Satellite
10/26/2009
Mobile Cellular
Mobile Cell
Ponsel merupakan gabungan dua teknologi, yaitu telepon (Alexander Graham Bell, 1876) dan Radio (Nikolai Tesla, 1880; Guglielmo Marconi, 1894) Sebelum ponsel, orang harus memasang telepon radio di mobil. Pada sistem sistem telepon-radio, terdapat tower antena pusat di tiap kota, mungkin ada 25 channel frekuensi tiap tower.
Teknologi Komunikasi dan Keamanan data
Artinya, telepon di mobil harus memiliki transmiter yang cukup kuat, selain itu dengan channel terbatas tidak semua orang dapat menggunakan pada saat bersamaan
Lalu, muncul sistem cellular, yang membagi suatu kota/daerah menjadi beberapa sel kecil.
Memungkinkan penggunakan frekuensi ulang pada satu kota, sehingga jutaan pemakai dapat menggunakan ponsel bersamaan.
Konsep Selular Analog Setiap sel (terdiri dari 7 sel hexagonal grid), menggunakan 1/7 dari channel suara duplex Sehingga: setiap sel memiliki frekuensi unik dan dapat menghindari tabrakan (collision)
Sebuah cell-phone carrier mendapat 832 frekuensi radio Setiap ponsel menggunakan 2 frekuensi/panggilan (channel duplex), sehinggal ada 395 kanal suara per carrier. (42 frekuensi lain untuk channel kontrol)
(832 – 42) / 2 = 395 Sehingga, setiap sel memiliki sekitar 56 kanal suara. 395 / 7 = 56,429 => 56
Jadi, pada setiap sel, 56 orang dapat saling berbicara pada saat sama.
Transmisi Selular Analog Ponsel memiliki transmisi berkekuatan rendah (0.6 watt dan 3 watt) Keuntungan:
Transmisi base station dan ponsel tidak akan berada diluar sel. Konsumsi listrik ponsel juga rendah, sehingga baterai dapat berukuran sekecil mungkin
Setiap carrier memiliki sebuah Mobile Telephone Switching Office (MTSO) yang menangani semua hubungan ponsel dengan PSTN dan mengontrol semua base station
10/26/2009 Area Splitting
Membagi satu sel menjadi beberapa subsel Tujuan: membagi suatu area yang terlalu padat agar layanan dapat masih tersedia
Prinsip kerja jaringan seluler Arsitektur Seluler
10/26/2009 SIM CARD Mekanisme Pemanggilan
Ketika On, akan dicari SID pada channel control Tahun 1991 -> munich card (Giesecke & Devrient) Channel control adalah frekuensi khusus dimana ponsel
3 digit = Mobile Country Code dan BTS berkomunikasi untuk setup panggilan dan pergantian channel
2 digit = Mobile Network Code Jika tidak ditemukan SID, akan muncul “no service”
10 digit = Mobile Station Identification Number Jika ditemukan SID, akan dicocokan dengan SID SIM = Subcriber Identity Module => Smart Card pada ponsel (pada kartu), jika ya, ponsel berada dalam jaringan sistem home. RUIM = Removable User Identity Module Dengan SID, ponsel mengirimkan registration
Untuk CDMA request, dan MTSO mencatat lokasi ponsel Jika ada panggilan masuk, MTSO akan mencari posisi ponsel Anda di sel mana.
Mekanisme Pemanggilan Roaming MTSO akan menyiapkan dua frekuensi agar ponsel dapat menerima panggilan
Apabila kode SID pada Control Channel tidak MTSO berkomunikasi dengan ponsel melalui kanal cocok dengan kode SID yang telah terprogram kontrol untuk memberitahu frekuensi mana yang digunakan. sebelumnya di dalam database MTSO
Dan ketika ponsel dan tower dapat berpindah ke frekuensi tersebut, koneksi terbentuk. MTSO dari sel dimana Anda sedang roaming akan Anda berbicara dua arah dengan teman Anda. menguji apakah SID Anda valid dengan Ketika Anda bergerak ke tepi sel, sinyal akan melemah. menghubungi MTSO jaringan home Anda
Sementara itu, BTS berikut yang Anda tuju mendeteksi Sistem home memverifikasi ponsel Anda ke lokal sinyal ponsel Anda menguat.
Kedua BTS akan berkoordinasi lewat MTSO, dan pada saat MTSO, kemudian menelusuri ponsel Anda yang itu, melalui kanal kontrol, ponsel Anda mendapat sedang berada pada selnya informasi perubahan kanal frekuensi.
Sangat cepat waktunya Inilah yang disebut hand off
10/26/2009 GSM (Global System for Mobile Communication)
GSM distandarisasi oleh “Groupe Spécial Mobile”.
