Current is a flow of electrical charge carriers, usually electrons. The common symbol  for current is the uppercase letter I. The standard unit is the ampere, symbolized by A.

 

One ampere of current represents one coulomb of electrical charge (6.24 x 10¹⁸   charge carriers) moving past a specific point in one second.

 

Physicists consider, Current to flow from relatively positive points to relatively negative points; this is   called conventional current or Franklin current.

 

Electrons, the most common charge carriers, are negatively charged. They flow from relatively negative points to relatively positive points.

 

Electric current can be either direct or alternating. Direct current (DC) flows in the same direction at all points in time, although the instantaneous magnitude of the current might vary.

 

In an alternating current (AC), the flow of charge carriers, reverses direction periodically. The number of complete AC cycles per second is the frequency, which is measured in hertz.

 

Current per unit cross-sectional area is known as current density. It is expressed in amperes per square meter.

 

In general, the current in a conductor have higher the current density. However, in some situations, current density varies in different parts of an electrical conductor.

 

An electric current always produces a magnetic field. The stronger current, have more intense the magnetic field. A pulsating DC, or an AC, characteristically produces an electromagnetic field. This is the principle by which wireless signal propagation occurs.

 

What is alternating Current (AC)?

In electricity, alternating current (AC) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement.

 

Household utility current in most countries is AC with a frequency of 60 hertz (60 complete cycles per second), although in some countries it is 50 Hz.

 

The radio-frequency (RF) current in antennas and transmission lines is an example of AC.

An AC waveform can be sinusoidal, square, or saw tooth-shaped, Some AC waveforms are irregular or complicated.

An example of sine-wave AC is common household utility current (in the ideal case). Square or saw tooth waves are produced by certain types of electronic oscillators.

The voltage of an AC power source can be easily changed by means of a power transformer. This allows the voltage to be stepped up (increased) for transmission and distribution. High-voltage transmission is more efficient than low-voltage transmission over long distances, because the loss caused by conductor resistance decreases as the voltage increases.

The voltage of an AC power source changes from instant to instant in time. The effective voltage of an AC utility power source is usually considered to be the DC voltage that would produce the same power dissipation as heat assuming a pure resistance. The effective voltage for a sine wave is not the same as the peak voltage.

To obtain effective voltage from peak voltage, multiply by 0.707.

To obtain peak voltage from effective voltage, multiply by 1.414.

For example, if an AC power source has an effective voltage of 117 V, typical of a household in the United States, the peak voltage is 165 V.

What is direct Current (DC)?

DC (direct current) is the unidirectional flow of electric charge carriers (which are usually electrons).

The intensity of the current can vary with time, but the general direction of movement stays the same at all times. As an adjective, the term DC is used in reference to voltage whose polarity never reverses.

In a DC circuit, electrons emerge from the negative, or minus, pole and move towards the positive, or plus, pole. Nevertheless, physicists define DC as traveling from plus to minus.

Direct current is produced by electrochemical and photovoltaic cells and batteries. In contrast, the electricity available from utility mains in most countries is AC (alternating current).

Utility AC can be converted to DC by means of a power supply consisting of a transformer, a rectifier (which prevents the flow of current from reversing), and a filter (which eliminates current pulsations in the output of the rectifier).

Virtually all electronic and computer hardware needs DC to function. Most solid-state equipment requires between 1.5 and 13.5 volts.

 Current demands can range from practically zero for an electronic wristwatch to more than 100 amperes for a radio communications power amplifier.

Equipment using vacuum tubes, such as a high-power radio or television broadcast transmitter or a CRT (cathode-ray tube) display, require from about 150 volts to several thousand volts DC.

 

 

 

 

3G Introduction

The Third Generation (3G) will be digital mobile multimedia offering broadband mobile communications with voice, video, and graphics, audio and other information.

The mobile communications industry has evolved in three stages, and correspondingly three generations of mobile phones have emerged thus far. Each one has provided more flexibility and usability than the previous ones.

• (1G) Analog: Analog phones helped to make voice calls inside one’s country without roaming facilities.
• (2G) Digital mobile phone systems added fax, data and messaging capabilities as well as voice telephone service in many countries offering worldwide roaming.
• (3G) Multimedia services add high speed data transfer to mobile devices, allowing new video, audio and other applications (including Internet services) through mobile phones.

During the first and second generations, different regions of the world pursued different mobile phone standards, such as NMT and TACS for analog and GSM for digital, North America pursued AMPS for analog and a mix of TDMA, CDMA and GSM for digital.

3G, based on CDMA technology, will bring these incompatible standards together.

3G Features

1.   With 3G, the information is split into separate but related packets before being transmitted and reassembled at the receiving end. Packet switched data formats are much more common than their circuit switched counterparts.

2.   The World Wide Web (WWW) is becoming the primary communications interface. People access the Internet for entertainment, services, and information collection, the intranet for accessing enterprise information and connecting with colleagues and the extranet for accessing customers and suppliers. These are all derivatives of the World Wide Web aimed at connecting different communities of interest. Information and other resources are being stored in remote Web servers, which serves the various needs of human beings through Web browsers at their ends.

3.  Speeds of up to 2 Megabits per second (Mbps) are achievable with 3G. The data transmission rates will depend upon the environment, the call is being made in, however, only indoors and in stationary environments that these types of data rates will be available. For high mobility, data rates of 144 kbps are expected to be available

3G  Applications

3G facilitates several new applications that have not previously been readily available over mobile networks due to the limitations in data transmission speeds. These applications range from Web Browsing to file transfer to Home Automation (the ability to remotely access and control in-house appliances and machines). Because of the bandwidth increase, these applications will be even more easily available with 3G than they were previously with interim technologies.

3G Services Access

To use 3G, users specifically need:

• A 3G Mobile Phone
• A subscription to a mobile telephone network that supports 3G,
• Use of 3G must be enabled for that user. Automatic access to the 3G may be allowed by some mobile network operators, others will charge a monthly subscription and require a specific opt-in to use the service as they do with other non-voice mobile services,
• Knowledge of how to send and/ or receive 3G information using their specific model of mobile phone, including software and hardware configuration (this creates a customer service requirement),
• A destination to send or receive information through 3G. From day one, 3G users can access any web page or other Internet applications - providing an immediate critical mass of users

Conclusion

The telecommunications world is changing as the trends of media convergence; industry consolidation, Internet and IP technologies and mobile communications collide into one. Significant change will be bought about by this rapid evolution in technology with the arrival of mobile Internet technology.

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