Poc

MULTIPLE ACCESS TECHNIQUES: There are different multiple access techniques. Some are: -TDMA -FDMA -CDMA -WDMA others are like satellite communication and etc.. TDMA: Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only the part of its bandwidth they require. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, and combat-net radio systems. FDMA: Frequency Division Multiple Access or FDMA is an access technology that is used by radio systems to share the radio spectrum. The terminology “multiple access” implies the sharing of the resource amongst users, and the “frequency division” describes how the sharing is done: by allocating users with different carrier frequencies of the radio spectrum. This technique relies upon sharing of the available radio spectrum by the communications signals that must pass through that spectrum. The terminology “multiple access” indicates how the the radio spectrum resource is intended to be used: by enabling more than one communications signal to pass within a particular band; and the “frequency division” indicates how the sharing is accomplished: by allocating individual frequencies for each communications signal within the band. In an FDMA scheme, the given Radio Frequency (RF) bandwidth is divided into adjacent frequency segments. Each segment is provided with bandwidth to enable an associated communications signal to pass through a transmission environment with an acceptable level of interference from communications signals in adjacent frequency segments.

CDMA: Code division multiple access (CDMA) is the current name for the cellular technology originally known as IS-95. This technology is in competition with GSM for leadership in the global cellular technology market. Developed by Qualcomm and enhanced by Ericsson, CDMA is characterized by high capacity and small cell radius, employing spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). By contrast, time division multiple access divides access by time, while frequency-division multiple access divides it by frequency. CDMA is a form of "spreadspectrum" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated. An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other. WDMA: In fibre-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to making it possible to perform bidirectional communications over one strand of fibre. "The true potential of optical fibre is fully exploited when multiple beams of light at different frequencies are transmitted on the same fibre. This is a form of frequency division multiplexing (FDM) but is commonly called wavelength division multiplexing."[1] The term wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency). However, since wavelength and frequency are inversely proportional, and since radio and light are both forms of electromagnetic radiation, the two terms are equal.

WIRELESS COMMUNICATION: The term wireless is normally used to refer to any type of electrical or electronic operation which is accomplished without the use of a "hard wired" connection. Wireless communication is the transfer of information over a distance without the use of electrical conductors or "wires".[1] The distances involved may be short (a few meters as in television remote control) or very long (thousands or even millions of kilometers for radio communications). When the context is clear the term is often simply shortened to "wireless". Wireless communications is generally considered to be a branch of telecommunications. The term wireless technology is generally used for mobile IT equipment. It encompasses cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of wireless technology include GPS units, garage door openers and or garage doors, wireless computer mice and keyboards, satellite television and cordless telephones.

TELECOMMUNICATION: Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process typically involves the sending of electromagnetic waves by electronic transmitters, but in earlier times telecommunication may have involved the use of smoke signals, drums or semaphore. Today, telecommunication is widespread and devices that assist the process, such as the television, radio and telephone, are common in many parts of the world. There are also many networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet is one of many examples of telecommunication. SMOKE SIGNALS: A smoke signal is a form of optical communication used over a long distance, developed both in the Americas and in China. By covering a fire with a blanket and quickly removing it, a puff of smoke can be generated. With some training, the sizes, shapes, and timing of these puffs can be controlled. Puffs may be observed from long distance, apparent to anyone within its visual range. With this in mind, signaling stations were often created to maximize the viewable distance. Stone bowls used by Native Americans and the towers of the Great Wall of China are examples of signaling stations. There is no standardized code for smoke signals; the signals are often of a predetermined pattern discerned by sender and receiver.[citation needed] Because of this, smoke signals tend to only convey simple messages, and are a limited form of communication.

Retrieved from "http://en.wikipedia.org/wiki/Smoke_signal" DRUMS: Developed and used by cultures living in forested areas, drums served as an early form of long distance communication, and were used during ceremonial and religious functions. In Africa, New Guinea and the tropical America, natives used drum telegraphy to communicate with each other from far away for centuries. When European expeditions came into the jungles to explore the primeval forest, they were surprised to find that the message of their coming and their intention was carried through the woods a step in advance of their arrival. Talking drums were also used in East Africa and are described by Andreus Bauer in the 'Street of Caravans' while acting as security guard in the Wissmann Truppe for the caravan of Charles Stokes. SEMAPHORE: The semaphore or optical telegraph is an apparatus for conveying information by means of visual signals, with towers with pivoting blades or paddles, shutters, in a matrix, or hand-held flags etc. Information is encoded by the position of the mechanical elements; it is read when the blade or flag is in a fixed position. In modern usage it refers to a system of signaling using two handheld flags. Other forms of optical telegraphy include ship flags, Aldis lamps, and Heliographs. Semaphore lines preceded the electrical telegraph. They were faster than post riders for bringing a message over long distances, but far more expensive and less private than the electrical telegraph lines which would replace them. The distance that an optical telegraph can bridge is limited by geography and weather, thus in practical use, most optical telegraphs used lines of relay stations to bridge longer distances. CELLULAR TELEPHONES: A mobile telephone or cellular telephone (commonly "mobile phone" or "cell phone") is a long-range, portable electronic device used for mobile communication. In addition to the standard voice function of a telephone, current mobile phones can support many additional services such as SMS for text messaging, email, packet switching for access to the Internet, and MMS for sending and receiving photos and video. Most current mobile phones connect to a cellular network of base stations (cell sites), which is in turn

interconnected to the public switched telephone network (PSTN) (the exception is satellite phones). PERSONAL DIGITAL ASSISTANTS: Personal digital assistants (PDAs) are handheld computers that were originally designed as personal organizers, but became much more versatile over the years. PDAs are also known as pocket computers or palmtop computers. PDAs have many uses: calculation, use as a clock and calendar, accessing the Internet, sending and receiving Emails, video recording, typewriting and word processing, use as an address book, making and writing on spreadsheets, scanning bar codes, use as a radio or stereo, playing computer games, recording survey responses, and Global Positioning System (GPS). Newer PDAs also have both color screens and audio capabilities, enabling them to be used as mobile phones (smartphones), web browsers, or portable media players. Many PDAs can access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). One of the most significant PDA characteristics is the presence of a touch screen. GPS: The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed/direction, and time.

