System Technologies

8/7/2019 System Technologies

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INTRODUCTION

Low cost business terminals with small antennas (generally less than

2m’s in diameter) are often termed very small aperture terminals

(VSAT). These are usually perceived as being 2 way data terminals,

though strictly speaking many of the systems used for data broadcast are

really one way VSATS. .

Taking the USA as an example approximately half of all installed

VSATS is only used for one-way data links.

A more general definition is that a network is a VSAT network if it

consists of a large high performance hub earth station and a large number

of smaller, lower performance terminals. Being completely general, these

small terminals can be received only /transmit only / transmit as well as

receive. Even this definition is not universal. Meshed VSAT networks

exist in which all terminals have the same size and performance.

VSAT evaluated as a wide area data communication network fit for

almost any application currently served by telephone lines and array of

devices such as modems and multiplexes.

DEPARTMENT OF ECE,SIST 1



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THE SATELLITE COMMUNICATION

A satellite is a specialized wireless receiver/transmitter – essentially a

radio-frequency repeater – that is launched by a rocket and placed in orbit

around the earth. Today, these are literally hundreds of commercialsatellites in operation around the world. These satellites are used for such

diverse purposes as wide-area network communication, weather

forecasting, television broadcasting, amateur radio communications,

Internet access and the Global positioning system. In communication, the

satellite communication has the wide and important role for national and

international information interchanges, which may be in the form of

telephone transmissions, television and radio broadcasting, computer

communication etc.

Satellite communication involves the transmission of information from a

ground station to the satellite to the satellite, which is termed as uplink,

or retransmission of the information from the satellite back to the ground,

which is termed as downlink. The downlink may either be to a selected

number of ground stations or it may be broadcast to every one in a large

areas.




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It consist of a satellite in space that links many earth stations in terrestrial

networks.

BASIC COMPONENTS OF COMMUNICATION

SATELLITE

The satellite have a receiver and a receive antenna, a transmitter and a

transmit Antenna. Every communications satellite in its simplest form

(whether low earth or geosynchronous) involves the transmission of

information from an originating ground station to the satellite (the

uplink), followed by the retransmission of information from the

satellite back to ground (the downlink). The downlink may either be

to a select number of ground stations or it may be broadcast to every

one to a large area. Hence the satellite must have a receiver and a

receive antenna, a transmitter and a transmit antenna, some method

for connecting the uplink to the downlink for retransmission, and

prime electrical power to run all of the electronics. The exact nature of




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these components will differ, depending on the orbit and the system

architecture, but every communications satellite must have these basiccomponents. This is illustrated in the drawing above.

Transmitter

The amount of power that a satellite transmitter needs to send out

depends a great deal on whether it is in low earth orbit or in

geosynchronous orbit. This is a result of the fact that the

geosynchronous satellite is at an altitude of 22,300miles, while the

low earth satellite is only a few hundred miles. The geosynchronous

satellite is nearly 100 times as far away as the low earth satellite. We

can show fairly easily that this means the higher satellite would need

almost 10,000 times as much power as the low-orbiting one, if

everything else were the same.(fortunately, of course, we change

some other things so that we don’t need 10,000 times as much power).

Antennas

One of the biggest differences between a low earth satellite and a

geosynchronous satellite is in their antennas. As mentioned earlier, the

geosynchronous satellite would require nearly 10,000 times more

transmitter power, if all other components were the same. One of the

most straightforward ways to make up the difference, however, is

through antenna design. Virtually all antennas in use today radiate




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energy preferentially in some direction. An antenna used by a

commercial terrestrial radio station, for example, is trying to reach people to the north, south, east and west. However, the commercial

station will use an antenna that radiates very little power straight up or

straight down. Since they have very few listeners in those directions

(except may be coal miners and passing airplanes) power sent out in

those directions would be totally wasted. The communications

satellite carries this principle even further. All of its listeners are

located in an even smaller area, and a properly designed antenna will

concentrate most of the transmitter power within that area, wasting

none in directions where there are no listeners. The easiest way to do

this is simply to make the antenna larger.

Doubling the diameter of a reflector antenna (a big “dish”) will

reduce the area of the beam spot to one fourth of what it would be

with a smaller reflector. we describe this in terms of the gain of the

antenna. Gain simply tells us how much more power will fall on I

square centimeter (or square meter or square mile) with this antenna

would fall on that same square centimeter (or square meter or square

meter or square mile) if the transmitter power were spread uniformly

(isotropic ally) over all directions. The larger antenna described

above would have four times the gain of the smaller one. This is one

of the primary ways that the geosynchronous satellite makes up for

the apparently larger transmitter power, which it requires.