Eropa & Asia menerapkan GSM 900 dan GSM 1800. Sedangkan untuk US, GSM 1900 Untuk dapat terhubung pada jaringan GSM, pemakai harus memiliki subscriber identification module (SIM) card. GSM 900 menyediakan 124 kanal full duplex, 25 MHz GSM1800 menyediakan 374 kanal full duplex, 25 MHz Roaming technology: complete communication from anywhere in world Providers establish roaming areas: higher cost for users when outside home area GSM offers SMS service infrastruktur GSM
Arsitektur GSM Switching Subsystem
HLR (Home Location Register), merupakan database yang digunakan untuk manajemen dan penyimpanan subcriptions MSC (Mobile services Switching Center), melakukan fungsi telephone switching
VLR (Visitor Location Register), database untuk memyimpan informasi mengenai subscribers yang diperlukan oleh MSC untuk melayani visiting subscribers AUC (Authentication Center), menyediakan fungsi authentikasi dan enkripsi EIR (Equipment Identity Register), merupakan database yang menyimpan informasi mengenai identitas mobile equipment (IMEI)
GSM Arsitektur Base Station Subsystem
BSC (Base Station Controller), menyediakan fungsi kontrol dan link antara MSC dan BTS BTS (Base Transceiver Station), merupakan radio equipment (transceiver dan antena). Sekelompok BTS dikontrol oleh satu BSC
Mobile Station (MS) Mobile Equipment (ME) => handset Subscriber Identity Module (SIM) card, merupakan card yang berisi informasi mengenai user subscription
10/26/2009
3 Class of Message Services of GSM/GPRS In GSM/GPRS network, conventional circuit switched services (speech, data, and SMS) and GPRS services can be used in parallel. Three classes are defined:
Class A mendukung GPRS dan GSM secara bersama- sama (2on) Class B mendukung GPRS dan GSM, namun hanya aktif salah satu saja pada suatu saat Class C mendukung GPRS dan GSM, namun harus di switch secara manual
CdmaOne: technologies and standards associated with CDMA Telecommunications Industry Association (TIA) and International CDMA Telecommunications Union (ITU) regulate standards
Ada 2 standard: 3G Standard: W-CDMA (CDMA 3G): CDMA2000 1xEV Qualcomm memperkenalkan merk cdmaOne (CDMA 2G), dan CDMA2000 Freedom of Mobile Multimedia Access (FOMA) ~ NTT DoCoMo dan Vodafone Universal Mobile Telecommunications System (UMTS) ~ Eropa/Jepang CDMA2000 is family of technology types CDMA 3xMC CDMA 1xEV CDMA 1xMC Similar to EDGE Upgrade 1xMC to 3G networks 1xEV-DV: expands DO to handle voice 1xEV-DO: data only transmissions Separates voice and data into two separate channels 1xMC upgrades voice and data capacity Similar to UMTS – voice and data
10/26/2009
4G
4G Working Group has defined following objectives of HSDPA (High-Speed Downlink Packet Access)
4G wireless communication standard Downlink speeds: 1.8, 3.6, 7.2 dan 14.4 Mbps Spectrally efficient system Included in UMTS Release 5 Specification High network capacity, more simultaneous users per cell HSUPA (High-Speed Uplink Packet Access) 100 Mbps in moving, 1000 Mbps while in fixed position Uplink speeds up to 5.76 Mbps Mendukung HDTV Included in UMTS Release 6 Specification All IP, packet-switched network
Evolusi Aplikasi Mobile
10/26/2009 Mobile Station Terdiri dari : Merupakan terminal yang dipakai oleh pelanggan untuk melakukan proses komunikasi Subscriber Identification Module (SIM) Mobile Equipment (ME)/HP Catatan : MS tidak akan dapat berhubungan tanpa SIM card ME +
MS = SIM Mobile Equipment SIM Card Subscriber Identity Module (SIM) adalah sebuah smart card yang berisiwhich
Merupakan terminal tranceiver stores seluruh informasi user dan beberapa feature dari GSM Diidentifikasikan dengan IMEI Informasi yang ada berupa : tertentu 2 algorithma enkripsi. Yaitu algoritma autentikasi A3 dan A8 sebagai cipher Authentication Key “Ki”
IMEI = International Mobile key Equipment Identity SIM card dilindungi oleh sebuah mekanisme Personal Identity Number (PIN) yang dimiliki user Service tambahan IMSI and TMSI
10/26/2009 Base Station Controller
BSC mengatur sumber radio untuk sebuah BTS atau lebih. BSC menangani radio-channel setup, frequency hopping, and handover intern BSC
Network Sub-system (NSS) NSS terdiri dari : Mobile Switching Center (MSC)
Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AuC) Equipment Identity Register (EIR)
Mobile Switching Center (MSC) Melakukan fungsi switching dasar Mengatur BSC melalui A-interface Sebagai penghubung antara satu jaringan GSM dengan jaringan lainnya melalui Internetworking Function (IWF)
Authentication Center (AuC)
Berisi parameter authentikasi pelanggan untuk mengakses jaringan GSM. AuC berisi parameter seperti Ki, algorithma A3 atau A8 AuC memproduksi tiga buah parameter autentikasi seperti (SRES, RAND, Kc) dan menyimpannya di VLR.
