REFERAT ENGLEZA: VERY SMALL APPERTURE TERMINAL

Publicat: 19 Mai 2009 00:00

A brief history of space communication The idea of radio transmission through space was first conceived in 1911. In 1945 British author-scientist Arthur C Clarke suggested the use of a geosynchronous earth satellite for the purpose. His assumption of a manned space station was later revised by a US engineer, J R Pierce, in April 1955, who was also the first one to analyze unmanned communication satellites. This idea later led to the great success of satellite communications. The first artificial satellite "SPUTNIK I" was launched by the erstwhile USSR, in 1957. This began a series of space initiatives by USA and USSR. The first satellite communication experiment was the US government's project SCORE (Signal Communication by Orbiting Relay Equipment), which launched a satellite on December 18, 1958. This satellite circled the earth in an elliptical orbit and retransmitted messages recorded on a magnetic tape. It lasted for about 13 days after which the batteries ran out!The US Army Signal Corp's Courier IB, launched in October 1960, lasted for about 17 days. It could handle typewriter data and voice and facsimile messages. It was a balloon, Echo 1, launched in August 1960, which led American Telephone & Telegraph Company (AT&T) to build Telstar. Communication tests carried out by reflecting radio signals from Echo 1's surface were completely successful. Telstar, launched on July 1962 was the first active satellite with a microwave receiver and transmitter to transmit live television and telephone conversations across the Atlantic. It was turned off in February 1963. Successive initiatives include NASA's Relay 1 satellite was launched in elliptical orbit in December 1962 and Syncom 2, the first synchronous communication satellite was launched in July 1963. In 1964 a global initiative was undertaken leading to the formation of INTELSAT, which has been one of the major driving forces for the large scale commercial exploitation of satellite technology for communications. Since then there has been no looking back. What is a VSAT? The term Very Small Aperture Terminal (VSAT) refers to a small fixed earth station. VSATs provide the vital communication link required to set up a satellite based communication network. VSATs can support any communication requirement be it voice, data, or video conferencing. The VSAT comprises of two modules - an outdoor unit and an indoor unit. The outdoor unit consists of an Antenna and Radio Frequency Transceiver. (RFT). The antenna size is typically 1.8 metre or 2.4 metre in diameter, although smaller antennas are also in use. The indoor unit functions as a modem and also interfaces with the end user equipment like stand alone PCs, LANs, Telephones or an EPABX. Outdoor Unit The antenna system comprises of a reflector, feedhorn and a mount. The size of a VSAT antenna varies from 1.8 metres to 3.8 metres. The feedhorn is mounted on the antenna frame at its focal point by support arms. The Feed Horn directs the transmitted power towards the antenna dish or collects the received power from it. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength. The RFT is mounted on the antenna frame and is interconnected to the feed horn. Also termed as outdoor electronics, RFT, in turn, consists of different subsystems. These include low noise Amplifiers (LNA) and down converters for amplification and down conversion of the received signal respectively. LNAs are designed to minimise the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of a LNA Upconverters and High Powered Amplifiers (HPA) are also part of the RFT and are used for upconverting and amplifying the signal before transmitting to the feedhorn. The Up/Down converters convert frequencies between intermediate frequency (Usually IF level 70 MHz) and radio frequency. For Extended C band, the downconverter receives the signal at 4.500 to 4.800 GHz and the upconverter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts Interlink Facility The outdoor unit is connected through a low loss coaxial cable to the indoor unit. Indoor Unit The IDU consists of modulators which superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for upconversion, amplification and transmission. It also consists of demodulators which receive the signal from the RFT in the IF range and demodulates the same to segregate the user traffic signal from the carrier. The IDU also determines the access schemes under which the VSAT would operate. The IDU also interfaces with various end user equipment, ranging from stand alone computers, LAN's, routers, multiplexes, telephone instruments, EPABX as per the requirement. It performs the necessary protocol conversion on the input data from the customer end equipment prior to modulation and transmission to the RFT. An IDU is specified by the access technique, protocols handled and number of interface ports supported. Features:Ability to target small-dish audiences and meet specialized service requirements Support for applications including LAN/WAN networking, Internet/intranet, video conferencing, remote site networkingLarge in-orbit capacity makes VSAT platforms cost-effectiveStar, multi-star, mesh wideband and remote solutionsNetwork ArchitecturesDifferent transmission architectures are used for interactive hubbed VSAT networks:TDM/TDMA ( def: TDMA = Time Division Multiple Access - a technology for delivering digital wireless service using time-division multiplexing (TDM). TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. ) Demand Assigned SCPC (single channel per carrier)Point-to-point SCPC links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.)CDMA -; contentionless, nr. of simultaneously transmitting users under a threshold.DAMA Of these TDM/TDMA is by far the dominant technique with only CDMA being used to a small extent. Demand assigned SCPC has been virtually abandoned as a transmission scheme for the present.TDM/TDMA Interactive VSAT NetworksAll the established interactive hubbed