit"). 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. 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 orbit and 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. The absolute minimum total amount of delay is variable, due to the satellite staying in one place in the sky, while ground based users can be directly below with a roundtrip latency of 239.6 ms, or far to the side of the planet near the horizon with a roundtrip latency of 279.0 ms. 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, and two orders of magnitude higher than the typical 15-40 ms latency experienced by users of other high-speed internet services, such as cable or VDSL. 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. 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. 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—such as virtual private networks—is working much better with the new generation of satellite Internet service than in the past. Medium Earth orbit (MEO) and low Earth orbit (LEO) satellites do not have such great delays. For example: 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 2013 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. Unlike geostationary satellites, low and medium Earth orbit satellites do not stay in a fixed position in the sky. Consequently, ground based antennas cannot be easily locked into communication with any one specific satellite. As with GPS, the small orbits may cause a low Earth orbit satellite to only be in the sky for an hour or less before it goes over the horizon and out of range, so a complex relaying and passing-off needs to be done to hand over the fixed-position terrestrial signal to other satellites passing overhead. Communications with MEO or LEO satellites that are moving in the sky can be done in two ways: More diffuse or completely omnidirectional ground antennas capable of communicating with one or more satellites visible in the sky at the same time, but at significantly higher transmit power than fixed geostationary dish antennas (due to the lower gain), and with much poorer signal to noise ratios for receiving the signal. Tracking, high-gain, narrow beam antennas, but the tracking requires more expensive solutions like a motorized antenna mount or a dynamic phased array antanna that can steer the beam electronically, and complex software that can predict the path of each satellite in the constellation. 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 margins, adaptive uplink power control and reduced 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. 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. In other words, increasing antenna gain through the use of a larger parabolic reflector is one way of increasing the overall channel gain and, consequently, the signal-to-noise (S/N) ratio, which allows for greater signal loss due to rain fade without the S/N ratio dropping below its minimum threshold for successful communication. Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost. However, it is often more economical to build a more expensive satellite and smaller, less expensive consumer antennas than to increase the consumer antenna size to reduce the satellite 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. Satellites typically use photovoltaic solar power, so there is no expense for the energy itself, but a more powerful satellite will require larger, more powerful solar panels and electronics, often including a larger transmitting antenna. The larger satellite components not only increase materials costs but also increase the weight of the satellite, and in general, the cost to launch a satellite into an orbit is directly proportional to its weight. (In addition, since satellite launch vehicles [i.e. rockets] have specific payload size limits, making parts of the satellite larger may require either more complex folding mechanisms for parts of the satellite like solar panels and high-gain antennas, or upgrading to a more expensive launch vehicle that can handle a larger payload.) 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. 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. Two-way satellite Internet service involves both sending and receiving data from a remote very-small-aperture terminal (VSAT) via satellite to a hub telecommunications port (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. 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. 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. 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. 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 (USB). 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. For many years satellite phones have been able to connect to the internet. Bandwidth varies from about 24 kbit/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 96 kbit/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 96 kbit/s dial-up connection is also possible, the 60 kbit/s service is always-on and the user is billed for data transferred. 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 Shell data 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. If you have information (a lot of information, usually) that should be transmitted to various (but usually not all) hosts over an internet, then Multicast is the answer. One common situation in which it is used is when distributing real time audio and video to the set of hosts which have joined a distributed conference. Multicast is much like radio or TV in the sense that only those who have tuned their receivers (by selecting a particular frequency they are interested on) receive the information. That is: you hear the channel you are interested in, but not the others. Unicast is anything that is not broadcast nor multicast. When you send a packet and there is only one sender process -yours- and one recipient process (the one you are sending the packet to), then this is unicast. TCP is, by its own nature, unicast oriented. UDP supports a lot more paradigms, but if you are sending UDP packets and there is only one precess supposed to receive them, this is unicast too. With today's technology it is possible to afford the "cost" of making a unicast connection with everyone who wants to see your web page. However, if you are to send audio and video, which needs a huge amount of bandwidth compared with web applications, you have -you had, until multicast came into scene- two options: to establish a separate unicast connection with each of the recipients, or to use broadcast. The first solution is not affordable: if we said that a single connection sending audio/video consumes a huge bandwidth, imagine having to establish hundreds or, may be, thousands of those connections. Both the sending computer and your network would collapse. Broadcast seems to be a solution, but it's not certainly the solution. If you want all the hosts in your LAN to attend the conference, you may use broadcast. Packets will be sent only once and every host will receive them as they are sent to the broadcast address. The problem is that perhaps only a few of the hosts and not all are interested in those packets. Furthermore: perhaps some hosts are really interested in your conference, but they are outside of your LAN, a few routers away. And you know that broadcast works fine inside a LAN, but problems arise when you want broadcast packets to be routed across different LANs. The best solution seems to be one in which you send packets to a certain special address (a certain frequency in radio/TV transmissions). Then, all hosts which have decided to join the conference will be aware of packets with that destination address, read them when they traverse the network, and