10/26/2009 Equipment Identity Register (EIR)
EIR merupakan register penyimpan data seluruh mobile stations EIR berisi IMEIs (international Mobile Equipment Identities), yang merupakan nomor seri perangkat + tipe code tertentu Mobile Equipment dibagi menjadi tiga kelompok :
Blacklist Grey list White list * catatan: EIR belum diterapkan di Indonesia.
Operation Sub-system (OSS) Operation and Maintenance Jaringan Pengaturan pelanggan dan tagihan Pengaturan Mobile Equipment
Interface MSC Transcoder BSC BTS A Interface Ater Interface Abis Interface
Konsep kanal pada GSM Kanal terdiri dari dua jenis :
1. Kanal fisik: Satu TimeSlot(TS) frameTDMA merupakan satu kanal fisik Setiap carrier RF terdiri dari 8 TS(CH 0 – 7)
2. Kanal Logic: Kanal Trafik (TCH) dapat membawa suara atau data untuk layanan komunikasi. TCH dibagi dua jenis, full rate channel dengan Bit rate 13 Kbps dan half rate channel dengan kecepatan bit 6,5 Kbps Kanal Kontrol digunakan untuk keperluan signalling Kanal logik ditumpangkan pada kanal fisik
10/26/2009 Kanal Logik BCH Kanal logik CCCH BCCH ( Broadcast Control Channel ) Arah downlink Point to Multipoint Arah downlink Informasi power output maksimum MS Informasi BCCH carrier sel yang berdekatan Informasi LAI (Location Area Identity) PCH ( Paging Channel ) RACH ( Random Access Channel ) Paging message ( IMSI/TMSI ) Point to Multipoint AGCH ( Access Grant Channel ) Point to Point Downlink MS call set up Point to Point Menyediakan kanal signalling (SDCCH) Uplink Kanal logik DCCH Kanal Logik DCCH SDCCH ( Stand Alone Dedicated Control Channel ) Arah downlink dan uplink Call set up Point to Point SACCH ( Slow Associated Control Channel ) Downlink : Downlink dan uplink Uplink : MS measurement data Point to Point Menyediakan TCH Short message dan cell broadcast Location Updating Authentication FACCH ( Fast Associated Control Channel ) Stealing mode ( pengganti sementara TCH) Handover Point to Point Downlink dan uplink Timing advanced MS power output
10/26/2009 Call Set Up MS BTS BSC MSC/VLR EIR HLR SADM(pag resp) AUTH RES UA (Pag Resp) Chan Req CIPH Mode CMD ENCR CMD
Paging req Permintaan panggilan akan diteruskan ke seluruh Base Station diseluruh lokasi
MM Ass Chan Active ACK Chan RGD EST IND (Pag Resp) CR(Pag Resp) Paging CMD MM ASS CMD Chan active CIPH Mode CMD Authentication parameters MSC akan mengomentari authentikasi dari MS dan parameter harus dicek di UDT (paging) Auth Req dan siap untuk menjawab panggilan. CC Send Parameters Call Set Up area. radio, dan BSC akan memberikannya. Ketika MS yang dituju ditemukan, MS akan meminta sebuah interface kanal Ketika kanal aktif, MS akan mengirim PAG RESP sebagai tanda bisa dipanggil, Ciph mod com ASS COM ASS COM Call Conf Connect Alert ID Res UA RF Chan REL ACK Chan Activ ACK RF Chan ASL Chan Activ Ass Req ASS CMD SETUP ID Req Check IMEI ACM IMEI check Recall (RAND,SRES,Kc) Jika sukses, panggilan akan dikirim ke MS, yang merespon dengan CALL Conf untuk menandai MS dapat merespon semua jenis panggilan. Jika sukses, sebuah kanal trafik akan dialokasikan dengan sinyal ASS, terdengar HLR, dengan mengirim permintaan ‘send parameter’. Atau juga, MSC akan mengecek IMEI MS Pada EIR(optional) Proses Encripsi diinisialisasi dengan sinyal CIPH MODE. alarm dan terjadi hubungan.Connect ACK ANU
Handover1 Handover2
Handover adalah proses perpindahan kanal trafik user Handover dibagi menjadi : 1. Intra-cell HandOver: pemindahan hubungan ke kanal yang pada saat user aktif tanpa terjadi pemutusan hubungan berbeda pada satu BTS yang sama Penyebab Handover antara lain pergerakan dari user dan 2. Intern-cell HandOver: pemindahan hubungan antar BTS yang melemahnya sinyal terima dari satu sel 4. MSC ekstern HandOver: Pemindahan hubungan antar BTS 3. MSC intern HandOver: pemindahan hubungan yang terjadi dari MSC yang berbeda antar BSC dalam satu MSC berbeda dalam satu BSC
National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology
Spacecraft Components
Pasadena, California 2-Way Communication High- and Low-gain Antennas
Arm Holding (1 of 2)
Device to Sense Magnetic Field Magnetometer on Boom (11 meters/ 36.3 feet long)
Brains Computers Skeleton Core Structure
Hand Sifting Dust Particles Cosmic Dust Analyzer
