Cell-phone Frequencies

In the dark ages before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit 40 or 50 miles (about 70 km). It also meant that not many people could use radio telephones -- there just were not enough channels.
The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.
A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.
Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once.
Channels - A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more!
Range - A walkie-talkie can transmit about 1 mile (1.6 km) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 km) using a 5-watt transmitter. Cell phones operate within cells, and they can switch cells as they move around. Cells give cell phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.
In half-duplex radio, both transmitters use the same frequency. Only one party can talk at a time.
In full-duplex radio, the two transmitters use different frequencies, so both parties can talk at the same time.Cell phones are full-duplex.

How Cell Phones Work

Cell Phone Image GalleryNokiaThe internal display of the Nokia 6555 has more colors than you can actually see at once. See more cell phone pictures.Millions of people in the United States and around the world use cellular phones. They are such great gadgets -- with a cell phone, you can talk to anyone on the planet from just about anywhere!
These days, cell phones provide an incredible array of functions, and new ones are being added at a breakneck pace. Depending on the cell-phone model, you can:
Store contact information
Make task or to-do lists
Keep track of appointments and set reminders
Use the built-in calculator for simple math
Send or receive e-mail
Get information (news, entertainment, stock quotes) from the Internet
Play games
Watch TV
Send text messages
Integrate other devices such as PDAs, MP3 players and GPS receivers Explore Cell PhonesiPhoneCell Phone Quiz10 Popular PhonesDiscovery.com: Cell Phone Users Tracked -->

Use of Mobile Phone in Schools

Some schools limit or restrict the use of mobile phones. Schools set restrictions on the use of mobile phones because of the use of cell phones for cheating on tests, harassing other people, causing threats to the schools security, and facilitating gossip and other social activity in school. Many mobile phones are banned in school locker room facilities and in public restrooms. New camera phones are required to have a shutter effect when a photo is taken.[citation needed]

Mobile phones and driving safety

Mobile phone use while driving is common but controversial. Being distracted while operating a motor vehicle has been shown to increase the risk of accident. Because of this, many governments have made the use of a mobile phone while driving illegal. Israel, Japan, Portugal and Singapore ban both hand-held and hands-free use of a mobile phone whilst many other countries ban hand-held phone use only.

Software and Applications of Cell Phone

The most commonly used data application on mobile phones is SMS text messaging, with 74% of all mobile phone users as active users (over 2.4 billion out of 3.3 billion total subscribers at the end of 2007). SMS text messaging was worth over 100 billion dollars in annual revenues in 2007 and the worldwide average of messaging use is 2.6 SMS sent per day per person across the whole mobile phone subscriber base (source Informa 2007). The first SMS text message was sent from a computer to a mobile phone in 1992 in the UK, while the first person-to-person SMS from phone to phone was sent in Finland in 1993.
The other non-SMS data services used by mobile phones were worth 31 Billion dollars in 2007, and were led by mobile music, downloadable logos and pictures, gaming, gambling, adult entertainment and advertising (source: Informa 2007). The first downloadable mobile content was sold to a mobile phone in Finland in 1998, when Radiolinja (now Elisa) introduced the downloadable ringing tone service. In 1999 Japanese mobile operator NTT DoCoMo introduced its mobile internet service, i-Mode, which today is the world's largest mobile internet service and roughly the same size as Google in annual revenues.
The first mobile news service, delivered via SMS, was launched in Finland in 2000. Mobile news services are expanding with many organisations providing "on-demand" news services by SMS. Some also provide "instant" news pushed out by SMS. Mobile telephony also facilitates activism and public journalism being explored by Reuters and Yahoo![14] and small independent news companies such as Jasmine News in Sri Lanka.

Handsets

There are several categories of mobile phones, from basic phones to feature phones such as musicphones and cameraphones, to smartphones. The first smartphone was the Nokia 9000 Communicator in 1996 which incorporated PDA functionality to the basic mobile phone at the time. As miniaturisation and increased processing power of microchips has enabled ever more features to be added to phones, the concept of the smartphone has evolved, and what was a high-end smartphone five years ago, is a standard phone today. Several phone series have been introduced to address a given market segment, such as the RIM BlackBerry focusing on enterprise/corporate customer email needs; the SonyEricsson Walkman series of musicphones and Cybershot series of cameraphones; the Nokia N-Series of multimedia phones; and the Apple iPhone which provides full-featured web access and multimedia capabilities.