pass them to the IP layer to be demultiplexed. This is similar to broadcasting in that you send only one broadcast packet and all the hosts in the network recognize and read it; it differs, however, in that not all multicast packets are read and processed, but only those that were previously registered in the kernel as being "of interest". Those special packets are routed at kernel level like any packet because they are IP packets. The only difference might reside in the routing algorithm which tells the kernel where to route or not to route them. The range of IP addresses is divided into "classes" based on the high order bits of a 32 bits IP address: Bit --> 0 31 Address Range: The one which concerns us is the "Class D Address". Every IP datagram whose destination address starts with "1110" is an IP Multicast datagram. The remaining 28 bits identify the multicast "group" the datagram is sent to. Following with the previous analogy, you have to tune your radio to hear a program that is transmitted at some specific frequency, in the same way you have to "tune" your kernel to receive packets sent to an specific multicast group. When you do that, it's said that the host has joined that group in the interface you specified. There are some special multicast groups, say "well known multicast groups", you should not use in your particular applications due the special purpose they are destined to: All this special multicast groups are regularly published in the "Assigned Numbers" RFC. In any case, range 224.0.0.0 through 224.0.0.255 is reserved for local purposes (as administrative and maintenance tasks) and datagrams destined to them are never forwarded by multicast routers. Similarly, the range 239.0.0.0 to 239.255.255.255 has been reserved for "administrative scoping". Hosts can be in three different levels of conformance with the Multicast specification, according to the requirements they meet. Level 0 is the "no support for IP Multicasting" level. Lots of hosts and routers in the Internet are in this state, as multicast support is not mandatory in IPv4 (it is, however, in IPv6). Not too much explanation is needed here: hosts in this level can neither send nor receive multicast packets. They must ignore the ones sent by other multicast capable hosts. Level 1 is the "support for sending but not receiving multicast IP datagrams" level. Thus, note that it is not necessary to join a multicast group to be able to send datagrams to it. Very few additions are needed in the IP module to make a "Level 0" host "Level 1-compliant". Level 2 is the "full support for IP multicasting" level. Level 2 hosts must be able to both send and receive multicast traffic. They must know the way to join and leave multicast groups and to propagate this information to multicast routers. Thus, they must include an Internet Group Management Protocol (IGMP) implementation in their TCP/IP stack. Broadcast is (in comparison) easier to implement than multicast. It doesn't require processes to give the kernel some rules regarding what to do with broadcast packets. The kernel just knows what to do: read and deliver all of them to the proper applications. With multicast, however, it is necessary to advise the kernel which multicast groups we are interested in. That is, we have to ask the kernel to "join" those multicast groups. Depending on the underlying hardware, multicast datagrams are filtered by the hardware or by the IP layer (and, in some cases, by both). Only those with a destination group previously registered via a join are accepted. Essentially, when we join a group we are telling the kernel: "OK. I know that, by default, you ignore multicast datagrams, but remember that I am interested in this multicast group. So, do read and deliver (to any process interested in them, not only to me) any datagram that you see in this network interface with this multicast group in its destination field". Some considerations: first, note that you don't just join a group. You join a group on a particular network interface. Of course, it is possible to join the same group on more than one interface. If you don't specify a concrete interface, then the kernel will choose it based on its routing tables when datagrams are to be sent. It is also possible that more than one process joins the same multicast group on the same interface. They will all receive the datagrams sent to that group via that interface. As said before, any multicast-capable hosts join the all-hosts group at start-up , so "pinging" 224.0.0.1 returns all hosts in the network that have multicast enabled. Finally, consider that for a process to receive multicast datagrams it has to ask the kernel to join the group and bind the port those datagrams were being sent to. The UDP layer uses both the destination address and port to demultiplex the packets and decide which socket(s) deliver them to. When a process is no longer interested in a multicast group, it informs the kernel that it wants to leave that group. It is important to understand that this doesn't mean that the kernel will no longer accept multicast datagrams destined to that multicast group. It will still do so if there are more precesses who issued a "multicast join" petition for that group and are still interested. In that case the host remains member of the group, until all the processes decide to leave the group. Even more: if you leave the group, but remain bound to the port you were receiving the multicast traffic on, and there are more processes that joined the group, you will still receive the multicast transmissions. The idea is that joining a multicast group only tells the IP and data link layer (which in some cases explicitly tells the hardware) to accept multicast datagrams destined to that group. It is not a per-process membership, but a per-host membership. Using a new technology usually carries some advantages and disadvantages. The advantages of multicast are -I think- clear. The main disadvantage is that hundreds of hosts and, specially, routers don't support it yet. As a consequence, people who started working on multicast, bought new equipment, modified their operating systems, and built multicast islands in their local places. Then they discovered that it was difficult to communicate with people doing similar things because if only one of the routers between them didn't support multicast there was nothing to do... The solution was clear: they decided to build a virtual multicast network in the top of the Internet. That is: sites with multicast routers between them could communicate directly. But sites joined across unicast routers would send their island's multicast traffic encapsulated in unicast packets to other multicast islands. Routers in the middle would not have problems, as they would be dealing with unicast traffic. Finally, in the receiving site, traffic would be de-encapsulated, and sent to the island in the original multicast way. Two ends converting from multicast to unicast, and then again to multicast define what is called a multicast tunnel. The MBone or Multicast Backbone is that virtual multicast network based on multicast islands connected by multicast tunnels. One trivial algorithm to make worldwide multicast traffic available everywhere could be to send it... everywhere, despite someone wants it or not. As this does not seem quite optimized, several routing algorithms and forwarding techniques have been implemented. DVMRP (Distance Vector Multicast Routing Protocol) is, perhaps, the one most multicast routers use now. It is a dense mode routing protocol, that is, it performs well in environments with high bandwidth and densely distributed members. However, in sparse mode