Eyes Magnetospheric Imaging Instrument
Eyes Cameras
Baby Huygens Probe National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California
Spacecraft Components
On October 15, 1997, the Cassini–Huygens spacecraft was launched on an almost 7-year journey to the Saturn system. On its way, Cassini– Huygens passes Venus (twice), Earth, and Jupiter — arriving at the Saturn system on July 1, 2004. On arrival, the Huygens probe will be released from the Cassini orbiter and will descend to the surface of Sat- urn’s largest moon, Titan, on November 27, 2004. During the Huygens probe mission, data about Titan’s atmosphere, winds, and surface condi- tions will be collected. These data will be sent back to Earth using the Cassini orbiter’s high-gain antenna as a relay. The Cassini orbiter will orbit Saturn for 4 years. The spacecraft’s 12 onboard instruments will collect data about Saturn, the rings, the magnetosphere, Titan, and Saturn’s smaller moons.
The Cassini–Huygens mission is managed for the National Aeronautics and Space Administration (NASA) by the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. The European Space Agency, the Italian Space Agency (Agenzia Spaziale Italiana), and many European and American academic and industrial partners have teamed with NASA to make the Cassini–Huygens mission a reality.
The Cassini orbiter stands 2 stories tall; at launch, it weighed 5,300 kilo- grams (11,594 pounds). Over half of the orbiter’s mass is propellant. The Huygens probe, built by ESA, is 2.7 meters (8.86 feet) in diameter and weighs approximately 350 kilograms (766 pounds).
Cassini–Huygens is a robotic spacecraft: that means that Cassini– Huygens is controlled by people on Earth. The spacecraft consists of many different components that work together to keep Cassini– Huygens functioning. Take a tour of the Cassini–Huygens space- craft and see how all of these different components work. Computers on board process commands sent from Earth. These commands tell the spacecraft what to do; whether it is burning fuel to change the speed of the spacecraft, triggering the camera to get a good image of Saturn, sending data that have been stored on the solid-state recorder back to Earth, or any other spacecraft activity. of electronics. This food is supplied by three radioisotope thermo- electric generators. To make the many maneuvers necessary to tour the Saturn system, Cassini–Huygens is equipped with two sets of legs. The main engine serves as the “walking legs,” providing the endurance and strength necessary to make the long journey from Earth to Saturn. Cassini–Huygens also has a set of “dancing legs” in the form of orientation thrusters. These small thrusters are used to point instruments at different targets and to aim the antenna toward Earth. Finally, the Huygens probe is like a baby to Cassini–Huygens. The probe is attached to the spacecraft during the long journey to Titan. Upon arrival, the probe will be separated from the spacecraft and it will make a solo journey through the atmosphere of Titan. The Huygens probe is built with a computer (brain), instruments (senses), radio (mouth), parachutes (wings), and heat shield (shell — like a hermit crab). Just like a human body, all of the spacecraft’s components are supported by a skeletal structure.
Building a robotic spacecraft that will travel hundreds of millions of miles to its destination takes careful planning. Spacecraft designers need to carefully assess all the objectives of the mission and then choose the electrical and mechanical systems that will be necessary to achieve those mission goals. The Cassini–Huygens spacecraft contains components that are typical of those required on a spacecraft. Components on a spacecraft like Cassini–Huygens often have an analogy with the human body. Besides the legs, hands, brains, eyes, ears, child, skeleton, and food detailed in this illustration, are there any other senses or human parts that Cassini–Huygens should have? Can Cassini–Huygens smell? If so, how? Now take a look at the Huygens probe. What “body parts” does the Huygens probe have that are similar to human body parts? What “parts” doesn’t it have?
