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
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are
WiMAX is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide multiple physical layer and Media Access Control options. The name "WiMAX" was created by the WiMAX Forum, formed in June 2001 to promote conformity and interoperability of the standard, including the definition of predefined system profiles for commercial vendors; the forum describes WiMAX as "a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL". IEEE 802.16m or WirelessMAN-Advanced was a candidate for the 4G, in competition with the LTE Advanced standard. WiMAX was designed to provide 30 to 40 megabit-per-second data rates, with the 2011 update providing up to 1 Gbit/s for fixed stations; the latest version of WiMAX, WiMAX release 2.1, popularly branded as/known as WiMAX 2+, is a smooth, backwards-compatible transition from previous WiMAX generations. It is compatible and inter-operable with TD-LTE.
WiMAX refers to interoperable implementations of the IEEE 802.16 family of wireless-networks standards ratified by the WiMAX Forum. WiMAX Forum certification allows vendors to sell fixed or mobile products as WiMAX certified, thus ensuring a level of interoperability with other certified products, as long as they fit the same profile; the original IEEE 802.16 standard was published in 2001. WiMAX adopted some of its technology from WiBro, a service marketed in Korea. Mobile WiMAX is the revision, deployed in many countries and is the basis for future revisions such as 802.16m-2011. WiMAX is sometimes referred to as "Wi-Fi on steroids" and can be used for a number of applications including broadband connections, cellular backhaul, etc, it is similar to Long-range Wi-Fi. The scalable physical layer architecture that allows for data rate to scale with available channel bandwidth and range of WiMAX make it suitable for the following potential applications: Providing portable mobile broadband connectivity across cities and countries through various devices Providing a wireless alternative to cable and digital subscriber line for "last mile" broadband access Providing data, telecommunications and IPTV services Providing Internet connectivity as part of a business continuity plan Smart grids and metering WiMAX can provide at-home or mobile Internet access across whole cities or countries.
In many cases, this has resulted in competition in markets which only had access through an existing incumbent DSL operator. Additionally, given the low costs associated with the deployment of a WiMAX network, it is now economically viable to provide last-mile broadband Internet access in remote locations. Mobile WiMAX was a replacement candidate for cellular phone technologies such as GSM and CDMA, or can be used as an overlay to increase capacity. Fixed WiMAX is considered as a wireless backhaul technology for 2G, 3G, 4G networks in both developed and developing nations. In North America, backhaul for urban operations is provided via one or more copper wire line connections, whereas remote cellular operations are sometimes backhauled via satellite. In other regions and rural backhaul is provided by microwave links. WiMAX has more substantial backhaul bandwidth requirements than legacy cellular applications; the use of wireless microwave backhaul is on the rise in North America and existing microwave backhaul links in all regions are being upgraded.
Capacities of between 34 Mbit/s and 1 Gbit/s are being deployed with latencies in the order of 1 ms. In many cases, operators are aggregating sites using wireless technology and presenting traffic on to fiber networks where convenient. WiMAX in this application competes with microwave radio, E-line and simple extension of the fiber network itself. WiMAX directly supports the technologies; these are inherent to the WiMAX standard rather than being added on as carrier Ethernet is to Ethernet. On May 7, 2008 in the United States, Sprint Nextel, Intel, Bright House, Time Warner announced a pooling of an average of 120 MHz of spectrum and merged with Clearwire to market the service; the new company hopes to benefit from combined services offerings and network resources as a springboard past its competitors. The cable companies will provide media services to other partners while gaining access to the wireless network as a Mobile virtual network operator to provide triple-play services; some analysts questioned how the deal will work out: Although fixed-mobile convergence has been a recognized factor in the industry, prior attempts to form partnerships among wireless and cable companies have failed to lead to significant benefits to the participants.