scenarios, it suffers from scalability problems. Together with DVMRP we can find other dense mode routing protocols, such as MOSPF (Multicast Extensions to OSPF - Open Shortest Path First -) and PIM-DM (Protocol-Independent Multicast Dense Mode). To perform routing in sparse mode environments, we have PIM-SM (Protocol Independent Multicast Sparse Mode) and CBT (Core Based Trees). All this routing protocols use some type of multicast forwarding, such as flooding, Reverse Path Broadcasting (RPB), Truncated Reverse Path Broadcasting (TRPB), Reverse Path Multicasting (RPM) or Shared Trees. So far we have been talking about multicast transmissions using UDP. This is the usual practice, as it is impossible to do it with TCP. However, intense research is taking place since a couple of years in order to develop some new multicast transport protocols. Several of these protocols have been implemented and are being tested. A good lesson from them is that it seems no multicast transport protocol is general and good enough for all types of multicast applications. If transport protocols are complex and difficult to tune, imagine dealing with delays (in multimedia conferences), data loss, ordering, retransmissions, flow and congestion control, group management, etc, when the receiver is not one, but perhaps hundreds or thousands of sparse hosts. Here scalability is an issue, and new techniches are implemented, such as not giving acknowledges for every packet received but, instead, send negative acknowledges (NACKs) for data not received. RFC 1458 gives the proposed requirements for multicast protocols. Real-Time Transport Protocol (RTP) is concerned with multi-partite multimedia conferences, Scalable Reliable Multicast (SRM) is used by the wb (the distributed White-Board tool), Uniform Reliable Group Communication Protocol (URGC) enforces reliable and ordered transactions based in a centralized control, Muse was developed as an application specific protocol: to multicast news articles over the MBone, the Multicast File Transfer Protocol (MFTP) is quite descriptive by itself and people "join" to file much in the same way they would join a conference, Log-Based Receiver-reliable Multicast (LBRM) is a curious protocol that keeps track of all packets sent in a logging server that tells the sender whether it has to retransmit the data or can drop it safely as all receivers got it. One protocol with a funny name -especially for a multicast protocol- is STORM (STructure-Oriented Resilient Multicast). Today, the advent of High Throughput Satellite (HTS) technology, coupled with rising demand for satellite communications, are expected to have a profound impact on the VSAT industry. With the huge influx of bandwidth capacity, HTS will bring both improved speeds and lower costs. Combined with major space segment and infrastructure advances, a prime opportunity exists to accelerate overall VSAT adoption on a much broader scale. High Throughput Satellites are a new breed of high-performance broadband satellites that today mostly use Ka-band frequencies, but not exclusively – with Intelsat’s announcement of EPIC we will see Ku HTS very soon. HTS is fundamentally different both in terms of design and the ground segment requirements. (For more details, see HTS Whitepaper.) Traditional satellites use large regional beams covering an entire footprint with fixed capacity. Any service provider could own a hub and teleport and offer services to customers as long as they were in the satellite footprint. By contrast, HTS employ frequency re-use across multiple spot beams to create a massive increase in capacity. A dedicated high bandwidth feeder link is required to serve to the spot beams. Hub infrastructure must be located within the feeder link to serve all the spot beams. See below: As more and more HTS are launched they are expected, over time, to provide a huge influx of bandwidth capacity that will deliver higher speeds at lesser cost. In fact, NSR states that high throughput satellites are expected to supply at least 1.34 TBps of capacity by 2020. Since bandwidth will no longer constrain how business is done, rapid adoption of satellite is expected to open doors to new opportunities in both the enterprise and government markets.
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Tvrtka XipLink uspješno demonstrirala LTE preko satelita Specijalist za optimizaciju bežičnih veza XipLink završio test sa utjecajnim azijskim davateljem usluga, isporuka ubrzanog glasa, podataka i videa sa prometom većim od 90 megabita u sekundi (Mbps) prema udaljenoj LTE baznoj stanici preko satelitske veze... 5 stvari koje smo naučili na Global MilSatCom-u Global MilSatCom je jedan od glavnih događaja za vojne i komunikacijske rukovoditelje i službenike... Cobham Satcom debitira sa laganim Sailor VSAT antenskim sustavom Cobham Satcom je otkrio dva nova 60cm Ka-band VSAT antenska sustava za pomorstvo... Eutelsat nagovještava nove dogovore u Broadcastu i IoT-u Eutelsat Communications je prijavila niz korisnika u Africi i Europi za broadcast usluge, te je učvrstila suradnju s Internet stvari (IoT) pionirom Sigfox... Avanti, Global Invacom prezentirali demo enkriptirani satelitski TV preko Ka-band IP mreže Avanti komunikacije i Global Invacom, u suradnji s Microsoftom i Europskom svemirskom agencijom (ESA) Advanced Research in Telecommunications Systems (ARTES) programu, demonstrirali su satelitsko emitiranje koristeći standardne IP-omogućene mreže, kao što su Ethernet ili Wi-Fi... Iridium GO! dobio pet novih aplikacija Iridium je učinio dostupnim pet mobilnih aplikacija koje su napravili vanjski suradnici za Iridium GO !, smartphone uređaj za globalni pristup...više Istražite neke od lokacija s našim vezamaviše dr.sc. Draško Marin, dipl.ing. «Elektroničke komunikacije-regulativa, sustavi, norme» Nakon lansiranja prvog satelita, Sputnik 1 Sovjetskog Saveza u listopadu 1957 g., SAD je uspješno lansirao satelit Explorer 1 1958. Prvi komercijalni komunikacijski satelit je Telstar 1, kojeg je izgradio Bell Labs, a pokrenut je u srpnju 1962 godine. Ideja geostacionarnog satelita - onaj koji bi mogao kružiti oko Zemlje iznad ekvatora i ostati na mjestu slijedeći rotaciju Zemlje - prvi je predložio pisac znanstvene fantastike Arthur C. Clarke u 1945 godini. Prvi satelit koji je uspješno dospio u geostacionarnu orbitu je bio Syncom3 kojeg su izgradili Hughes Aircraft za NASA-u lansiran 19. kolovoza 1963. Usvojene su sljedeće generacije komunikacijskih satelita koje imaju veće kapacitete i poboljšane karakteristike performansi za upotrebu isporuke televizije, vojne primjene i telekomunikacijske svrhe. Nakon izuma Interneta i World Wide Weba, geostacionarni je satelit privukao interes kao potencijalno sredstvo pružanja pristupa Internetu. Značajan osposobljivač satelitskog Interneta je otvaranje Ka - banda za satelite. U prosincu 1993. Hughes Aircraft Co. podnosi zahtjev za dozvolu Federal Communications Commission-u za pokretanje prvog Ka - band satelita, Spaceway. Godine 1995., FCC je objavio poziv više Ka - band satelitskih prijava, što je privuklo prijave 15 tvrtki. Među kojima su Echostar, Lockheed Martin, GE - Americom, Motorola i KaStar Satelit, koji je kasnije postao WildBlue. Među istaknutim kandidatima u ranom stadiju sektora satelitskog Interneta je bio Teledesic, ambiciozan ali na kraju neuspio projekt financiran u sklopu Microsoft-a koji je na kraju koštao više od 9 milijardi dolara. Teledesic-ova ideja je bila stvoriti konstelaciju stotina širokopojasnih satelita niske orbite u Ka - band frekvencijskom području, pružajući jeftin pristup internetu s download brzinama do 720 Mbit/s. Projekt je napušten 2003 godine. Teledesic-ov neuspjeh, zajedno