A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is popularly called a dish antenna or parabolic dish; the main advantage of a parabolic antenna is. It functions to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used. Parabolic antennas are used as high-gain antennas for point-to-point communications, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite communications and spacecraft communication antennas.
They are used in radio telescopes. The other large use of parabolic antennas is for radar antennas, in which there is a need to transmit a narrow beam of radio waves to locate objects like ships and guided missiles, for weather detection. With the advent of home satellite television receivers, parabolic antennas have become a common feature of the landscapes of modern countries; the parabolic antenna was invented by German physicist Heinrich Hertz during his discovery of radio waves in 1887. He used cylindrical parabolic reflectors with spark-excited dipole antennas at their focus for both transmitting and receiving during his historic experiments; the operating principle of a parabolic antenna is that a point source of radio waves at the focal point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point.
A typical parabolic antenna consists of a metal parabolic reflector with a small feed antenna suspended in front of the reflector at its focus, pointed back toward the reflector. The reflector is a metallic surface formed into a paraboloid of revolution and truncated in a circular rim that forms the diameter of the antenna. In a transmitting antenna, radio frequency current from a transmitter is supplied through a transmission line cable to the feed antenna, which converts it into radio waves; the radio waves are emitted back toward the dish by the feed antenna and reflect off the dish into a parallel beam. In a receiving antenna the incoming radio waves bounce off the dish and are focused to a point at the feed antenna, which converts them to electric currents which travel through a transmission line to the radio receiver; the reflector can be of sheet metal, metal screen, or wire grill construction, it can be either a circular "dish" or various other shapes to create different beam shapes.
A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than one-tenth of a wavelength, so screen reflectors are used to reduce weight and wind loads on the dish. To achieve the maximum gain, it is necessary that the shape of the dish be accurate within a small fraction of a wavelength, to ensure the waves from different parts of the antenna arrive at the focus in phase. Large dishes require a supporting truss structure behind them to provide the required stiffness. A reflector made of a grill of parallel wires or bars oriented in one direction acts as a polarizing filter as well as a reflector, it only reflects linearly polarized radio waves, with the electric field parallel to the grill elements. This type is used in radar antennas. Combined with a linearly polarized feed horn, it helps filter out noise in the receiver and reduces false returns. Since a shiny metal parabolic reflector can focus the sun's rays, most dishes could concentrate enough solar energy on the feed structure to overheat it if they happened to be pointed at the sun, solid reflectors are always given a coat of flat paint.
The feed antenna at the reflector's focus is a low-gain type such as a half-wave dipole or more a small horn antenna called a feed horn. In more complex designs, such as the Cassegrain and Gregorian, a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point; the feed antenna is connected to the associated radio-frequency transmitting or receiving equipment by means of a coaxial cable transmission line or waveguide. At the microwave frequencies used in many parabolic antennas, waveguide is required to conduct the microwaves between the feed antenna and transmitter or receiver; because of the high cost of waveguide runs, in many parabolic antennas the RF front end electronics of the receiver is located at the feed antenna, the received signal is converted to a lower intermediate frequency so it can be conducted to the receiver through cheaper coaxial cable. This is called a low-noise block downconverter. In transmitting dishes, the microwave transmitter may be located at the feed point.
An advantage of parabolic antennas is that most of the structure of the antenna is nonresonant, so it can function over a wide range of frequencies, a wide bandwidth. All, necessary to change the frequency of operation is to replace the feed antenna with one that works at the new frequen
In telecommunications, point-to-multipoint communication is communication, accomplished via a distinct type of one-to-many connection, providing multiple paths from a single location to multiple locations. Point-to-multipoint telecommunications is used in wireless Internet and IP telephony via gigahertz radio frequencies. P2MP systems have been designed without a return channel from the multiple receivers. A central antenna or antenna array broadcasts to several receiving antennas and the system uses a form of time-division multiplexing to allow for the return channel traffic. In contemporary usage, the term point-to-multipoint wireless communications relates to fixed wireless data communications for Internet or voice over IP via radio or microwave frequencies in the gigahertz range. Point-to-multipoint is the most popular approach for wireless communications that have a large number of nodes, end destinations or end users. Point to Multipoint assumes there is a central base station to which remote subscriber units or customer premises equipment are connected over the wireless medium.
