Wireless Communications: Assignment 1 (By Akshay Garg) 1.14 In early 1980's when analog cellular telephone system was experiencing rapid growth in Europe, each country developed its own system which was incompatible with others in equipment and operation. This created the problems of limited markets for each type of equipment as well as inability to use a technology outside national boundaries. To resolve the issue the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop a pan-European public land mobile system. The proposed system had to meet the following criteria: 1 Good subjective speech quality 2 Low terminal and service cost per subscriber 3 Support for international roaming: customer could use a single mobile unit throughout Europe. 4 Ability to support handheld terminals 5 Support for range of new services and facilities like caller id, call forwarding, call waiting, multipart conversations, speech security and data communication. 6 Spectral efficiency: reduced adjacent channel and co channel interference, better power control and handoff. 7 ISDN compatibility A new radio band was introduced for the GSM service (935-960 MHz downlink and 890-915 MHz uplink) to insure its compatibility with the then existing analog systems. Some major factors that led to the development of US Digital Cellular System were: 1. Reduce the mobile equipment size and weight. 2. Improvement in system capacity: ability to support a larger number of subscribers. USDC offered a 3 time capacity improvement over AMPS, which was further improved by CDMA. 3. Quality of Service improvement. 4. Ability to provide services like messaging, data transmission capabilities, voice dispatch etc. American digital cellular systems were designed to ensure compatibility with the analog system mobile equipments. American digital cellular, first called IS-54 and then IS-136, still accepts the earliest analog phones. American cellular networks evolved slowly, dragging a legacy of underperforming equipment with it. 1.21 Wireless Local Loop ----------------------------- Definition: ---------------- A telephone system where subscribers are connected to the Public Switched Telephone Network using radio signals rather than copper wire for part or all of the connection between the subscriber and the switch. WLL includes cordless access systems, proprietary fixed radio access and fixed cellular systems. Synonymous with Fixed Radio Access and Radio in the Loop Systems [1]. Architecture: -------------------- A Wireless Local Loop architecture consists of three major components:

1. The handset that communicates with the WLL through the air interface; 2. The base station that provides radio communications with the handset and the wireline line communications with the control unit 3. The control unit that performs call control to and from the public switched Benefits of using WLL: ----------------------------------- 1. Compared to the deployment of copper lines, WLL offers rapid deployment, reduced construction costs, low network maintenance, low network extension costs, and improved survivability against natural disasters and vandalism [3]. 2. WLL also proves it worth at places where it is impossible to lay copper cables. 3. WLL systems also allow for lower extension costs. The cost for each incremental subscriber is low. Most WLL systems are designed to be modular and scalable and are flexible enough to meet uncertain levels of penetration and rates of growth. Copper wire networks require continuous maintenance [3]. 3. It offers the mobility advantage which makes WLL a competitive replacement to wireline networks for POTS and data access. The Technology: --------------------------- The WLL systems are typically based on one of four technologies [2]: I. Cellular systems that provide large power, large range, median subscriber density and median circuit quality WLL services. Cellular WLL technologies were primarily used to expand the basic telephony services. E.g. IS-136 (Digital TDMA), IS-95 (Digital CDMA) and PCS-1900 (GSM) based WLL Systems. Issues: 1. Limited user bandwidth: cellular systems are optimized for high-tier coverage. Support for mobiles traveling in excess of 100 mph and cell sizes up to 10 mi in radius is required. To achieve the above goals, extensive signal processing is required, which translates into high delay, high overhead and low user bandwidth 2. These systems are not well suited to deployment indoors and in picocells. Additional complexity of the air interface with the same low user bandwidth is required. In developing countries, such data capability limitations can be more than adequately compensated by the large coverage areas and consequent rapid deployment [4]. II. Cordless or microcellular systems that provide low power, small range, high subscriber density and high circuit quality WLL services. Cordless technologies are used to facilitate rapid market entry and to expend the capacity of the existing infrastructure. e.g. Digital European Cordless Telephone (DECT), PACS, DCS-1800, PHS. Issues: 1. Compared with the cellular-based WLL, more base stations are required to cover the same service area. Operators may consider low-tier WLL technologies when an existing infrastructure is in place to support backhaul from many base stations to the switch, or when wireline-like services and quality are essential. Microwave backhaul should be seriously considered to reduce the transmission cost. For densely populated urban environments, WLL technologies based on existing low-tier PCS radio technologies, by supporting higher user bandwidths, can offer features and quality more commonly associated with conventional wireline access. 2. Overlapping coverage areas and support of limited handover between neighboring base stations or radio ports is desirable in WLL systems as it improves the ability to perform maintenance, increases the robustness of the system, improves blocking statistics and provides