Other analysts point out that as wireless progresses to higher bandwidth, it competes more directly with cable and DSL, inspiring competitors into collaboration. As wireless broadband networks grow denser and usage habits shift, the need for increased backhaul and media service will accelerate, therefore the opportunity to leverage cable assets is expected to increase. IEEE 802.16REVd and IEEE 802.16e standards support both Time Division Duplexing and Frequency
A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the airing of telephone callers, the broadcast-quality telephone hybrid is known as a broadcast telephone hybrid or telephone balance unit; the need for hybrids comes from the nature of analog plain old telephone service home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network and transmission are always four-wire circuits with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links. In today's digital systems, each speech direction must be transported independently; the line cards in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers.
The search for better telephone hybrids and echo cancelers was an important motive for the development of DSP algorithms and hardware at Bell Labs, NEC, other sites. The fundamental principle is that of impedance matching; the incoming signal is applied to both the telephone line and a "balancing network", designed to have the same impedance as the line. The outgoing signal is derived by subtracting the two, thus canceling the incoming signal from the outgoing signal. Early hybrids were made with transformers configured as hybrid coils that had an extra winding that could be connected out of phase; the name hybrid comes from these special mixed-winding transformers. An effective hybrid would have high trans-hybrid loss, which means that little of the incoming audio would appear on the outgoing port. Too much leakage can cause echoes when there is a delay in the transmission path, as there is with satellite, mobile phone, VoIP links; this is a result of a talker's voice traversing to the far-end hybrid and returning to his own receiver with insufficient attenuation.
ITU-T Recommendation G.131 describes the relationship of echo delay vs. amplitude to listener annoyance. At 100ms, 45 dB return loss is required for less than 1% of test subjects to express dissatisfaction. Good cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that matches the line. Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation. For this reason, modern hybrids use digital signal processing to implement an adaptive least mean squares filter that automatically detects the line's impedance across the voice frequency range and adjusts to it; these may reach greater than 30 measured with white noise as the send signal. DSP hybrids are called "line echo cancellers". Hybrids and cancellers are sometimes combined with echo suppressors; these work on the assumption that only one of the two parties to a conversation is speaking at a given time.
The suppressor switches a loss into the inactive speech path, thus enhancing the echo-cancelling effect of the hybrid at the expense of simultaneous two-way conversation. Despite being inherently four-wire, VoIP systems require hybrids when they interface to two-wire lines. A VoIP-to-Telco gateway used to interface a VoIP PBX to analog lines would contain hybrids to perform the required conversion. End-end VoIP needs no hybrids. In broadcast studio facilities, the name for the functional part has come to refer to the whole, a telephone hybrid is the device that packages all the functions needed to connect telephone lines to studio audio systems, providing electrical and physical interface between the telco lines and studio equipment; these devices include processing in addition to the hybrid function, such as dynamics control and equalization. Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from varying source audio; some incorporate acoustic echo cancellation to allow setups with acoustic paths between loudspeakers carrying phone audio and microphones feeding the phone lines.
In studio applications, a hybrid needs good send-to-receive isolation. When too much of the host audio appears at the hybrid's output, there will be a number of defects. Distortion of the host's voice can result from the telephone line's changing the phase of the send audio before it returns, with varying shifts at different frequencies; the original and leakage audio are mixed at the console and combine in and out of phase at the various frequencies. When this occurs, the host sounds either hollow or tinny as the phase cancellation affects some frequencies more than others. Audio feedback can result from the acoustic coupling created when callers must be heard on a loudspeaker; when lines are conferenced and the gain around the loop of the multiple hybrids is greater than unity, feedback "singing" will be audible. If the leakage is high, operators will not be able to control the relative levels of the host audio
G.fast is a digital subscriber line protocol standard for local loops shorter than 500 m, with performance targets between 0.1 and 1 Gbit/s, depending on loop length. High speeds are only achieved over short loops. Although G.fast was designed for loops shorter than 250 meters, Sckipio in early 2015 demonstrated G.fast delivering speeds over 0.1 Gbit/s nearly 500 meters and the EU announced a research project. Formal specifications have been published as ITU-T G.9700 and G.9701, with approval of G.9700 granted in April 2014 and approval of G.9701 granted on December 5, 2014. Development was coordinated with the Broadband Forum's FTTdp project; the letter G in G.fast stands for the ITU-T G series of recommendations. Limited demonstration hardware was demonstrated in mid-2013; the first chipsets were introduced in October 2014, with commercial hardware introduced in 2015, first deployments started in 2016. In G.fast, data is modulated using discrete multi-tone modulation, as in VDSL2 and most ADSL variants.