Connections between the base station and subscriber units can be either line-of-sight or, for lower-frequency radio systems, non-line-of-sight where link budgets permit. Lower frequencies can offer non-line-of-sight connections. Various software planning tools can be used to determine feasibility of potential connections using topographic data as well as link budget simulation; the point to multipoint links are installed to reduce the cost of infrastructure and increase the number of CPE's and connectivity. Point-to-multipoint wireless networks employing directional antennas are affected by the hidden node problem in case they employ a CSMA/CA medium access control protocol; the negative impact of the hidden node problem can be mitigated using a time-division multiple access based protocol or a polling protocol rather than the CSMA/CA protocol. The telecommunications signal in a point-to-multipoint system is bi-directional, TDMA or channelized. Systems using frequency-division duplexing offer full-duplex connections between base station and remote sites, time-division duplex systems offer half-duplex connections.
Point-to-multipoint systems can be implemented in licensed, semi-licensed or unlicensed frequency bands depending on the specific application. Point-to-point and point-to-multipoint links are popular in the wireless industry and when paired with other high-capacity wireless links or technologies such as free space optics can be referred to as backhaul; the base station may have a single omnidirectional antenna or multiple sector antennas, the latter of which allowing greater range and capacity. Backhaul Broadcasting Local Multipoint Distribution Service Multichannel Multipoint Distribution Service Wireless access point
The Advanced Research Projects Agency Network was an early packet-switching network and the first network to implement the TCP/IP protocol suite. Both technologies became the technical foundation of the Internet; the ARPANET was founded by the Advanced Research Projects Agency of the United States Department of Defense. The packet-switching methodology employed in the ARPANET was based on concepts and designs by Leonard Kleinrock, Paul Baran, Donald Davies, Lawrence Roberts; the TCP/IP communications protocols were developed for the ARPANET by computer scientists Robert Kahn and Vint Cerf, incorporated concepts from the French CYCLADES project directed by Louis Pouzin. As the project progressed, protocols for internetworking were developed by which multiple separate networks could be joined into a network of networks. Access to the ARPANET was expanded in 1981, when the National Science Foundation funded the Computer Science Network. In 1982, the Internet protocol suite was introduced as the standard networking protocol on the ARPANET.
In the early 1980s the NSF funded the establishment of national supercomputing centers at several universities and provided interconnectivity in 1986 with the NSFNET project, which created network access to the supercomputer sites in the United States from research and education organizations. The ARPANET was decommissioned in 1989. Voice and data communications were based on methods of circuit switching, as exemplified in the traditional telephone network, wherein each telephone call is allocated a dedicated, end to end, electronic connection between the two communicating stations; such stations might be computers. The temporarily dedicated line comprises many intermediary lines which are assembled into a chain that reaches from the originating station to the destination station. With packet switching, a network could share a single communication link for communication between multiple pairs of receivers and transmitters; the earliest ideas for a computer network intended to allow general communications among computer users were formulated by computer scientist J. C. R. Licklider of Bolt and Newman, in April 1963, in memoranda discussing the concept of the "Intergalactic Computer Network".
Those ideas encompassed many of the features of the contemporary Internet. In October 1963, Licklider was appointed head of the Behavioral Sciences and Command and Control programs at the Defense Department's Advanced Research Projects Agency, he convinced Ivan Sutherland and Bob Taylor that this network concept was important and merited development, although Licklider left ARPA before any contracts were assigned for development. Sutherland and Taylor continued their interest in creating the network, in part, to allow ARPA-sponsored researchers at various corporate and academic locales to utilize computers provided by ARPA, and, in part, to distribute new software and other computer science results. Taylor had three computer terminals in his office, each connected to separate computers, which ARPA was funding: one for the System Development Corporation Q-32 in Santa Monica, one for Project Genie at the University of California and another for Multics at the Massachusetts Institute of Technology.
