Femur Bone in Human Body

Femur Bone in Human Body

The Femur - Traits and Bony Features

Skeletal

Organs  

The Femur

Function  

It keeps mobility of our body and facilitates movement of legs.

The femur is the largest and strongest bone in our body. It is a long bone making up about one fourth of our height and is the attachment point for some powerful muscles. At the hip (proximally), the femur's ball-shaped head joins (articulates) with the pelvis in the acetabulum and is secured by a strong ligament (ligamentum teres) attached to the fovea captitis on the head. The head attaches out (laterally) to a short neck that then attaches to the vertical shaft of the femur. Because of the way the head of the femur attaches to the side of the shaft, the neck is the weakest point and most prone to a break. Where the neck meets the shaft of the femur, we will find the greater trochanter on the outside (lateral) and the medial trochanter on the inside (medial) which are connected by the intertrochanteric line in front (anteriorly) and the intertrochanteric crest in the back (posteriorly). The trochanters are attachment points for the powerful muscles of the thigh and butt. More muscle attachment sites are found on the back side of the femur (posteriorly) – the gluteal tuberosity which leads down the shaft to the ridge-like linea aspera. At the bottom (distally), the femur spreads into a wide base with the medial and lateral condyles joining (articulating) with the tibia. Between the condyles is the intercondylar notch. Outside of them are the lateral and medial epicondyles, which are attachment sites for more muscles.  The patellar surface in the front (anteriorly) joins (articulates) with the kneecap (patella).

Posted on 31.01.2013

DETAILS OF COAL PULVERISERS FOR POWER PLANTS

DETAILS OF COAL PULVERISERS FOR POWER PLANTS

Types of Pulverisers

Coal pulverisers may be classified by speed, as follows:

• Low Speed
• Medium Speed
• High Speed

Low Speed

Ball and Tube Mills

A ball mill is a pulveriser that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods.

A tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as slime (slurry).

Both types of mill include liners that protect the cylindrical structure of the mill from wear. Thus the main wear parts in these mills are the balls themselves and the liners. The balls are simply "consumed" by the wear process and must be re-stocked, whereas the liners must be periodically replaced. The ball and tube mills are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. Due to its shape only, people call it as Tube Mill and due to use of Grinding Balls for crushing, it is called Ball Mill. Hence, is the name Ball Tube Mill. These Mills are also designated as BBD 4772; where, B - Broyer (Name of Inventor). B - Boulet (French word for Balls). D - Direct firing. 47 - Diameter of shell (in Decimetre) i.e. 4.7 metres is the shell diameter. 72 - Length of shell (in Decimetre) i.e. 7.2 metres is the length of shell.

By the name the grinding in the ball and tube mill is produced by rotating quantity of steel balls by their fall and lift due to rotation of tube. The ball charge may occupy one third to half of the total internal volume of the shell. The significant feature incorporated in the BBD mills is its double end operation, each end catering to one elevation of a boiler. The system facilitated entry of raw coal and outlet of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit.

Mill Internals and Other Details

A ball tube mill may be described as a cylinder made of steel plate having separate heads or trunion attached to the ends with the trunion resting on suitable bearings for supporting the machine. The trunion are hollow to allow for the introduction of discharge of the materials undergoing reduction in size. The mill shell is lined with chilled iron, carbon steel, manganese steel, high chrome liners attached to shell body with counter sunk bolts. These liners are made in different shapes so that the counter inside surface of the mill is suited for requirement of application.

The Shells are of three pieces. The Intermediate shell connects to the end shells by flange joints and the total length of shell is 7.2 metres. The liners are fastened to the inner side of mill shell (cylindrical part) to protect the shell from the impact of steel balls. There are 600 Nos. of liners of ten variants in each shell weighing around 60.26 MT. The original lift value of the liners is around 55 mm and the minimum allowable lift is 20 mm.

