What is Telecommunication and network?

What is Telecommunication?

Telecommunication is a process of transmitting or receiving information over a distance by any electrical or electromagnetic medium. It enables to pass information to one or more given correspondents, or possible correspondents.

Components of Telecom System:-

1. Audio Signal.

2. Carrier Signal.

3. Modulator.

4. Transmitter.

5. Transmission Media.

6. Receiver.

7.Demodulator.

8. Received Audio Signal.

Telecommunication Transmission Types:-

1. Asynchronous Transmission.

2.Plesiochronous Transmission.

3. Synchronous Transmission.

Asynchronous Transmission:-

In" Telecommunication" asynchronous transmission is transmission of data in which time intervals between transmitted characters may be of  unequal length. Transmission is controlled by starting and stopping bits at the beginning and end of each character.

Transition of signals do not occur at the same nominal rate generally free running quartz derive the clock, there is no timing pulses sent from transmitter to receiver.

Plesiochronous Transmission:-

In "Telecom system"Plesiochronous is a Greek word meaning Almost Synchronous, but not fully synchronous. The digital transition in the signals occur at almost the same rate. There may be a phase difference between the transition of the two signals, and this would lie on specified limits.

Synchronous Transmission:-

The digital transition within the signals occur at precisely the same rate in "Telecommunication System". there'll be no part distinction between the transition of the two signals, and this may lie on mere limits.

In a synchronous network, all the clocks are traceable to 1 primary reference clock. The accuracy of the PRC is derived from a cesium atomic standard.

Types of Signals:-

1. Analog Signal.

2. Digital Signal.

Analog Signal:-

An analog signal is a continuous electrical signal that varies continuously in amplitude and frequency. Analog signals can have infinite number of amplitude values or states within a specified range. In an analog system it is difficult to remove noise and wave distortions during the transmission. For this reason, analog signals cannot perform high-quality data transmission in transmission media.

Digital Signal:-

Digital Signal is an electrical signal, which possesses two distinct states,on/off or positive/negative. The most fundamental and widely used form of digital signals are binary signal, in which one amplitude condition represent a binary digit 1,and another amplitude condition represents a binary digit 0. Noise and distortions have little effect, making high-quality data transmission possible.

What is multiplexing and its types:-

It is the process of combining a number of individual channels into a common frequency band or into a common bit stream for transmission.

Different  multiplexing techniques are:-

1. Frequency Division Multiplexing.

2. Time Division Multiplexing.

3. Wavelength Division Multiplexing.

Frequency Division Multiplexing (FDM)

The FDM techniques is the process of translating individual speech circuits (300 - 3400 Hz) into pre-assigned frequency slots within the bandwidth of the transmission medium.The frequency translation is done by amplitude modulation of the audio frequency with an appropriate carrier frequency.At the output of the modulator a filter network is connected to select either a lower or an upper side band since the intelligence is carried in either side band single side band suppressed carrier mode of AM is employed.This ends up in substantial saving of bandwidth mid also permits the utilization of low power amplifiers.FDM Technology usually find their application in analogue transmission systems.An analogue transmission system is one which is used for transmitting continuously varying signals.

Time Division Multiplexing (TDM)

Basically Time division multiplexing involves nothing quite sharing.A transmission medium by variety of circuits in time domain by establishing a sequence of time slots during which individual channels (circuits) are often transmitted.Thus the entire bandwidth is periodically available to each channel.Normally all time slots are equal in length.Each channel is assigned a time slot with a specific common repetition period called a frame interval.

Each channel is sampled at a specified rate and transmitted for a fixed duration.All channels are sampled one by the cycle is repeated once more and once more.The channels are connected to individual gates that opened one by during a fastened sequence.At the receiving end  also similar gates are opened in unision with the gates at the transmitting end. The signal received at the receiving  finish are going to be within the style of distinct samples and these are combined to reproduce the original signal.Thus at a given instant of time ,only one channel is transmitted through the medium and by  sequent sampling variety of  channels  are often staggered in time as  against sending all the channel at constant time as in EDM systems.This staggering of channels in time sequence for transmission over a typical medium is named Time Division Multiplexing.

Pulse Code Modulation (PCM) system.

It was only in 1938  USA developed a Pluse Code Modulation system to transmit the spoken word in digital form.since then didgital speech transmission has become  an alternative to the analogue systems.PCM systems use TDM technique to provide a number of circuits on the same transmission medium via open wire or underground cable pair or a channel provided by carrier ,coaxial , microwave or satellite system.

Necessities for PCM system

To develop a PCM signal from multiple analogue signals the following processing steps are follows and given below.

• Filtering.
• Sampling.
• Quantization.
• Encoding.
• Line Coding.

Wavelength Division Multiplexing(WDM)

WDM (Wavelength Division Multiplex) Technology is a very effective means of increasing the transmission of Fiber ,as it demands neither the installation of new links, nor any increasing in transmission speed.The data are transmitted along the fiber at different wavelengths each waves-length  or channel transmitting a signal.The Channels are defined according to the G-692 recommendations of the ITU-T.