Taylor recalls the circumstance: "For each of these three terminals, I had three different sets of user commands. So, if I was talking online with someone at S. D. C. and I wanted to talk to someone I knew at Berkeley, or M. I. T. about this, I had to get up from the S. D. C. Terminal, log into the other terminal and get in touch with them. I said, "Oh Man!", it's obvious what to do: If you have these three terminals, there ought to be one terminal that goes anywhere you want to go. That idea is the ARPANET". Meanwhile, since the early 1960s, Paul Baran at the RAND Corporation had been researching systems that could survive nuclear war and developed the idea of distributed adaptive message block switching. Donald Davies at the United Kingdom's National Physical Laboratory independently invented the same concept in 1965, his work, presented by a colleague caught the attention of ARPANET developers at a conference in Gatlinburg, Tennessee, in October 1967. He gave the first public demonstration, having coined the term packet switching, on 5 August 1968 and incorporated it into the NPL network in England.
Elizabeth Feinler created the first Resource Handbook for ARPANET in 1969 which led to the development of the ARPANET directory. The directory, built by Feinler and a team made it possible to navigate the ARPANET. Larry Roberts at ARPA applied Davies' concepts of packet switching for the ARPANET; the NPL network followed by the ARPANET were the first two networks in the world to use packet switching, were themselves connected together in 1973. Bob Taylor convinced ARPA's Director Charles M. Herzfeld to fund a network project in February 1966, Herzfeld transferred a million dollars from a ballistic missile defense program to Taylor's budget. Taylor hired Larry Roberts as a program manager in the ARPA Information Processing Techniques Office in January 1967 to work on the ARPANET. In April 1967, Roberts held a design session on technical standards; the initial standards for identification and authentication of users, transmission of characters, error checking and retransmission procedures were discussed.
At the meeting, Wesley Clark proposed minicomputers called Interface Message Processors should be used to interface to the network rather than the large mainframes that would be the nodes of the ARPANET. Roberts modified the ARPANET plan to incorporate Clark's suggestion; the plan was presented at the ACM Symposium in Gatlinburg, Tennessee, in October 1967. Donald Davies' work on packet switc
In telecommunications, RS-232, Recommended Standard 232 refers to a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a DTE such as a computer terminal, a DCE, such as a modem; the standard defines the electrical characteristics and timing of signals, the meaning of signals, the physical size and pinout of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997; the RS-232 standard had been used in computer serial ports. A serial port complying with the RS-232 standard was once a standard feature of many types of computers. Personal computers used them for connections not only to modems, but to printers, computer mice, data storage, uninterruptible power supplies, other peripheral devices. RS-232, when compared to interfaces such as RS-422, RS-485 and Ethernet, has lower transmission speed, short maximum cable length, large voltage swing, large standard connectors, no multipoint capability and limited multidrop capability.
In modern personal computers, USB has displaced RS-232 from most of its peripheral interface roles. Many computers no longer come equipped with RS-232 ports and must use either an external USB-to-RS-232 converter or an internal expansion card with one or more serial ports to connect to RS-232 peripherals. Thanks to their simplicity and past ubiquity, RS-232 interfaces are still used—particularly in industrial machines, networking equipment, scientific instruments where a short-range, point-to-point, low-speed wired data connection is adequate; the Electronic Industries Association standard RS-232-C as of 1969 defines: Electrical signal characteristics such as voltage levels, signaling rate and slew-rate of signals, voltage withstand level, short-circuit behavior, maximum load capacitance. Interface mechanical characteristics, pluggable connectors and pin identification. Functions of each circuit in the interface connector. Standard subsets of interface circuits for selected telecom applications.
The standard does not define such elements as the character encoding, the framing of characters, transmission order of bits, or error detection protocols. The character format and transmission bit rate are set by the serial port hardware a UART, which may contain circuits to convert the internal logic levels to RS-232 compatible signal levels; the standard does not define bit rates for transmission, except that it says it is intended for bit rates lower than 20,000 bits per second. RS-232 was first introduced in 1960 by the Electronic Industries Association as a Recommended Standard; the original DTEs were electromechanical teletypewriters, the original DCEs were modems. When electronic terminals began to be used, they were designed to be interchangeable with teletypewriters, so supported RS-232; because the standard did not foresee the requirements of devices such as computers, test instruments, POS terminals, so on, designers implementing an RS-232 compatible interface on their equipment interpreted the standard idiosyncratically.