Working Principles

Primary air in the case of Tube Mill has dual function to perform. It is used as drying as well as transporting media and by regulating the same, the Mill output is regulated. Governed by the pulverized fuel outlet temperature requirement, the combination of cold air and hot air dampers are suitably regulated to have proper primary air temperature. Apart from raising the coal temperature inside the Mill for drying and better grinding, the same air works in tandem as a carrying media for pulverised coal through annular space between fixed trunion tube and rotating hot air tube on way to classifier. Coal-laden air passing through double cone static classifiers with adjustable classifier vanes for segregation into pulverised fuel of desired fineness and coarse particles continues its journey towards nozzle of coal burners for combustion. Coarse particles rejected in classifier find their way back to mill for another cycle of grinding. In order to avoid excess sweeping of coal from Mill only, part of the primary air, directly proportional to the boiler load demand is passed through Mill. Further to ensure and maintain sufficient velocity of pulverized fuel and to avoid settling in pulverised fuel pipes, an additional quantity of primary air is fed in to mixing box on raw coal circuit. This by-pass air tapped from the primary air duct going in to Mill makes appreciable contribution for drying of raw coal by flash drying effect in addition to picking up the pulverised fuel from Mill outlet for its transportation towards classifiers. Tube mill output while responding to boiler load demand is controlled by regulating primary air-flow. Such regulation by sweeping away of pulverized fuel from Mill being very fast rather well comparable with oil firing response, needs coal level to be maintained in the Mill. Mill level control circuit sensing the decreased coal level in Mill automatically increases the speed of raw coal feeder and vice versa. Maintaining the coal level in Mill offers built-in-capacity cushion of pulverised fuel to take care of small interruption in raw coal circuit. The mill is pressurized and the tightness is ensured by plenum chambers around the rotating trunion filled with pressurized seal air. Bleeding seal air from plenum chamber to Mill provides air cushion between pulverized fuel in the Mill and the outside atmosphere. Inadequacy or absence of seal air will allow escape of pulverized fuel into atmosphere creating nuisance. On the other hand excess of seal air leaking into Mill will affect the desired Mill outlet temperature. As such the seal air is controlled by a local control damper by maintaining just sufficient differential pressure for sealing of pulverized fuel.

Medium Speed

Ring and Ball Mill/Ball and Race Mill

This type of mill consists of two rings separated by a series of large balls, like a thrust bearing. The lower ring rotates, while the upper ring presses down on the balls via a set of spring and adjuster assemblies, or nitrogen gas pressurised rams/loading cylinders. The material to be pulverized is introduced into the centre or side of the pulveriser (depending on the design). As the lower ring rotates, the balls to orbit between the upper and lower rings, and balls roll over the bed of coal on the lower ring. The pulverized material is carried out of the mill by the flow of air moving through it. The size of the pulverized particles released from the grinding section of the mill is determined by a classifier separator - if the coal is fine enough to be picked up by the air, it is carried through the classifier. Coarser particles return to the grinding zone for further pulverization.

Vertical Spindle Roller Mill

Similar to the ring and ball mill, this mill uses large "tyres" to crush the coal. These are usually found in utility plants.

Raw coal is gravity-fed through a central feed pipe to the grinding table where it flows outwardly by centrifugal action and is ground between the rollers and table. Hot primary air for drying and coal transport enters the wind box plenum underneath the grinding table and flows upward through a swirl ring having multiple sloped nozzles surrounding the grinding table. The air mixes with and dries coal in the grinding zone and carries pulverized coal particles upward into a classifier.

Fine pulverized coal exits the outlet section through multiple discharge coal pipes leading to the burners, while oversized coal particles are rejected and returned to the grinding zone for further grinding. Pyrites and extraneous dense impurity material fall through the nozzle ring and are ploughed, by scraper blades attached to the grinding table, into the pyrites chamber to be removed. Mechanically, the vertical roller mill is categorized as an applied force mill. There are three grinding roller wheel assemblies in the mill grinding section, which are mounted on a loading frame via pivot point. The fixed-axis roller in each roller wheel assembly rotates on a segmentally-lined grinding table that is supported and driven by a planetary gear reducer directly coupled to a motor. The grinding force for coal pulverization is applied by a loading frame. This frame is connected by vertical tension rods to three hydraulic cylinders secured to the mill foundation. All forces used in the pulverizing process are transmitted to the foundation via the gear reducer and loading elements. The pendulum movement of the roller wheels provides a freedom for wheels to move in a radial direction, which results in no radial loading against the mill housing during the pulverizing process.