The Cassini–Huygens Spacecraft: A Special Robot
This paper is intended to give an overview of that methods of and uses for satellite communications, in
addition to presenting recent trends and future directions in the fieldSatellite Comunications
Satellite Communications
Table of Contents
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Satellite Comunications
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Introduction
In 1962, the American telecommunications giant AT&T launched the world's first true communications
satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and
the technology being applied to them is forever growing in sophistication.Basic Elements
Satellite communications are comprised of 2 main components:
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The Satellite
The satellite itself is also known as the space segment, and is composed of three separate units, namely the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the case of a telecom satellite, the primary task is to receive signals from a ground station and send them down to another ground station located a considerable distance away from the first. This relay action can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is the case with television broadcasts, the ground station's uplink is then downlinked over a wide region, so that it may be received by many different customers possessing compatible equipment. Still another use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and Satellite Comunications
Various Uses of Satellite Communications
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Traditional Telecommunications
Since the beginnings of the long distance telephone network, there has been a need to connect the
telecommunications networks of one country to another. This has been accomplished in several ways. Submarine cables have been used most frequently. However, there are many occasions where a largelong distance carrier will choose to establish a satellite based link to connect to transoceanic points,
geographically remote areas or poor countries that have little communications infrastructure. Groups like the international satellite consortium Intelsat have fulfilled much of the world's need for this type of service.●
Cellular
Various schemes have been devised to allow satellites to increase the bandwidth available to ground
based cellular networks. Every cell in a cellular network divides up a fixed range of channels which
consist of either frequencies, as in the case of FDMA systems, or time slots, as in the case of TDMA. Since a particular cell can only operate within those channels allocated to it, overloading can occur. Byusing satellites which operate at a frequency outside those of the cell, we can provide extra satellite
channels on demand to an overloaded cell. These extra channels can just as easily be, once free, usedby any other overloaded cell in the network, and are not bound by bandwidth restrictions like those
used by the cell. In other words, a satellite that provides service for a network of cells can allow its
own bandwidth to be used by any cell that needs it without being bound by terrestrial bandwidth and location restrictions.●
Television Signals
Satellites have been used for since the 1960's to transmit broadcast television signals between the
network hubs of television companies and their network affiliates. In some cases, an entire series of
programming is transmitted at once and recorded at the affiliate, with each segment then beingbroadcast at appropriate times to the local viewing populace. In the 1970's, it became possible for
private individuals to download the same signal that the networks and cable companies were transmitting, using c-band reception dishes. This free viewing of corporate content by individuals led to scrambling and subsequent resale of the descrambling codes to individual customers, which started the direct-to-home industry. The direct-to-home industry has gathered even greater momentum since the introduction of digital direct broadcast service.Ku Band (11.7 - 12.2 GHz) - Satellites operating in this band can be spaced as closely as two degrees apart in space, and carry from 12 to 24 transponders that operate at a wide range of powers from 20 to 120 watts each. Typical receive antennas are three to six feet in diameter. More than 20 FSS Ku-Band satellites are in operation over North America today, including several "hybrid" satellites which carry both C-Band and Ku-Band transponders. PrimeStar currently operates off Satcom K-2, an FSS or so-called "medium-power" Ku-Band satellite. AlphaStar also uses an FSS-Ku Band satellite,
Ku-Band (12.2 - 12.7 GHz) - Satellites operating in this band are spaced nine degrees apart in space, and normally carry 16 transponders that operate at powers in excess of 100 watts. Typical receive antennas are 18 inches in diameter. The United States has been allocated eight BSS orbital positions, of which three (101, 110 and 119 degrees) are the so-called prime "CONUS" slots from which a DBS provider can service the entire 48 contiguous states with one satellite. A total of 32 DBS "channels" are available at each orbital position, which allows for delivery of some 250 video signals when digital
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DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct to the end user. More than four million households in the United States enjoy C-Band DBS. Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS launched in 1994.
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In the maritime community, satellite communication systems such as Inmarsat provide good communication links to ships at sea. These links use a VSAT type device to connect to geosynchronous satellites, which in turn link the ship to a land based point of presence to the respective nations telecommunications system.
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Along the same lines as the marine based service, there are VSAT devices which can be used to
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Satellite Comunications
Broadcasting Satellite Service (BSS)
DBS
Marine Communications
Spacebourne Land Mobile
Satellite Comunications satellites.
Technological Overview Satellites for Data
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Characteristics
Incorporating satellites into terrestrial networks is often hindered by three characteristics possessed by satellite communication.