The resulting common problems were non-standard pin assignment of circuits on connectors, incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes, patch boxes, test equipment and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage; some manufacturers therefore built transmitters that supplied +5 V and −5 V and labeled them as "RS-232 compatible". Personal computers started to make use of the standard so that they could connect to existing equipment. For many years, an RS-232-compatible port was a standard feature for serial communications, such as modem connections, on many computers, it remained in widespread use into the late 1990s. In personal computer peripherals, it has been supplanted by other interface standards, such as USB. RS-232 is still used to connect older designs of peripherals, industrial equipment, console ports, special purpose equipment.
The standard has been renamed several times during its history as the sponsoring organization changed its name, has been variously known as EIA RS-232, EIA 232, most as TIA 232. The standard continued to be revised and updated by the Electronic Industries Association and since 1988 by the Telecommunications Industry Association. Revision C was issued in a document dated August 1969. Revision D was issued in 1986; the current revision is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. Changes since Revision C have been in timing and details intended to improve harmonization with the CCITT standard V.24, but equipment built to the current standard will interoperate with older versions. Related ITU-T standards include V.24 and V.28. In revision D of EIA-232, the D-subminiature connector was formally included as part of the standard; the voltage range was extended to ±25 volts, the circuit capacitance limit was expressly stated as 2500 pF.
Revision E of EIA-232 introduced a new, standard D-shell 26-pin "Alt A" connector, made other changes to improve compatibility w
A ring network is a network topology in which each node connects to two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travels from node to node, with each node along the way handling every packet. Rings can be unidirectional, with all traffic travelling either clockwise or anticlockwise around the ring, or bidirectional; because a unidirectional ring topology provides only one pathway between any two nodes, unidirectional ring networks may be disrupted by the failure of a single link. A node failure or cable break might isolate every node attached to the ring. In response, some ring networks add a "counter-rotating ring" to form a redundant topology: in the event of a break, data are wrapped back onto the complementary ring before reaching the end of the cable, maintaining a path to every node along the resulting C-Ring; such "dual ring" networks include the ITU-T's PSTN telephony systems network Signalling System No. 7, Spatial Reuse Protocol, Fiber Distributed Data Interface, Resilient Packet Ring.
802.5 networks - known as IBM token ring networks - avoid the weakness of a ring topology altogether: they use a star topology at the physical layer and a media access unit to imitate a ring at the datalink layer. All Signalling System No. 7, some SONET/SDH rings have two sets of bidirectional links between nodes. This allows maintenance or failures at multiple points of the ring without loss of the primary traffic on the outer ring by switching the traffic onto the inner ring past the failure points. Orderly network where every device has access to the token and the opportunity to transmit Performs better than a bus topology under heavy network load Does not require a central node to manage the connectivity between the computers Due to the point to point line configuration of devices with a device on either side, it is quite easy to install and reconfigure since adding or removing a device requires moving just two connections. Point to point line configuration makes it easy to isolate faults.
Reconfiguration for line faults of bidirectional rings can be fast, as switching happens at a high level, thus the traffic does not require individual rerouting. One malfunctioning workstation can create problems for the entire network; this can be solved by using a switch that closes off the break. Moving and changing the devices can affect the network Communication delay is directly proportional to number of nodes in the network Bandwidth is shared on all links between devices More difficult to configure than a Star: node adjunction = Ring shutdown and reconfiguration Rings can be used to carry circuits or packets or a combination of both. SDH rings carry circuits. Circuits are set up with out-of-band signalling protocols, whereas packets are carried via a Medium Access Control Protocol; the purpose of media access control is to determine. As in any MAC protocol, the aims are to provide fairness. There are three main classes of media access protocol for ring networks: slotted and register insertion.
The slotted ring treats the latency of the ring network as a large shift register that permanently rotates. It is formatted into so-called slots of fixed size. A slot is either empty, as indicated by control flags in the head of the slot. A station that wishes to transmit puts data in. Other stations can copy out the data and may free the slot, or it may circulate back to the source who frees it. An advantage of source-release, if the sender is banned from re-using it, is that all other stations get the chance to use it first, hence avoiding bandwidth hogging; the pre-eminent example of the slotted ring is the Cambridge Ring. "Token Ring is an example of a ring topology." 802.5 networks do not use a ring topology at layer 1. As explained above, IBM Token Ring networks imitate a ring at layer 2 but use a physical star at layer 1. "Rings prevent collisions." The term "ring" only refers to the layout of the cables. It is true that there are no collisions on an IBM Token Ring, but this is because of the layer 2 Media Access Control method, not the physical topology Token passing, not rings, prevent collisions.