sa stečajnim zahtjevima pružatelja satelitskih komunikacija Iridium Communications Inc. i Globalstar, usporio je tržišni entuzijazam za razvoj satelitskog interneta. Sve do rujna 2003 godine, kada je pokrenut prvi Eutelsat-ov internet – spreman satelit za potrošače. Sa lansiranjem Anik F2 2004. godine, prvog visoko propusnog satelita, počeo je rad nove generacije satelita sa povećanim kapacitetom i širinom propusnosti. U novije vrijeme, visoko propusni sateliti poput Viasatovog Viasat-1 satelita u 2011. i HughesNetovog Jupiter u 2012. godini postigli su daljnja poboljšanja, podižući silazne brzine prijenosa podataka od 1-3Mbit/s do 12-15Mbit/s pa i više. Usluge Internet pristupa vezane za te satelite su usmjerene uglavnom na ruralno stanovništvo kao alternativa za internet usluge preko dial-up, ADSL ili klasičnih FSSes. Viasat-1, komunikacijski satelit sa najvećim kapacitetom u svijetu, lansiran je 19. listopada 2011. iz Baikonour Kazahstana. Sa 140 Gbit/s ukupne propusne moći, nova satelitska usluga, Exede Internet usluga, sa download i upload brzinama mnogo višim nego išto ranije ponuđeno u satelitskoj industriji. Echostar XVII satelit s JUPITER visoko propusnom tehnologijom, kojeg je izgradio Space Systems/Loral, pokrenut je 5. srpnja 2012. od Arianespace i postavljen je u svoju stalnu geosinkronu orbitu na 107.1° Zapadno. Pružatelj satelitskih usluga je HughesNet Gen4 satelitska Internet usluga. Upošljava multi-spot zraku, arhitekturu savijene cjevi, ovaj Ka-band satelit ima više od 100 Gbit/s kapacitet propusnosti. SkyTerra-1 lansiran je sredinom studenog 2010, te će pružati usluge u cijeloj Sjevernoj Americi, dok je Hylas-1 lansiran krajem studenog 2010, te će ciljati Europu. 26. prosinca 2010. Eutelsat-ov KA-SAT satelit uspješno je lansiran pomoću ILS Proton Breeze M vozila iz Baïkonour kozmodroma u Kazahstanu. Posljednji satelit je postavljen u službu sredinom 2011. godine. Pokriva europski kontinent sa 80 zraka fokusiranih signala koji pokrivaju područje nekoliko stotina kilometara diljem Europe i Mediterana. Spot zrake omogućuju da se frekvencija učinkovito ponovno koristiti na više područja bez međusobnih smetnji. Rezultat je povećani kapacitet. Svaka zraka će imati ukupni kapacitet od 1064 Mbit/s, a čitav satelit će imati kapacitet od 90 Gbit/s. Satelitski pristup internetu je pristup internetu osiguran kroz komunikacijske satelite. Moderna satelitska Internet usluga se obično pruža korisnicima putem geostacionarnih satelita koji imaju visoke brzine prijenosa podataka, a noviji sateliti postižu silazne brzine prijenosa podataka do 20 Mbps. Satelitski Internet se uglavnom oslanja na tri osnovne komponente: satelit u geostacionarnoj orbiti (ponekad se naziva geosinkrona Zemljina orbita; eng. geosynchronous Earth orbit, ili GEO), broj zemaljskih stanica poznat kao gateway koji prenose podatake na i sa Interneta preko satelita putem radio valova (mikrovalnih ), i VSAT (very-small-aperture terminal) antene s primopredajnikom koje se nalaze na lokaciji pretplatnika. Ostale komponente satelitskog internet sustava uključuju modem na korisničkom kraju koji povezuje korisnikovu mrežu i primopredajnik sa centraliziranim mrežnim nadzornim središtem (NOC) za nadzor cjelokupnog sustava. Kako rade u suradnji sa širokopojasnim pristupnikom (gateway), satelit radi u Zvijezda mrežnoj topologiji gdje sve mrežne komunikacije prolaze kroz hub procesor mreže, koji je u središtu zvijezde. Uz ovu konfiguraciju, broj udaljenih VSAT-a koji se mogu priključiti na hub je gotovo neograničen., U središtu novih širokopojasnih satelitskih mreža su nova generacija snažnih GEO satelita pozicioniranih na 35.786 km (22.236 milja) iznad ekvatora, koji rade u Ka-band (18,3-30 GHz) modu. Ovi novo namjenski sateliti su dizajnirani i optimizirani za širokopojasne primjene sa mnogo uskih zraka, koje ciljaju puno manju površinu od širokih zraka koje koriste raniji komunikacijski sateliti. Ova tehnologija uskih zraka omogućuje satelitima da ponovno dodijele širinu pojasa više puta, omogućujući im da postignu puno veći kapacitet od konvencionalnih satelita široke zrake. Uske zrake također povećavaju učinkovitost i posljedični kapacitet fokusiranjem više snage i veće osjetljivosti prijemnika na koncentrirana područja. Uske zrake su označene kao jedna od dvije vrste: pretplatnička uska zraka, kojom se odašilje prema i od pretplatničke strane terminala; i gateway uske zrake, koje odašilju prema/od zemaljske stanice davatelja usluga. U suradnji sa tehnologijom uske zrake satelita, arhitektura savijene cijevi je puštena u mrežu u kojoj satelit funkcionira kao most u svemiru, spajajući dvije komunikacijske točake na zemlji. Pojam "savijena cijev" se koristi za opis oblika putanje podataka između prijamne i predajne antene sa satelitom postavljenim na mjestu savijanja. Jednostavno rečeno, uloga satelita u ovom mrežnom rasporedu je relej signala od terminala krajnjeg korisnika do gateway-a ISP-a i natrag. Satelit prima, pojačava i preusmjerava signale nošene na određenoj frekvenciji kroz transponder. Satelit ima svoj set antena za primanje komunikacijskog signala sa Zemlje i za prijenos signala do ciljane lokacije. Ove antene i transponderi su dio satelitskog „tereta", koji je dizajniran za primanje i prijenos signala prema i sa različitih mjesta na Zemlji. Ono sto omogućuje ovaj prijenos i prijam u sadržaju transpondera je „repeater“ podsustav ( RF ( radio frekvencija ) oprema ) koja se koristi za promjenu frekvencije, filtriranje, odvajanje, pojačavanje i grupiranje signala prije nego što ih se usmjeri na njihovu odredišnu adresu na Zemlji. Satelitska prijemna antena visoke dobiti propušta podatke na transponder koji ih filtrira, pretvara i pojačava, a zatim ih preusmjerava na prijenosnu antenu na satelitu. Signal se zatim usmjeri preko kanala nosioca na određeno mjestu na tlu. Osim tog tereta, druga glavna komponenta komunikacijskog satelita se zove satelitska sabirnica, koja obuhvaća svu potrebnu opremu za pomicanje satelita u položaj, opskrba napajanja, regulira temperaturu opreme, pruža informacije o putanji i zdravlju sustava, te obavlja i brojne druge operativne poslove. Zajedno s dramatičnim napretkom u posljednjih deset godina u satelitskoj tehnologiji, zemaljska oprema je slično evoluirala dobivajući veću razinu integracije i povećanje procesorske snage šireći pritom granice performansi i kapaciteta. Gateway ili Gateway Earth Station ( puni naziv ) također se spominje kao zemaljska stanica ili teleport ili hub. Izraz se ponekad koristi za opisivanje tanjura antene ili se može odnositi na kompletan sustav sa svim pripadajućim komponentama. Ukratko, gateway prima signal radio valova sa satelita u zadnjem koraku sa povratne ili uzlazne komponente, noseći zahtjev krajnjeg korisnika. Satelitski modem na gateway-u demodulira dolazni signal iz vanjske antene u IP pakete koje dalje šalje na lokalnu mrežu. Prijamni server/gateway upravlja prometom koji se prenosi sa i na Internet. Nakon što je početni zahtjev obradio server gateway-a, koji je poslan i vraćen s Interneta, zatražene informacije se šalju natrag kao proslijeđeni ili silazni sadržaj do krajnjeg korisnika preko satelita koji usmjerava signal do pretplatničkog terminala. Svaki gateway pruža priključak na internet okosnicu za pristupne točke - zrake koje opslužuje. Sustav gateway-a koji tvori satelitski sustav na tlu pruža sve mrežne usluge za satelit i pripadajuću zemaljsku povezivost. Svaki gateway pruža višeuslužnu pristupnu mrežu za priključak pretplatničkih terminalnala na Internet. U kontinentalnim Sjedinjenim Američkim Državama, jer su sjeverno od ekvatora, sva gateway i pretplatniče antene moraju imati nesmetan pogled prema južnom nebu. Zbog