G.fast modulates up to 12 bit per DMT frequency carrier, reduced from 15 in VDSL2 for complexity reasons. The first version of G.fast will specify 106 MHz profiles, with 212 MHz profiles planned for future amendments, compared to 8.5, 17.664, or 30 MHz profiles in VDSL2. This spectrum overlaps the FM broadcast band between 87.5 and 108 MHz, as well as various military and government radio services. To limit interference to those radio services, the ITU-T G.9700 recommendation called G.fast-psd, specifies a set of tools to shape the power spectral density of the transmit signal. To enable co-existence with ADSL2 and the various VDSL2 profiles, the start frequency can be set to 2.2, 8.5, 17.664, or 30 MHz, respectively. G.fast uses time-division duplexing, as opposed to ADSL2 and VDSL2, which use frequency-division duplexing. Support for symmetry ratios between 90/10 and 50/50 is mandatory, 50/50 to 10/90 is optional; the discontinuous nature of TDD can be exploited to support low-power states, in which the transmitter and receiver remain disabled for longer intervals than would be required for alternating upstream and downstream operation.
This optional discontinuous operation allows a trade-off between power consumption. The forward error correction scheme using trellis coding and Reed–Solomon coding is similar to that of VDSL2. FEC does not provide good protection against impulse noise. To that end, the impulse noise protection data unit retransmission scheme specified for ADSL2, ADSL2+, VDSL2 in G.998.4 is present in G.fast. To respond to abrupt changes in channel or noise conditions, fast rate adaptation enables rapid reconfiguration of the data rate. Performance in G.fast systems is limited to a large extent by crosstalk between multiple wire pairs in a single cable. Self-FEXT cancellation called vectoring, is mandatory in G.fast. Vectoring technology for VDSL2 was specified by the ITU-T in G.993.5 called G.vector. The first version of G.fast will support an improved version of the linear precoding scheme found in G.vector, with non-linear precoding planned for a future amendment. Testing by Huawei and Alcatel shows that non-linear precoding algorithms can provide an approximate data rate gain of 25% compared to linear precoding in high frequencies.
Since all current G.fast implementations are limited to 106 MHz, non-linear precoding yields little performance gain. Instead, current efforts to deliver a gigabit are focusing on bonding and more bits per hertz. In tests performed in July 2013 by Alcatel-Lucent and Telekom Austria using prototype equipment, aggregate data rates of 1.1 Gbit/s were achieved at a distance of 70 m and 800 Mbit/s at a distance of 100 m, in laboratory conditions with a single line. On older, unshielded cable, aggregate data rates of 500 Mbit/s were achieved at 100 m. A A straight loop is a subscriber line without bridge taps. B The listed; the Broadband Forum is investigating architectural aspects of G.fast and has, as of May 2014, identified 23 use cases. Deployment scenarios involving G.fast bring fiber closer to the customer than traditional VDSL2 FTTN, but not quite to the customer premises as in FTTH. The term FTTdp is associated with G.fast, similar to how FTTN is associated with VDSL2. In FTTdp deployments, a limited number of subscribers at a distance of up to 200–300 m are attached to one fiber node, which acts as DSL access multiplexer.
