Chapter 27: Low−Earth−Orbit Satellites (LEOs)

Low-Earth-Orbits

     Private companies are striving to provide truly seamless global communications to the public, making today’s personal communication systems (PCS) a proving ground for new technologies. This global approach has sparked the development of several new communication satellite systems, which abandon the traditional use of geostationary earth orbit (GEO) in favor of medium earth orbit (MEO) and low earth orbit (LEO) satellite systems. LEO and MEO satellite networks increase the service regions of their designers, providing services to regions of the world where there is little or no telecommunication infrastructure, such as Asia, Africa, Eastern Europe, South America, and the polar regions. These LEO and MEO satellite networks provide global coverage to their users, which a typical GEO satellite system cannot provide. One such LEO satellite system, Motorola’s IRIDIUM system, was completely deployed in May 1998.

     In December 1990, Motorola filed an application with the FCC for the purposes of constructing, launching, and operating a LEO global mobile satellite system known as Iridium. This was the hot button that sparked the world into a frenzy. Iridium was a concept of launching a series of 66 satellites around the world to provide global coverage for a mobile communications service operating in the 1.610 to 1.6265 GHz frequency bands. The concept was to use a portable or mobile transceiver with low profile antennas to reach a constellation of 66 satellites. Each of the satellites would be interconnected to one another through a radio communications system as they traversed the globe at 413 nautical miles above the earth in multiple polar orbits.This would provid worldwide coverage, much similar to an orange slice concept . This would provide a continuous line−of−sight coverage area from any point on the globe to virtually any other point on the globe, using a spot beam from the radio communications services on−board each of the satellites.

    The main component s of the IRIDIUM system are the satellites, gateways, and user handsets. The satellites utilize inter-satellite links (ISLs) to route network traffic. Regional gateways will handle call setup procedures and interface IRIDIUM with the existing public switched telephone network (PSTN). A dual-mode handset will allow users to access either a compatible cellular telephone network or IRIDIUM.  IRIDIUM will give the user the capability to receive persona l communications worldwide using a single telephone number. It is designed to augment the existing terrestrial and cellular telephone networks. IRIDIUM is expected to provide cellular-like service in areas where terrestrial cellular service is unavailable, or where the PSTN is not well developed.

Network Connectivity

    Communication networks are commonly represented by graphs of nodes, which represent communication locations,and links,which represent communication transmission paths. The IRIDIUM network essentially has two planes of nodes, the satellites and the earth stations, which are moving with respect to each other. As a result, the links connecting earth stations to satellites change over time. 

    In the IRIDIUM network, a link is established from an earth station to the satellite with the strongest signal. The satellites are moving much faster than the mobile users. Mobile user scan be considered stationary with respect to the velocity of the satellites, as even a mobile user in an airplane is travelling much slower than a satellite. As the satellites pass overhead, the link from earth station to satellite is handed off from a satellite leaving the user’s area to one entering the user’s area.

System Capacity

   The IRIDIUM system uses a combination of time division multiple access ( TDMA) and frequency division  multiple access (FDMA). The TDMA frame is 90 ms long and it contains four full-duplex user channels at a burst data rate of 50 kb/s.The four full-duplex channels consist of four uplink time slots and four downlink time slots. The IRIDIUM system will support full-duplex voice channels at 4800 b/s and half-duplex data channels at 2400 b/s. The specific details of the TDMA frame, such as the number of framing bi ts and the length of a user time slot, are not published in open literature. In addition, the type of voice encoding that will be used to provide acceptable voice quality at 2400 b/s is proprietary and is not published in open literature. 

Call Processing

     The IRIDIUM system will allow users to roam worldwide and still utilize a single subscriber number. To accomplish this, each user will have a home gateway that normally provides his service. The gateways in this system will be regional and will support large geographical areas. An IRIDIUM subscriber is uniquely identified by three numbers: the mobile subscriber integrated services digital network number (MSISDN), the temporary mobile subscriber identification (TMSI), and the IRIDIUM mobile subscriberidentity (IMSI). 

      The MSISDN is the telephone number of an IRIDIUM subscriber. The MSISDN is five digits long, and makes up part of the twelve-digit number dialed to reach a subscriber. The first field of the twelve-digit number is the four-digit country codeThe second field of the number is a three-digit geographical code. This code will be used to identify a user’s home country in  regions where one gateway services mor e than one country. The third and final field of the number is the MSISDN. The TMSI is a temporary number that is transmitted over the network during call setup. This number is changed periodically to protect subscriber confidentiality.

Watch this video for more information.

Source:  http://earthobservatory.nasa.gov/Features/OrbitsCatalog/

             http://www.deetc.isel.ipl.pt/sistemastele/ST1/arquivo/IRIDIUM%203.pdf

             Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf

Chapter 25: Third−Generation (3G) Wireless Systems

As the convergence of wireless technology and the Internet continue at an escalating pace, the new possibilities created by 3G and 4G technologies appear endless. Preparing for the revolution, existing Time Division Multiple Access (TDMA) operators must evolve their networks to take advantage of Mobile Multimedia applications and the eventual shift to an all−IP architecture. One way to do that is through the evolution of General Packet Radio Services (GPRS). However, soon after we see the installation of GPRS, some operators will begin the next step in the evolution process to Enhanced Data for Global Environment (EDGE).