GARAGE DOOR OPENER: The first garage door opener remote controls were simple and consisted of a simple transmitter (the remote) and receiver which controlled the opener mechanism. The transmitter would transmit on designated frequency; the receiver would listen for the radio signal, then open or close the garage, depending on the door position. The basic concept of this can be traced back to World War II. This type of system was used to detonate remote bombs. While novel at the time, the technology would run its course when garage door openers would become widely available and used. Then, not only did a person open their garage door, they opened their neighbor’s garage door as well. While the garage door remote is low in power and in range, it was powerful enough to interfere with other receivers in the area. The second stage of the wireless garage door opener system deals with the shared frequency problem. To rectify this, systems required a garage door owner to preset a digital code via dip switches on the receiver and transmitter. While these switches provided garage door systems with 28 = 256 different codes they were not really designed with security in mind, the main idea was to avoid interference with similar systems nearby.

The current garage door opener market uses a frequency spectrum range between 300400 MHz and most of the transmitter/receivers rely on hopping or rolling code technology. This approach prevents perpetrators from recording a code and replaying it to open a garage door. Since the signal is supposed to be significantly different from that of any other garage door remote control, manufacturers claim it is impossible for someone other than the owner of the remote to open the garage. When the transmitter sends a code, it generates a new code using an encoder. The receiver, after receiving a correct code, uses the same encoder with the same original seed to generate a new code that it will accept in the future. Because there is a high probability that someone might accidentally push the open button while not in range and desynchronize the code, the transmitter and receiver generate look-a-head codes ahead of time. SATELLITE TELEVISION: Satellites used for television signals are generally in either highly elliptical (with inclination of +/-63.4 degrees and orbital period of about 12 hours) or geostationary orbit 37,000 km (22,300 miles) above the earth’s equator. Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility. Uplink satellite dishes are very large, as much as 9 to 12 meters (30 to 40 feet) in diameter. The increased diameter results in more accurate aiming and increased signal strength at the satellite. The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite. The transponder 'retransmits' the signals back to Earth but at a different frequency band (to avoid interference with the uplink signal), typically in the Cband (4–8 GHz) or Ku-band (12–18 GHz) or both. The leg of the signal path from the satellite to the receiving Earth station is called the downlink. A typical satellite has up to 32 transponders for Ku-band and up to 24 for a C-band only satellite, or more for hybrid satellites. Typical transponders each have a bandwidth between 27 MHz and 50 MHz. Each geo-stationary C-band satellite needs to be spaced 2 degrees from the next satellite (to avoid interference). For Ku the spacing can be 1 degree. This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites and 360/1 = 360 geostationary Ku-band satellites. C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected by rain (as water is an excellent absorber of microwaves). The downlinked satellite signal, quite weak after traveling the great distance (see inversesquare law), is collected by a parabolic receiving dish, which reflects the weak signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the flared front-end of a section of waveguide that gathers the signals at or near the focal point and 'conducts' them to a probe or pickup connected to a low-noise block downconverter or LNB. The LNB amplifies the relatively weak signals, filters the block of frequencies in which the satellite TV signals are

transmitted, and converts the block of frequencies to a lower frequency range in the Lband range. The evolution of LNBs was one of necessity and invention. The original C-Band satellite TV systems used a Low Noise Amplifier connected to the feedhorn at the focal point of the dish. The amplified signal was then fed via very expensive 50 Ohm impedance coaxial cable to an indoor receiver or in other designs fed to a downconverter (a mixer and a voltage tuned oscillator with some filter circuitry) for downconversion to an intermediate frequency. The channel selection was controlled, typically by a voltage tuned oscillator with the tuning voltage being fed via a separate cable to the headend. But this simple design evolved. Designs for microstrip based converters for Amateur Radio frequencies were adapted for the 4 GHz C-Band. Central to these designs was concept of block downconversion of a range of frequencies to a lower, and technologically more easily handled block of frequencies (intermediate frequency). The advantages of using an LNB are that cheaper cable could be used to connect the indoor receiver with the satellite TV dish and LNB, and that the technology for handling the signal at L-Band and UHF was far cheaper than that for handling the signal at C-Band frequencies. The shift to cheaper technology from the 50 Ohm impedance cable and NConnectors of the early C-Band systems to the cheaper 75 Ohm technology and FConnectors allowed the early satellite TV receivers to use, what were in reality, modified UHF TV tuners which selected the satellite television channel for down conversion to another lower intermediate frequency centered on 70 MHz where it was demodulated. This shift allowed the satellite television DTH industry to change from being a largely hobbyist one where receivers were built in low numbers and complete systems were expensive (costing thousands of Dollars) to a far more commercial one of mass production. WHY WIRELESS TECHNOLOGY?? The following situations justify the use of wireless technology: • • • • • •

To span a distance beyond the capabilities of typical cabling, To avoid obstacles such as physical structures, EMI, or RFI, To provide a backup communications link in case of normal network failure, To link portable or temporary workstations, To overcome situations where normal cabling is difficult or financially impractical, or To remotely connect mobile users or networks.

ENVIRONMENTAL CONCERNS AND HEALTH HAZARD: In recent time there are concerns and research linking usage of wireless communications with poor concentration, memory loss, nausea, premature senility and even cancer.