Satellite Systems can be classified based upon their orbits as low earth orbit, medium earth orbit & geostationary earth orbit systems. Geostationary is also the highest earth orbit and hence, also provides the greatest visibility using only a few satellites. The coverage region of a satellite is called its footprint. This is the region from which the satellite is visible. Three geostationary satellite footprints ensure complete coverage of the earth as shown:

Hence, there is permanent or 24 hour visibility of geostationary satellites without the need of handoffs. While LEO & MEO satellites do not have 24 hour visibility as the satellites have smaller footprints since they are closer to the earth (low satellite height). Hence, a larger number of satellites are needed to cover the earth. Also, since each satellite has a small footprint, handoffs are also required between satellites.

Major differences between LEO, MEO & GEO satellite systems:

Parameter LEO MEO GEO

Satellite Height 500-1500 km 5000-12000 km 35,800 km

Orbital Period 10-40 minutes 2-8 hours 24 hours

Number of Satellites 40-80 8-20 3

Satellite Life Short Long Long

Number of Handoffs High Low Least(none)

Gateway Cost Very Expensive Expensive Cheap

Propagation Loss Least High Highest

A geosynchronous satellite is a satellite in geosynchronous orbit, with an orbital period the same as the Earth's rotation period. Such a satellite returns to the same position in the sky after each sidereal day, and over the course of a day traces out a path in the sky that is typically some form of analemma. A special case of geosynchronous satellite is the geostationary satellite, which has a geostationary orbit – a circular geosynchronous orbit directly above the Earth's equator. Another type of geosynchronous orbit used by satellites is the Tundra elliptical orbit.

Geosynchronous satellites have the advantage of remaining permanently in the same area of the sky, as viewed from a particular location on Earth, and so permanently within view of a given ground station. Geostationary satellites have the special property of remaining permanently fixed in exactly the same position in the sky, meaning that ground-based antennas do not need to track them but can remain fixed in one direction. Such satellites are often used for communication purposes; a geosynchronous network is a communication network based on communication with or through geosynchronous satellites.

There are approximately 600 geosynchronous satellites, some of which are not operational.[1]

A geostationary satellite above a marked spot on the Equator. An observer on the marked spot will see the satellite

remain directly overhead unlike the other heavenly objects which sweep across the sky. This novel phenomenon, a

straightforward consequence of Newton's theory of motion and gravity, is made possible by the fact that the earth

spins.

Geostationary satellites appear to be fixed over one spot above the equator. Receiving and transmitting. antennas on the earth do not need to track such a satellite. These antennas can be fixed in place and are much less expensive than tracking antennas. These satellites have revolutionized global communications,television broadcasting and weather forecasting, and have a number of importantdefense and intelligence applications.

One disadvantage of geostationary satellites is a result of their high altitude: radiosignals take approximately 0.25 of a second to reach and return from the satellite, resulting in a small but significant signal delay. This delay increases the difficulty oftelephone conversation and reduces the performance of common network protocolssuch as TCP/IP, but does not present a problem with non-interactive systems such as television broadcasts. There are a number of proprietary satellite data protocols that are designed to proxy TCP/IP connections over long-delay satellite links—these are marketed as being a partial solution to the poor performance of native TCP over satellite links. TCP presumes that all loss is due to congestion, not errors, and probes link capacity with its "slow-

start" algorithm, which only sends packets once it is known that earlier packets have been received. Slow start is very slow over a path using a geostationary satellite.

Another disadvantage of geostationary satellites is the incomplete geographical coverage, since ground stations at higher than roughly 60 degrees latitude have difficulty reliably receiving signals at low elevations. Satellite dishes at such high latitudes would need to be pointed almost directly towards the horizon. The signals would have to pass through the largest amount of atmosphere, and could even be blocked by land topography, vegetation or buildings. In the USSR, a practical solution was developed for this problem with the creation of special Molniya / Orbita inclined path satellite networks with elliptical orbits. Similar elliptical orbits are used for the Sirius Radio satellites.