"Token passing happens on rings." Token passing is a way of managing access to the cable, implemented at the MAC sublayer of layer 2. Ring topology is the cable layout at layer one, it is possible to do token passing on a ring. Token passing is not restricted to rings
A two-way radio is a radio that can both transmit and receive a signal, unlike a broadcast receiver which only receives content. It is an audio transceiver designed for bidirectional person-to-person voice communication with other users with similar radios using the same radio frequency. Two-way radios are available in stationary base and hand-held portable configurations. Hand-held two-way radios are called walkie-talkies, handie-talkies or hand-helds. Two-way radio systems operate in a half-duplex mode: the operator can talk, or he can listen, but not at the same time. A push-to-talk or Press To Transmit button activates the transmitter. Other Full-duplex is achieved by the use of two different frequencies or by frequency-sharing methods to carry the two directions of the conversation simultaneously. Methods for mitigating the self interference caused by simultaneous transmission and reception on different but close-spaced frequencies include using two antennas, or dynamic solid-state filters.
Time-division technologies are used for mitigating self interference by simultaneous transmission and reception on the same frequency. Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly; as early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available. By 1912, commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship, out of sight of land; the first mobile two-way radio was developed in Australia in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police were the first in the world to use wireless communication in cars, putting an end to the inefficient status reports via public telephone boxes, used until that time; the first sets took up the entire back seat of the Lancia patrol cars. As radio equipment became more powerful and easier to use, smaller vehicles had two-way radio communication equipment installed.
Installation of radio equipment in aircraft allowed scouts to report back observations in real-time, not requiring the pilot to drop messages to troops on the ground below or to land and make a personal report. In 1933, the Bayonne, New Jersey police department operated a two-way system between a central fixed station and radio transceivers installed in police cars. During World War II walkie-talkie hand-held radio transceivers were extensively used by air and ground troops, both by the Allies and the Axis. Early two-way schemes allowed only one station to transmit at a time while others listened, since all signals were on the same radio frequency – this was called "simplex" mode. Code and voice operations required a simple communication protocol to allow all stations to cooperate in using the single radio channel, so that one station's transmissions were not obscured by another's. By using receivers and transmitters tuned to different frequencies and solving the problems introduced by operation of a receiver next to a transmitter, simultaneous transmission and reception was possible at each end of a radio link, in so-called "full duplex" mode.
The first radio systems could not transmit voice. This required training of operators in use of Morse code. On a ship, the radio operating officers had no other duties than handling radio messages; when voice transmission became possible, dedicated operators were no longer required and two-way radio use became more common. Today's two-way mobile radio equipment is nearly as simple to use as a household telephone, from the point of view of operating personnel, thereby making two-way communications a useful tool in a wide range of personal and military roles. Two-way radio systems can be classified in several ways depending on their attributes. Conventional radios operate on fixed RF channels. In the case of radios with multiple channels, they operate on one channel at a time; the proper channel is selected by a user. The user operates a channel selector on the radio control panel to pick the appropriate channel. In multi-channel systems, channels are used for separate purposes. A channel may be reserved for a geographic area.
In a functional channel system, one channel may allow City of Springfield road repair crews to talk to the City of Springfield's road maintenance office. A second channel may allow road repair crews to communicate with state highway department crews. In a wide-area or geographic system, a taxi company may use one channel to communicate in the Boston, Massachusetts area and a second channel when taxis are in Providence, Rhode Island; this is referred to as Multisite operation. In this case, the driver or the radio must switch channels to maintain coverage when transitioning between each area. Most modern conventional digital radios and systems are capable of automatic "roaming" where the radio automatically switches channels on a dynamic basis; the radio accomplishes this based on the received signal strength of the radio repeater's recurring "beacon" signal and a "site" or "roam" list that identifies available geographic channels. Some analog conventional systems can be equipped with a feature called "vote-scan" that provides more limited roaming.