for alternative access during exceptional propagation activity. Such limited handover can easily be supported by a radio port or base station controller and need not have any impact on the switch [4]. III. Fixed Wireless Access Systems - These systems are proprietary radio systems designed specifically for fixed wireless applications, which may or may not be extensible to PCS or cordless. The primary disadvantage of the cellular approach is its limitation on toll quality voice and signaling transparency. The primary disadvantage of low-tier PCS and microcellular approaches is their range. Nonstandard fixed wireless access technology can address these issues and become more efficient. Such systems include the Interdigital TDMA system, which is a Bellcore standard; the new broadband CDMA standard by Okidata, which is a TR.45 interim standard from the Joint Technical Committee (JTC); and the Interdigital broadband CDMA technology, which is proprietary. FWA systems for zonal areas are designed to cover the local telephone area directly from the PSTN switches. The systems for rural areas provide connection at the remote ends of rural links to the end users [4]. IV. Satellite-Based Systems: These systems provide telephony services for rural communities and isolated areas such as islands. These systems can be of two types: a) Technology designed specifically for WLL applications b) Technology piggybacked onto mobile satellite systems as an adjunct service Of these, the former offers quality and grade of service comparable to wireline access, but it may be expensive. The latter promises to be less costly but, due to bandwidth restrictions, may not offer the quality and grade of service comparable to plain old telephone service (POTS). There are many proposed systems for mobile satellite service, including the Inmarsat International Circular Orbit (ICO) system, Iridium, Globalstar, Odyssey, American Mobile Satellite Corporation (AMSC), Asia Cellular Satellite (ACeS), and Thuraya mobile satellite system. These systems are specialized to support low-cost mobile terminals primarily for low bit rate voice and data applications. Fixed applications are a possible secondary use to mobile applications [4]. Key Organizations supporting WLL related work [5]: --------------------------------------------------------------------------

1. CDMA Development Group (CDG): The CDG supports the deployment of CDMA WLL systems worldwide. URL: http://www.cdg.org/technology/cdma_technology/wll.asp

2. National Telecommunications and Information Administration (NTIA): They maintain a WLL forum.

3. IEEE 802.16 Group: To provide a standardized approach to WLL, the IEEE 802 committee set up the 802.16 working group in 1999 to develop broadband wireless standards. In essence, IEEE 802.16 standardizes the air interface and related functions associated with WLL. Three working groups have been chartered to produce standards:

• IEEE 802.16.1: Air Interface for 10 to 66 GHz

• IEEE 802.16.2: Coexistence of Broadband Wireless Access Systems

• IEEE 802.16.3: Air Interface for Licensed Frequencies, 2 to 11 GHz

URL: http://grouper.ieee.org/groups/802/16/index.html

4. Wireless Communications Association International: Represents the fixed broadband wireless access industry worldwide. URL: www.wcai.com

5. U.S. National Wireless Electronic Systems Testbed: A measurements and standards

resource for the broadband wireless access industry.

URL: http://ieeexplore.ieee.org/iel5/4119994/4119995/04120006.pdf

6. Broadband Wireless Internet Forum BWIF: An industry consortium to promote fixed wireless access.

References: ------------------- [1] Spread Spectrum Scene Glossary, Page 4, URL: http://sss-

mag.com/glossary/page4.html#WLL [2] "The Wireless Local Loop", Yi-Bing Lin, IEEE Potentials, Volume 16, Issue 3, Aug-Sep

1997, Pages 8-10.

[3] "A SURVEY OF WIRELESS LOCAL LOOP TECHNOLOGIES", URL: www.bandwidthmarket.com/speeches/sat/patel/pjp.doc

[4] "Wireless local loop: architecture, technologies and services", Neorpel, A.R, Yi-Bing Lin, IEEE Personal Communications, Jun 1998, Vol 5, Issue 3, Pages 74-80

[5] "IEEE 802.16 for Broadband Wireless Local Loop", By William Stallings, October 26, 2001, URL: http://www.informit.com/articles/article.aspx?p=23764