Medium Earth orbit (MEO), sometimes called intermediate circular orbit(ICO), is the region of space around the Earth above low Earth orbit (altitude of 2,000 kilometres (1,243 mi)) and below geostationary orbit (altitude of 35,786 kilometres (22,236 mi)).[1]

The most common use for satellites in this region is for navigation,communication, and geodetic/space environment science.[1] The most common altitude is approximately 20,200 kilometres (12,552 mi)), which yields an orbital period of 12 hours, as used, for example, by the Global Positioning System(GPS).[1] Other satellites in Medium Earth Orbit include Glonass (with an altitude of 19,100 kilometres (11,868 mi)) and Galileo (with an altitude of 23,222 kilometres (14,429 mi)) constellations.[citation needed] Communications satellites that cover the North and South Pole are also put in MEO.[2]

The orbital periods of MEO satellites range from about 2 to nearly 24 hours.[1] Telstar  1, an experimental satellite launched in 1962, orbits in MEO.[3]

The orbit is home to a number of satellites.[1]

Low earth orbits (LEO) are satellite systems used in telecommunication, which orbit between 400 and 1,000 miles above the earth's surface. They are used mainly for data communication such as email, video conferencing and paging. They move at extremely high speeds and are not fixed in space in relation to the earth.

LEO-based telecommunication systems provide underdeveloped countries and territories with the ability to acquire satellite telephone service in areas where it otherwise would be too costly or even impossible to lay land lines.A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi), with an orbital period of about 88 minutes, and 2,000 kilometers (1,200 mi), with an orbital period of about 127 minutes. Objects below approximately 160 kilometers (99 mi) will experience very rapid orbital decay and altitude loss.[1][2] With the exception of the manned lunar flights of the Apollo program, all human spaceflights have taken place in LEO (or were suborbital). The altitude record for a human spaceflight in LEO was Gemini 11with an apogee of 1,374.1 kilometers (853.8 mi). All manned space stations to date, as well as the majority of artificial satellites, have been in LEO.

HiperLAN (High Performance Radio LAN) is a Wireless LAN standard.[1] It is a European alternative for the IEEE 802.11standards (the IEEE is an international organization). It is defined by the European Telecommunications Standards Institute(ETSI). In ETSI the standards are defined by the BRAN project (Broadband Radio Access Networks). The HiperLAN standard family has four different versions.

Contents  [hide] 

1 HiperLAN/1 2 HiperLAN/2 3 Failure in the Market 4 See also 5 References

HiperLAN/1[edit]

Planning for the first version of the standard, called HiperLAN/1, started 1991, when planning of 802.11 was already going on. The goal of the HiperLAN was the high data rate, higher than 802.11. The standard was approved in 1996. The functional specification is EN300652, the rest is in ETS300836.

The standard covers the Physical layer and the Media Access Control part of the Data link layer like 802.11. There is a new sublayer called Channel Access and Control sublayer (CAC). This sublayer deals with the access requests to the channels. The accomplishing of the request is dependent on the usage of the channel and the priority of the request.

CAC layer provides hierarchical independence with Elimination-Yield Non-Preemptive Multiple Access mechanism (EY-NPMA). EY-NPMA codes priority choices and other functions into one variable length radio pulse preceding the packet data. EY-NPMA enables the network to function with few collisions even though there would be a large number of users.Multimedia applications work in HiperLAN because of EY-NPMA priority mechanism. MAC layer defines protocols for routing, security and power saving and provides naturally data transfer to the upper layers.

On the physical layer FSK and GMSK modulations are used in HiperLAN/1.

HiperLAN features:

range 50 m

slow mobility (1.4 m/s)

supports asynchronous and synchronous traffic

Bit rate - 23.2 Mbps

Description- Wireless Ethernet

Frequency range- 5 GHz

HiperLAN does not conflict with microwave and other kitchen appliances, which are on 2.4 GHz. An innovative feature of HIPERLAN 1, which may other wireless networks do not offer, is its ability to forward data packets using several relays. Relays can extend the communication on the MAC layer beyond the radio range. For power conservation, a node may set up a specific wake up pattern. This pattern determines at what time the node is ready to receive, so that at other times, the node can turn off its receiver and save energy. These nodes are called p-savers and need so called p-supporters that contain information about wake up patterns of all the p-savers they are responsible for. A p-supporter only forwards data to a p-saver at the moment p-saver is awake. This action also requires buffering mechanisms for packets on p-supporting forwaders.

