In telecommunications, wireless communication may be used to transfer information over short distances or long distance. The term is often shortened to “wireless”. it encompasses various types of fixed , mobile, cellular telephones etc. now -a – days people who need to be online all the time. For these mobile users, twisted pair, coax, and fiber optics are of no use. They need to get their hits of data for their laptop, notebooks, shirt pocket or wrist watch computers without being tethered to the terrestrial communication infrastructure. For these users, wireless communication is the answer.
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THE ELECTROMAGNETIC SPECTRUM
Wireless transmission can provide special services and conveniences to people, such as connection to the internet or other networks without connecting to wire directly. It can also facilitate the creation of networks in special situations, such as terrain that is unfriendly to ground cables. Wireless communication began, in fact, in the Hawaiian island precisely the problem of terrain separated by large stretched of ocean. Political organizations determine which sections of the spectrum of light are all to which wireless transmission purpose. Because the existing organizations are always in agreement with current practice or with each other, some products manufactured for one country may not work in another.
Radio waves are easy to generate and are omnidirectional, but have low transmission rates. Also, depending on their frequency, radio waves either cannot travel very far, or are absorbed by the earth. In some cases, though, high frequency waves are reflected back to earth by the ionosphere. Ionosphere is a layer of the atmosphere.
Microwave transmission is popular for its ability to travel in straight lines. A source can be directly focused on its destination without interfering with neighboring transmissions. Because they travel in straight lines, though, the curvature of the earth can interfere with the microwave transmitters. The solution to this is the addition of repeaters in between the source and destination to redirect the data path. Microwaves are used for long distance communication like cellular phones, garage door openers and so on..
INFRARED AND MILLIMETER WAVES
Infrared light is used for close- range communication, such as remote controls, because if does not pass through objects well. This is also a plus because infrared communications in one room de not interfere with the infrared communications in another room. Infrared communication is more secure than other option, such as radio, but it cannot be used outside due to interference by the sun.
Lasers can be used for wireless communication. It is a relatively low cost way to connect two buildings LAN, but it has drawbacks. The laser is difficult to target on the destinations receiver because the beam is so small. Laser light also diffuses easily in poor atmospheric conditions like rain, fog and so on.
TERMS OF WIRELESS TRANSMISSION
The theoretical basis for data communication
The telephone system
Broadband ISDN and ATM
THE THEORETICAL BASIS FOR DATA COMMUNICATION
An infinite series, such as a Fourier series, is just the addition of an infinite number of terms. Each term is called a harmonic.
Signals are used to send data over a write. If we want to indicate a data change, there is a corresponding signal change on the wire. The maximum possible number of signal changes per second is called the baud.
A periodic function is one that repeats itself over time. Sine and cosine are periodic functions. Fourier proved that any reasonably behaved periodic function could be written as a sum of sine and cosine functions. This is important because sine and cosine are easily represented and recreated. The Fourier series allows periodic signals to be sent over a wire.
A voice-grade line has certain restrictions that limits the maximum number of signal changes per second.
Signal-To-Noise Ratio Decibels
A way to measure to measure the thermal noise that is present on a wire.
Here more than ten terms are there so I am going to explain five terms:
Material on which to store data. It is used in diskettes and magnetic tape. Magnetic media is a common way to transport data quickly.
The oldest and still most common transmission medium. Two insulated copper wires. Twisting the wires reduces electrical interferences from nearby wires. It also counters the antenna effect caused by parallel wires.
In a dual cable system, there are two cables which only transmit data in one direction each. At one end of the network, data is collection from one line for re-transmission on the line going the opposite direction. The end of the network responsible for the data collection and retransmission is called the head-end.
When two cables are not used, but there is need for simulation of dual cable system, bandwidth on a single cable can be split up, with one portion representing one cable, and the other portion representing the second cable. Splitting the frequencies so that the lower frequencies are used for one purpose and the higher for another is called a subsplit system.
Characteristic of a light wave associated with the reflection of the wave through the silica fiber.
Light and electricity travel in the form of waves. Waves are periodic in that they repeat themselves, so the frequency of a wave is just the number of times the wave repeats itself in one second.
The unit of measure of frequency. The name comes from the german physicist who first produced electromagnetic waves.
Wavelength is the measure of the length of a wave that is it is used ti find the distance between two consecutive maxima or minima.
Speed Of Light (C)
The speed that electromagnetic waves travel in a vacuum approximately 3*108 m/sec. electromagnetic waves do not travel as fast through a medium as they do through a vacuum.
A pattern for EM transmission which is popular in the military for its ability to avoid jamming. The transmitter hops from frequency to frequency across a wide frequency band.
THE TELEPHONE SYSTEM
In this there are more than ten terms so here I am going to explain six terms
Public Switched Telephone Network
The currently existing network used for telephone communication. It was designed specifically for voice, and is not well suited for use by computers for transmitting data to one another.
The place to which your telephones lines connect to receive the telephone service. The distance from the telephone to the end office is usually small around 1 to 10km).
Switching centers that connect several end offices.
Similar to the toll office, except it connects the end offices that are within the same local area.
Toll Connecting Trunks
The hardware which connects the end office to the toll of offices.
Integrated Services Digital Network
Fully digital, circuit switched telephone system that is designed to accommodate both voice and data services.
Digital Bit Pipe
A bi-directional logical pipe which will pass bits on from source to destination without concern about whether they are voice.
Device placed between the user and the nearest end office allows ISDN connection.
Plain Old Telephone Exchange or NT2
Device placed between the end user which can provide a variety of ISDN services.
ISDN services on 64-kbps channels.
BROADBAND ISDN AND ATM
It is more recent design for a digital network on which data can travel rates of 155Mbps. It is based on ATM technology.
Permanent Virtual Circuits
Virtual circuits that remains in place for extended periods of time.
Switched Virtual Circuits
Virtual circuits that remains in place for short periods of time, typically related to the session time.
If a questionable cell tries to come into ATM switch, it will be stalled, effectively stalling the cells behind it unfairly. This is called head-of -line blocking.
To solve the head-of-line blocking problem, queuing on the output side has been proposed. The knockout switch does just this, by stimulating a single output queue by having several output queues that are activated on a round- robin type basis.
THE THEORETICAL BASIS FOR DATA COMMUNICATION
The baud rate is NOT the same as the data rate. Baud tells how many signals are sent per second, but there are ways of encoding more than one bit per signal change. AVOID getting this confused.
Baud= number of signal changes/second
Maximum Data Rate Of A Channel (Nyquist’s Theorem)
Max data rate=2 Hlog2 V
H=bandwidth in HZ
This formula shows the maximum number of bits that can be sent per second on a data line with a bandwidth of H, is V bits are sent per signal. The max data rate should be in bits per second.
This formula is used to quantify the quality of a line. It is not usually presented as a ratio, but instead it is given in the units decibels.
Maximum Number of Bits Per Second (Shannon’s Result)
Max number of bits per second=H log2 (1+(S/N))
This formula shows the maximum achievable data rate on a noisy line. The difference between shannon’s result and Nyquist’s theorem is that Shannon’s result takes the noise on a line into consideration. The noise can drastically reduce a line’s capacity to send data. It is measured in bits per second.
10 log10 (transmitted power/ received power)
Attenuation is a way to measure the amount of power lost in a signal’s strength from when it was sent to when it was received. Attenuation is measured in decibels.
This formula is used to find the relationship between frequency, wavelength, and the speed of light.
C=speed of light
THE TELEPHONE SYSTEM
This formula is used to fine the bits per second.
Bits per second= number of bits per signal change*baud.
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