Local Multipoint Distribution Service ---------------------------------------------------- Definition: --------------- Local multipoint distribution systems (LMDS) represent a new radio-based access technology with cellular architecture offering flexible high capacity connections to private users and organizations. The systems employ a point-to-multipoint broadcast downlink with a total capacity of 34–38 Mb/s per transport stream, giving high flexibility for inclusion of any type of data. Benefits of using LMDS ---------------------------------- 1. The advantages of LMDS are easy operation and deployment, flexibility in on demand capacity allocation, and potential support for a broad spectrum of applications, allowing for future development. 2. LMDS provides a wireless alternative to fiber, coax, and asynchronous/very high-rate digital subscriber line (ADSL/VDSL) and offers a high capacity locally compared with other radio solutions like interactive satellite systems and stratospheric platforms. Architecture ------------------ A typical LMDS system has a point-to-multipoint downlink and a point-to-point uplink, as illustrated in Fig. 1 [1]. The transmitter site should be on top of a tall building or on a high pole overlooking the service area. The transmitter covers a sector typically 60–90° wide. Full coverage thus requires 4–6 transmitters. The streams transmitted contain 34–38 Mb/s of data addressed to everybody (typical TV) in the coverage zone, subgroups or individuals (typical communication, Internet). The capacity of the point to point return channels is determined by the needs of the individual user.

Operation of LMDS in an area will normally require a cluster of cells with separate base stations for co-located transmitter/receiver sites. One of the base station sites will serve as coordination center for the franchise area and connect the LMDS cells to external networks. Intercell networking may be implemented using fiber or short hop radio relay connections. Collocation with mobile base stations allows for infrastructure sharing. Issues ---------- 1. Precipitation effects lead to severe attenuation and limit the reliable range of operation to 3–5 km depending on the climatic zone and the frequency of operation. 2. Line of sight is also required. Use of repeaters and reflectors are possibilities, but that requires some additional equipment. 3. The most severe restriction may be the attenuation caused by transmission through vegetation. Buildings completely shielded by vegetation need an elevated rooftop antenna or some broadband connection to an unshielded site. Frequencies of Operation ------------------------------------- In the United States 1.3 GHz in the 28–29 GHz band has been allocated, while European countries are allocating frequencies in different bands. The main high-capacity band is presently 40.5–42.5 GHz with a possible extension to 43.5 GHz. The frequency band 24.5–26.6 GHz with sub bands of 56 MHz has been opened for point-to multipoint applications in many European countries. Indications are given that typically LMDS and related systems would be in different frequency bands from 24 GHz up to 43.5 GHz. The Technology ----------------------- In LMDS the high-capacity broadcast-based downlink is shared among several users in a flexible way. For the front end technology, high electron mobility transistor (HEMT) modules offer the required performance. The output power level needed per transport beam of 36 Mb/s is about 25 dBm. A technology allowing for final stage amplification of several transport beams would reduce equipment complexity and cost. The hub transmitters, however, are shared by many users, and cost is not directly critical. The front-end technology at 40 GHz is more expensive than at 28–29 GHz, and attenuation by precipitation increases with frequency, favoring the lower frequency ranges. The higher capacity offered at 40 GHz may compensate for these effects in the long run.

The transmission format for Digital Video Broadcasting (DVB) satellite transmission based on quadrature phase shift keying (QPSK) modulation has been adopted by both Digital Audio/Visual Council (DAVIC) and the DVB project, and with the same IF interface, 950–2150 MHz, between the outdoor and indoor units. This allows for application of set-top boxes developed for reception of digital TV by satellite with data included in the transport multiplex. The IF is then fed into a set-top box interfacing a TV or a PC or both in parallel, depending on user orientation. Both options allow for interactivity since set-top boxes are also equipped with a return channel connection to the public switched telephone network (PSTN)/ISDN. The uplink is the individual connection, and different technologies may be used depending on demand. Two of the broadband driving applications, interactive TV and the Internet, will require only low-capacity return links, and technologies like General Packet Radio Service (GPRS) and PSTN/ISDN will suffice. Applications ------------------ The flexibility offered by LMDS with regard to high on-demand capacity in both directions makes it well suited to home offices and teleteaching in the local domain. The first major applications are TV and Internet thus combining professional and entertainment use. Key Organizations supporting LMDS related work [5]: -----------------------------------------------------------------------------

LMDS Wireless: LMDS Wireless is the leading resource for information concerning the LMDS industry. Founded in 1999 as a resource for CLEC's to compete against LEC's, the website is owned and operated by Jim Rossi. LMDS Wireless is based in San Jose California strategically positioned within the market place to be on the cutting edge of technology and is focused on delivering solutions for the current demand for faster more reliable Internet services.

