Electronic mail is a method of exchanging messages between people using electronic devices. Invented by Ray Tomlinson, email first entered limited use in the 1960s and by the mid-1970s had taken the form now recognized as email. Email operates across computer networks, which today is the Internet; some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging. Today's email systems are based on a store-and-forward model. Email servers accept, forward and store messages. Neither the users nor their computers are required to be online simultaneously. An ASCII text-only communications medium, Internet email was extended by Multipurpose Internet Mail Extensions to carry text in other character sets and multimedia content attachments. International email, with internationalized email addresses using UTF-8, has been standardized, but as of 2017 it has not been adopted; the history of modern Internet email services reaches back to the early ARPANET, with standards for encoding email messages published as early as 1973.
An email message sent in the early 1970s looks similar to a basic email sent today. Email had an important role in creating the Internet, the conversion from ARPANET to the Internet in the early 1980s produced the core of the current services; the term electronic mail was used generically for any electronic document transmission. For example, several writers in the early 1970s used the term to describe fax document transmission; as a result, it is difficult to find the first citation for the use of the term with the more specific meaning it has today. Electronic mail has been most called email or e-mail since around 1993, but variations of the spelling have been used: email is the most common form used online, is required by IETF Requests for Comments and working groups and by style guides; this spelling appears in most dictionaries. E-mail is the format that sometimes appears in edited, published American English and British English writing as reflected in the Corpus of Contemporary American English data, but is falling out of favor in some style guides.
Mail was the form used in the original protocol standard, RFC 524. The service is referred to as mail, a single piece of electronic mail is called a message. EMail is a traditional form, used in RFCs for the "Author's Address" and is expressly required "for historical reasons". E-mail is sometimes used, capitalizing the initial E as in similar abbreviations like E-piano, E-guitar, A-bomb, H-bomb. An Internet e-mail consists of an content. Computer-based mail and messaging became possible with the advent of time-sharing computers in the early 1960s, informal methods of using shared files to pass messages were soon expanded into the first mail systems. Most developers of early mainframes and minicomputers developed similar, but incompatible, mail applications. Over time, a complex web of gateways and routing systems linked many of them. Many US universities were part of the ARPANET, which aimed at software portability between its systems; that portability helped make the Simple Mail Transfer Protocol influential.
For a time in the late 1980s and early 1990s, it seemed that either a proprietary commercial system or the X.400 email system, part of the Government Open Systems Interconnection Profile, would predominate. However, once the final restrictions on carrying commercial traffic over the Internet ended in 1995, a combination of factors made the current Internet suite of SMTP, POP3 and IMAP email protocols the standard; the diagram to the right shows a typical sequence of events that takes place when sender Alice transmits a message using a mail user agent addressed to the email address of the recipient. The MUA formats the message in email format and uses the submission protocol, a profile of the Simple Mail Transfer Protocol, to send the message content to the local mail submission agent, in this case smtp.a.org. The MSA determines the destination address provided in the SMTP protocol, in this case firstname.lastname@example.org, a qualified domain address. The part before the @ sign is the local part of the address the username of the recipient, the part after the @ sign is a domain name.
The MSA resolves a domain name to determine the qualified domain name of the mail server in the Domain Name System. The DNS server for the domain b.org responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a message transfer agent server run by the recipient's ISP. smtp.a.org sends the message to mx.b.org using SMTP. This server may need to forward the message to other MTAs before the message reaches the final message delivery agent; the MDA delivers it to the mailbox of user bob. Bob's MUA picks up the message using either the Post Office Protocol or the Internet Message Access Protocol. In addition to this example and complications exist in the email system: Alice or Bob may use a client connected to a corporate email system, such as IBM Lotus Notes or Microsoft Exchange; these systems have their own internal email format and their clients communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which does any necessary reformatt
A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced intelligible replication of the human voice; this instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones became indispensable to businesses and households and are today some of the most used small appliances; the essential elements of a telephone are a microphone to speak into and an earphone which reproduces the voice in a distant location. In addition, most telephones contain a ringer to announce an incoming telephone call, a dial or keypad to enter a telephone number when initiating a call to another telephone.