Radio "simulcast" technology can be used in adjacent areas, where each site is equipped with the same channel. Here, the transmitters must be synchronized, a centralized voter or receiver comparator device is required to select the best quality sign
A telephone call is a connection over a telephone network between the called party and the calling party. The first telephone call was made on March 1876 by Alexander Graham Bell. Bell demonstrated his ability to "talk with electricity" by transmitting a call to his assistant, Thomas Watson; the first words transmitted were "Mr Watson, come here. I want to see you."This event has been called Bell's "greatest success", as it demonstrated the first successful use of the telephone. Although it was his greatest success, he refused to have one in his own home because it was something he invented by mistake and saw it as a distraction from his main studies. A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile transmission when they are using fax machines; the call may use mobile phone, satellite phone or any combination thereof. When a telephone call has more than one called party it is referred to as a conference call.
When two or more users of the network are sharing the same physical line, it is called a party line or Rural phone line. If the caller's wireline phone is connected directly to the calling party, when the caller takes their telephone off-hook, the calling party's phone will ring; this is called a hot ringdown. Otherwise, the calling party is given a tone to indicate they should begin dialing the desired number. In some cases, if the calling party cannot dial calls directly, they will be connected to an operator who places the call for them. Calls may be placed through a public network provided by a commercial telephone company or a private network called a PBX. In most cases a private network is connected to the public network in order to allow PBX users to dial the outside world. Incoming calls to a private network arrive at the PBX in two ways: either directly to a users phone using a DDI number or indirectly via a receptionist who will answer the call first and manually put the caller through to the desired user on the PBX.
Most telephone calls through the PSTN are set up using ISUP signalling messages or one of its variants between telephone exchanges to establish the end to end connection. Calls through PBX networks are set up using DPNSS or variants; some types of calls are not charged, such as local calls dialed directly by a telephone subscriber in Canada, the United States, Hong Kong, United Kingdom, Ireland or New Zealand. In most other areas, all telephone calls are charged a fee for the connection. Fees depend on the provider of the service, the type of service being used and the distance between the calling and the called parties. In most circumstances, the calling party pays this fee. However, in some circumstances such as a reverse charge or collect call, the called party pays the cost of the call. In some circumstances, the caller pays a flat rate charge for the telephone connection and does not pay any additional charge for all calls made. Telecommunication liberalization has been established in several countries to allows customers to keep their local phone provider and use an alternate provider for a certain call in order to save money.
A typical phone call using a traditional phone is placed by picking the phone handset up off the base and holding the handset so that the hearing end is next to the user's ear and the speaking end is within range of the mouth. The caller rotary dials or presses buttons for the phone number needed to complete the call, the call is routed to the phone which has that number; the second phone makes a ringing noise to alert its owner, while the user of the first phone hears a ringing noise in its earpiece. If the second phone is picked up the operators of the two units are able to talk to one another through them. If the phone is not picked up, the operator of the first phone continues to hear a ringing noise until they hang up their own phone. One of the main struggles for Alexander Graham Bell and his team was to prove to non-English speakers that this new phenomenon "worked in their language." It was a concept, hard for people to understand at first. In addition to the traditional method of placing a telephone call, new technologies allow different methods for initiating a telephone call, such as voice dialing.
Voice over IP technology allows calls to be made through a PC. Other services, such as toll-free dial-around enable callers to initiate a telephone call through a third party without exchanging phone numbers. No phone calls could be made without first talking to the Switchboard operator. Using 21st century mobile phones does not require the use of an operator to complete a phone call; the use of headsets is becoming more common for receiving a call. Headsets can either be wireless. A special number can be dialed for operator assistance, which may be different for local vs. long-distance or international calls. Preceding and after a traditional telephone call is placed, certain tones signify the progress and status of the telephone call: a dial tone signifying that the system is ready to accept a telephone number and connect the call either: a ringing tone signifying that the called party has yet to answer the telephone a busy signal signifying that the called party's telephone is being used in a telephone call to another person a fast busy signal (also called reorder tone or overflow bu