URL: http://www.lmdswireless.com/index.php Additional Organizations: ABI Research, ADC Corporation Angle Technologies Corporation, Broadband Wireless Exchange Magazine, Cable AML, CCG Consulting LLC, CMP Technology, Competitive Local Exchange Carriers, Corsair, CTIA, Dudley Lab, Federal Communications Commission, General Dynamics, IEEE, Integrity Communications Incorporated, Nexyn Corporation, Paratek, PCIA - The Wireless Infrastructure Association, Radiantnetworks.com, Telcordia Technologies, Inc, The Insight Research Corporation, The Strategis Group Inc, TRI Publications, Unique Broadband Systems Ltd, WCA International

LMDS Equipment Manufacturers: Alcatel-Lucent, Allta Partners, Andrew, Ceeragon Networks Ltd, dmcstratexnetworks.com, Endwave Corporation, Ericsson, Galleonwireless.com, Harmonicdata, Harris Stratex, Hughes Network Systems LLC, Netro-corp.com, North East Toy Review Organization, Nortel Networks, P-com, Provigent, Remec Magnum Inc, Siemens, Sierra Digital Communications, TalkSwitch, Triton Network, vyyo, Winnetmcs.com

References: ------------------ [1] LMDS systems and their application, Nordbotten. A, IEEE Communications Magazine,

Volume 38, Issue 6, June 2000 Page(s):150 - 154 [2] LMDS, Wikipedia, URL: en.wikipedia.org/wiki/LMDS [3] LMDS Wireless, URL: www.lmdswireless.com

Third Generation Wireless Systems --------------------------------------------------- Definition -------------- 3G refers to the third generation of developments in wireless technology, especially mobile communications. The third generation, as its name suggests, follows the first generation (1G) and second generation (2G) in wireless communications. 3G includes capabilities and features such as:

• Enhanced multimedia (voice, data, 3G-324M video, PTT and remote control). • Usability on all popular modes (cellular telephone, e-mail, paging, fax, videoconferencing,

and Web browsing). • Broad bandwidth and high speed (upwards of 2 Mbps). IP Multimedia Services. • Location based services. • Roaming capability throughout Europe, Japan, and North America.

Technologies that come under 3G ------------------------------------------------ 3G is a generic term covering a range of future wireless network technologies, including WCDMA, CDMA2000, UMTS and EDGE [2].

1. WCDMA - Wideband Code Division Multiple Access A technology for wideband digital radio communications of Internet, multimedia, video and other capacity-demanding applications. WCDMA has been selected for the third generation of mobile telephone systems in Europe, Japan and the United States.

Voice, images, data, and video are first converted to a narrowband digital radio signal. The signal is assigned a marker (spreading code) to distinguish it from the signal of other users. WCDMA uses variable rate techniques in digital processing and it can achieve multi-rate transmissions.

WCDMA has been adopted as a standard by the ITU under the name IMT-2000 direct spread.

2. CDMA 2000 - Code Division Multiple Access 2000 Commercially introduced in 1995, CDMA quickly became one of the world's fastest-growing wireless technologies. In 1999, the International Telecommunications Union selected CDMA as the industry standard for new "third-generation" (3G) wireless systems. Many leading wireless

carriers are now building or upgrading to 3G CDMA networks in order to provide more capacity for voice traffic, along with high-speed data capabilities.

Today, over 100 million consumers worldwide rely on CDMA for clear, reliable voice communications and leading-edge data services.

3. CDMA2000 1X for Voice and Data CDMA2000 1X technology supports both voice and data services over a standard (1X) CDMA channel, and provides many performance advantages over other technologies. First, it provides up to twice the capacity of earlier CDMA systems (with even bigger gains over TDMA and GSM), helping to accommodate the continuing growth of voice services as well as new wireless Internet services. Second, it provides peak data rates of up to 153 kbps (and up to 307 kbps in the future), without sacrificing voice capacity for data capabilities.

CDMA2000 1X phones also feature longer standby times. And because it's backwards-compatible with earlier CDMA technology, CDMA2000 1X provides an easy and affordable upgrade path for both carriers and consumers.

4. CDMA2000 1xEV-DO for Faster Data For those who want higher-speed or higher capacity data services, a data-optimized version of CDMA2000 called 1xEV-DO provides peak rates of over 2 Mbps, with an average throughput of over 700 kbps - comparable to wireline DSL services and fast enough to support even demanding applications such as streaming video and large file downloads. CDMA2000 1xEV-DO also delivers data for the lowest cost per megabyte, an increasingly important factor as wireless Internet use grows in popularity. 1xEV-DO devices will provide "always-on" packet data connections, helping to make wireless access simpler, faster and more useful than ever.