The receiver and transmitter are built into a handset, held up to the ear and mouth during conversation. The dial may be located either on a base unit to which the handset is connected; the transmitter converts the sound waves to electrical signals which are sent through a telephone network to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices; the first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were replaced by manually operated centrally located switchboards; these exchanges were soon connected together forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973.
In decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation, they may be able to record spoken messages and receive text messages and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. A traditional landline telephone system known as plain old telephone service carries both control and audio signals on the same twisted pair of insulated wires, the telephone line; the control and signaling equipment consists of three components, the ringer, the hookswitch, a dial. The ringer, or beeper, light or other device, alerts the user to incoming calls; the hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.
A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used exclusively the rotary technology, replaced by dual-tone multi-frequency signaling with pushbutton telephones. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference and crosstalk better than a single wire or an untwisted pair; the strong outgoing speech signal from the microphone does not overpower the weaker incoming speaker signal with sidetone because a hybrid coil and other components compensate the imbalance. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths; the line voltages are negative compared to earth. Negative voltage attracts positive metal ions toward the wires.
The landline telephone contains a switchhook and an alerting device a ringer, that remains connected to the phone line whenever the phone is "on hook", other components which are connected when the phone is "off hook". The off-hook components include a transmitter, a receiver, other circuits for dialing 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 switchhook, which powers the telephone by connecting the transmitter and related audio components to the line; the off-hook circuitry has a low resistance which causes a direct current, which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, sends a dial tone to indicate readiness. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the called party; the keys control a tone generator circuit. A rotary-dial telephone uses pulse
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
Audio editing software
Audio editing software is software which allows editing and generating of audio data. Audio editing software can be implemented or as library, as computer application, as Web application or as a loadable kernel module. Wave Editors are digital audio editors and there are many sources of software available to perform this function. Most can apply effects and filters, adjust stereo channels etc.. A digital audio workstation consists of software to a great degree, is composed of many distinct software suite components, giving access to them through a unified graphical user interface using GTK+, Qt or some other library for the GUI widgets. Editors designed for use with music allow the user to do the following: The ability to import and export various audio file formats for editing. Record audio from one or more inputs and store recordings in the computer's memory as digital audio Edit the start time, stop time, duration of any sound on the audio timeline Fade into or out of a clip, or between clips Mix multiple sound sources/tracks, combine them at various volume levels and pan from channel to channel to one or more output tracks Apply simple or advanced effects or filters, including compression, flanging, audio noise reduction and equalization to change the audio Playback sound that can be sent to one or more outputs, such as speakers, additional processors, or a recording medium Conversion between different audio file formats, or between different sound quality levelsTypically these tasks can be performed in a manner, non-linear.
Audio editors may process the audio data non-destructively in real-time, or destructively as an "off-line" process, or a hybrid with some real-time effects and some off-line effects. Destructive editing modifies the data of the original audio file, as opposed to just editing its playback parameters. Destructive editors are known as "sample editors". Destructive editing applies edits and processing directly to the audio data, changing the data immediately. If, for example, part of a track is deleted, the "deleted" audio data is removed from that part of the track. Real-time editing does not apply changes but applies edits and processing on the fly during playback. If, for example, part of a track is deleted, the "deleted" audio data is not removed from the track, but is hidden and will be skipped on playback. In graphical editors, all changes to the audio is visible as the visible waveform is updated to match the audio data; the number of effects that may be applied is unlimited. Editing is precise down to exact sample intervals.
Effects may be applied to a specified selected region. Mixing down or exporting the edited audio is relatively quick as little additional processing is required. Once an effect has been applied, it cannot be changed; this is mitigated by the ability to "undo" the last performed action. A destructive audio editor will maintain many levels of "undo history" so that multiple actions may be undone in the reverse order that they were applied. Edits can only be undone in the reverse order. Effects can be adjusted during playback, or at any other time. Edits may be adjusted at any time in any order. Multiple effects and edits may be ` stacked'. A stack of effects may be changed so that effects are applied in a different order, or effects inserted or removed from the chain; some real-time editors support effect automation so that changes to effect parameters may be programmed to occur at specified times during audio playback. The waveform does not show the effect of processing until the audio has been mixed-down or "bounced" to another track.