Depending on the required coal fineness, there are two types of classifier that may be selected for a vertical roller mill. The dynamic classifier, which consists of a stationary angled inlet vane assembly surrounding a rotating vane assembly or cage, is capable of producing micron fine pulverized coal with a narrow particle size distribution. In addition, adjusting the speed of the rotating cage can easily change the intensity of the centrifugal force field in the classification zone to achieve coal fineness control real-time to make immediate accommodation for a change in fuel or boiler load conditions. For the applications where a micron fine pulverized coal is not necessary, the static classifier, which consists of a cone equipped with adjustable vanes, is an option at a lower cost since it contains no moving parts. With adequate mill grinding capacity, a vertical mill equipped with a static classifier is capable of producing a coal fineness less than 50 mesh up to 99.5 percent or more and less than 200 mesh up to 80 percent or more, while one equipped with a dynamic classifier produces coal fineness less than 100 mesh up to 100 percent and less than 200 mesh up to 95 percent or more.

In 1954, a Jet Pulveriser was developed which operates like a Vertical Pulverizer; the coal is pulverized by the high speed air action only; forcing/pressurizing coal against coal. 

Bowl Mill

Similar to the vertical roller mill, it also uses “tyres” to crush coal. There are two types, a deep bowl mill, and a shallow bowl mill.

High Speed

Attrition Mill

Rotor, Stationary Pegs

Beater Wheel Mill

Hammer Mill

It is used in farms for grinding grain and chaff for feed.

Demolition Pulveriser

An attachment fitted to an excavator. Commonly used in demolition work to break up large pieces of concrete.

Posted in Blog on 22 January, 2013

JOURNEY OF TELEPHONE FROM 19^TH - 21^ST CENTURY

Telephone

A telephone, or phone, is a telecommunications device that transmits and receives sound, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other. Developed in the mid-1870s by Alexander Graham Bell and others, the telephone has long been considered indispensable to businesses, households and governments, is now one of the most common appliances in the developed world. The word "telephone", which has been adapted to many languages and is now recognized around the world, is derived from the Greek word: tēle, "far" and phōnē, "voice", together meaning "distant voice".

All modern telephones have a microphone to speak into, an earphone (or 'speaker') which reproduces the voice of the other person, a ringer which makes a sound to alert the owner when a call is coming in, and a keypad (or on older phones a telephone dial) to enter the telephone number of the telephone to be called. The microphone and earphone are usually built into a handset which is held up to the face to talk. The keypad may be part of the handset or of a base unit to which the handset would be connected. A land line telephone is connected by a pair of wires to the telephone network, while a mobile phone (also called a cell phone) is portable and communicates with the telephone network by radio. A cordless telephone has a portable handset which communicates by radio transmission with the handset owner’s base station which is connected by wire to the telephone network, and can only be used within about 50 feet from the base station.

The microphone converts the sound waves to electrical signals and then these are sent through the telephone network to the other phone and there converted by an earphone, or speaker, back into sound waves. Telephones are a duplex communications medium, meaning they allow the people on both ends to talk simultaneously. The telephone network, consisting of a worldwide net of telephone lines, fiber optic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, allows any telephone in the world to communicate with any other. Each telephone line has an identifying number called its telephone number. To initiate a telephone call the user enters the other telephone's number into a numeric keypad on the phone.

Although originally designed for simple voice communications, most modern telephones have many additional capabilities. They may be able to record spoken messages, send and receive text messages, take and display photographs or video, play music, and surf the Internet. A current trend is phones that integrate all mobile communication and computing needs; these are called smart phones.

History

Credit for the invention of the electric telephone is frequently disputed, and new controversies over the issue have arisen from time to time. As with other influential inventions such as radio, television, the light bulb, and the computer, there were several inventors who did pioneering experimental work on voice transmission over a wire and improved on each other's ideas. Innocenzo Manzetti, Antonio Meucci, Johann Philipp Reis, Elisha Gray, Alexander Graham Bell, and Thomas Edison, among others, have all been credited with pioneering work on the telephone. An undisputed fact is that Alexander Graham Bell was the first to be awarded a patent for the electric telephone by the United States Patent and Trademark Office (USPTO) in March 1876. That first patent by Bell was the master patent of the telephone, from which other patents for electric telephone devices and features flowed. 