VSAT systems use TDM/TDMA access as the primary access technique (TDM on the outbounds and TDMA on the inbounds).A typical TDMA network employs a large satellite hub system that manages all network terminal access and routing. Data is transmitted to and from the hub in short bursts on satellite channels that are shared with 30 to 40 other terminals (depending upon network loading parameters). The hub communicates with these VSAT terminals over a higher-speed "outbound" TDM satellite carrier. The terminals transmit back to the hub on their assigned "inbound" carriers using TDM protocols.This is how a star data, TDM/TDMA VSAT network works using a hub station, usually six metres or more in size and small VSAT antennas (between 75 centimetres and 2.4 metres). All the channels are shared and the remote terminals are online, offering fast response times. Consequently, TDM/TDMA systems are comparable with terrestrial X.25 or frame relay connections:Initial systems were designed to offer fast response times for predicable, bursty traffic patterns - typified by credit verification transactions and lottery systems. As the internet and broadband generally began to drive demand, the manufacturers introduced completely new IP-centric platforms designed to serve broadband applications. In essence, current systems are now also able to trade a short initial delay to allow the hub to allocate dedicated capacity within the inbound (return) channel to a VSAT. This capacity is dynamically sized by the system based on the traffic demand seen by the VSAT. In addition, all systems also incorporate frequency hopping, allowing the network to be load-balanced by moving VSATs between inbound carriers to ensure that capacity is used efficiently and congestion does not occur.Broadband VSAT systems are sometimes criticised for poor performance with the blame often being laid at the door of the product. However, almost every VSAT platform of which we are aware, is very capable of meeting the demands of the most demanding user - the culprit for poor performance is usually a result of the amount of bandwidth a subscriber is allocated as part of the service. High rates of over-subscription are mostly a consequence of low prices - satellite bandwidth is both a finite and expensive resource unfortunately.On a brighter note, the industry has been working hard towards introducing greater efficiencies at all levels - the satellite (Ka-band and on-board processing), the inbound/return links (turbo coding and modulation), the outbound/forward channel (DVB-S2) and with more advanced techniques to manage IP and web traffic (acceleration, compression and caching).Network ConfigurationTDMA is a Contentionless ( the users transmit in an orderly scheduled manner), fixed assignment ( each user has a predetermined and fixed allocation of resources) fashion protocol.Signal Types and CharacteristicsThe outbound data stream from the hub is transmitted at a relatively high data rate (typically 56 to 1024 kb/s) using TDM. The bit stream consists of a synchronisation word followed by a series of messages in time slots directed towards individual VSAT terminals. Broadcast messages to all remote VSAT terminals are also generally permitted.Outbounds are transmitted continuously (i.e. duty cycle 100%) as a TDM stream. The number of outbounds per network is determined by the traffic statistics, packet length as well as the outbound data rate.The outbounds for a network are generally grouped together at either the top or the bottom of the leased bandwidth.The inbound carrier is often accessed using ALOHA or Slotted ALOHA -; contention ( possibility of collision exists), random access ( there is no distributed scheduling of transmission) . If a higher capacity is required, a separate channel can be dedicated to ALOHA or Slotted ALOHA access requests and a demand assigned TDMA access scheme established.Inbound slotted ALOHA carriers information rates are usually between 2.4 and 16 kb/s. Inbound TDMA or SCPC carriers used for file transfer usually have information data rates between 56 kb/s and 256 kb/s. All carriers are BPSK or QPSK modulated and have rate 1/2 or 2/3 Forward Error Correction (FEC). This ensures that bit error rates are low (typically 10-6 or 10-7 which is comparable to ISDN).Remote terminals transmit in TDMA bursts in either a pre-assigned inbound channel slot or in any inbound channel slot depending on the manufacturer.Several different inbound TDMA access systems are used depending on traffic characteristics and the manufacturer.In a shared hub network, individual customers are often, but not always, allocated one or more dedicated outbounds and several inbounds.If the traffic mix is a combination of short interactive messages and long file transfers it is often worthwhile to use a technique called Adaptive ALOHA/TDMA. VSATs which have large blocks of data to transmit request dedicated TDMA time slots and use TDMA. The other VSAT terminals in the network use slotted ALOHA and avoid the assigned time slots. Alternatively, dedicated SCPC carriers can be temporarily assigned for file transfer.Typical Interactive Hubbed VSAT Network SpectrumTypical Interactive Hubbed VSAT Frame and Packet FormatEach TDM outbound carries a continuously transmitted bitstream which is divided into frames.The start of a frame is denoted by a framing packet contain a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information.The rest of the frame is filled by (generally) fixed length data packets which each contain: F preamble HDR header - giving IDU address and control information FCS frame check sequence F postambleOutbound data packets typically contain between 50 and 250 bytes in transactional networks.Each TDMA inbound contains frames which are synchronised to the outbound frames. Each inbound frame is divided into slots. Individual IDUs transmit in these slots in a manner depending on the access modes available to the particular system and how the network has been set up.Each inbound packet consists of: F preamble HDR