History of mobile phones

This history of mobile phones chronicles the development of radio telephone technology from two-way radios in vehicles to handheld cellular communicating devices.
In the beginning, two-way radios (known as mobile rigs) were used in vehicles such as taxicabs, police cruisers, ambulances, and the like, but were not mobile phones because they were not normally connected to the telephone network. Users could not dial phone numbers from their vehicles. A large community of mobile radio users, known as the mobileers, popularized the technology that would eventually give way to the mobile phone. Originally, mobile phones were permanently installed in vehicles, but later versions such as the so-called transportables or "bag phones" were equipped with a cigarette lighter plug so that they could also be carried, and thus could be used as either mobile or as portable two-way radios. During the early 1940s, Motorola developed a backpacked two-way radio, the Walkie-Talkie and later developed a large hand-held two-way radio for the US military. This battery powered "Handie-Talkie" (HT) was about the size of a man's forearm.
In Europe, radio telephony was first used on the first-class passenger trains between Berlin and Hamburg in 1926. At the same time, radio telephony was introduced on passenger airplanes for air traffic security. Later radio telephony was introduced on a large scale in German tanks during the Second World War. After the war German police in the British zone of occupation first used disused tank telephony equipment to run the first radio patrol cars.[citation needed] In all of these cases the service was confined to specialists that were trained to use the equipment. In the early 1950s ships on the Rhine were among the first to use radio telephony with an untrained end customer as a user.

Recording units connected to handset wire

Some commercial telephone recording devices are connected between telephone base part and handset. Those devices are small boxed which are placed between telephone ant the modular wire coming from telephone base part to handset. The telephone conversation can be taken from that wire, because it is carrying the microphone and speaker signas which are connected to base part of the telephone. The signals going in this wire between base part and handset are no fully standardized, which means problems to those who are trying to make devices which connect to this interface. But this interface has the advantage that it can be also used with those digital telephones, where recording from line wires is not possible.
Typically the handset of the telephone is connected to the base of the telephone using 4 wire connector. Two of those wires carry the signals to the handset spaker and two other wires on the other side of the connector carry the microphone signals. The standard handser wiring used in telephone in USA has Green and White leads connected to earpiece and Black and Red wires are connected to microphone.
There are differences on the signal levels used by deferent telephones, so typically the equipment which connect to handset connector have some adjustments to make them suit to many telephone types. Well adjusted recording interface which is connected to handset interface gives good balance between the local voice and voice from distant end (much better than recording devices which connect to telephone line).

Telephone line current sensing method

The most straightforward way of adding telephone controlling function to telephone recording unit is to add a relay which senses the telephone line operation current and controls the recorder. That relay coil is simply connected in series with telephone in on of the wires going to telephone. The basic idea is very easy and works reliably. All you have to find is suitable relay, which will pull at normal telephone operation current, can handle the maximum line current, has good enough insulation to withstand telephone line overvoltages and low enough resistamce to not to disturb normal telephone operation. The relay should also be selected so that it does not trip at normal telephone ring currents. There is usually a few microfarad capacitor (usually bibolar electrolytic) in parallel with relay coil to allow audio to pass easily the high inductance relay coil. One commercial circuit used a relay with 150 ohm coil resistance and there was 47 uF 25V bipolar electrolytic capacitor in parallel with relay coil. Here is a schematic of one simple telephone controlling circuit: To <----------------------+
Tape <==== Relay Contacts
Recorder <------------------o/ o
mmmmm DC 3v 20 mA Relay Coil

Line >------------------------+---+----------> To Phone
47 uF Capacitor
Line >-----------------------------------------> To Phone

Recording of telephone conversations

Sometimes it is useful to record telephone conversations. The easiest way is to go to a shop and buy a telephone conversation recording adapter which is connected between telephone line and the tape recorder. Those recording adapters are usually connected between telephone line and the telephone, but there are also other type available.
The adapter has two functions: it connects the audio from telephone line to recorder input at correct leveal and it isolates the recorder electrically from telephone line. The isolation is very important part of the adapter, because connecting tape recorder directly to the telephone line is against telephone regulations, causes all kinds of noises to telephone line and will damage your recorder. Some adapters have also tape recorder controlling function enabling recorder to start automatically when phone is picked up and stop when phone conversation ends.