As a comparison, in ADSL2 deployments the DSLAM may be located in a central office at a distance of up to 5 km from the subscriber, while in some VDSL2 deployments the DSLAM is located in a street cabinet and serves hundreds of subscribers at distances up to 1 km. VDSL2 is widely used in fiber to the basement. A G.fast FTTdp fiber node has the approximate size of a large shoebox and can be mounted on a pole or underground. In a FTTB deployment, the fiber node is in the basement of a multi-dwelling unit and G.fast is used on the in-building telephone cabling. In a fiber to the front yard scenario, each fiber node serves a single home; the fiber node may be reverse-powered by the subscriber modem. For the backhaul of the FTTdp fiber node, the Broadband Forum's FTTdp architecture provides GPON, XG-PON1, EPON, 10G-EPON, point-to-point fiber Ethernet, and
A baby monitor known as a baby alarm, is a radio system used to remotely listen to sounds made by an infant. An audio monitor consists of a transmitter unit, equipped with a microphone, placed near to the child, it transmits the sounds by radio waves to a receiver unit with a speaker carried by, or near to, the person caring for the infant. Some baby monitors provide two-way communication; some allow music to be played to the child. A monitor with a video camera and receiver is called a baby cam. One of the primary uses of baby monitors is to allow attendants to hear when an infant wakes, while out of immediate hearing distance of the infant. Although used, there is no evidence that these monitors prevent SIDS, many doctors believe they provide a false sense of security. Infants and young children can be heard over a baby monitor in crib talk, in which they talk to themselves; this is a normal part of practising their language skills. The first baby monitor was the Zenith Radio Nurse in 1937; this Zenith Radio product was developed by Eugene F. McDonald, designed by Japanese-American sculptor and product designer Isamu Noguchi.
Some baby monitors use a video camera to show pictures on the receiver, either by plugging the receiver into a television or by including a portable LCD screen. This type of surveillance camera is called a baby cam; some baby cams can work at night with low light levels. Most video baby monitors today have a night vision feature. Infrared LEDs attached on the front of the camera allow a user to see the baby in a dark room. Video baby monitors that have night vision mode will switch to this mode automatically in the dark; some advanced baby cams now work over Wi-Fi so parents can watch babies through their smartphone or computer. Baby monitors continue to evolve and now can utilize features such as night lights and built-in lullabies; these are not available in all monitors. Some include temperature and movement monitoring devices to sit underneath a mattress or close to the baby within a cot. A baby movement monitor uses sensor pads placed under the crib mattress to detect movement, if movement stops for more than 20 seconds an alarm will sound.
Baby monitors use wireless systems, but can use wires or may operate over existing household wiring such as X10. Wireless systems use radio frequencies. For example, in North America frequencies near 49 MHz, 902 MHz or 2.4 GHz are available. While these frequencies are not assigned to powerful television or radio broadcasting transmitters, interference from other wireless devices such as cordless telephones, wireless toys, computer wireless networks, Smart Power Meters and microwave ovens is possible. Digital audio wireless systems using DECT, are resistant to interference and have a range up to 300 m. Analog audio transmissions can be picked up at a distance from the home by a scanner receiver or other baby monitor receivers, so present a risk to privacy as long as the transmitter is switched on. Digital transmission such as Frequency-hopping spread spectrum provides a level of protection from casual interception; some wireless baby monitors support multiple cameras on one handheld monitor-receiver.
These systems are compatible with a standard wireless security camera. FM transmitters, paired with a microphone can be an inexpensive solution to a DIY baby monitor, since clock radios can be used as one. Smartphone apps allow a user to monitor a camera-equipped device, such as another smartphone or a tablet. Alternatively, Wi-Fi or Bluetooth can link a camera to a dedicated app on a tablet; this means. Portable battery-operated receivers can be carried by the parent around the house; the transmitter stays near the infant crib and is plugged into a socket. Some baby monitor. Baby monitors may have a visible signal as well as repeating the sound; this is in the form of a set of lights to indicate the noise level, allowing the device to be used when it is inappropriate or impractical for the receiver to play the sound. Other monitors have a vibrating alert on the receiver making it useful for people with hearing difficulties. Systems with several transmitters can monitor several rooms in the home at once.