geostacionarnog satelita u orbiti, gateway antena može ostati u fiksnom položaju. Na samom kraju vanjske jedinice je mali (promjera 2-3 stope) reflektirajući tanjur radio antene konstruiran i obložen različitim materijalima. Kao što je navedeno ranije, isto kao antena koju koristi gateway, VSAT antene moraju također imati nesmetan pogled prema južnom nebu kako bi imali liniju vidljivi (LOS – line of sight) prema satelitu. Postoje četiri karakteristične postavke koje se koriste da se osigura pravilno namještanje antene prema satelitu, a to su: azimut, elevacija, polarizacija i namještanje polarizacije. Kombinacija tih postavki daje vanjskoj jedinici LOS za odabrani satelit i omogućuje prijenos podataka. Ovi podaci se većinom postavljaju u vrijeme instalacije opreme zajedno sa odabirom zrake (samo Ka-band); ovi koraci se poduzimaju prije stvarnog aktiviranja usluge. Prijamne i predajni dijelovi su montirani u žarišnoj točki antene koja prima/šalje podatke od/do satelita. Glavni dijelovi su: Ulijev (Feed) - Ovaj sklop je dio VSAT prijemnog i predajnog lanca koji se sastoji od nekoliko dijelova s različitim funkcijama, uključujući lijevkasti ulaz na prednjoj strani uređaja koji podsjeća na dimnjak i ima zadatak fokusiranja satelitskih mikrovalnih signala sa cijele površine tanjura. Lijevkasti ulaz (feed horn) isto tako prima signal koji se reflektira od površine tanjura i šalje izlazni signal natrag na satelit. Blok predajni pretvarač (Block upconverter - BUC) - Ova jedinica se nalazi iza feed-a i može biti dio iste jedinice, ali isto tako veći (više wata) BUC može biti odvojeni komad priključen na bazu antene. Njegov zadatak je da pretvori signal iz modema na višu frekvenciju i pojača ga, te ga pošalje nazad ka satelitu. Blok prijemni pretvarač (Low-noise block downconverter - LNB) - To je prijemni element terminala. Zadatak LNB-a je da pojača primljeni satelitski radio signal i filtrira šum, a šum je bilo koji signal koji ne nosi valjanu informaciju. LNB prenosi pojačan, filtrirani signal na satelitski modem na lokaciji korisnika. Satelitski modem služi kao sučelje između vanjske jedinice i opreme kupca (tj. računalo, router) i kontrolira satelitski prijenos i prijem. Od uređaja koji šalje (računalo, usmjerivač, itd.) prima ulazni bitstream i pretvara ga ili modulira u radio val i obratno za dolazne signale što se zove demodulacija. Omogućuje dvije vrste povezivanja: Koaksijalni kabel (COAX) povezivanje na satelitsku antenu. Kabel koji nosi elektromagnetske satelitske signale između modema i antene općenito je ograničen na dužinu ne više od 150 feet-a. Ethernet povezivost s računalom, noseći podatke klijenta prema i od servera internetskog sadržaja. Satelitski modemi koriste DOCSIS (Data Over Cable Service Interface Specification) ili WiMAX (World Interoperability for Microwave Access) telekomunikacijske standarde kako bi komunikacirali sa dodijeljenim gateway-om. Latencija je kašnjenje između traženog podatka i primitka odgovora, ili u slučaju jednosmjerne komunikacije, između stvarnog trenutka emitiranja signala i vremena dolaska na odredište. Količina latencije ovisi o pređenoj udaljenosti brzine svjetlosti. Svjetlu, uključujući i bežično zračenje, treba oko 0.12 sekundi da dosegne geostacionarni satelit (na 36.000 km iznad ekvatora), tako da je potrebno gotovo četvrtina sekunde za cijelokupno putovanje. Latencija je glavna razlika između standardne zemaljske mreže i geostacionarne satelitske mreže. Cijelokupno vrijeme latencije geostacionarne satelitske mreže je gotovo 20 puta veće od zemaljske mreže. Geostacionarna orbita (ili geostacionarna orbita Zemlje / GEO) je geosinkrona orbita točno iznad Zemljinog ekvatora (0 ° širine), sa periodom jednakoj periodi Zemljine vrtnje i orbitalne ekscentričnosti od oko nula (tj. "kružna orbita"). Objekt u geostacionarnoj orbiti se čini nepomičan, na fiksnom položaju na nebu, u odnosu na promatrača na tlu. Komunikacijski sateliti i vremenski sateliti su često u geostacionarnim orbitama, tako da se satelitske antene koje komuniciraju s njima ne moraju pomicati kako bi ih pratili, već su nepomično učvršćene u istom položaju prema nebu. Zbog konstantnog 0° zemljopisne širine i cirkularnosti geostacionarne orbite, sateliti u GEO se samo razlikuju u zemljopisnoj dužini. U usporedbi sa komunikacijama na tlu sve geostacionarne satelitske komunikacije imaju visoke latencije zbog signala koji putuje 35.786 km (22.236 milja) na satelit u geostacionarnoj orbiti i ponovo nazad na Zemlju. Čak i pri brzini svjetlosti (oko 300.000 km / s ili 186.000 milja u sekundi) to kašnjenje može biti značajano. Ako se otklone sve ostale signalizacijske prepreke signalu još uvijek treba oko 250 milisekundi (ms), ili oko četvrtine sekunde, putovanja do satelita i natrag na zemlju. Apsolutni ukupni minimum kašnjenja je promjenjiv s obzirom da satelit boravi na jednom mjestu na nebu, dok korisnici na tlu mogu biti izravno ispod sa latencijom od 239,6 ms, ili daleko na rubu planete blizu horizonta sa cjelokupnom latencijom od 279,0 ms. Za internet paket, kašnjenje se udvostručuje prije nego dođe odgovor. To je teoretski minimum. Faktoriranje u ostalim normalnim kašnjenjima iz mrežnih izvora daje tipičnu jednosmjernu latenciju od 500-700 ms od korisnika do ISP-a, ili oko 1,000-1,400 ms latencije za ukupno vrijeme obilaska (RTT) nazad do korisnika. To je puno više nego što većina dial-up korisnika doživljava sa prosjekom od 150-200 ms ukupne latencije ili nekoliko reda veličine veće od tipičnih 15-40 ms latencija kod korisnika internet usluga velike brzine, kao što su kabel ili VDSL . Ne postoji način da se eliminiraju latencije za geostacionarne satelite, ali problem se može donekle ublažiti sa značajkom TCP ubrzanja koja skraćuje vrijeme povratnog putovanja (RTT) po paketu razdvajanjem povratne veze između pošiljatelja i primatelja. Takva ubrzanja su obično prisutna u najnovijem razvoju tehnologije ugrađene u nove usluge satelitskog interneta. Latencija također utječe na iniciranje sigurne internetske veze, kao što su SSL koja zahtijeva razmjenu brojnih dijelova podataka između web poslužitelja i web klijenta. Iako su ti dijelovi podataka mali, višestruka ukupna putovanja (RTT) koja obuhvaćaju handshake proizvode velika kašnjenja u odnosu na druge oblike povezivanja s Internetom, kao što je dokumentirano Stephen T. Cobb u izvješću objavljenom 2011. u Rural Mobile and Broadband Alliance. Ova smetnja se proširuje na unos i uređivanje podataka pomoću nekih Software as a Service ili SaaS aplikacija, kao i drugih oblika online rada. Funkcionalnost uživo interaktivnog pristupa udaljenog računala, kao što su virtualne privatne mreže, radi puno bolje s novom generacijom usluga satelitskog Interneta nego u prošlosti. Sateliti srednje Zemljine orbite (MEO – medium earth orbit) i niske Zemljine orbite (LEO – low earth orbit) nemaju takva velika kašnjenja. Na primjer: Trenutni položaj LEO satelita Globalstar i Iridium imaju kašnjenja manja od 40 ms za ukupno vrijeme obilaska, ali je njihov promet manji od širokopojasnog od 64 kbit / s po kanalu. Globalstar konstelacija kruži na oko 1.420 km iznad Zemlje a Iridium na visini od 670 km. Predložena O3b Mreže MEO konstelacija zakazana za pogon u 2013. bi kružila na 8062 km, s RTT latencijom od oko 125 ms. Predložila nova mreža također je dizajnirana za znatno višu propusnost sa linkovima i više od 1 Gbit / s (gigabita u sekundi). Planirani COMMStellation, zakazan za lansiranje u 2015., će kružiti oko Zemlje na 1000 km s kašnjenjem od oko 7 ms. Ova konstelacija u polarnoj orbiti od 78 mikrosatelita osigurava globalnu okosnicu s protokom većim od 1,2 Gbit / s. Za razliku od geostacionarnih satelita, sateliti u niskoj i srednjoj zemljinoj orbiti nisu u