Main parts of Telephone

A telephone has two main parts to it, one is the 'Transmitter' and the other is the 'Receiver'. The transmitter is placed behind the mouthpiece of the instrument. It has an eardrum which is thin, round metal disc called a diaphragm. This diaphragm vibrates when the sound waves strike it. This vibration takes place at different speed levels. This depends on the air pressure caused by the voice. Behind this metal disk there is a container filled with tiny grains of carbon. Subsequently, the diaphragm presses on these carbon grains. This pressure varies as the diaphragm vibrates, due to sound waves. The louder the sound the harder is the diaphragm pushed. The receiver is known as an electric mouth. In the receiver there is a diaphragm located which has two magnets located at the edge of it. One magnet is an electromagnet which consists of an electric wired coil which is wound around it. If the vocal levels are high, the electric force passed through this wire is stronger and attracts the diaphragm towards it. The diaphragm vibrates depending on the vocal levels. Since the diaphragm keeps moving in and out it pushes the air around it, this air sets up the sound waves. These waves are what strike the ear of the listener.

Very large image demonstrating how a telephone works

The telephone is an electrical instrument. Speaking into the handset's transmitter or microphone makes its diaphragm vibrate. This varies the electric current, causing the receiver's diaphragm to vibrate. This duplicates the original sound. Take a look at this image to make this point much more clear.
Modern telephones don't use carbon in their handsets. They use electret microphones for the transmitter and piezoelectric transducers for receivers but the principle described in the image linked above is the same. Sound waves picked up by an electret microphone causes "a thin, metal-coated plastic diaphragm to vibrate, producing variations in an electric field across a tiny air gap between the diaphragm and an electrode."[Britannica definition] A piezoelectric transducer uses material which converts the mechanical stress of a sound wave upon it into a varying electrical signal.
Telephone history begins at the start of human history. Man has always wanted to communicate from afar. People have used smoke signals, mirrors, jungle drums, carrier pigeons and semaphores to get a message from one point to another. But a phone was something new. Some say Francis Bacon predicted the telephone in 1627, however, his book New Utopia only described a long speaking tube. A real telephone could not be invented until the electrical age began. And even then it didn't seem desirable. The electrical principles needed to build a telephone were known in 1831 but it wasn't until 1854 that Bourseul suggested transmitting speech electrically. And it wasn't until 22 years later in 1876 that the idea became a reality. But before then, a telephone might have been impossible to form in one's consciousness.

Bell's Telephone

A pioneer in the field of telecommunications, Alexander Graham Bell was born in 1847 in Edinburgh, Scotland. He moved to Ontario, and then to the United States, settling in Boston, before beginning his career as an inventor. Throughout his life, Bell had been interested in the education of deaf people. This interest lead him to invent the microphone and, in 1876, his "electrical speech machine," which we now call a telephone. News of his invention quickly spread throughout the country, even throughout Europe. By 1878, Bell had set up the first telephone exchange in New Haven, Connecticut. By 1884, long distance connections were made between Boston, Massachusetts and New York City.
Bell imagined great uses for his telephone, like this model from the 1920s, but would he ever have imagined telephone lines being used to transmit video images? Since his death in 1922, the telecommunication industry has undergone an amazing revolution. Today, non-hearing people are able to use a special display telephone to communicate. Fiber optics are improving the quality and speed of data transmission. Actually, your ability to access this information relies upon telecommunications technology. Bell's "electrical speech machine" paved the way for the Information Superhighway.