One other big difference between the geosynchronous antenna and low

earth antenna is the difficulty of meeting the requirement that the satellite




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antennas always be “pointed” at the earth. For the geosynchronous

satellite, of course, it is relatively easy. As seen from the earth station, thesatellite never appears to move any significant distance. As seen from the

satellite, the earth station never appears to move. We only need to

maintain the orientation of the satellite. The low earth orbiting satellite,

on the other hand, as seen from the ground is continuously moving. It

zooms across our field of view in 5 or 10 minutes. Likewise, the earth

station, as seen from the satellite is a moving target. As a result, both the

earth station and the satellite need some sort of tracking capability, which

will allow its antennas to follow the target during the time that it is

visible. The only alternative is to make that antenna beam so wide that

the intended receiver (or transmitter) is always within it. Of course,

making the beam spot larger area, which in turn increases the amount of

power, which the transmitter must provide.

Power Generation

You might wonder why we don’t actually use transmitters with

thousands of watts of power, like your favorite radio station does. You

might also have figured out the answer already. There is no line from the

power company to the satellite must generate all of its own power. For a

communication satellite, that power usually is generated by large solar

panels covered with solars cells – just like the ones in your solar-powered

calculator. These convert sunlight into electricity. Since there is a

practical limit to the amount of power which can generate. In addition,




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unfortunately, transmitters are not very good at converting input power to

radiated power so that 1000 watt s of power into the transmitter will probably result in only 100 or 150 watts of power being radiated. We say

that transmitters are only 10 or 15% efficient. In practice the solar cells

on the most “powerful” satellites generate only a few thousand watts of

electrical power.

Must also be prepared for those periods when the sun is not visible, usually

because the earth is passing between the satellite and the sun. This requires

that the satellite have batteries on board which can supply the required

power for the necessary time and then recharge by the time of the next

period of eclipse.




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CONCEPT OF VSAT

VSAT technology has emerged as a very useful. Every day

application of modern telecommunications. VSAT systems can

provide a variety of services including broadband communication

systems, satellite-based video, audio, internet and data distribution

networks as well as worldwide customer service and support.

VSAT stands for “Very Small Aperture Terminal;” it refers to

receive\transmit Terminals, installed at dispersed sites and connecting

to a central hub via satellite using small diameter antenna dishes (0.6

to 3.8 meter).

VSAT technology represents a cost effective solution for users

seeking an independent communication network connecting a large

number of geographically dispersed sites. VSAT networks offer

value-added satellite-based services capable of supporting the

internet, data, LAN, and voice\fax communications, and can provide

powerful, dependable private and public network communications

solutions.




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COMMUNICATION BETWEEN TWO TERMINALS

VSAT Networks

As mentioned earlier. Due to high performance requirements, design of

earth station is quite complicated. This increases the costs and the need

for maintenance. Very Small Aperture Terminals (VSAT) provides a

solution to this problem. The key point in VSAT networks that either the

transmitter or the receiver antenna on a satellite link must be larger. In

order to simplify VSAT design, a lower performance microwave




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transceiver and lower gain dish antenna (smaller size) is used. They act

as bidirectional earths stations that are small, simple and cheap enoughto be installed in the end user’s premises.

How does a VSAT network?

A VSAT network has three components:

A central hub (also called a master earth station)

The Satellite

A virtually unlimited number of VSAT earth stations in various

locations-across a country or continent.

Content originates at the hub, which features a very large-15 to 36-foot

(4,5-11m)-antenna. The hub controls the network through a network

management system (NMS) server, which allows a network operator to

monitor and control all components of the network. The NMS operator

can view, modify and download individual configuration information to

the individual VSATs.

Outbound information from the hub to the VSATs) is sent up to the

communications satellite’s transponder, which receives it, amplifies it

andBeams it back to earthy for reception by the remote VSATs. The

VSATs at remote locations send information inbound (from the VSATs

to the hub) via the same satellite transponder to the hub station.

This arrangement, where all network communication passes through

the network’s hub processor, is called a “star” configuration, with the hub




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station at the center of the star. One major advantage of this configuration

is that there is virtually no limit on the number of remote VSATs that can be connected the hub “Mesh” configurations also allow for direct

communication between VSATs.

Operation of VSAT Network

VSAT networks are typically arranged in a star based topology, where

each remote user 9s supported by a VSAT. The Earth hub station acts as

the central node and employs a large size dish antenna with a high quality

transceiver. The Satellite provides a broadcast medium acting as a

common connection point for All the remote VSAT earth stations. VSAT

networks are ideal for centralized networks with a central host and a

number of geographically dispersed terminals. Typical examples are

small and medium businesses with a central office, Banking Institutions

with branches all over the country, backbone links for an ISP and Airline

ticketing system.

The weaker signal from the remote ES is amplified at the satellite

acting as a bent pipe and received by the hub ES. Thus the lower gain at

the uplink is compensated at the downlink by the high performance Hub

ES.

The down side of this arrangement is that in case two VSATs need to

communicate, two satellite hopes are required, since all connections must




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pass through the hub ES node. The data link supported from the hub to

the VSAT is typically slower (19.2 kbps) than that in the reversedirection (512kbps). Figure shows how two VSAT terminals can

communicate in simple VSAT network.

Fig shows the Communication between two VSAT terminals.

Recent advances in satellite technology have enhanced the

functionality of the Satellites above the bent pipe operation. Modern

satellites now employ on-board processing with switching functions.

This means that the satellite repeater is capable of demodulation,

amplifying in baseband and retransmitting at fullpower. Thus the basic

function of the Hub ES can be eliminated and the satellite can provide

full point-point mesh connectivity between VSAT ES with

largerbandwidth in both directions. The trend towards multiple high

power spot beamsin the Ku band would further reduce the VSAT cost

and size (60 cm dia antenna dishes). All of the above, combined with the

improved hardware and communication schemes will make it possible to

provide end user’s with several Mbps links.




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VSAT SYSTEM TOPOLOGIES

Existing VSAT network are classified into 3 categories

Star Topology

Mesh Topology

Hybrid Network Topology




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STAR TOPOLOGY

The networks of VSAT’s at different locations adopt different topologies

depending on the end applications.

The most popular of these is star topology. Here we have a big central

earth station known as the hub. Generally the hub antennas in the range

of 6-11m are in diameter. This hub station controls, monitors and

communicate with a large number of dispersed VSAT’s.Since all

VSAT’s communicate with the central hubstation only,this network is

more suitable for centralized data applications.Large organizations like

banks,with centralized datd processing is a case point.

There are two reasons for the use of a single high performance hub.

It optimizes the use of satellite capacity from one point to

multipoints.




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One powerful hubstation offer a cost effective solution,allowing

the operation of low cost remote VSAT terminals.

MESH TOPOLOGY

In mesh topology a group of VSAT’s communicate directly with any

other VSAT in the network without going through a central hub. A hub

station in a mesh network performs only the monitoring and control

function. These networks are more suitable for telephony applications.

These have also been adopted to deploy point to point high speed links.




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HYBRID NETWORK TOPOLOGY

In actual practice a number of requirements are centered to by a hybrid

network topology.Under hybrid networks a part of the network operates

as star topology while some sites operate on a mesh topology.




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INTERACTIVE VSAT NETWORK

Overview Of Technology

The most common VSAT configuration is the TDM/TDMA star network.

These have a high bit rate outbound carrier (TDM) from the hub to the

remote earth stations, and one or more low or medium bit rate Time

Division Multiple Access(TDMA) inbound carriers. With its star

configuration network architecture, interactive VSAT technology is

appropriate for any organization with centralized management and data

processing.




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This configuration has been developed to minimize overall lifetime costs

for the complete network including satellite transmission costs. The useof a single high performance hub allows the use of low cost remote

VSAT terminals and optimizes use of satellite capacity. Even so, in most

VSAT networks, the cost of the VSAT terminals usually far exceeds the

cost of the hub(typically a VSAT terminal is 0.1 to 0.2% of the price of

the hub).

In a typical VSAT network, remote user sites have a number of personal

computers, dumb terminals and printers connected to the VSAT terminal

which connects them to centralized host computer either at the

organization’s head office or data processing centre.Datas sent to the

VSAT terminal from the DTEs is buffered and transmitted to the hub in

packets.

How does a VSAT network work? A VSAT network has three

components:

A central hub(also called a master earth station)

The satellite

A virtually unlimited number of VSAT earth station in the various

location across a country or continent

Content originates at the hub, which features a very large -15 to 36-foot

(4, 5-11m) antenna. The hub controls the network through a network




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management system (NMS) server, which allows a network operator to

monitor and control all components of the network. The NMS operator can view, modify and download individual configuration information to

the individual VSATs.

Outbound information (from the hub of the VSATs) is sent up to the

communication satellite’s transponder, which receives it, amplifies it and

beams it back to earth for reception by the remote VSATs. The VSATs at

the remote locations sent information inbound (from the VSATs to the

hub) via the same satellite transponder to the hub station.

VSAT’s can typically be divided into two part:- An Outdoor Unit and An

Indoor Unit

Outdoor Unit

The antenna system comprises of a reflector, feed horn and a mount. The

size of a VSAT antenna varies from 1.8m to 3.8m.The feed horn is

mounted on the antenna frame at its focal point by support arms. The

feed horn directs transmitter power towards the antenna dish or collects

the received power from it. It consists of an array of microwave passive

components. Antenna size is used to describe the ability of the antenna to

amplify the signal strength.




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These include low noise amplifiers(LNA)and down converters for

amplification and down conversion of the received signalrespectively.LNAs are designed to minimize the noise added to the

signal during this first stage of the converter as the noise performance of

this stage determines the overall noise performance of the

Converter unit. The noise temperature is the parameter used to describe

the performance of a LNA Up converters and High Powered Amplifiers

(HPA) are also part of the network and are used for uncovering and

amplifying the signal before transmitting to the feed horn. The Up/Down

converters convert frequency.

Inter link facility

The outdoor unit is connected through a low loss coaxial cable to the

indoor unit. The typical limit of an IFLL cable is about 300 feet.

Indoor Unit

The IDU consists of modulators whish superimpose the user traffic signal

on a carrier signal. This is then sent to the RFT for up conversion,

amplification and transmission. It also consists of demodulators, which

receive the signal from the outdoor unit, and demodulates the same to

segregate the user traffic signal from the carrier. The IDU also

determines the access schemes under which the VSAT would operate.

The IDU also interfaces with various end user equipment from stand




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along computers, LAN’s, routers, multiplexers, telephone instruments,

EPABX as per the requirement. It performs the necessary protocolconversion on the input data from the customer end equipment prior to

modulation and transmission to the RFT.An IDU is specified by the

access technique, protocols handled and number of interface ports

supported.




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SHARED HUB NETWORKS

To make VSAT networks affordable it is possible to share the hub

between several users, thereby spreading the cost. In this case the hub is

usually owned by a service provider who retains overall control of the

network and who manages the hub itself.

Each user, however, is allocated his own time slots or carriers and can so

operate his own private network using the shared hub facility without any

loss of privacy. The operation and management of these sub networks is




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performed by the users themselves completely independently of the

service supplier.

VSAT BANDS

There are 3 major signal bands available on the VSAT system. They are

the C-band,Ku-band and the Ka-band. The C-band is the lowest

frequency signal based transponder, which exhibits reliable capability to

withstand weather interference. It also allows the largest bandwidth up

gradability.The Ku band is subject to weather change interference. A

prolonged rain or cloudy weather sometimes affects transmission.

Typically, interactive Ku-band antenna sizes range from 75centimetres to

1.8 meters and C-band from 1.8metres to 2.4 meters.

The various frequency bands are as below:-

Frequency band Uplink(GHz)earth

station to satellite

Downlink(Ghz)satellite

to earth station

C Band 5.925 to 6.425 3.700 to 4.200

Extended C

Band

6.725 to 7.025 4.500 to 4.800




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Ku Band 14.00 to 14.500 10.950 to 11.700

Internationally Ku-band is a popular frequency band in use. The Ku-

Band by virtue of its higher frequency can support traffic with smaller

antenna sizes in comparison to C/Ext-c Band. It is, however, susceptible

to rain outages making it unsuitable for use in South East Asian regions.

Indian service providers are presently allowed to hire space segment only

on the INSAT series and operate in Ext-C band only.Ext_C band is

available only on the INSAT series of satellite and is not a standard band

available internationally.




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ADVANTAGES OF VSAT

If by now you believe that VASTS provide an edge over terrestrial lines

only in cases where the land lines are difficult to install, say in the case of

remote locations, then consider this. Close to 50 percent of the total

VSAT population is installed in the US which also boasts of world’s best

terrestrial communications.Networking of business activities, processes

and divisions is essential to gain a competitive edge in any

industry.VSATs are an ideal option for networking because they enable

enterprise wide networking with high reliability and a wide reach which

extends even to remote sites.

Last Mile Problem

Let us begin with the situation where you have reliable high speed links

between city exchanges for meeting your communication requirements.

But before you begin to feel comfortable, connections from the nearest

exchange to your company’s office often fail.Consequently, stretching




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what is technically called the last mile problem into much longer

distances. VSATs located at your premises guarantee seamlesscommunication even across the last mile.