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Latency (propagation delay): Due to the high altitudes of satellite orbits, the time required for a transmission to navigate a satellite link (more than 2/10ths of a second from earth station to earth station) could cause a variety of problems on a high speed terrestrial network that is waiting for the packets.
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Due to radio spectrum limitations, there is a fixed amount of bandwidth Poor Bandwidth: allocable to satellite transmission.
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Noise: A radio signals strength is in proportion to the square of the distance traveled. Due to the distance between ground station and satellite, the signal ultimately gets very weak. This problem
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Error Correction
Due to the high noise present on a satellite link, numerous error correction techniques have been tested in on such links. They fall into the two categories of forward-error-correction (FEC) and automatic-repeat-request (ARQ):
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Forward-error-correction (FEC)
In this method a certain number of information symbols are mapped to new information symbols, but in such a way as to get more symbols than were original had. When these new symbols are checked on the receiving end, the redundant symbols are used to decipher the original symbols, as well as to check for data integrity. The more redundant symbols that are included in the mapping, the better the reliability of the error correction. However it should be Satellite Comunications
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Stop and Wait (SW)
With this form of ARR, the sender must wait for an acknowledgement of each packet before it can send a new one. This can take upwards of 4/10ths of a second per packet since it takes 2/10ths seconds for the receiver to get the packet an another 2/10th seconds for the sender to receive the acknowledgement.
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Go-back-N (GBN)
This method of ARR is an improvement over stop and wait in that it allows the sender to keep sending packets until it gets a request for a resend. When the sender gets such a request, it sends packets starting at the requested packet over again. It can again send packets until it receives another retransmit request, and so on.
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Selective-repeat (SR)
This ARR protocol is an improvement over GBN in that it allows the receiver to request a retransmit of only that packet that it needs, instead of that packet and all that follows it. The receiver, after receiving a bad packet and requesting a retransmit, can continue to accept any good packets that are coming. This method is the most efficient method for satellite transmissions of the three ARR methods discussed. ARR methods can be demonstrated to provide a usable error correction scheme, but it is also the most expensive, in terms of hardware. This is in part due to the buffering memory that is required, but more importantly to the cost of the receiver, which needs to be able to transmit re-requests. Systems such as the Digital Broadcast Satellites used for television signal distribution would become inordinately expensive if they had to make use of ARR, since the home based receiver would now need to be a transmitter, and the 18 inch dish would be inadequate for the requirements of transmitting back to a satellite.
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Hybrid Networks
In today's global networking landscape, there are many ways to transmit data from one place to
another. It is desirable to be able to incorporate any type of data transmission media into a network,
especially in networks that encompass large areas. A hybrid network is one that allows data to flow
across a network, using many types of media, either satellite, wireless or terrestrial, transparently.
Since each type of media will have different characteristics, it is necessary to implement a standard transmission protocol. One that is normally used in hybrid networks is TCP/IP. In addition, much work Satellite Comunications In fact, a product currently being marketed by Direct PC called Turbo Internet uses a form of hybrid network. The system uses two network interfaces; one connects via a special ISA bus PC adapter to a receive-only Very Small Aperture Terminal (VSAT), while the other is a modem attached to a serial port. Inbound traffic comes down to the VSAT, while outbound traffic goes through the modem link. The two interfaces are combined to appear as a single virtual interface to upper layer TCP/IP protocol stacks by a special NDIS compliant driver. The Serial Line Internet Protocol (SLIP) is used to connect the modem-based link with an internet service provider. Packets, which are encapsulated by the terminal such that the desired ip address of the destination host is embedded underneath the IP address of the Direct PC Gateway, to which all packets leaving the terminal must go. Once at the gateway, the outer packet is stripped, and the gateway contacts the destination address within. Upon the gateway's receiving the request from the host, it then prepares the packet for satellite transmission, which is then used to send the packet back to the terminal.
Satellite Comunications
The group that is currently working to develop interoperability specifications that facilitate ATM
access and ATM network interconnect in both fixed and mobile satellite networks is known as The
TIA/SCD/CIS - WATM group. As of March, 1997 they have proposed the following standards: SATATM Type 1 - Fixed ATM Direct AccessFixed network access via satellite that is characterized by a large number of small inexpensive user terminals and a small number of gateway earth stations. Provides for a radio interface of 64 kbit/s - NxE1 and a service interface of 2.4 kbit/s - NxE1 while providing no mobility support
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SATATM Type 2 - Fixed ATM Network Interconnect High speed interconnections using PNNI, B-ICI, or Public UNI between earth stations and fixed ATM networks. Allows for a radio interface of T1 - 1.2 Gbit/s but provides no mobility support.