The number of effects that may be applied is limited by the available processing power of the computer or editing hardware. In some editors this may be mitigated by "freezing" the track, it is not possible to have an effect only on part of a track. To apply a real-time effect to part of a track required that the effect is set to turn on at one point and turn off at another. In multi-track editors, if audio is copied or moved from one track to another, the audio in the new track may sound different from how it sounded in the original track as there may be different real-time effects in each track. In some applications, mixing down or exporting the edited audio may be slow as all effects and processing needs to be applied. Editors designed for use in speech research add the ability to make measurements and perform acoustic analyses such as extracting and displaying a fundamental frequency contour or spectrogram, they lacks most or all of the effects that interest musicians. Audio signal processing Digital audio workstation Comparison of digital audio editors Comparison of free software for audio Music sequencer Software synthesizer Software effect processor List of music software
Online shopping is a form of electronic commerce which allows consumers to directly buy goods or services from a seller over the Internet using a web browser. Consumers find a product of interest by visiting the website of the retailer directly or by searching among alternative vendors using a shopping search engine, which displays the same product's availability and pricing at different e-retailers; as of 2016, customers can shop online using a range of different computers and devices, including desktop computers, tablet computers and smartphones. An online shop evokes the physical analogy of buying products or services at a regular "bricks-and-mortar" retailer or shopping center; when an online store is set up to enable businesses to buy from another businesses, the process is called business-to-business online shopping. A typical online store enables the customer to browse the firm's range of products and services, view photos or images of the products, along with information about the product specifications and prices.
Online stores enable shoppers to use "search" features to find specific models, brands or items. Online customers must have access to the Internet and a valid method of payment in order to complete a transaction, such as a credit card, an Interac-enabled debit card, or a service such as PayPal. For physical products, the e-tailer ships the products to the customer; the largest of these online retailing corporations are Alibaba, Amazon.com, eBay. Alternative names for the activity are "e-tailing", a shortened form of "electronic retail" or "e-shopping", a shortened form of "electronic shopping". An online store may be called an e-web-store, e-shop, e-store, Internet shop, web-shop, web-store, online store, online storefront and virtual store. Mobile commerce describes purchasing from an online retailer's mobile device-optimized website or software application; these websites or apps are designed to enable customers to browse through a companies' products and services on tablet computers and smartphones.
One of the earliest forms of trade conducted online was IBM's online transaction processing developed in the 1960s and it allowed the processing of financial transactions in real-time. The computerized ticket reservation system developed for American Airlines called Semi-Automatic Business Research Environment was one of its applications. Here, computer terminals located in different travel agencies were linked to a large IBM mainframe computer, which processed transactions and coordinated them so that all travel agents had access to the same information at the same time; the emergence of online shopping as we know today developed with the emergence of the Internet. This platform only functioned as an advertising tool for companies, providing information about its products, it moved on from this simple utility to actual online shopping transaction due to the development of interactive Web pages and secure transmissions. The growth of the internet as a secure shopping channel has developed since 1994, with the first sales of Sting album'Ten Summoner's Tales'.
Wine and flowers soon followed and were among the pioneering retail categories which fueled the growth of online shopping. Researchers found that having products that are appropriate for e-commerce was a key indicator of Internet success. Many of these products did well as they are generic products which shoppers did not need to touch and feel in order to buy, but importantly, in the early days, there were few shoppers online and they were from a narrow segment: affluent, male, 30+. Online shopping has come along way since these early days and -in the UK- accounts for significant percents; as the revenues from online sales continued to grow researchers identified different types of online shoppers, Rohm & Swaninathan identified four categories and named them "convenience shoppers, variety seekers, balanced buyers, store-oriented shoppers". They focused on shopping motivations and found that the variety of products available and the perceived convenience of the buying online experience were significant motivating factors.
This was different for offline shoppers, who were more motivated by time saving and recreational motives. Digital High Street 2020 English entrepreneur Michael Aldrich was a pioneer of online shopping in 1979, his system connected a modified domestic TV to a real-time transaction processing computer via a domestic telephone line. He believed that videotex, the modified domestic TV technology with a simple menu-driven human–computer interface, was a'new, universally applicable, participative communication medium — the first since the invention of the telephone.' This enabled'closed' corporate information systems to be opened to'outside' correspondents not just for transaction processing but for e-messaging and information retrieval and dissemination known as e-business. His definition of the new mass communications medium as'participative' was fundamentally different from the traditional definitions of mass communication and mass media and a precursor to the social networking on the Internet 25 years later.