Alexander Graham Bell - Evolution of the Telegraph into the Telephone

The telegraph and telephone are both wire-based electrical systems, and Alexander Graham Bell's success with the telephone came as a direct result of his attempts to improve the telegraph.
When Bell began experimenting with electrical signals, the telegraph had been an established means of communication for some 30 years. Although a highly successful system, the telegraph, with its dot-and-dash Morse code, was basically limited to receiving and sending one message at a time. Bell's extensive knowledge of the nature of sound and his understanding of music enabled him to conjecture the possibility of transmitting multiple messages over the same wire at the same time. Although the idea of a multiple telegraph had been in existence for some time, Bell offered his own musical or harmonic approach as a possible practical solution. His "harmonic telegraph" was based on the principle that several notes could be sent simultaneously along the same wire if the notes or signals differed in pitch.

The History of the Telephone - Alexander Graham Bell

In the 1870s, two inventors Elisha Gray and Alexander Graham Bell both independently designed devices that could transmit speech electrically (the telephone). Both men rushed their respective designs to the patent office within hours of each other, Alexander Graham Bell patented his telephone first. Elisha Gray and Alexander Graham Bell entered into a famous legal battle over the invention of the telephone, which Bell won.

Telephone operating companies

In some countries, many telephone operating companies (commonly abbreviated to telco in American English) are in competition to provide telephone services. The above Main article lists only facilities based providers and not companies which lease services from facilities based providers in order to serve their customers.

IP telephony

Internet Protocol (IP) telephony (also known as Voice over Internet Protocol, VoIP), is a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. As of January 2005, up to 10% of telephone subscribers in Japan and South Korea have switched to this digital telephone service. A January 2005 Newsweek article suggested that Internet telephony may be "the next big thing." [1] As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses an Internet connection and hardware IP Phones or softphones installed on personal computers to transmit conversations encoded as data packets. In addition to replacing POTS (plain old telephone service), IP telephony services are also competing with mobile phone services by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private networks which may or may not have a connection to the global telephone network.
IP telephones have two notable disadvantages compared to traditional telephones. Unless the IP telephone's components are backed up with an uninterruptible power supply or other emergency power source, the phone will cease to function during a power outage as can occur during an emergency or disaster, exactly when the phone is most needed. Traditional phones connected to the older PSTN network do not experience that problem since they are powered by the telephone company's battery supply, which will continue to function even if there's a prolonged power black-out. A second distinct problem for an IP phone is the lack of a 'fixed address' which can impact the provision of emergency services such as police, fire or ambulance, should someone call for them. Unless the registered user updates the IP phone's physical address location after moving to a new residence, emergency services can be, and have been, dispatched to the wrong location

Telephone

The telephone (from the Greek: τῆλε, tēle, "far" and φωνή, phōnē, "voice") is atelecommunications device that is used to transmit and receive electronically or digitally encodedsound (most commonly speech) between two or more people conversing. It is one of the most common household appliances in the developed world today. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other user. Graphic symbols used to designate telephone service or phone-related information in print, signs, and other media include ℡, ☎, ☏, and ✆.

Digital telephony

The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems. Later methods such as SONET and fiber optic transmission further advanced digital transmission. Although analog carrier systems existed, digital transmission allowed lower cost and more channels multiplexed on a single transmission medium. Today the end instrument remains analog but the analog signals are typically converted to digital signals at the (Serving Area Interface (SAI), central office (CO), or other aggregation point. Digital loop carriers (DLC) place the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.

IP telephony

Hardware-based IP phone

Internet Protocol (IP) telephony (also known as Voice over Internet Protocol, VoIP), is a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. As of January 2005, up to 10% of telephone subscribers in Japan and South Korea have switched to this digital telephone service. A January 2005 Newsweek article suggested that Internet telephony may be "the next big thing."[1] As of 2006 many VoIP companies offer service to consumers and businesses.

IP telephony uses an Internet connection and hardware IP Phones or softphones installed on personal computers to transmit conversations encoded as data packets. In addition to replacing POTS (plain old telephone service), IP telephony services are also competing with mobile phone services by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private networks which may or may not have a connection to the global telephone network.