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SATATM Type 3 - Mobile ATM Direct Access ATM network access by mobile terminals. The radio interface provides for 64 kbit/s - E1 for moving, 64 kbit/s - NxE1 for portable terminals and the rervice interfaceallows for 75 bit/s - E1 for moving, 75 bits/s - NxE1 for portable terminals.
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SATATM Type 4 - Mobile ATM Network Interconnect High speed interconnections between mobile and fixed networks or between two mobile networks providing fast moving land-mobile data rates < NxE1 and slow-moving airborne data rates of < 622 Mbit/s.
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The group also has established requirements for dealing with the physical layer, the media access
control layer and the data link control layer.The goal of SATIN is to create a fully integrated hybrid network in which the method of communication, which can incorporate networks of local, metropolitan and wide area scope, Broadband ISDN, Integrated Network Management, AIN (Advanced Intelligent Networks) and PCS (Personal Communications Services), in addition to ATM (Asynchronous Transfer Mode) over satellite, is totally transparent to the user. The difficulties inherent in this are obvious.
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Satellite Comunications
SATIN - Satellite Integrated Terrestrial Network:
GEO stands for Geostationary Earth Orbit. This refers to satellites that are placed in orbit such that they remain stationary relative to a fixed spot on earth. If a satellite is placed at 35,900 km above the earth, its angular velocity is equal to that of the earth, thereby causing it to appear to be over the same point on earth. This allows for them to provide constant coverage of the area and eliminate blackout periods of ordinary orbiting satellites, which is good for providing
television broadcasting. However their high altitude causes a long delay, so two way
communications, which would need to be uploaded and then downloaded over a distance of 72,000 km, are not often used with this type of orbit.❍
LEO stands for Low Earth Orbit, and it refers to satellites in orbit at less that 22300 miles above the earth. This type of an orbit reduces transmission times as compared to GEO. A LEO orbit can also be used to cover a polar region, which the GEO cannot accomplish. Since it does not appear stationary to earth stations, however, earth stations need an antenna assembly that will track the motion of the satellite.
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The idea behind a constellation is to use to acheive global simultaneous satellite coverage by placing
enough satellites into orbit so that (nearly) every point on earth is covered. There are currently two
main types of service being planned at the moment, global voice and global data.There are currently several consortiums that are working on global voice via satellite. One of the most prominant is the IRIDIUM constellation, which will consist of 66 interconnected satellites orbiting 420 nautical miles above the earth. The satellites will use a LEO orbit so that very small handheld terminals can be used by ground-based cutomers. The system will use intersatellite crosslink transmissions that will take place in the Ka frequency band between 23.18 and 23.38 GHz. The IRIDIUM system will use a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) signal multiplexing to make the most efficient. The L-Band (1616-1626.5 MHz), is used to link the satellite and IRIDIUM the subscribers equipment. The Ka-Band (19.4-19.6 GHz for downlinks and 29.1-29.3 GHz for uplinks) links the satellite and the gateways and earth terminals.
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Satellite Comunications
GEO
LEO
Constellations
Global Voice Communications
Satellite Comunications The Teledesic Network uses a constellation of 840 operational interlinked low-Earth orbit satellites. The system is planned to provide "on-demand" channel rates from 16 Kbps up to 2.048 Mbps ("E1"), and for special applications up to 1.24416 Gbps ("OC-24"). The network uses fast packet switching technology based on the Asynchronous Transfer Mode (ATM) using fixed-length (512) bit packets.. Each satellite in the constellation is a node in the fast packet switch network, and has intersatellite communication links with eight adjacent satellites. Each satellite is normally linked with four satellites within the same plane (two in front and two behind) and with one in each of the two adjacent planes on both sides. Each satellite keeps the same position relative to other satellites in its orbital plane. The Teledesic Network uses a combination of multiple access methods to ensure efficient use of the spectrum. Each cell within a supercell is assigned to one of nine equal time slots. All communication takes place between the satellite and the terminals in that cell during its assigned time slot . Within each cell’s time slot, the full frequency allocation is available to support communication channels. The cells are scanned in a regular cycle by the satellite’s transmit and receive beams, resulting in time division multiple access (TDMA) among the cells in a supercell. Since propagation delay varies with path length, satellite transmissions are timed to ensure that cell N (N=1, 2, 3,...9) of all supercells receive transmissions at the same time. Terminal transmissions to a satellite are also timed to ensure that transmissions from the same numbered cell in all supercells in its coverage area reach that satellite at the same time. Physical separation (space division multiple access (SDMA) and a checkerboard pattern of left and right circular polarization eliminate interference between cells scanned at the same time in adjacent supercells. Guard time intervals eliminate overlap between signals received from time-consecutive cells.