In March 1980 he launched Redifon's Office Revolution, which allowed consumers, agents, distributors and service companies to be connected on-line to the corporate systems and allow business transactions to be completed electronically in real-time. During the 1980s he
A web browser is a software application for accessing information on the World Wide Web. Each individual web page and video is identified by a distinct Uniform Resource Locator, enabling browsers to retrieve these resources from a web server and display them on the user's device. A web browser is not the same thing as a search engine, though the two are confused. For a user, a search engine is just a website, such as google.com, that stores searchable data about other websites. But to connect to a website's server and display its web pages, a user needs to have a web browser installed on their device; the most popular browsers are Chrome, Safari, Internet Explorer, Edge. The first web browser, called WorldWideWeb, was invented in 1990 by Sir Tim Berners-Lee, he recruited Nicola Pellow to write the Line Mode Browser, which displayed web pages on dumb terminals. 1993 was a landmark year with the release of Mosaic, credited as "the world's first popular browser". Its innovative graphical interface made the World Wide Web system easy to use and thus more accessible to the average person.
This, in turn, sparked the Internet boom of the 1990s when the Web grew at a rapid rate. Marc Andreessen, the leader of the Mosaic team, soon started his own company, which released the Mosaic-influenced Netscape Navigator in 1994. Navigator became the most popular browser. Microsoft debuted Internet Explorer in 1995. Microsoft was able to gain a dominant position for two reasons: it bundled Internet Explorer with its popular Microsoft Windows operating system and did so as freeware with no restrictions on usage; the market share of Internet Explorer peaked at over 95% in 2002. In 1998, desperate to remain competitive, Netscape launched what would become the Mozilla Foundation to create a new browser using the open source software model; this work evolved into Firefox, first released by Mozilla in 2004. Firefox reached a 28% market share in 2011. Apple released its Safari browser in 2003, it remains the dominant browser on Apple platforms. The last major entrant to the browser market was Google, its Chrome browser, which debuted in 2008, has been a huge success.
Once a web page has been retrieved, the browser's rendering engine displays it on the user's device. This includes video formats supported by the browser. Web pages contain hyperlinks to other pages and resources; each link contains a URL, when it is clicked, the browser navigates to the new resource. Thus the process of bringing content to the user begins again. To implement all of this, modern browsers are a combination of numerous software components. Web browsers can be configured with a built-in menu. Depending on the browser, the menu may be named Options, or Preferences; the menu has different types of settings. For example, users can change their home default search engine, they can change default web page colors and fonts. Various network connectivity and privacy settings are usually available. During the course of browsing, cookies received from various websites are stored by the browser; some of them contain login credentials or site preferences. However, others are used for tracking user behavior over long periods of time, so browsers provide settings for removing cookies when exiting the browser.
Finer-grained management of cookies requires a browser extension. The most popular browsers have a number of features in common, they allow users to browse in a private mode. They can be customized with extensions, some of them provide a sync service. Most browsers have these user interface features: Allow the user to open multiple pages at the same time, either in different browser windows or in different tabs of the same window. Back and forward buttons to go back to the previous page forward to the next one. A refresh or reload button to reload the current page. A stop button to cancel loading the page. A home button to return to the user's home page. An address bar to display it. A search bar to input terms into a search engine. There are niche browsers with distinct features. One example is text-only browsers that can benefit people with slow Internet connections or those with visual impairments. Mobile browser List of web browsers Comparison of web browsers Media related to Web browsers at Wikimedia Commons
Personal digital assistant
A personal digital assistant known as a handheld PC, is a variety mobile device which functions as a personal information manager. PDAs were discontinued in the early 2010s after the widespread adoption of capable smartphones, in particular those based on iOS and Android. Nearly all PDAs have the ability to connect to the Internet. A PDA has an electronic visual display. Most models have audio capabilities, allowing usage as a portable media player, enabling most of them to be used as telephones. Most PDAs can access intranets or extranets via Wi-Fi or Wireless Wide Area Networks. Sometimes, instead of buttons, PDAs employ touchscreen technology; the technology industry has recycled the term personal digital assistance. The term is more used for software that identifies a user's voice to reply to the queries; the first PDA, the Organizer, was released in 1984 by Psion, followed by Psion's Series 3, in 1991. The latter began to resemble the more familiar PDA style, including a full keyboard; the term PDA was first used on January 7, 1992 by Apple Computer CEO John Sculley at the Consumer Electronics Show in Las Vegas, referring to the Apple Newton.