Capabilities & service attributes that are expected from 3G systems

Time line for 3G/UMTS

GPRS

     Probably the most important aspects of GPRS are that it enables data transmission speeds up to 170 Kbps, it is packet based, and it supports the leading data communications protocols (IP and X. 25). GPRS operates at much higher speeds than current networks, providing advantages from a software perspective. Wireless middleware currently is required to enable slow speed mobile clients to work with fast networks for applications such as e−mail, databases, groupware, or Internet access. With GPRS, wireless middleware will probably be unnecessary, making it easier to deploy wireless solutions.

Because there is minimal delay before sending data, GPRS is ideally suited for applications such as

              · Extended communications sessions

              · E−mail communications

              · Chat

              · Database queries

              · Dispatch

              · Stock updates

                                 GPRS Network Architecture

EDGE

     EDGE, Enhanced Data rates for GSM Evolution, is a further step for GSM to migrate to 3G. It uses a new air-interface technology -- 8 Phase Shift Keying Modulation (8-PSK) to offer 48 kbits/s per GSM timeslot. The overall offered data speeds of 384Kbps places EDGE as an early pre-taste of 3G and it is actually labeled as 2.75G by the industry.

     EDGE is occasionally referred to as Enhanced GPRS (EGPRS) because it increases the capacity and data throughput of GPRS by three to four times. Like GPRS, EDGE is a packet-based service, which provides customers with a constant data connection.

     For EDGE to be effective it should be installed along with the packet-switching upgrades used for GPRS. This entails the addition of two types of nodes to the network: the gateway GPRS service node (GGSN) and the serving GPRS service node (SGSN). The GGSN connects to packet-switched networks such as internet protocol (IP) and X.25, along with other GPRS networks, while the SGSN provides the packet-switched link to mobile stations.

     By providing an upgrade route for GSM/GPRS and TDMA networks, EDGE forms part of the evolution to IMT-2000 systems. Since GPRS is already being deployed, and IMT-2000 is not expected until 2002, there is a definite window of opportunity for EDGE systems to fill in as a stop-gap measure.

EDGE Network Architecture

UMTS

     Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) cell phone technologies. Currently, the most common form of UMTS uses W-CDMA as the underlying air interface. It is standardized by the 3GPP, and is the European answer to the ITU IMT-2000 requirements for 3G cellular radio systems.

      UMTS is packet-based and it allows transmission of text, digitized voice, video, and multimedia at data rates up to 2 megabits per second (Mbps). UMTS offers a consistent set of services to mobile computer and phone users, no matter where they are located in the world.

EDGE Network Architecture

     A UMTS network consists of three domains; Core Network (CN), UMTS Terrestrial Radio Access Network (UTRAN) and User Equipment (UE). The main function of the core network is to provide switching, routing and transit for user traffic. Core network also contains the databases and network management functions.

      The basic Core Network architecture for UMTS is based on GSM network with GPRS. All equipment has to be modified for UMTS operation and services. The UTRAN provides the air interface access method for User Equipment. Base Station is referred as Node-B and control equipment for Node-B's is called Radio Network Controller (RNC).

Source:   http://www.slideshare.net/Dominque23/3g-wireless-systemsdoc

              http://www.geekzone.co.nz/content.asp?contentid=207

              http://www.mobilecomms-technology.com/projects/edge/

              http://conningtech.blogspot.com/2010_07_01_archive.html

              Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf

Chapter 24:General Packet Radio Service (GPRS)

     The introduction of wireless communication has allowed many people around the world to live their lives and conduct business in ways that were never before possible.Millions of cellular subscribers have become accustomed to always having a telephone with them wherever they go.Now, businesses are wanting to be able to connect to the office when they are out of the office so they can check their email, search on the Internet, access company files, send faxes and data whenever and wherever it is needed.Currently, there are numerous wireless data services available, but a new technology, General Packet Radio Service, offers much excitement to consumers. Indeed, the demand for communications and new technology is greatly growing based on the needs of the people and business especially in the Internet.

What is GPRS?

     General Packet Radio Service, more commonly known as GPRS, is a new non-voice, value added, high-speed, packet-switching technology, for GSM (Global System for Mobile Communications) networks. It makes sending and receiving small bursts of data, such as email and web browsing, as well as large volumes of data over a mobile telephone network possible. A simple way to understand packet switching is to relate it to a jigsaw puzzle. Image how you buy a complete image or picture that has been divided up into many pieces and then placed in a box. You purchase the puzzle and reassemble it to form the original image. Before the information is sent, it is split up into separate packets and it is then reassembled at the receivers end. The main benefits of GPRS are that it reserves radio resources only when data is available to send, and it reduces the reliance on traditional circuit−switched networks.