The early history of the telephone became and still remains a confusing morass of claims and counterclaims, which were not clarified by the large number of lawsuits that hoped to resolve the patent claims of many individuals and commercial competitors. The Bell and Edison patents, however, were forensically victorious and commercially decisive.

A Hungarian engineer, Tivadar Puskás, quickly invented the telephone switchboard in 1876, which allowed for the formation of telephone exchanges, and eventually networks.

Basic Principles

Schematic of a landline telephone installation

A traditional land line telephone system, also known as "plain old telephone service" (POTS), commonly carries both control and audio signals on the same twisted pair of insulated wires: the telephone line. The signaling equipment, or ringer, consists of a bell, beeper, light or other device to alert the user to incoming calls, and number buttons or a rotary dial to enter a telephone number for outgoing calls. Most of the expense of wire-line telephone service is the wires, so telephones transmit both the incoming and outgoing voice channels on a single pair of wires. A twisted pair line rejects electromagnetic interference (EMI) and crosstalk better than a single wire or an untwisted pair. The strong outgoing voice signal from the microphone does not overpower the weaker incoming speaker signal with a side tone because a hybrid coil subtracts the microphone's signal from the signal sent to the local speaker. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line's length. Telephones have similar adjustments for inside line lengths. The wire's voltages are negative compared to earth, to reduce galvanic corrosion. Negative voltage attracts positive metal ions toward the wires.

Details of Operation

The land line telephone contains a switch hook and an alerting device, usually a ringer, that remains connected to the phone line whenever the phone is "on hook" (i.e. the switch is open), and other components which are connected when the phone is "off hook". The off-hook components include a transmitter (microphone), a receiver (speaker), and other circuits for dialing, filtering, and amplification.

A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switch hook, which powers the telephone by connecting the transmitter (microphone), receiver (speaker), and related audio components to the line. The off-hook circuitry has a low resistance (less than 300 ohms) which causes a direct current (DC), which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness. On a modern push-button telephone, the caller then presses the number keys to send the telephone number of the called party. The keys control a tone generator circuit (not shown) that makes DTMF tones that the exchange receives. A rotary-dial telephone uses pulse dialing, sending electrical pulses, that the exchange can count to get the telephone number (as of 2010 many exchanges were still equipped to handle pulse dialing). If the called party's line is available, the exchange sends an intermittent ringing signal (about 90 volts alternating current (AC) in North America and UK and 60 volts in Germany) to alert the called party to an incoming call. If the called party's line is in use, the exchange returns a busy signal to the calling party. However, if the called party's line is in use but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.

The phone's ringer is connected to the line through a capacitor, a device which blocks direct current but passes alternating current. So, the phone draws no current when it is on hook (a DC voltage is continually connected to the line), but exchange circuitry can send an AC voltage down the line to ring for an incoming call. (When there is no exchange, telephones often have hand-cranked magnetos to make the ringing voltage.) When a land line phone is inactive or "on hook", the circuitry at the telephone exchange detects the absence of direct current and therefore "knows" that the phone is on hook (therefore, only AC current will go through) with only the alerting device electrically connected to the line. When a party initiates a call to this line, the exchange sends the ringing signal. When the called party picks up the handset, they actuate a double-circuit switch hook (not shown) which simultaneously disconnects the alerting device and connects the audio circuitry to the line. This, in turn, draws direct current through the line, confirming that the called phone is now active. The exchange circuitry turns off the ring signal, and both phones are now active and connected through the exchange. The parties may now converse as long as both phones remain off hook. When a party "hangs up", placing the handset back on the cradle or hook, direct current ceases in that line, signaling the exchange to disconnect the call.

Calls to parties beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections. Satellite technology may be used for communication over very long distances.

In most land line telephones, the transmitter and receiver (microphone and speaker) are located in the handset, although in a speakerphone these components may be located in the base or in a separate enclosure. Powered by the line, the microphone produces a modulated electrical current which varies its frequency and amplitude in response to the sound waves arriving at its diaphragm. The resulting current is transmitted along the telephone line to the local exchange then on to the other phone (via the local exchange or via a larger network), where it passes through the coil of the receiver. The varying current in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the original sound waves present at the transmitter.