5. UMTS - Universal Mobile Telecommunications System The name for the third generation mobile telephone standard in Europe, standardized by ETSI.

6. EDGE - Enhanced Data for Global Evolution A technology that gives GSM the capacity to handle services for the third generation of mobile telephony. EDGE was developed to enable the transmission of large amounts of data at a high speed, 384 kilobits per second. EDGE uses the same TDMA (Time Division Multiple Access) frame structure, logic channel and 200 kHz carrier bandwidth as today's GSM networks, which allows existing cell plans to remain intact.

3G Frequency Ranges [3] -------------------------------------

Key Organizations supporting 3G related works: ------------------------------------------------------------------- 1. International Telecommunications Union, ITU, 2. Federal Communications Commission, FCC, URL: http://www.fcc.gov/3G/ 3. National Telecommunications and Information Administration, NTIA URL: http://www.ntia.doc.gov/osmhome/reports/imt2000/ 4. Department of Defense (DOD). 5. 3g Wireless Tech, URL: www.3gwirelesstech.com 6. NMS Communications, URL: www.nmss.com References ---------------- [1] 3G Wireless Systems, Wikipedia, URL: en.wikipedia.org/wiki3G [2] URL: http://www.3g.co.uk/All%20About%203G.htm [3] 3G Tutorial, URL: http://www.nmscommunications.com/file/3G_Tutorial.pdf Wireless Local Area Networks ------------------------------------------- A wireless local area network (WLAN) is two or more computers joined together using radio frequency (RF) transmissions. This differs from a wired LAN, which uses cabling to link together computers in a room, building, or site to form a network. Currently, Wireless LAN technology is significantly slower than wired LAN. Wireless LANs have a nominal data transfer rate of between 11 and 54 Megabits per second (Mbps) compared to most wired LANs in schools which operate at 100Mbps or 1000Mbps. Architecture ------------------ To access a wireless network all devices will need to have a wireless network interface card (NIC) either built in, or installed separately. There are two main types of wireless network configuration: ad-hoc mode and infrastructure mode. Ad-hoc networks are the simplest form of wireless network created by two or more wireless enabled computers communicating with each other directly. These types of WLANs are useful for creating small dynamic networks. Infrastructure mode requires one or more access points (APs) through which the network cards communicate. In a typical wireless LAN, a transmitter/receiver (transceiver) device, called an access point, is physically connected to the wired network using standard Ethernet cabling. It acts as a bridge between the wired network and the remote computer(s). At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure, using radio frequencies to transmit data to each user.’ Access points can have a varying amount of intelligence and functionality built-in. There are two main types of AP. “Thick” APs are fully functional and can handle all processes. “Thin” APs only include radios and antennas and rely on controllers (WLAN switches/appliances) for other functionality including managing APs, security and authentication. There is also a third hybrid category with some limited radio frequency management functionality, but that still need controllers to function fully.

The vast majority of WLANs use fully functional (thick) APs in a decentralized architecture. The APs are usually deployed in stand-alone mode, but in larger networks where the communication between APs poses an unacceptable load, a controller can be used to handle load balancing and roaming. There is a management overhead in configuring and managing each access point, although overlay management tools are available. Centralized architectures are less common. In these networks all traffic passes through the controllers (WLAN switches/WLAN Appliances), which handle load balancing and other management functions. The APs deal with RF access and often enforce policies set by the controller. Various manufacturers balance the functionality between controllers and APs differently. Benefits of using WLAN ---------------------------------- A wireless LAN has some specific advantages over wired LAN:

1. Access to the network can be from anywhere in the school within range of an access point, giving users the freedom to use ICT where and when it is needed.

2. It is typically easier and quicker to add or move devices on the network (once in place, a wired LAN can be difficult to move and expensive to change.) Increasing the overall network coverage of the wireless LAN can often be achieved by adding further access points.

3 Small dynamic ad hoc networks can be created very quickly and relatively easily. 4 It is typically easier and quicker to provide wireless connectivity to the network in areas where it is difficult or undesirable to lay cable or drill through walls.