fiksnom položaju na nebu. Prema tome, antene na tlu se nemogu lako uspostaviti komunikaciju sa bilo kojim određenim satelitom. Kao i sa GPS-om, male orbite uzrokuju da sateliti u niskoj Zemljinoj orbiti budu na nebu samo sat vremena ili manje prije nego što odu iza horizonta i izvan dosega, tako da takav složeni prelaz mora biti učinjen kako bi se prenio fiksni zemaljski signal na druge satelite. Komunikacija sa MEO ili LEO satelitima se može obaviti na dva načina: Više difuznim ili potpuno omni usmjerenim antenama na tlu koje mogu komunicirati u isto vrijeme s jednim ili više vidljivih satelita na nebu, ali sa znatno većom snagom odašiljanja od fiksnih geostacionarnih antena (zbog niske dobiti) i uz dosta lošiji signal šum odnos za prijem signala. Praćenjem, visoko – dobitnih antena uske zrake, ali to praćenje zahtijeva skuplja rješenja poput motoriziranih antena ili antene sa dinamičnim fazama polja koje mogu usmjeriti zraku elektronski i kompleksnim softverom koji može predvidjeti putanju svakog satelita u konstelaciji. Na satelitske komunikacije utječe vlaga i različiti oblici padalina (kao što su kiša ili snijeg) na putu signala između krajnjih korisnika ili zemaljskih postaja i satelita koji se koristi. Ovakva smetnja na signalu je poznata kao kišna smetnja (rain fade). Učinci su manje izraženi u nižim frekvencijeama 'L' i 'C' banda, ali mogu postati vrlo veliki na višoj frekvenciji 'Ku' i 'Ka' banda. Za usluge satelitskog interneta u tropskim područjima s jakom kišom, popularno je korištenje C-banda (4/6 GHz) s kružnom polarizacijom satelita. Satelitske komunikacije na Ka bandu (19/29 GHz) mogu koristiti posebne tehnike kao što su margine obilatih kiša, prilagodljivu uplink kontrolu snage i smanjenje brzine prijenosa tijekom padalina. Kišne margine su dodatni uvjeti komunikacijske veze koji se uzimaju u obzir pri signalnoj degradaciji zbog vlage i oborina, te su od velike važnosti za sve sustave koji rade na frekvencijama iznad 10 GHz. Vrijeme kada usluga nije dostupna se može smanjiti povećanjem satelitskog komunikacijskog tanjura kako bi prikupili više satelitskog signala na silaznoj vezi i pružili jači signal na uzlaznoj vezi. Drugim riječima, povećanje dobitka antene pomoću većeg tanjura je jedan od načina povećanja ukupne dobiti kanala i shodno tome signal-šum (S / N) odnosa, što omogućuje bolji signal pri utjecaju kiše bez da S / N omjer padne ispod minimalnog praga za uspješnu komunikaciju. Moderne potrošačke antene imaju tendenciju biti prilično male, što smanjuje kišnu marginu ili povećava potrebnu satelitsku silaznu snagu i troškove. Međutim, često je isplativije izgraditi skuplji satelit i manju, jeftiniju potrošačku antenu nego povećati veličinu potrošačke antene kako bi smanjili troškove. Veliki komercijalni tanjuri od 3,7 m do 13 m promjera se koriste za postizanje velike kišne margine i za smanjenje troškova po bitu zahtijevajući daleko manjoj energiji iz satelita. Sateliti obično koriste fotonaponski solarni izvor energije tako da nema trošaka za samu energiju, ali moćniji sateliti zahtijevaju snažnije solarne panele i elektroniku, često uključujući veću odašiljačku antenu. Veće satelitske komponente ne samo da povećavaju troškove, već i težinu satelita i općenito, troškovi lansiranja satelita u orbitu su izravno proporcionalni njegovoj težini. (Osim toga, budući da lansirna vozila (rakete) imaju određena ograničenja nosivosti, veći dijelovi satelita mogu zahtijevati složenije sklopive mehanizme za dijelove satelita poput solarnih panela i high-gain antena ili u krajnjem slučaju nadogradnja na skuplje lansirno vozilo koje može podnijeti veći teret.) Obično je potpuno jasna vidljivost između tanjura i satelita potrebna da sustav radi. Osim što je signal osjetljiv na apsorpciju i rasipanja uslijed vlage, također utječe i prisutnost drveća i druge vegetacije na putu signala. Kako se radio frekvencija smanjuje, na ispod 900 MHz, prodor kroz vegetaciju se povećava, ali većina satelitskih komunikacija je iznad 2 GHz što ih čini osjetljivim na sitne zapreke poput lišća. Širina radio signala između bilo koje dvije antene nije savršeno ravna i ravnomjerna, kao što je snop svjetlosti. Umjesto toga, kako se signal širi dalje od odašiljačke antene, širi se prema središtu između dvije antene, a zatim se opet sužava kada prilazi prijemnoj anteni. To je poznato kao Fresnel zona (nazvano po fizičaru Augustin-Jean Fresnel) i ograničava korisnost satelitske antene na mjestima gdje je izuzetno ograničeno čisto nebo za prijem signala. Put signala kroz prostor mora biti čist ne samo za izravnu liniju vidljivosti, već i za širenje Fresnelove zone. Dvosmjerna satelitska internet usluga uključuje slanje i primanje podataka s udaljenog vrlo-malog-otvorom (VSAT) terminala putem satelita na telekomunikacijski hub (teleport), koji zatim prosljeđuje podatake zemaljskim Internetom. Na svakoj lokaciji satelitska antena mora biti precizno usmjerena kako bi se izbjegle smetnje sa drugih satelita. Tržište dvosmjerne satelitske komunikacije se može podijeliti na one sustave koji podržavaju profesionalne aplikacije, kao što su bankarstvo, maloprodaja i sl. i na onekoje pružaju malim i poslovnim korisnicima pristup internetu. Ključna razlika između ovih sustava može se vidjeti u njihovoj sposobnosti da podržavaju napredno upravljanje kvalitetom servisa. Dok sustavi za profesionalce kao što su oni iz iDirect-a omogućuju operaterima da definiraju i zadovolje ugovorenu kvalitetu usluge. Postoji nekoliko tipova dvosmjernog satelitskog interneta, uključujući i pristup s više vremenskom podjelom (TDMA) i jedan kanal po nosiocu (SCPC). Dvosmjerni sustavi mogu biti jednostavni VSAT terminali sa 60-100 cm tanjurom i izlaznom snagom od samo nekoliko wata namijenjeni za potrošače i male tvrtke ili veće sustave kojima pružaju veću propusnost. Takvi sustavi često se označavaju kao "satelitski širokopojasni pristup", a mjesečna cijena im može biti dva do tri puta viša od zemaljskih sustava kao što su ADSL. Modemi koji se koriste za ovu uslugu često su vlasnički zaštićeni, ali neki od njih su kompatibilni s nekoliko drugih pružatelja usluga. Kupci satelitskog interneta se razlikuju od pojedinačnih kućnih korisnika s jednim računalom do velikih udaljenih poslovnih lokacija s nekoliko stotina računala. Kućni korisnici imaju tendenciju da koriste zajednički satelitski kapacitet kako bi smanjili troškove, a pri tome još uvijek imaju visoke brzine prijenosa kad nema zagušenja. Uobičajeno postoje vremenski ograničene kvote propusnosti, tako da svaki korisnik dobiva svoj udio, prema njihovoj uplati. Kada korisnik prekorači dopuštenje, tvrtka može usporiti njihov pristup, deklasirati njihov promet ili zaračunati naknadu za iskorištenu prekomjernu potrošnju. Uplink smjer za zajedničke korisnike je obično višestruki pristup s vremenskom podjelom (TDMA), koji uključuje prijenos povremene kratke bljeskove paketa u odnosu na ostale korisnike (slično kao što mobitel dijeli pristupni toranj) Svaka udaljena lokacija može također biti opremljena telefonskim modemom. Tu je veza kao i kod uobičajenih dial-up ISP-a. Dvosmjerni satelitski sustavi ponekad mogu koristiti modem kanal u oba smjera za podatke gdje je latencija važnija od propusnosti, a rezerviraju satelitski kanal za preuzimanje podataka gdje je propusnost važnija od latencije, npr. za prijenos datoteka. Obično dolaze u obliku kompaktnih ravnih pravokutnih kućišta koje treba odprilike usmjeriti u smjeru satelita – za razliku od VSAT-a ne moraju biti vrlo precizno usmjereni, a modemi imaju ugrađene mjerače jačine signala kako bi korisnik ispravno usmjerio uređaj. Modemi najčešće koriste Ethernet ili Universal Serial Bus (USB) priključke. Neki također imaju integrirani