header - giving IDU address and control information FCS frame check sequence F postambleInbound data packets typically contain between 50 and 250 bytes in transactional networks.The main inbound transmission modes used are:Aloha, in which an IDU can transmit data packets at any time in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 10 to 15%.Slotted Aloha, in which an IDU can transmit data packets in any slot (or any of a predetermined number of slots) in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 25 to 30%.Fixed Assignment, in which specific time slots in an inbound frequency slot are permanently, or for the duration of a particular transmission, assigned to a particular IDU. This is often used for batch transmission and for telephony. Transmissions in any particular frequency slot are intermittent but can have a peak traffic duty cycle of 100% if that particular inbound is carrying telephony traffic or several batch file transfers from different IDUs.Dynamic Assignment, in which time slots in an inbound frequency slot are dynamically assigned to a particular IDU in line with ongoing traffic demands. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of from 25 to 30% to approaching 100%, depending on the traffic mix.Demand assigned SCPC (point -;to -;multipoint)SCPC is used for economical distribution of broadcast data, digital audio and video materials, as well as for full-duplex or two-way data, audio or video communications. In an SCPC system, user data is transmitted to the satellite continuously on a single satellite carrier. The satellite signal is received at a single location, in the case of a point-to-point system, or at many locations in a broadcast application, providing hubless connectivity among multiple sites.Mesh networks which use capacity on a demand assigned multiple access (DAMA) basis take a different approach. The master control station merely acts as a controller and facilitator rather than a hub through which traffic passes as in a star network. However, these connections take a little time to set-up and thus, mesh/DAMA systems are often equated to a terrestrial dial-up connection.There are also mesh systems which use a TDMA access scheme where all of the terminals in a network receive and transmit to the same channel, selecting different time slots because each terminal is aware of what the others have reserved. In the past this type of system has been costly and therefore, reserved for large scale trunking applications, but, more recently, costs have come down considerably and now they can be cost competitive with SCPC/DAMA systems for thin route applications as well.Point -;to -; point SCPC Point-to-point SCPC (single channel per carrier) links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.All other systems are usually a variation on one of the themes described above, either in a star, mesh or hybrid (star and mesh) configuration. Most of the TDM/TDMA manufacturers also offer a mesh product which can be deployed in a hybrid-ised configuration, sharing common components such as antennas and RF units, at a remote site.DAMA Demand Assigned Multiple Access (DAMA)A DAMA system is typically a single hop satellite transmission network which allows direct connection between any two nodes in the network among many users sharing a limited "pool" of satellite transponder space. DAMA supports full mesh, point-to-point or point-to-multipoint communications -- any user can connect directly to any other user anywhere within the network. The result is economical and flexible bandwidth sharing with any mix of voice, FAX, video and data traffic. DAMA optimizes the use of satellite capacity by automatically allocating satellite resources to each active node upon demand. Users typically pay for satellite capacity used, plus a network maintenance fee.Network Control System....In a DAMA system, the network allocates communications bandwidth to each call from a pool of frequency channels on a demand-assigned basis. When a caller at a remote terminal requests service (e.g. picks up the telephone to make a call) the request is made to a Network Control System (NCS) over the shared DAMA common signaling channel. The NCS functions as a "switchboard in the sky". The NCS determines if the call is valid and then establishes the channel (including bandwidth) between the originating site and the called number. Circuits remain active only as long as needed, then are broken to free bandwidth for other users. When the call is completed, the NCS is informed by the remote terminals and the freed bandwidth is returned to the frequency pool. By using a DAMA system a single transponder can support several thousand subscribers. Any remote unit can be configured to perform as the Network Control System with the addition of some hardware and Network Management System (NMS) software. The hardware, together with the NMS, serves as the single focal point for system level control within the satellite communications network. The NCS can be located anywhere within the satellite footprint.What are the benefits of using a DAMA system?DAMA systems quickly and transparently assign communication links or circuits to users on a call-by-call basis. After use (hang up), channels are immediately returned to the central pool, for reuse by others. By using DAMA, many subscribers can be served using only a fraction of the satellite resources required by dedicated, point-to-point Single-Channel-Per-Carrier (SCPC) networks. The business case for DAMA is a strong one. DAMA saves satellite users money, optimizes use of scarce satellite resources, and can increase service provider revenues. Service providers using DAMA are able to pack more services into the same satellite bandwidth, with a resulting increase in their profit-making opportunities. For military applications, many users compete for very limited communications resources. DAMAs inherent network management capabilities provide positive control of those resources.

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