This Technology demands new measurements to check the parameters.The most important item of equipment for carrying out these tests on WDM system is the Optical Spectrum Analyzer(OSA).It can be connected at critical measurement points in the WDM system to the ends of the links or to the amplifier location.

Types of Transmission medium:-

1.Twisted paire(point to point).

----telephone lines.

----low data rate.

----high EMI.

----lossy at Rf.

2. Coaxial Cable(point to pint).

----LAN.

----data rate few Mbps.

----low EMI.

----moderate loss.

3.Micro wave link(point to point).

----long distance.

----large B/W.

----Line of sight.

----high free space loss.

4.Satellite communication(point to multipoint).

----larger B/W.

---data monitoring.

---large delay.

---mobile enviroment.

---moderate life.
5.Fiber Optical Cable
---Larger B/W.
---Avoid EMI.
---Low loss in media.
---Light weight.

Coaxial Cable Transmission

In bands up to 2GHz coaxial cable is usually used and except for very short runs it is usually of the air dielectric type.typical size are 2.2cm diameter andrew type HJ5-5 with attenuation of about 6dB per 100 meters at 2GHz and 4.1 cm diameter andrew type HJ7-50 with an attenuation of about 3dB per 1 meters.In any coaxial cable system where VSWR is important the number of connections should be kept to a minimum.However at the equipment end it may be necessary to reduce the large size cable if used to the smaller size cable with a suitable runs with suitable transitions for flexibility in connection to the radio equipment.

Satellite versus Fiber optics:-

1. Point to multipoint   1.Point to point.

2. B/w GHz                    2. B/w THz.

3. Maintenance free      3. Needs maintenance.

4. Short life 7~8 yr        4. Long life.

5. No Upgrade ability   5.Upgradable.

6.Mobile,air,sea             6.On ground only.

Two will co exist due their complementary nature.

Advantages of Satellite Communications

1. Large Coverage: Almost one-third of the earth with exception of  polar  regions is visible from geostationary orbit.it is thus possible to cover about 10,000 Kms distance irrespective of intervening terrain  with a single satellite.
2. High quality:Satellite links can be designed for high quality performance.The link performance is highly stable since it is free from ionospheric disturbances multipath effects or fading.
3. High reliability:Reliability is high since there is only one repeater in the link.
4. High capacity:With microwave frequencies wide bandwidths are available and large communication capacity can be obtained.
5. Flexibility:In a terrestrial system communication is tied down to the links installed on the other hand satellite communication is well suited for changing traffic requirements locations and channel capacities.
6. Speed of installation:Installation of earth terminals can be achieved in a short time as compared to laying of cables or radio relay links.
7. Mobile,short term or emergency communications:With ariliftable or road transportable terminals short term or emergency communications can be quickly provided.Reliable long distance land mobile,maritime mobile and aeronautical mobile services are feasible only by means of satellite.
8. Satellite communication is ideally suited for point to multipoint transmission on broadcasting over large areas.
9. All types of common services are possible.

Two Important Elements of a satellite communication network are

• Space Segment.
• Ground Segment.

Uplinks

Uplinks is the radio path from Ground Segment i.e earth station to the Space segment.

Downlinks

Downlink is the radio path from space segment i.e satellite to the ground segment.

Time Delay

The total earth-satellite-earth path length may be as much as 74000 Km thus giving  a one way propagation delay of 250ms.The effect of this delay on telephone conversations where a 500ms gap can arise between one person asking question and hearing the other person reply has been widely investigated and was found to be less of a problem than had been anticipated with geostationary satellites two hop operation sometimes unavoidable and gives rise to a delay of over one second.

Satellite communication systems classify that

(a)Communication system

1.Multiplexed telephone channels with one

• Carrier frequency system.
• Single channel pre carrier system.

(b)Modulation System

• Analog modulation (frequency modulation system) and
• Digital modulation system.

(c) System configuration

• Pre-assignment system
• Demand assignment system
• Various other sytems combined with those above.

Microwaves

Microwaves are very short frequency radio waves that have many of the characteristics of light wave in that they travel in line-of-sight paths and can be reflected boomed and focused.By focusing these ultra high radio waves into a narrow beam,their energies are concentrated and relatively low transmitting power is required for reliable transmission over long distance.
Microwave communication systems are used to carry telephony,television and data signals.Majority of the systems , however carry multi-channel telephone signals.The spectrum of the multichannel telephone signal is also called base band.System with capacities up to 300 channels is called narrow band system and the system providing more than 300 channels are called wide band system.Microwave system used to provide communication on major trunk routes with high traffic density and serving long distances are classified as long haul m/w system.2 , 4 and 6 GHz systems are long haul systems.Systems used to provide communication over short distances for trunk routes with light traffic density are classified as short haul system. 7 and 811 GHz systems are short haul systems.