Transmitters with movement sensors such as a pressure-sensitive mat placed beneath the child's mattress give additional warning of restless activity by the infant. The new voluntary ASTM International F2951 standard has been developed to address incidents associated with strangulations that can result from infant entanglement in the cords of baby monitors; this standard for baby monitors includes requirements for audio and motion sensor monitors. It provides requirements for labeling, instructional material and packaging and is intended to minimize injuries to children resulting from normal use and reasonably foreseeable misuse or abuse of baby monitors. Signals of baby monitors can be received by third parties. Frequency-hopping spread spectrum Nanny cam Spy video car Title 47 CFR Part 15
A walkie-talkie is a hand-held, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, engineering teams at Motorola. First used for infantry, similar designs were created for field artillery and tank units, after the war, walkie-talkies spread to public safety and commercial and jobsite work. Typical walkie-talkies resemble a telephone handset, with a speaker built into one end and a microphone in the other and an antenna mounted on the top of the unit, they are held up to the face to talk. A walkie-talkie is a half-duplex communication device. Multiple walkie-talkies use a single radio channel, only one radio on the channel can transmit at a time, although any number can listen; the transceiver is in receive mode. Canadian inventor Donald Hings was the first to create a portable radio signaling system for his employer CM&S in 1937, he called the system a "packset", although it became known as a "walkie-talkie".
In 2001, Hings was formally decorated for the device's significance to the war effort. Hings' model C-58 "Handy-Talkie" was in military service by 1942, the result of a secret R&D effort that began in 1940. Alfred J. Gross, a radio engineer and one of the developers of the Joan-Eleanor system worked on the early technology behind the walkie-talkie between 1938 and 1941, is sometimes credited with inventing it; the first device to be nicknamed a "walkie-talkie" was developed by the US military during World War II, the backpacked Motorola SCR-300. It was created by an engineering team in 1940 at the Galvin Manufacturing Company; the team consisted of Dan Noble. The first handheld walkie-talkie was the AM SCR-536 transceiver from 1941 made by Motorola, named the Handie-Talkie; the terms are confused today, but the original walkie-talkie referred to the back mounted model, while the handie-talkie was the device which could be held in the hand. Both devices were powered by high voltage dry cell batteries.
Following World War II, Raytheon developed the SCR-536's military replacement, the AN/PRC-6. The AN/PRC-6 circuit used 13 vacuum tubes; the unit was factory set with one crystal which could be changed to a different frequency in the field by replacing the crystal and re-tuning the unit. It used a 24-inch whip antenna. There was an optional handset. An adjustable strap was provided for support while operating. In the mid-1970s, the United States Marine Corps initiated an effort to develop a squad radio to replace the unsatisfactory helmet-mounted AN/PRR-9 receiver and receiver/transmitter handheld AN/PRT-4; the AN/PRC-68, first produced in 1976 by Magnavox, was issued to the Marines in the 1980s, was adopted by the US Army as well. The abbreviation HT, derived from Motorola's "Handie-Talkie" trademark, is used to refer to portable handheld ham radios, with "walkie-talkie" used as a layman's term or to refer to a toy. Public safety and commercial users refer to their handhelds as "radios". Surplus Motorola Handie-Talkies found their way into the hands of ham radio operators following World War II.
Motorola's public safety radios of the 1950s and 1960s were loaned or donated to ham groups as part of the Civil Defense program. To avoid trademark infringement, other manufacturers use designations such as "Handheld Transceiver" or "Handie Transceiver" for their products; some cellular telephone networks offer a push-to-talk handset that allows walkie-talkie-like operation over the cellular network, without dialing a call each time. However, the cellphone provider must be accessible. Walkie-talkies for public safety and industrial uses may be part of trunked radio systems, which dynamically allocate radio channels for more efficient use of limited radio spectrum; such systems always work with a base station that acts as a repeater and controller, although individual handsets and mobiles may have a mode that bypasses the base station. Walkie-talkies are used in any setting where portable radio communications are necessary, including business, public safety, outdoor recreation, the like, devices are available at numerous price points from inexpensive analog units sold as toys up to ruggedized analog and digital units for use on boats or in heavy industry.
Most countries allow the sale of walkie-talkies for, at least, marine communications, some limited personal uses such as CB radio, as well as for amateur radio designs. Walkie-talkies, thanks to increasing use of miniaturized electronics, can be made small, with some personal two-way UHF radio models being smaller than a deck of cards. In addition, as costs come down, it is possible to add advanced squelch capabilities such as CTCSS and DCS to inexpensive radios, as well as voice scrambling and trunking capabilities; some units include DTMF keypads for remote