5. Where wireless enabled laptop computers are used, any classroom in range of an access point(s) can become a ‘computer suite’, potentially increasing the use of ICT across the curriculum. 6. While the initial investment required for wireless LAN hardware can be similar to the cost of wired LAN hardware, installation expenses can be significantly lower. Issues ---------- Wireless LANs also have some issues:

1. The current data rates of wireless networks means that high bandwidth activities are better done on wired networks. 2. As the number of devices using the network increases, the data transfer rate to each device will decrease accordingly. 3. As wireless standards change, it may be necessary, or at least desirable, to upgrade to higher specifications of wireless which could mean replacing wireless equipment (wireless NICs, access points etc). Currently, wireless standards are changing more quickly than wired standards. 4. Security is more difficult to guarantee. 5. Devices will only operate at a limited distance from an access point, with the distance largely determined by the standard used. Obstacles between the access point and the user, like walls, glass, water, trees and leaves can also determine the distance of operation. Poor signal reception has been experienced around reinforced concrete school buildings; these may require higher numbers of access points which in turn increases overall cost.

6. In practice, a wireless LAN on its own is not a complete solution and will still require a wired LAN to be in place to provide a network backbone. 7. Data speeds drop as the user moves further away from the access point 8. It is generally easier to make a wired network ‘future proof’ for future requirements 9. As the number of people using wireless devices increases, there is the risk that certain radio frequencies used for wireless will become congested and prone to interference; particularly the 2.4GHz frequency. Standards --------------- In the field of wireless LAN there are currently three main operational standards: IEEE 802.11a, 802.11b and 802.11g. There are also a number of other standards relating to security, functionality and interoperability. The table below shows the various amendments to the 802.11 standard and their significance:

Organizations supporting WLAN related work: ------------------------------------------------------------------ 1. Institute of Electrical and Electronics Engineer (IEEE), URL: http://www.ieee.org http://standards.ieee.org/wireless 2. Wi-Fi Alliance, URL: http://www.wi-fi.org 3. Ofcom: URL: http://www.ofcom.org.uk 4. European Telecommunications Standards Institute (ETSI): URL: http://www.etsi.org 5. Communications Electronics Security Group (CESG): URL: http://www.cesg.gov.uk References: ----------------- [1] Introduction to Wifi (802.11 or Wifi) URL: http://en.kioskea.net/contents/wifi/wifiintro.php3 [2] “Wireless Local Area Networks (WLANS)”, Technical Paper, Becta, URL:

http://foi.becta.org.uk/content_files/corporate/resources/technology_and_education_research/w_lans.pdf

[3] “IEEE 802.11: Wireless Local Area Networks”, Brian P. Crow, The MITRE Corporation,

Indra Widjaja, Fujitsu Network Communications, Jeong Geun Kim, University of Arizona, Prescott T. Sakai, Cypress Semiconductor, URL: http://inst.eecs.berkeley.edu/~ee122/sp07/ieee80211_overview.pdf

[4] “Wireless LAN”, Wikipedia: URL: en.wikipedia.org/wiki/Wireless_LAN Satellite Communications ------------------------------------- Definition --------------

A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits. [1]

For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology complementary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting, for which application of other technologies, such as cable, is impractical or impossible.

Architecture --------------- Satellite communications are comprised of 2 main components [2]: 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 it merely downlinks any information it picks up from its vantage point. l The Ground Station: This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately converting signals received through the parabolic antenna to base band signal. Network Topologies -----------------------------

Depending on the application, satellites can be used with different ground network designs or network topologies. At its simplest, satellite can support one-direction or two-direction links between two earth stations (called respectively simplex transmission and duplex transmission). More complex communications needs can also be addressed with more sophisticated network topologies, such as star and mesh.

The following examples show some of the options available to customers for configuring their satellite networks [4]:

Simplex Transmission

Applications for simplex services include broadcast transmissions such as:

• TV and video services • Radio services

Point-to-Point Duplex Transmission

Applications for duplex services include:

• Voice Telephony transport

• Data and IP transport (especially in asymmetric configurations) • Corporate networks • TV and Broadcast program contribution and distribution

Point-to-Multipoint Transmission

(May be simplex or duplex, symmetric or asymmetric). Applications for point-to-multipoint services include:

• Corporate networks, including VSAT services and business television • Video and broadcast distribution, including Direct-to-Home Internet services

Mobile Antenna Service

Applications for mobile antenna services include:

• Satellite News Gathering • Special Event Backhaul and Broadcasting • Maritime services

Star Network

Applications for Star Networks include:

• Corporate Networks • Distance Learning

Mesh Network

Applications for Mesh Networks include:

• National and International Telephony and Data networks • Rural Telephony

Frequency Ranges: ----------------------------

The use of different bands for different satellite applications has been agreed upon through various international agencies [3]:

Given below is a table from 4990 to 7075 MHz allocation.