IEEE-802.11Wireless connectivity for computers is now well established and virtually all new laptops contain a Wi-Fi capability. Of the WLAN solutions that are available the IEEE 802.11 standard, often termed Wi-Fi has become the de-facto standard. With operating speeds of systems using the IEEE 802.11 standards of around 54 Mbps being commonplace, Wi-Fi is able to compete well with wired systems. As a result of the flexibility and performance of the system, Wi-Fi "hotpots" are widespread and in common use. These enable people to use their laptop computers as they wait in hotels, airport lounges, cafes, and many other places using a wire-less link rather than needing to use a cable.

In addition to the 802.11 standards being used for temporary connections, and for temporary Wireless Local Area Network, WLAN applications, they may also be used for more permanent installations. In offices WLAN equipment may be used to provide semi-permanent WLAN solutions. Here the use of WLAN equipment enables offices to be set up without the need for permanent wiring, and this can provide a considerable cost saving. The use of WLAN equipment allows changes to be made around the office without the need to re-wiring.

As a result the Wi-Fi, IEEE 802.11 standard is widely used to provide WLAN solutions both for temporary connections in hotspots in cafes, airports, hotels and similar places as well as within office scenarios.

IEEE 802.11 StandardsThere is a plethora of standards under the IEEE 802 LMSC (LAN / MAN Standards Committee). Of these even 802.11 has a variety of standards, each with a letter suffix. These cover everything from the wireless standards themselves, to standards for security aspects, quality of service and the like:

802.11a - Wireless network bearer operating in the 5 GHz ISM band with data rate up to 54 Mbps

802.11b - Wireless network bearer operating in the 2.4 GHz ISM band with data rates up to 11 Mbps

802.11e - Quality of service and prioritisation

802.11f - Handover

802.11g - Wireless network bearer operating in 2.4 GHz ISM band with data rates up to 54 Mbps

802.11h - Power control

802.11i - Authentication and encryption

802.11j - Interworking

802.11k - Measurement reporting

802.11n - Wireless network bearer operating in the 2.4 and 5 GHz ISM bands with data rates up to 600 Mbps

802.11s - Mesh networking

802.11ac - Wireless network bearer operating below 6GHz to provide data rates of at least 1Gbps per second for multi-station operation and 500 Mbps on a single link

802.11ad - Wireless network bearer providing very high throughput at frequencies up to 60GHz

802.11af - Wi-Fi in TV spectrum white spaces (often called White-Fi)

Of these the standards that are most widely known are the network bearer standards, 802.11a, 802.11b, 802.11g and now 802.11n.

802.11 Network bearer standardsAll the 802.11 Wi-Fi standards operate within the ISM (Industrial, Scientific and Medical) frequency bands. These are shared by a variety of other users, but no license is required for operation within these frequencies. This makes them ideal for a general system for widespread use.

There are a number of bearer standards that are in common use. These are the 802.11a, 802.11b, and 802.11g standards. The 802.11n standard is the latest providing raw data rates of up to 600 Mbps.

Each of the different standards has different features and they were launched at different times. The first accepted 802.11 WLAN standard was 802.11b. This used frequencies in the 2.4 GHz Industrial Scientific and Medial (ISM) frequency band, this offered raw, over the air data rates of 11 Mbps using a modulation scheme known as Complementary Code Keying (CCK) as well as supporting Direct-Sequence Spread Spectrum, or DSSS, from the original 802.11 specification. Almost in parallel with this a second standard was defined. This was 802.11a which used a different modulation technique, Orthogonal Frequency Division Multiplexing (OFDM) and used the 5 GHz ISM band. Of the two standards it was the 802.11b variant that caught on. This was primarily because the chips for the lower 2.4 GHz band were easier and cheaper to manufacture.

The 802.11b standard became the main Wi-Fi standard. Looking to increase the speeds, another standard, 802.11g was introduced and ratified in June 2003. Using the more popular 2.4 GHz band and OFDM, it offered raw data rates of 54 Mbps, the same as 802.11b. In addition to this, it offered backward compatibility to 802.11b. Even before the standard was ratified, many vendors were offering chipsets for the new standard, and today the vast majority of computer networking that is shipped uses 802.11g.

Then in January 2004, the IEEE announced it had formed a new committee to develop an even higher speed standard. With much of the work now complete, 802.11n is beginning to establish itself in the same way as 802.11g. The industry came to a substantive agreement about the features for 802.11n in early 2006. This gave many chip manufacturers sufficient information to get their developments under way. As a result it is anticipated that before long, with ratification of 802.11n expected in 2007, that some cards and routers will find their way into the stores.