Bluetooth primopredajnik koji služi i kao satelitski telefon. Modemi također imaju svoje baterije, tako da se mogu spojiti na laptop. Najčešći takav sustav je Inmarsat BGAN - ovi terminali su odprilike veličine aktovke i imaju gotovo simetrične brzine spajanja oko 350-500 kbit / s. Postoje manji modemi koje nudi Thuraya ali samo na brzinama od 444 kbit / s na ograničenom području pokrivenosti. Već dugi niz godina satelitski telefoni su u mogućnosti da se spoje na internet. Propusnost varira od oko 24 kbit/s za Iridium satelitske mreže i ACeS temeljene telefone do 15 kbit/s uzlazno i 60 kbit/s nizlazno za Thuraya uređaje. Globalstar također omogućuje pristup internetu do 96 kbit/s - kao i za Iridium i ACeS dial - up veza je potrebna i naplaćuje se po minuti, no oba Globalstar i Iridium planiraju lansirati satelite koji će nuditi uvijek dostupne podatkovne usluge na višim razinama. Kod Thuraya telefona također je moguća 96 kbit/s dial - up veza, 60 kbit/s servis je uvijek dostupan, a korisnik se naplaćuje po količini prenesenih podataka. Telefoni se mogu spojiti na laptop ili drugo računalo pomoću USB ili RS - 232 sučelja. Zbog niske propusnosti izuzetno je sporo pregledavanje weba s takvim vezama, ali korisno za slanje e-pošte, Secure Shell podatke i druge nisko - propusne protokole. Budući da satelitski telefoni imaju omnidirekcionalne antene nema potrebe za usmjeravanje dokle god postoji optička vidljivost između telefona i satelita. Ako imate podatke (puno informacija, obično) koje treba odaslati na različite (ali obično ne sve) host-ove preko interneta, onda je Multicast odgovor. Jedna od najčešćih situacija u kojima se koristi je prilikom distribucije u realnom vremenu audia i videa na set host-ova koji su se pridružili distribuiranoj konferenciji. Multicast je kao i radio ili TV u smislu da samo oni koji su ugodili svoje prijemnike (odabirom određene frekvencije) primaju informacije. Ustvari: čujete kanal koji vam je interesantan, ali ne i druge. Unicast je sve ono što nije broadcast niti multicast. Kada šaljete paket, a postoji samo jedan pošiljatelj proces – vaš – i jedan proces primatelj (onaj koji šalje paket prema), onda je to unicast. TCP je, po vlastitoj prirodi, unicast orijentirani. UDP podržava puno više paradigme, ali ako šaljete UDP pakete i postoji samo jedan proces koji bi ih trebalo primiti, to je također unicast. S današnjom tehnologijom moguće je priuštiti "trošak" izrade Unicast veze sa svima koji žele vidjeti vašu web stranicu. Međutim, ako šaljete audio i video, koji trebaju veliku količinu propusnosti u usporedbi s web aplikacijama – imate, sve dok multicast nije došao na scenu – dvije opcije: uspostaviti zasebnu Unicast vezu sa svakom od primatelja, ili da koristite broadcast. Prvo rješenje nije pristupačno: ako smo rekli da je jedna veza za slanje audia / videa troši veliku propusnost, zamislite da uspostavljate stotine ili možda tisuće tih veza. I vaše računalo i vaša mreže bi se srušila. Broadcast je čini se rješenje, ali to nije pravo rješenje. Ako želite da svi host-ovi u vašem LAN-u sudjeluju u konferenciji, možete koristiti broadcast. Paketi će se slati samo jednom i svaki host će ih primiti jer su poslani na adrese prijenosa. Problem je u tome što su možda samo neki host-ovi, ali ne svi, zainteresirani za te pakete. Nadalje: možda su neki host-ovi stvarno zainteresirani za vašu konferenciju, ali su izvan vašeg LAN-a, nekoliko routera daleko. Znate da broadcast radi dobro unutar LAN-a, ali problemi nastaju kada želite broadcast pakete preusmjeriti na drugi LAN. Čini se da je najbolje rješenje ono u kojem šaljete pakete do određene posebne adrese (određene frekvencije u radio / TV prijenosu). Zatim, svi host-ovi koji su se odlučili da se pridruže konferenciji će biti svjesni paketa sa te odredišne adrese, pročitat će ih kada prolaze preko mreže, te će ih prenijeti na IP sloj kako bi se demultipleksirali. To je slično s broadcast-om na naćin da šaljete samo jedan broadcast paket, a svi host-ovi u mreži ga prepoznaju i čitaju; razlikuje se, međutim, u tome da nisu svi multicast paketi pročitani i obrađeni, već samo oni koji su prethodno registrirani u kernelu kao "interesantni". Ti posebni paketi se usmjeravaju na kernel razinu kao i svaki paket jer su to IP paketi. Jedina razlika se nalazi u algoritmu usmjeravanja koji govori kernelu gdje da ih usmjeri ili ne usmjeri. Raspon IP adresa je podijeljen u "klase" koje se temelje na veličini reda bitova od 32 bita IP adrese: Bit --> 0 31 Address Range: Ona koja se nas tiče je "Class D adresa". Svaki IP datagram čija odredišna adresa počinje s "1110" je IP Multicast datagram. Preostalih 28 bitova identificiraju multicast "grupe" na koje se datagram šalje. Nasljedno prethodnoj analogiji, morate podesiti svoj radio kako bi čuli program koji se emitira u nekoj određenoj frekvenciji, na isti način morate "naštimati" kernel da prima pakete poslane na određenu multicast grupu. Kada to učinite, kaže se da se host pridružio toj skupinu na sučelju koje ste naveli. Postoje neke posebne multicast skupine, recimo "dobro poznate multicast skupine", koje ne bi trebali koristiti u određenim aplikacijama zbog njihove posebne namjene: Sve ove posebane multicast skupine se redovito objavljuju u " Assigned Numbers " RFC. U svakom slučaju, raspon 224.0.0.0 kroz 224.0.0.255 je rezerviran za lokalne potrebe (kao administrativni zadaci i zadaci održavanja), a datagrami namijenjeni njima se nikada ne proslijede od strane multicast usmjerivača. Slično je raspon 239.0.0.0 do 239.255.255.255 rezerviran za "administrativno područje“. Host-ovi mogu biti u tri različite razine skladnosti sa multicast specifikacijama, prema postojećim zahtjevima. Razina 0 je "nema podrške za IP Multicasting" razina. Puno host-ova i usmjerivača na internetu su u tom stanju, jer multicast podrška nije obvezna u IPv4 (međutim je u IPv6). Ne treba previše ovo objašnjavati: host-ovi na ovoj razini ne mogu niti slati niti primati multicast pakete. Moraju ignorirati one poslane od strane drugih multicasting sposobnih host-ova. Razina 1 je "podrška za slanje, ali ne i primanje multicast IP datagrama" razina. Dakle, imajte na umu da nije nužno pridružiti se multicast grupi kako bi joj mogli poslati datagrame. Vrlo malo treba dodati IP modulu da se od "Razine 0" host-a napravi "Razina 1 podržana". Razina 2 je "puna podrška za IP multicast" razina. Razina 2 host-ovi moraju biti u mogućnosti i da šalju i da primaju multicast promet. Moraju znati način kako se pridružiti i napustiti multicast grupe i tu informaciju proslijediti multicast usmjerivačima. S toga, moraju uključiti Internet Group Management Protocol (IGMP) implementaciju u TCP/IP složaj. Broadcast je (u usporedbi) lakše implementirati nego multicast. Ne zahtijeva od procesa da daju kernel-u pravila o tome što učiniti s broadcast paketima. Kernel jednostavno zna što treba učiniti: učita i dostavi ih sve odgovarajućim aplikacijama. Kod multicasta, međutim, potrebno je savjetovati kernel koje su nam multicast grupe interesantne. Odnosno, moramo pitati kernel da se "pridruži" tim multicast grupama. Ovisno o podlozi hardvera, multicast datagrame filtriraju hardver ili IP sloj (a u nekim slučajevima objoe). Samo oni koji su se prethodno registrirali u destinacijsku grupu putem pridruživanja su prihvaćeni. U biti, kada se pridružujemo grupi govorimo kernel-u: "OK. Znam da, po defaultu, ignoriraš multicast datagram, ali sjeti se da sam zainteresiran za ovu multicast grupu. Dakle, učitaj i isporuči (na bilo koji zainteresirani proces, ne samo meni) bilo koji datagram koji vidiš u ovom mrežnom sučelju na ovoj multicast grupi u svojem odredišnom polju". Neka razmatranja: prvo, imajte na umu da se ne pridružujete samo