Region 1: Europe, Africa, N Asia; Region 2: N & S America; Region 3: rest of Asia

Fixed indicates a definite allocation for the service in the frequency band. Lower case entries show services that may be allowed.

Numbers such as 795 refer to regulations which apply to the frequency band.

Benefits: -------------

Satellite communications have distinct benefits over terrestrial alternatives [5]:

• UNIVERSAL: Satellite communications are available virtually everywhere. A small constellation of satellites can cover the Earth's entire surface. And even the reach of a single satellite is far more extensive than what any terrestrial network can achieve.

• VERSATILE: Satellites can support all of today's communications needs - transactional and multimedia applications, video, voice, cellular networks, entertainment and breaking news.

o Bring broadband to the last mile of residences and businesses.

o Overcome regulatory issues that make alternative carriers dependent on incumbents.

o Deliver a communications infrastructure to areas where terrestrial alternatives are unavailable, unreliable or simply too expensive.

• RELIABLE: Satellite is a proven medium for supporting a company's communications needs. Whereas terrestrial IP networks are often a mix of different networks and topologies, with different level of congestion and latency. Satellite networks are extremely predictable allowing constant and uniform quality of service to hundreds of locations, regardless of geography.

• SEAMLESS: Satellite's inherent strength as a broadcast medium makes it ideal for the simultaneous distribution of bandwidth-intensive information to hundreds or thousands of locations.

• FAST: Unlike most terrestrial alternatives, satellite networks can be rolled out quickly and inexpensively to hundreds or thousands of locations, connecting cities or remote locations across a large landmass, where copper or fiber is cost prohibitive. Since satellite networks can be set up quickly, companies can be fast-to-market with new services.

• EXPANDABLE: Satellite networks are easily scalable, allowing users to expand their communications networks and their available bandwidth easily. In coordination with local vendors, expanding a network on the ground requires the ordering of new terminal components and the commissioning of increased bandwidth at each site.

• FLEXIBLE: Satellites can be easily integrated to complement, augment or extend any communications network, helping overcome geographical barriers, terrestrial network limitations and other constraining infrastructure issues.

Issues: -----------

Limitations of a satellite communications system are determined by the technical characteristics of the satellite and its orbital parameters. Active communications satellite systems are limited by two things. Satellite transmitter power on the down links and receiver sensitivity on the up links. Some early communications satellites have been limited by low-gain antennas

Applications: -------------------

Telephony: In early days comsats were used for long distance telephony. The fixed Public Switched Telephone Network relays telephone calls from land line telephones to an earth station, where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Current comsats still serve remote islands.

Satellite phones connect directly to a constellation of either geostationary or low-earth-orbit satellites. Calls are then forwarded to a satellite teleport connected to the Public Switched Telephone Network or to another satellite phone system.

Satellite television: Two satellite types are used for North American television and radio: Direct Broadcast Satellite (DBS), and Fixed Service Satellite (FSS).

Fixed Service Satellite: These use the C band, and the lower portions of the Ku bands. They are normally used for broadcast feeds to and from television networks and local affiliate stations (such as program feeds for network and syndicated programming, live shots, and backhauls), as well as being used for distance learning by schools and universities, business television (BTV), Videoconferencing, and general commercial telecommunications. FSS satellites are also used to distribute national cable channels to cable television headends.

Direct broadcast satellite: A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in the upper portion of the microwave Ku band. DBS technology is used for DTH-oriented (Direct-To-Home) satellite TV services, such as DirecTV and DISH Network in the United States, Bell TV And Star Choice in Canada, Freesat in the UK and Sky Digital in the UK, the Republic of Ireland, and New Zealand.

Satellite radio: Satellite radio offers audio services in some countries, notably the United States. Mobile services allow listeners to roam a continent, listening to the same audio programming anywhere.

Amateur radio: Amateur radio operators have access to the OSCAR satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as space borne repeaters, and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas.

Satellite Internet: After the 1990s, satellite communication technology has been used as a means to connect to the Internet via broadband data connections. This can be very useful for users who are located in very remote areas, and cannot access a broadband connection.

Military uses: Communications satellites are used for military communications applications, such as Global Command and Control Systems. Many military satellites operate in the X-band, and some also use UHF radio links, while MILSTAR also utilizes Ka band.