  802.11A 802.11B 802.11G 802.11N

Date of standard approval

July 1999

July 1999 June 2003 Oct 2009

Maximum data rate (Mbps)

54 11 54 ~600

Modulation OFDM CCK or DSSS

CCK, DSSS, or OFDM

CCK, DSSS, or OFDM

RF Band (GHz) 5 2.4 2.4 2.4 or 5

Number of spatial streams

1 1 1 1, 2, 3, or 4

Channel width (MHz)nominal

20 20 20 20, or 40

Summary of major 802.11 Wi-Fi StandardsBandwidths of nominal 20 MHz are usually quoted, although the actual bandwidth allowed is generally 22 MHz.

802.11 NetworksThere are two types of WLAN network that can be formed: infrastructure networks; and ad-hoc networks.

The infrastructure application is aimed at office areas or to provide a "hotspot". The WLAN equipment can be installed instead of a wired system, and can provide considerable cost savings, especially when used in established offices. A backbone wired network is still required and is connected to a server. The wireless network is then split up into a number of cells, each serviced by a base station or Access Point (AP) which acts as a controller for the cell. Each Access Point may have a range of between 30 and 300 metres dependent upon the environment and the location of the Access Point.

The other type of network that may be used is termed an Ad-Hoc network. These are formed when a number of computers and peripherals are brought together. They may be needed when several people come together and need to share data or if they need to access a printer without the need for having to use wire connections. In this situation the users only communicate with each other and not with a larger wired network. As a result there is no Access Point and special algorithms within the protocols are used to enable one of the peripherals to take over the role of master to control the network with the others acting as slaves.

Link Calculation for GEO Satellite

The satellite link is probably the most basic in microwave communications since a line-of-sight path typically exists between the Earth and space.

This means that an imaginary line extending between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not

ground obstacles.

Such a link is governed by free-space propagation with only limited variation with respect to time due to various constituents of the atmosphere.

The satellite link is probably the most basic in microwave communications since a line-of-sight path typically exists between the Earth and space.

This means that an imaginary line extending between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not

ground obstacles.

Such a link is governed by free-space propagation with only limited variation with respect to time due to various constituents of the atmosphere.

The satellite link is probably the most basic in microwave communications since a line-of-sight path typically exists between the Earth and space.

This means that an imaginary line extending between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not

ground obstacles.

Such a link is governed by free-space propagation with only limited variation with respect to time due to various constituents of the atmosphere.

The satellite link is probably the most basic in microwave communications since a line-of-sight path typically exists between the Earth and space.

This means that an imaginary line extending between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not

ground obstacles.

Such a link is governed by free-space propagation with only limited variation with respect to time due to various constituents of the atmosphere.

The satellite link is probably the most basic in microwave communications since a line-of-sight path typically exists between the Earth and space.

This means that an imaginary line extending between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not

ground obstacles.

Such a link is governed by free-space propagation with only limited variation with respect to time due to various constituents of the atmosphere.

The example shows a large hub type Earth station in the uplink and a small VSAT in the downlink; the satellite is represented by a simple frequency translating type

repeater (e.g., a bent pipe).

Most geostationary satellites employ bent-pipe repeaters since these allow the widest range of services and communication techniques.

Bidirectional (duplex) communication occurs with a separate transmission from each Earth station.

Due to the analog nature of the radio frequency link, each element contributes a gain or loss to the link and may add noise and interference as well.

The result in the overall performance is presented in terms of the ratio of carrier power to noise (the carrier-to-noise ratio, C/N) and, ultimately, information quality (bit

error rate, video impairment, or audio fidelity).

Done properly, this analysis can predict if the link will work with satisfactory quality based on the specifications of the ground and space components.

Any uncertainty can be covered by providing an appropriate amount of link margin, which is over and above the C/N needed to deal with propagation effects and

nonlinearity in the Earth stations and satellite repeater.

The link between the satellite and Earth station is governed by the basic microwave radio link equation:

where pr is the power received by the receiving antenna; pt is the power applied to the transmitting antenna; gt is the gain of the transmitting antenna; gr is the gain of the receiving antenna; c is the speed of light (i.e., approximately 300 × 106 m/s); R is the range (path length) in meters; and f is the frequency in hertz.