Along with the microphone and speaker, additional circuitry is incorporated to prevent the incoming speaker signal and the outgoing microphone signal from interfering with each other. This is accomplished through a hybrid coil. The incoming audio signal passes through a resistor and the primary winding of the coil which passes it to the speaker. Since the current path has a far lower impedance than the microphone, virtually all of the incoming signal passes through it and bypasses the microphone.

At the same time the DC voltage across the line causes a DC current which is split between the resistor-coil branch and the microphone-coil  branch. The DC current through the resistor-coil branch has no effect on the incoming audio signal. But the DC current passing through the microphone is turned into AC current (in response to voice sounds) which then passes through only the upper branch of the coil's primary winding, which has far fewer turns than the lower primary winding. This causes a small portion of the microphone output to be fed back to the speaker, while the rest of the AC current goes out through the phone line.

A Lineman's handset is a telephone designed for testing the telephone network, and may be attached directly to aerial lines and other infrastructure components.

Early Commercial Instruments

Early telephones were technically diverse. Some used a liquid transmitter, some had a metal diaphragm that induced current in an electromagnet wound around a permanent magnet, and some were "dynamic" - their diaphragm vibrated a coil of wire in the field of a permanent magnet or the coil vibrated the diaphragm. The sound-powered dynamic kind survived in small numbers through the 20th century in military and maritime applications, where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil which was an impedance matching transformer to make it compatible with the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument.

Early telephones were locally powered, using either a dynamic transmitter or by the powering of a transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the telephone exchange over the same wires that carried the voice signals.

Early telephones used a single wire for the subscriber's line, with ground return used to complete the circuit (as used in telegraphs). The earliest dynamic telephones also had only one port opening for sound, with the user alternately listening and speaking (or rather, shouting) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but also more expensive.

At first, the benefits of a telephone exchange were not exploited. Instead telephones were leased in pairs to a subscriber, who had to arrange for a telegraph contractor to construct a line between them, for example between a home and a shop. Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Francisco, and Bell was not slow in appreciating the potential.

Signaling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell in a ringer box, first operated over a second wire, and later over the same wire, but with a condenser (capacitor) in series with the bell coil to allow the AC ringer signal through while still blocking DC (keeping the phone "on hook"). Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push-button and two for speaking. Large wall telephones in the early 20th century usually incorporated the bell, and separate bell boxes for desk phones dwindled away in the middle of the century.

Rural and other telephones that were not on a common battery exchange had a magneto or hand-cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator. Some local farming communities that were not connected to the main networks set up barbed wire telephone lines that exploited the existing system of field fences to transmit the signal.

In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switch hook." Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In phones connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate bell box or "ringer box". In phones connected to common battery exchanges, the ringer box was installed under a desk, or other out of the way place, since it did not need a battery or magneto.

Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, now called a handset, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.

A Swedish Ericsson 1001 Model, from 1939

Disadvantages of single wire operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest technology.

What turned out to be the most popular and longest lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1–10 times for each digit, and the hook switch (in the center of the circuit diagram) disconnected the line and the transmitter battery while the handset was on the cradle.

After the 1930s, the base also enclosed the bell and induction coil, obviating the old separate ringer box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the telephone dial was added the instrument itself changed little until American Telephone & Telegraph Company (AT&T) introduced touch-tone dialing in the 1960s.

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, designed to support the basic 3 kHz voice channel by sampling the bandwidth-limited analog voice signal and encoding using PCM. While digitization allows wide band voice on the same channel, the improved quality of a wider analog voice channel did not find a large market in the PSTN.

Later transmission methods such as SONET and fiber optic transmission further advanced digital transmission. Although analog carrier systems existed that multiplexed multiple analog voice channels onto a single transmission medium, digital transmission allowed lower cost and more channels multiplexed on the transmission medium. Today the end instrument often 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

A hardware-based IP Phone, with touch-tone dialing

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." As of 2006 many VoIP companies offer service to consumers and businesses.

IP telephony uses an Internet connection and hardware IP Phones or soft phones 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.

Fixed telephone lines per 100 inhabitants 1997-2007

Usage

By the end of 2009, there were a total of nearly 6 billion mobile and fixed-line subscribers worldwide. This included 1.26 billion fixed-line subscribers and 4.6 billion mobile subscribers.

Posted on 07.01.2013