Organizations supporting Satellite Communication related work [6]: ----------------------------------------------------------------------------------------------- 1. Global VSAT Forum: an association of companies involved in the business of delivering advanced digital fixed satellite systems and services, URL: www.gvf.org 2. Satcoms UK: The VSAT & Satellite Communications community, URL: http://www.satcoms.org.uk/satellite/community/default.asp 3. Satcom Online Satellite School: Online Satcom related encyclopedia, URL: http://www.satcom.co.uk/ 4. IEEE Communications Society: Promotes developments toward meeting new market demands in systems, products, and technologies such as personal communications services, multimedia communications systems, enterprise networks, and optical communications systems. URL: www.comsoc.org 5. Independent US government agency directly responsible to Congress regulates interstate and international communications by radio, television, wire, satellite and cable. URL: www.fcc.gov 6. International Engineering Consortium: The International Engineering Consortium (IEC) conducts a broad range of university and industry cooperative programs consisting of educational forums and workshops, research studies, publications, Web education, and management services. URL: www.iec.org 7. The International Institute for Communication and Development (IICD): IICD acts as an independent broker between countries in development and the stakeholders that drive the international market of ICTs. URL: www.iicd.org 8. The Center for Satellite and Hybrid Communication Networks (CSHCN) : The goal of the center is to become a leader in the crucial convergence of satellite and terrestrial communications technologies. URL: www.isr.umd.edu 9. US Department of Commerce Office of Telecommunications : Provides information regarding various forms of transpiration such as cable, cellular and satellite. URL: infoserv2.ita.doc.gov 10. International Telecommunications Union (ITU): An international organization within which governments and the private sector coordinate global telecom networks and services. Headquartered in Geneva, Switzerland. URL: www.itu.ch 11. Satellite Communications Systems and Technology Group: A study covering emerging systems concepts, and applications. They studied the international status of satellite communications systems and technology. URL: itri.loyola.edu 12. Mobile Satellite Users Association: A non-profit association to promote the interests of users of mobile satellite communications worldwide. URL: www.msua.org 13. NAB - National Association of Broadcasters: Based in Washington DC, NAB represents the radio and television industries before Congress, the FCC and federal agencies, the courts, and internationally. URL: www.nab.org 14. National Telecommunications and Information Administration: An agency of the U.S. Department of Commerce, NTIA is the Executive Branch's principal voice on domestic and international telecommunications and information technology issues.

URL: www.ntia.doc.gov 15. Satellite Industry Association: Represents the U.S. commercial satellite industry. Member companies are the leading satellite service providers, satellite manufacturers, launch services companies and ground equipment suppliers in America. URL: www.sia.org 16. VITA: Volunteers In Technical Assistance: VITA's offers information dissemination techniques such a communications technologies of digital radio networks and a low-earth orbiting satellite system, VITAsat. URL: www.vita.org . References ---------------- [1] “Communication Satellite”, Wiki, URL:

http://en.wikipedia.org/wiki/Communications_satellite [2] “Satellite communications”, David Hart,

URL: http://www.cs.wustl.edu/~jain/cis78897/ftp/satellite_nets.pdf [3] “Satellite Communications tutorial”, J. P. Silver,

URL: www.odyseus.nildram.co.uk/Systems_And_Devices_Files/Sat_Comms.pdf [4] “Satellite Basics: How it works”, URL: http://www.intelsat.com/resources/satellite-

basics/how-it-works.asp [5] “Satellite Basics: Solution Benefits”, URL: http://www.intelsat.com/resources/satellite-

basics/benefits.asp [6] “Satellite Communications Organization Information | Business.com”

http://www.business.com/directory/telecommunications/satellite_communications/organizations/

Small Office/Home Networks ----------------------------------------- Definition: --------------- The modern concept of small office/home office, or SoHo, refers to the category of business which can be from 1 to 10 workers. SOHO can also stand for small or home office or single office/home office. A larger business enterprise, one notch up the size scale, is often categorized as a small business. When a company reaches 100 or more employees, it is often referred to as a Small and Medium-sized Enterprise (SME).

Typical Small Office Network -----------------------------------------

Organizations Supporting SOHO Related work: 1. SOHO Online, www.soho.org 2. Aruba Networks, URL: www.arubanetworks.com 3. The Small Office / Home Office Certificate (SOHO) References: ------------------- [1] http://www.cccc.edu/registrar/catalog/2006/pdfs/Network-office-C25340SH.pdf [2] SOHO Networks, Wikipedia, URL: http://en.wikipedia.org/wiki/SOHO_network