grupi. Pridružujete se grupi na određenom mrežnom sučelju. Naravno, moguće je pridružiti se istoj grupi na više od jednog sučelja. Ako ne odredite konkretno sučelje, onda će ga kernel odabrati na temelju svojih tablica usmjeravanja kada će datagrami biti poslani. Također je moguće da se više od jednog postupka pridruži istoj multicast grupi na istom sučelju. Svi će dobiti datagrame poslane toj skupini preko tog sučelja. Kao što je već rečeno, bilo koji multicast – sposoban host se pridružuje all – host grupi na start-up, tako da se "ping" 224.0.0.1 vraća od svih računala u mreži koja imaju multicast omogućeno. Na kraju, razmotrite, da bi proces mogao primiti multicast datagrame mora pitati kernel da se pridruži grupi i povezati port na koje su ti datagrami poslani. UDP sloj koristi i odredišnu adresu i port kako bi demultipleksirao pakete i odlučuje na koju ih priključnicu dostaviti. Kada proces više nije zainteresiran za multicast grupu, informira kernel da želi napustiti tu grupu. Važno je shvatiti da to ne znači da kernel više neće prihvatiti multicast datagrame namijenjene toj multicast grupi. To će i dalje činiti, ako postoji više precesa koji su izdala "multicast pridruženje" peticiju za tu grupu i koji su još uvijek zainteresirani. U tom slučaju host ostaje član grupe, dok se svi procesi ne odluče da napuste grupu. Još više: ako napusti grupu, ali ostane vezan za port na koji je primao multicast promet, a postoje još procesa koji su se pridružili grupi, i dalje će primati multicast prijenose. Ideja je ta da pridruživanje multicast grupi samo govori IP-u i podatkovnom sloju (koji u nekim slučajevima izričito govori hardveru) da prihvatiti multicast datagrame namijenjene toj grupi. To nije članstvo po procesu, već članstvo po host-u. Korištenje nove tehnologije obično nosi neke prednosti i nedostatke. Prednosti su multicasta su – Ja mislim – čisto. Glavni nedostatak je da ga stotine host-ova, a posebno routera ne podržava još. Kao posljedica toga, ljudi koji su počeli raditi na multicastu, kupili su novu opremu, modificirali su svoje operativne sustave i izgradili multicast otoke u svojim lokalnim mjestima. Tada su otkrili da je teško komunicirati s ljudima koji rade slične stvari, jer ako samo jedan od routera između njih ne podržava multicast ne može se ništa učinit... Rješenje je jasno: odlučili su izgraditi virtualnu multicast mrežu na vrhu Interneta. Odnosno: stranice sa multicast usmjerivačima između njih mogu komunicirati izravno. Ali, stranice pridružene preko unicast usmjerivača će poslati svoj otočni multicast prometa upakiran u unicast pakete na druge multicast otoke. Usmjerivači u sredini neće imati problema, jer se bave s unicast prometom. Konačno, u prijamnom mjestu, promet će biti odpakiran i poslan na otok u izvornom multicast načinu. Dva kraja koja pretvaraju multicast u unicast i nazad u multicast definiraju ono što se zove multicast tunel. Jedan trivijalni algoritam koji bi cijelom svijetu pružio multicast promet bi bio da ga samo pošalje... svugdje, unatoč što to netko želi ili ne. Zato što se to ne čini posve optimizirano, implementirano je nekoliko algoritama usmjeravanja i tehnika prosljeđivanja. DVMRP (Distance Vector Multicast Routing Protocol) je možda onaj koji multicast usmjerivači najviše koriste danas. To je protokol gustog moda usmjeravanja, to jest, djeluje dobro u okruženjima s visokom propusnošću i gusto raspoređenim članovima. Međutim, u scenariju rijetkog moda, pati od problema skalabilnosti. Zajedno s DVMRP možemo naći i druge guste modove protokola usmjeravanja, kao što je MOSPF (Multicast Extensions to OSPF – Open Shortest Path First –) i PIM - DM (Protocol – Independent Multicast Dense Mode). Za obavljanje usmjeravanja u rijetkim mod sredinama, imamo PIM -SM (Protocol Independent Multicast Sparse Mode) i CBT (Core Based Trees). Svi ovi protokoli usmjeravanja koriste neku vrstu multicast prosljeđivanja, kao što su poplave, Reverse Path Broadcasting (RPB), Truncated Reverse Path Broadcasting (TRPB), Reverse Path Multicasting (RPM) ili Shared Trees. Do sada smo govorili o multicast prijenosu pomoću UDP-a. To je uobičajena praksa, jer je nemoguće to učiniti s TCP-om. Međutim, u zadnjih nekoliko godina odvijaju se brojna istraživanja kako bi se razvile neki novi multicast transport protokoli. Nekoliko od tih protokola su implementirani i testiraju se. Dobra lekcija je čini se da nema općenitog multicast transport protokola i dovoljno dobrog za sve tipove multicast aplikacija. Ako su transportni protokoli komplicirani i teško ugodljivi, zamislite kako je sa kašnjenjem (u multimedijalnim konferencijama), gubitak podataka, naređivanje, ponovno slanje, tok i kontrola zagušenja, upravljanja skupinama, itd., kada na prijemu nije jedan, već možda i stotine ili tisuće host-ova. Ovdje je skalabilnost problem, a nove tehnike se uvode kao što je ne davanje priznanja za svaki primljeni paket nego slanjene negativnog priznaja (NACK) za podatke koji nisu primljeni. RFC 1458 nudi predložene uvjete za multicast protokole. Real-Time Transport Protocol (RTP) se bavi višestranim multimedijskim konferencijama, Scalable Reliable Multicast (SRM) koristi wb (distribuirani White – Board alat), Uniform Reliable Group Communication Protocol (URGC) provodi pouzdane i naređene transakcije sa temeljene na centraliziranom kontrolom, Muse je razvijen kao posebna protokol aplikacija: za multicasting vijesti preko MBone-a, Multicast File Transfer Protocol (MFTP) govori samo o sebi i ljudi se "pridružuju" na isti način na koji bi se pridružiti konferenciji, Log-Based Receiver-reliable Multicast (LBRM) je znatiželjni protokol koji prati sve pakete koji su poslani na log server, a koji govori pošiljatelju da li mora ponovo poslati podatke ili ne mora jer su ih svi prijamnici dobili. Jedan protokol sa smiješnim imenom, pogotovo za multicast protokole, je STORM (STructure-Oriented Resilient Multicast). Danas, dolaskom Satelita visoke propusnosti (HTS – High Throughput Satellite) i tehnologije, u kombinaciji s porastom potražnje za satelitskim komunikacijama, očekuje se da će imati dubok utjecaj u VSAT industriji. Uz ogroman priliv propusnosti kapaciteta, HTS će donijeti poboljšane brzine i niže troškove. U kombinaciji s velikim svemirskim segmentom i infrastrukturnim napretkom, postoji velika mogućnost da se ubrza i usvoji cjelokupni VSAT na mnogo širem planu . Sateliti visoke propusnosti su nova vrsta širokopojasnih satelita visokih performansi koji danas uglavnom koriste Ka - band frekvencije, ali ne i isključivo - jer Intelsat najavljuje EPIC pa bi mogli vidjeti Ku HTS vrlo brzo. HTS je bitno različit u pogledu dizajna i zahtjevima na tlu. (Za više detalja, pogledajte HTS Whitepaper). Tradicionalni sateliti koriste velike regionalne zrake koje pokrivaju cijelo područje sa fiksnim kapacitetom. Svaki davatelj usluga može posjedovati hub i teleport i nuditi usluge kupcima sve dok god su u satelitskom području pokrivanja. Za razliku od toga, HTS iskorištava ponovno korištenje frekvencije na više nasumičnih zraka za stvaranje golemog povećanja kapaciteta. Dedicirana visoko propusna „feed“ veza je potrebna da opsluži zrake. Hub infrastruktura mora biti smještena unutar „feed“ linka kako bi opslužila sve zrake. Pogledajte ispod : Kako je lansirano sve više i više HTS-a od njih se očekuje, tijekom vremena, da osiguraju ogroman priliv propusnosti kapaciteta koji će isporučiti veće brzine po manjoj cijeni. U stvari, NSR navodi da se od satelita visoke propusnosti očekuje da će dostaviti najmanje 1,34 Tbps kapaciteta do 2020 godine. Budući da propusnost više neće ograničavati kako se posluje, očekuje se brzo usvajanje satelita i otvaranje vrata za nove mogućnosti i u privatnom i državnom tržištu.