Satellite Comunications Within each cell’s time slot, terminals use Frequency Division Multiple Access (FDMA) on the uplink and Asynchronous Time Division Multiple Access (ATDMA) on the downlink. On the uplink, each active terminal is assigned one or more frequency slots for the call’s duration and can send one packet per slot each scan period (23.111 msec). The number of slots assigned to a terminal determines its maximum available transmission rate. One slot corresponds to a standard terminal’s 16 Kbps basic channel with its associated 2 Kbps signaling and control channel. A total of 1800 slots per cell scan interval are available for standard terminals. The terminal downlink uses the packet’s header rather than a fixed assignment of time slots to address terminals. difficulties of intersatellite communications are avoided. The problem arises due to the latency delays caused by the high orbit. Applications which rely on steady bandwidth, like multimedia, will definately be affected.
References Web Sites
Books and Papers Satellite Comunications
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Lloyd's satellite constellations
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High Bandwidth Web Page: http://www.specialty.com/hiband/satellite_index.html
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The Center for Satellite and Hybrid Communication Networks:
Low Earth Orbiting Satellites and Internet-Based Messaging Services:
SATELLITE COMMUNICATIONS IN THE 21ST CENTURY:
Big LEO Overview: http://www.idt.unit.no/
Centre for Satellite Engineering Research:
Satellite Comunications
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Deepak Ayyagari and Anthony Ephremides, "Enhancement of Cellular Service via the use of Satellite Capacity," University of Maryland, 1995
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Pelton, Joseph N., "Wireless & Satellite Telecommunications: The Technology, the Market, & the Regulations", Prentice Hall 1995
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Cochetti, Roger, "Mobile Satellite Communications Handbook", Quantum Publishing, Incorporated 1995
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Michael J. Miller (Editor),Branka Vucetic (Editor),Les Berry (Editor) , "Satellite Communications: Mobile & Fixed Services" Kluwer Academic Publishers, 1993
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Gerard Maral,Michel Bousquet, "Satellite Communication System: Systems, Techniques & Technology", John Wiley & Sons, Incorporated, 1993
❍ Wood, James, Satellite communications and DBS systems. Boston : Focal Press, 1992. ❍
Elbert, Bruce R, The satellite communication applications handbook, Boston, MA : Artech House, 1997.
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Miller, Michael J., Vucetic, Branka., Satellite communications : mobile and fixed services, Boston : Kluwer Academic Publishers, c1993
❍ Satellite communications systems and technology--Europe, Japan, Russia / Burton I. Edelson ...
[et al.], Park Ridge, N.J., U.S.A. : Noyes Data Corp., c1995
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Broadband communications : global infrastructure for the information age / proceedings of the International IFIP-IEEE Conference on Broadband Communications, Canada, 1996 ; edited by Lorne Mason and Augusto Casaca., London : Chapman and Hall on behalf of the International Federation for Information Processing, 1996
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Litva, J. (John), Digital beamforming in wireless communications, Boston : Artech House, c1996.
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Lindberg, Bertil C., Digital broadband networks and services, New York : McGraw-Hill, 1995
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International journal of satellite communications., Chichester, Sussex : Wiley, c1983-
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Feldman, Phillip M., An overview and comparison of demand assignment multiple access (DAMA) concepts for satellite communications networks Santa Monica, CA : RAND, 1996
❍ Maral, Gerard., VSAT networks, Chichester, West Sussex, England ; New York : Wiley, c1995.
Last Modified: 14 August 1997
Satellite Communication Lecture # 11
Channel Characteristics Transmission
Source Coding Encryption Multiplexing Channel Coding Interleaver Modulation Frequency Conversion De-Interleaving Channel Decoding Demultiplexing Decryption Display Frequency Conversion Demodulation Digitization Source Decoding The sequence of signal processing and transmission Signal processing and transmission Digitisation
Source Coding higher reliability, low cost, less susceptible to nois Encryption for communications privacy to reduce bit rate for transmission Multiplexing for efficient transmission of multiple channels ChannelCoding for error free transmission
Interleaving Modulation for robust error correction Frequency Conversion to operate at radio frequencies imparting baseband information to a carrier
Multiplexing and Multiple Access
Multiple Access and Multiplexing Multiple Access and Multiplexing
Multiple Access Multiplexing Multiple Access:is the ability for several earth at radio frequency at baseband stations to transmit their respective carriers simultaneously into the same satellite
TDMA - TDM transponder
- FDMA FDM
Multiplexing:is the reversible operation of
- CDMA CDM combining several information-bearing signals to form a single, more complex signal.