       GPRS offers a continuous connection to the Internet for mobile phone and computer users. Users may need to be connected to a data communication network (such as a LAN, WAN, the Internet, or a corporate Intranet), but that does not mean they are sending and receiving data at all times. Data transfer needs are not generally balanced.In the majority of cases, users will tend to send out small messages but receive large downloads.Therefore, most of the data transfer is in one direction. GPRS is expected to provide a significant boost to mobile data usage and usefulness. It is expected to greatly alter and improve the end-user experience of mobile data computing, by making it possible and cost-effective to remain constantly connected, as well as to send and receive data at much higher speeds than today.Its main innovations are that it is packet based, that it will increase data transmission speeds, and that it will extend the Internet connection all the way to the mobile PC – the user will no longer need to dial up to a separate ISP. 

Method of Operation

       GPRS gives GSM subscribers access to data communication applications such as e-mail, corporate networks, and the Internet using their mobile phones.The GPRS service uses the existing GSM network and adds new packet-switching network equipment. GPRS employs packet switching, which means that the GPRS mobile phone has no dedicated circuit assigned to it.Only when data is transferred is a physical channel created.After the data has been sent, it can be assigned to other users.This allows for the most efficient use of the network.

       When packet-switched data leaves the GPRS/GSM network, it is transferred to TCP-IP networks such as the Internet or X.25. Thus, GPRS includes new transmission and signaling procedures as well as new protocols for interworking with the IP world and other standard packet networks. Mobile phones currently available do not work with the new GPRS technology.The industry’s mobile phone vendors are working on new phones that will support both GSM and packet switching.There is also a possibility in the future, that laptops and PDA’s (Personal Digital Assistants) will have GPRS phone integrated in them.

GPRS Network Architecture

A Comparison of Data Transfer Speeds (in Kbps)

    GPRS could possibly be the technology that will allow consumers to really begin to sue the mobile Internet.GPRS is considered one step ahead of HSCSD (High Speed Circuit Switched Data) and a step towards 3G (Third-generation) networks. It is the step to 2.5G for GSM and TDMA (Time Division Multiple Access) service providers.Cingular and AT&T are both currently the standard.

      GPRS is ideal for Wireless Application Protocol (WAP) services because of the cost saving WAP over GPRS bring to mobile operators and cellular consumers.Costs are reduced because GPRS radio resources are only needed while the message is being transferred.For the end user, that means you only pay for the time it takes to download the data and information that you need.For the GSM operator, that means that you will be able to provide high speed Internet access to consumers at a reasonable cost, because you will bill mobile phone users for only the amount of data that they transfer rather than billing them for the length of them that they are connected to the network.

    With GPRS-enabled mobile phones, services are received faster than with traditional GSM phones.GPRS offers an increase in data throughput rates, so information retrieval and database access is faster, more usable and more convenient.At its best, GPRS is transparent, allowing the user to concentrate on the task in hand rather than on the technology.

GPRS History

DATEMILESTONE

User Features

3 TO 10 TIMES THE SPEED

     The maximum speed of 171.2 kbps, available through GPRS, is nearly three times as fast as the data transmission speeds of fixed telecommunications networks and ten times as fast as the current GSM network services.

INSTANT CONNECTIONS – IMMEDIATE TRANSFER OF DATA

    GPRS will allow for instant, continuous connections that will allow information and data to be sent whenever and wherever it is needed. GPRS users are considered to be always connected, with no dial-up needed.Immediacy is one of the advantages of GPRS (and SMS) when compared to Circuit Switched Data.High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds. 

NEW AND BETTER APPLICATIONS

     General Packet Radio Service offers many new applications that were never before available to users because of the restrictions in speed and messaged length.Some of the new applications that GPRS offers is the ability to perform web browsing and to transfer files from the office or home and home automation, which is the ability to use and control in-home appliances.

Network Features

PACKET SWITCHING

     From a network operator perspective, GPRS involves overlaying packet based air interference on the existing circuit switched GSM network.This gives the user an option to use a packet-based data service.To supplement a circuit switched network architecture with packet switching is quite a major upgrade.The GPRS standard is delivered in a very elegant manner – with network operators needing only to add a couple of new infrastructure nodes and making a software upgrade to some existing network elements.

 SPECTRUM EFFICIENCY

     Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data.Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users.This efficient use of scarce radio resources means that large number of GPRS users can potentially share the same bandwidth and be served from a single cell.

INTERNET AWARE

     For the first time, GPRS fully enables Mobile Internet functionality by allowing interworking between the existing Internet and the new GPRS network. Any service that is used over the fixed Internet today – File Transfer Protocol (FTP), web browsing, chat, email, telnet – will be as available over the mobile network because of GPRS.In fact, many network operators are considering the opportunity to use GPRS to help become wireless Internet Service Providers in their own right.

     The World Wide Web is becoming the primary communications interface – people access the Internet for entertainment and information collection, the intranet for accessing company information and connecting with colleagues and the extranet for accessing customers and suppliers.Web browsing is a very important application for GPRS. Because it uses the same protocols, the GPRS network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts.This means that each GPRS terminal can potentially have its own IP address and will be addressable as such.

SUPPORTS TDMA AND GSM

     It should be noted that the General Packet Radio Service is not only a service designed to be deployed on mobile networks that are based on the GSM digital phone standard.


Source: http://misnt.indstate.edu/harper/Students/GPRS/GPRS.html
            Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf