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106 Data Communication

Figure 5.1

(a) Functional

diagram

(b) Top view

(QAM) techniques are used to transmit and receive the digital data. A secondary handshake chan-

nel used for the establishment of a communications dialogue (request and acknowledgments)

operates at 300 bps using FSK modulation.

Document Drive

Aperture Slot

Light Source

Lens

Charge Coupled Device

Electrical Signal

Document

Mirrors

(a) Functional Diagram

(b) Top View

Mirrors

Light Source

Lens

Slot

Flatbed Scanner

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Data Communication Systems 107

Figure 5.2

Fascimile sys-

tem

Figure 5.2 shows an overall block layout for a tax system. The Voice/FAX/Mux block differ-

entiates between a fax signal and regular voice call on the telephone line. In this block, voice

communications are directed to a regular telephone handset, while fax data are sent to an

appropriate modem. The modem receives and demodulates the fax data, which are decoded and

processed by the processor block and sent to a specific peripheral printer for hard copy and termi-

nal for video display. Fax data can be entered into the system through a scanner or from data pre-

viously scanned and stored on disk or in a computers memory. The data are encoded and sent to

the modem. These are modulated and sent out onto the telephone lines to their destination.

5.3 SATELLITE COMMUNICATION

A satellite communication system basically consists of a satellite in space and many earth stations

on the ground which are linked with each other through the satellite. Baseband signal from the

user is transmitted to the earth station through a terrestrial network and is modulated by an RF

carrier at the earth station and transmitted to the satellite. The satellite receives the modulated RF

carrier in its uplink frequency spectrum form all the earth in the downlink frequency spectrum,

which is different from the uplink frequency spectrum. The satellites therefore can be thought of

as large repeater stations in space. The bandwidth of a typical commercial satellite is 500 MHz on

both uplink and downlink frequencies. The most widely used frequency spectrum on C-band is 6/4

GHz band with uplink frequency of 5.925-6.425 GHz and down link frequency of 3.700-4.200

GHz. The 6/4 GHz band, however, is getting overcrowded as it is being used by terrestrial micro-

wave links. DoT has been making use of extended C-band (at no extra cost) due to crowding on

C-band with uplink of 6.725-7.025 GHz and downlink of 4.500-4.800 GHz.

Satellite are also being operated on Ku band, i.e. 14/12 GHz band with an uplink of 12.75-14.8

GHz and a downlink of either 10.7-12.3 GHz or 12.5 to 12.7 GHz. But Ku band is affected by rain

droplets, the size of which is comparable to the wavelength of the radio frequency, thus simulating

antenna which absorb energy from transmitted/received signals. The solution to this lies in trans-

mitting high power, but this would burden the solar panels as more energy would be required for

transmit and receive signals. As mentioned earlier, the bandwidth of a typical satellite is 500 MHz

and accordingly it is distributed over the transponders. HCL Comnet, a private satellite operator,

Voice/Fax

Multiplexer

Telephone

LinesModem Processor

Interface

I/O

Telephone

Auto

Dialer

ROM RAMTelephone

Digital Bus

Printer

Scanner

Keyboard and

Display Terminal

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makes use of satellite INSAT-2D, transponder No. 13 whose bandwidth is 36 MHz. For better

utilization of transponders, frequency reuse is employed in a number of cases using orthogonal

polarisation, i.e. vertical and horizontal polarisation, the isolation between the two being main-

tained at 30 dB or more by staggering frequencies. With this the number of transponders could be

doubled, thus doubling its capacity.

A satellite with a large part of the earths surface in its line of sight, can communicate virtually

simultaneously with many ground stations. If it is in geostationary orbit, it can maintain constant

line-of-sight contact. In other orbits, satellites rise and set and can communicate with ground sta-

tions only at certain times.

Figure 5.3

Three geo-stationary satel-

lites covering

the entire

globe

The ideal orbit for a communications satellite is geostationary or motionless relative to the

ground. This occurs at an altitude of roughly 22,300 miles, where the satellite revolves once

around the earth in exactly the time it takes for the planet to turn once on its axis. To prevent the

satellite from rising and setting over the horizon as the moon does, the orbit must also be in the

same plane as the equator. The three geostationary satellites could in theory be distributed so that

each of their "footprints" the area that can receive their signals covers more than 40 percent

of the globe. Overlapping footprints would permit continuous communications world wide, exceptfor regions near to the poles. In practice, however, satellites with much greater transmission

capacity than now possible would be needed to satisfy communications demands with only three

artificial moons.

5.3.1 Components of Satellite Communication

The components are:

(a) Ground segment equipment

North Pole

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(b) Free space

(c) Spaced segment

Ground segment equipment

Ground segment equipment is basically a digital earth station. The digital signal through the ter-

restrial network is processed. Error correction coding is performed by the encoder which reduces

error rate to an acceptable level. Thereafter it is modulated using intermediate frequency (IF)

carrier of 70 MHz, a standard frequency in satellite communications for a communication channel

using 36MHz transponder bandwidth. The modulated IF carrier is further modulated to a satellite

uplink radio frequency (RF) carrier and is sent to HPA for transmission to satellite. Similarly, on

the receiver side, low level modulated RF is received by low noise amplifier (LNA) which ampli-

fies, it, keeping carrier-to-noise ratio at an acceptable level to meet the error rate requirement. Thedown converter translates it to the IF level which is fed to the demodulator where the digital

stream of data is extracted. Personal earth station (PES)/micro earth station segment equipment

working on the same basic principle.

Free space

Free space is the medium between the satellite and earth station which offers certain obstructions

for RF in both the uplink and downlink paths. The major obstructions could be: (i) presence of

AWGN (additive white gaussian noise), (ii) contamination by signals transmitted by other satel-

lites to adjacent earth stations, and (iii) rain that can severely attenuate signals around 10 GHz and

also reduce the isolation in frequency reuse systems. The transmitted/received signals undergo

energy loss as they pass through the free space, termed as free space loss.

Space segment

The space segment is the entire satellite system which is rotating around the earth in its geosta-

tionary orbit. It consists of the satellite which has two major subsystems--the antenna and the

communication repeater. The main function of the antenna is to provide shaped uplink and

downlink beams for reception and transmission of communication signals in the operating fre-

quency band.

5.4 MULTIPLE ACCESS TECHNIQUES

The term Multiple Access has been derived from the ability of the satellite to link all earth stations

simultaneously, thus providing point-to-multipoint communication. A satellite transponder can be

accessed by many earth stations and hence it is extremely essential to use an appropriate techniquefor allocating transponder capacity to each of them. For example, a transponder with a bandwidth

of 72 MHz may have the capacity equivalent to 120 mbps which can handle around 3562 voice

channels at 32 kbps, assuming 95 percent transponder efficiency. Therefore transponder capacity

must be wisely and systematically allocated to other earth stations. This is called multiple access.

The most commonly used multiple access technique are frequency division multiple access

(FDMA), time division multiple access (TDMA) and code division multiple access (CDMA).

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5.4.1 Frequency Division Multiple Access (FDMA)

This technique has been used since the inception of satellite communication and hence is the most

common of the various multiple access techniques. Each earth station is assigned one carrier with

a small guard band to avoid interference with the adjacent carrier. All such carriers are received by

the satellite trasponder and retransmitted back to earth. Modulation scheme could be either ana-

logue, i.e. frequency modulation (FM) or digital, i.e. phase shift keying (PSK). A major problem

in case of FDMA is the presence of intermodulation product due to amplification of multiple

carrier by the same TWTA in the satellite transponder which exhibits both amplitudes as well as

phase non-linearity. TWTAS are used to amplify low level downlink signals for transmission to

earth. To reduce the effect of downlink thermal noise, each carrier must be supplied with adequate

power. But this forces TWTA to go into saturation, consequently increasing the effect of intermo-dulation products. Therefore carrier to intermodulation ratio is an important parameter to be

determined to adjust the output power of TWTA in order to take care of the above problem.

FDMA has been making use of frequency modulations since the beginning and is still in vogue

despite advancement made in digital satellite technology.

5.4.2 Time Division Multiple Access (TDMA)

Transmission from satellite takes the form of a series of frames, usually 2 milliseconds in length.

Each frame is divided into as many as 35 slots. Assigned a particular slot within each frame the

second slot say or the fifth and seventh ground stations divide their messages into short signal

bursts, sending a burst every time the appropriate slot opens up on the systems transmission

schedule. A master station synchronizes the system by providing a reference burst at the beginning

of each frame. Because the satellite retransmits all incoming signals, ground station pick up trans-mission intended to them according to their assigned time slots. So satellites, operating at higher

frequencies, can effectively double their capacity bu using the same frequency for separate

transmission. They simply transmit the narrow beam signals in different directions. These systems

often employ a more complex techniques known as satellite-switched Time Division Multiplexing

Access (TDMA) to transmit messages between ground stations in different footprints. The trans-

mit timing of the bursts are synchronized such that overlapping does not occur. The satellite

receives one burst at a time and retransmits to earth after amplification so that the earth station can

extract the burst meant for it. The carrier modulation technique in TDMA is always digital modu-

lation.