Almost all link calculations are performed after converting from products and ratios to decibels.

The same formula, when converted into decibels, has the form of a power balance.

The received power in this formula is measured in decibel relative to 1W, which is stated as dBW.

The last two terms represent the free-space path loss (A0) between the Earth station and the satellite.

If we assume that the frequency is 1 GHz and that the distance is simply the altitude of a GEO satellite (e.g., 35,778 km), then the path loss equals 183.5 dB; that is,

for f = 1000000000 Hz and R = 35,788,000 m.

Multiple Access Collision Avoidance

Multiple Access with Collision Avoidance (MACA) is a slotted media access control protocol used in wireless LAN data transmission to avoid collisions caused by the hidden station problem and to simplify exposed station problem.

The basic idea of MACA is a wireless network node makes an announcement before it sends the data frame to inform other nodes to keep silent. When a node wants to transmit, it sends a signal called Request-To-Send (RTS) with the length of the data frame to send. If the receiver allows the transmission, it replies the sender a signal called Clear-To-Send (CTS) with the length of the frame

that is about to receive.Meanwhile, a node that hears RTS should remain silent to avoid conflict with CTS; a node that hears CTS should keep silent until the data transmission is complete.

WLAN data transmission collisions may still occur, and the MACA for Wireless (MACAW) is introduced to extend the function of MACA. It requires nodes sending acknowledgements after each successful frame transmission, as well as the additional function of Carrier sense.

Carrier sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by transmitting only when the channel is sensed to be "idle".[1][2] When they do transmit, nodes transmit their packet data in its entirety.

It is particularly important for wireless networks, where the collision detection of the alternative CSMA/CD is unreliable due to the hidden node problem.

CSMA/CA is a protocol that operates in the Data Link Layer (Layer 2) of the OSI model.[3]

Simplified Algorithm of CSMA/CA

Contents  [hide] 

1 Details 2 IEEE 802.11 RTS/CTS Exchange 3 Performance 4 Usage 5 See also 6 References 7 External links

Details[edit]

Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.

1. Carrier Sense: prior to transmitting, a node first listens to the shared medium (such as listening for wireless signals in a wireless network) to determine whether another node is transmitting or not. Note that the hidden node problemmeans another node may be transmitting which goes undetected at this stage.

2. Collision Avoidance: if another node was heard, we wait for a period of time for the node to stop transmitting before listening again for a free communications channel.

Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access to the shared medium. This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a time. However, wireless 802.11implementations do not typically implement RTS/CTS for all transmissions; they may turn it off completely, or at least not use it for small packets (the overhead of RTS, CTS and transmission is too great for small data transfers).

Transmission: if the medium was identified as being clear or the node received a CTS to explicitly indicate it can send, it sends the frame in its entirety. Unlike CSMA/CD, it is very challenging for a wireless node to listen at the same time as it transmits (its transmission will dwarf any attempt to listen). Continuing the wireless example, the node awaits receipt of an acknowledgement packet from the Access Point to indicate the packet was received and checksummed correctly. If such acknowledgement does not arrive after a timely manner, it assumes the packet collided with some other transmission, causing the node to enter a period of binary exponential backoffprior to attempting to re-transmit.

Although CSMA/CA has been used in a variety of wired communication systems, it is particularly beneficial in a wireless LANdue to a common problem of multiple stations being able to see the Access Point, but not each other. This is due to differences in transmit power, and receive sensitivity, as well as distance, and location with respect to the AP.[4] This will cause a station to not be able to 'hear' another station's broadcast. This is the so-called 'hidden node', or 'hidden station' problem. Devices utilizing 802.11 based standards can enjoy the benefits of collision

avoidance (RTS / CTS handshake, also Point coordination function), although they do not do so by default. By default they use a Carrier sensing mechanism called 'exponential backoff', or (Distributed coordination function) that relies upon a station attempting to 'listen' for another station's broadcast before sending. CA, or PCF relies upon the AP (or the 'receiver' for Ad hoc networks) granting a station the exclusive right to transmit for a given period of time after requesting it (Request to Send / Clear to Send).[5]

IEEE 802.11 RTS/CTS Exchange[edit]

CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange. Implementation of RTS/CTS helps to partially solve the hidden node problem that is often found in wireless networking.[6][7]