Reach : You must be well aware of the limitation faced by terrestrial

lines 8b reaching remote and other difficult locations. VSATs, on

the other hand, offer you unrestricted and unlimited reach.

Reliability

Uptime of uptown 99.5 percent is achievable on a VSAT network. This is

Significantly higher than the typical leased line uptime of approximately

80 to 85 percent.

Time

VSAT deployment takes no more than 4-6 weeks as compared to 4 to 6

months for leased lines.

Network Management

Network monitoring and control of the entire VSAT network is much

simpler than a network of leased lines, involving multiple carriers at




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multiple locations. A much smaller number of elements needs to be

monitored incase of a VSAT network and also the number of vendors andcarriers involved in between any two user terminals in a VSAT network

is typically one. This results in a single point of contact for resolving all

your VSAT networking issues. A VSAT NMS easily

Integrates end-to-end monitoring and configuration control for all

network Subsystems.

Maintenance

A single point contact for operation, maintenance, rapid isolation and

trouble shooting makes things very simple for a client, using VSAT

services. VSATsalso enjoy a low mean time to repaid (MTTR) of a

fewhours, which extends up to afew days in the case of leased line.

Essentially, lesser elements imply lower MTTR

Flexibility

VSAT network offer enormous expansion capability .this feature factor

in change in the business environment and traffic loads that can be

easily accommodated on technology migration path. Addition VSATs

can be rapid installed to support the network expansion to any site, no

matter how ever remote.




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Cost

A comparison of costs between a VSAT network and a leased line

network reveals that a VSAT network offers significant savings over two

to three years time frame. This does not take into account the cost of

downtime, inclusion of which would result in the VSAT network being

much more cost effective. Pay-by-mile concept in case of leased line

sends the costs spiraling upwards. More so if the locations to be linked

are dispersed all over the country. Compare this to VSATs where the

distance has nothing to do with the cost.Additionlly, in case of VSATs;

the service charges depend on the bandwidth which is allocated to your

network in line with your requirement. Whereas with a leased line you

get a dedicated circuit in multiples of 64Kbps whether you need that

amount of bandwidth or not.

Quick Establishment

A VSAT earth station can be placed any where as long as it has an

unobstructed view of satellites.




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DISADVANTAGES OF VSAT

Satellite internet is generally more expensive than terrestrial.

Outdoor Unit (antenna and cabling) are more prone to weather

condition.

Requires professional supports.

Common people could not work. Only professionals who are having the

right knowledge about VSAT can work.




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APPLICATIONS OF VSAT

Data communication network

Banking and financial service.

Credit card verifications.

LANS.

ATM machines.

Remote area voice network

National wide telephone/data service.

Satellite news gathering.

Rural communication.

Environmental monitoring

E-mail networks

Document transfer.




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Internet service

Data receive

Stock information.

News.

POPULARITY OF VSAT IN INDIA

In India, VSAT network for government organization and department of

telecommunication(DOT) have being in use in since 1980s.But the use of

VSATs by private organization in closed used groups was only permitted

in early 1992.Currently VSAT service are extensively used to provide

voice, data and for services to various private cooperative business and

service organization.

At present there are about 500 earth station of various sizes and 10000

VSATs in operation. Currently there are about 30 VSAT network in

service in the country.

WHY VSAT IS POPULAR?




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Quick establishment of new sites.

Flexibility.

Transportability.

Distance independent.

Reduced cost.

Maintenance.

GROWTH OF VSAT SERVICE

During the last few years, the following sectors have being increasingly

using VSAT services in Indian subcontinent.

Financial sector-including stock exchanges.

Banking sector for on line transaction.

Corporate communication for data, voice, video conference.

Rural communications and internet.

Educational trainings through distant learning.




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CONCLUSION

The major economic cost factor of any VSAT service in the large cost of

sophisticated hub station and require several full time staff members. To

make the service cost effective their must be a large number of micro

terminals in the network (in hundreds) and the shared hub must be used

to apportion costs among several users.

The hub equipment is basically the centre of all activity during satellite

communication the various VSATS.It carries out health checks on all

VSATs and undertakes all kinds of configuration at each station to

introduce different types of techniques as per the requirement of the

users.




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REFERENCES

⇒ Wireless Digital Communications:Dr.Kamilo Feher

⇒ [email protected]

⇒ www@GEAmerican communications

⇒ IEEE magazines

⇒ Satellite communications

⇒ Data communications and satellite network: William Stalling


mailto:[email protected]

mailto:[email protected]



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DEPARTMENT OF ECE SIST 35

System Technologies

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