TDMA System

Time division multiple access is a technique where a number of earth stations sharing the sametransponder transmit on a single carrier on a time-division basis. Each earth station transmits traf-

fic burst in a periodic time frame called TDMA frame.

Figure 5.4 shows the uplink of the TDMA simplified system. The concentric bands represent

transmission time slots assigned to three ground stations. Station 1 has the first and fourth periods,

Station 2 has the second, and Station 3 the third. By dividing their slots into subslots, each station

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Figure 5.4

TDMA

Uplink sys-

tem

can transmit to two separate receivers in the allotted period. Station 1, for example, has used each

of the slots to transmit to both Station 2 and Station 3. Slots are separated by brief "guard" times to

prevent transmission from overlapping.

Figure 5.5 shows the downlink of TDMA system. Receiving signals sent from terrestrial sta-

tions, the satellite retransmits them in the order in which they arrived. Here, the message sent up in

the first part of the first time slot (form Station 1 to Station 3) is the first to reach the earth.

Because ground stations are assigned time slots for receiving as well as for sending, each stationgets its messages by processing only specific moments of the satellites transmission. Uplink and

downlink transmission operate on different frequencies to avoid interfering with each other.

5.4.3 Code Division Multiple Access

The signals are encoded in such a fashion that the information from an individual transmitter can

be recovered only by the receiving station has its own coded address and the transmitting station

modulates its transmission with the address of the receiving station. CDMA transmission can

Guard Time

Station 1

Station 2

Station 3

SubslotTo2

To3

To3To2

To2

To1

To3

To1

Time Slot 1

Time Slot 2

Time Slot 3

Time Slot 4

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access a transponder on demand and hence may occupy the transponders entire bandwidth for

small intervals. This kind of transmission is more suited to military communication where mobile

stations may communicate among themselves for brief time periods.

5.4.4 Earth Stations

Earth station can be very inexpensive if they are intended only to receive satellite signals, thus

saving the cost of a transmitter. In the years 1970s, several companies began experimenting with

small, receive-only earth stations. A few years later, home owners began to receive buy receive-

only earth to intercept television signals relayed by satellite.

Figure 5.5

TDMAUplink sys-

tem

With the proliferation of small earth stations able to receive data from the Clarke belt, more

and more companies began offering information by satellite. Especially popular have been large,

Station 1

Station 2

Station 3

To 2

To 3

To 1

To 2

To 1

To 3

To 2

Transmission from Statin 1




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Figure 5.6

A PBX Local

Area Net-

work

74

12

36

85 9

0*

#

D

T

DCE

Host Computer

A

Terminal

Data VoiceSwitch

G

D

D

I

IVoice

Data

Data

Voice

Integrated

Voice/Data

D

Host Computer

DCE

DCE D

DCE D

DCE Remote Hosts

X.25 Network

DCE

DCE

Gateway

Mainframe

T1

Host Computer

T : Trunk InterfaceD : Data Interface

DCE : Data Communication Equipment

I : Integrated Interface A : Analog Interface

T1 : T1 at 1.544 MbpsG : Gateway

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Digital PABXs can serve as a local area network or may connect to a local area network. They

can also server as a gateway between a local area network and an external network, providing the

necessary protocol conversion. The terms digital PABX and digital PBX are used synonymously.

REVIEW QUESTIONS WITH ANSWERS

State True or False.

1. Twisted wire pair is effected by the electromagnetic interference.

2. Attenuation in a bounded media changes the power value at the receiving end.

3. Coaxial cable can be used for data rates over 10 Mbps and frequencies up to 400 MHz.

4. Single mode used in fiber optics does not have any dispersion problem.5. When the same number of channels are to be multiplexed for transmission, FDM always

requires a greater bandwidth than TDM.

Answers

1. True 2. True 3. True 4. True 5. False

Select the correct answer.

1. PCs in a computer communication networks are usually connected by:

(a) Telephone lines only (b) Satellite only

(c) Either satellite or telephone line or (d) None of the above

both2. The meaning of a digital channel means that the channel:

(a) is digitized (b) is carrying digital data

(c) accepts digital modulation tech- (d) None of the above

niques

3. Data networks for the efficiency of communication reasons, uses:

(a) Simplex transmission (b) Half-duplex transmission

(c) Full-duplex transmission (d) None of the above

4. Coaxial cables can be used for:

(a) Telephone networks only (b) Cable TV networks only

(c) Both in telephone and cable TV (d) None of the abovenetworks

5. Evesdropping is not possible in:

(a) UTP (b) STP

(c) Coaxial cable (d) Fiber optics

Answers

1. (c) 2. (b) 3. (c) 4. (c) 5. (d)

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