In 1994, IBM introduced the first PDA with full telephone functionality, the IBM Simon, which can be considered the first smartphone. In 1996, Nokia introduced a PDA with telephone functionality, the 9000 Communicator, which became the world's best-selling PDA. Another early entrant in this market was Palm, with a line of PDA products which began in March 1996. A typical PDA has a touchscreen for navigation, a memory card slot for data storage, IrDA, Bluetooth and/or Wi-Fi. However, some PDAs may not have a touchscreen, using softkeys, a directional pad, a numeric keypad or a thumb keyboard for input. To have the functions expected of a PDA, a device's software includes an appointment calendar, a to-do list, an address book for contacts, a calculator, some sort of memo program. PDAs with wireless data connections typically include an email client and a Web browser, may or may not include telephony functionality. Many of the original PDAs, such as the Apple Newton and Palm Pilot, featured a touchscreen for user interaction, having only a few buttons—usually reserved for shortcuts to often-used programs.
Some touchscreen PDAs, including Windows Mobile devices, had a detachable stylus to facilitate making selections. The user interacts with the device by tapping the screen to select buttons or issue commands, or by dragging a finger on the screen to make selections or scroll. Typical methods of entering text on touchscreen PDAs include: A virtual keyboard, where a keyboard is shown on the touchscreen. Text is entered by tapping the on-screen keyboard with stylus. An external keyboard connected via Infrared port, or Bluetooth; some users may choose a chorded keyboard for one-handed use. Handwriting recognition, where letters or words are written on the touchscreen with a stylus, the PDA converts the input to text. Recognition and computation of handwritten horizontal and vertical formulas, such as "1 + 2 =", may be a feature. Stroke recognition allows the user to make a predefined set of strokes on the touchscreen, sometimes in a special input area, representing the various characters to be input.
The strokes are simplified character shapes, making them easier for the device to recognize. One known stroke recognition system is Palm's Graffiti. Despite research and development projects, end-users experience mixed results with handwriting recognition systems; some find it frustrating and inaccurate, while others are satisfied with the quality of the recognition. Touchscreen PDAs intended for business use, such as the BlackBerry and Palm Treo also offer full keyboards and scroll wheels or thumbwheels to facilitate data entry and navigation. Many touchscreen PDAs support some form of external keyboard as well. Specialized folding keyboards, which offer a full-sized keyboard but collapse into a compact size for transport, are available for many models. External keyboards may attach to the PDA directly, using a cable, or may use wireless technology such as infrared or Bluetooth to connect to the PDA. Newer PDAs, such as the HTC HD2, Apple iPhone, Apple iPod Touch, Palm Pre, Palm Pre Plus, Palm Pixi, Palm Pixi Plus, Google Android include more advanced forms of touchscreen that can register multiple touches simultaneously.
These "multi-touch" displays allow for more sophisticated interfaces using various gestures entered with one or more fingers. Although many early PDAs did not have memory card slots, now most have either some form of Secure Digital slot, a CompactFlash slot or a combination of the two. Although designed for memory, Secure Digital Input/Output and CompactFlash cards are available that provide accessories like Wi-Fi or digital cameras, if the device can support them; some PDAs have a USB port for USB flash drives. Some PDAs use microSD cards, which are electronically compatible with SD cards, but have a much smaller physical size. While early PDAs connected to a user's personal computer via serial ports or another proprietary connection, many today connect via a USB cable. Older PDAs were unable to connect to each other via USB, as their implementations of USB didn't support acting as the "host"; some early PDAs were able to connect to the Internet indirectly by means of an external modem connected via the PDA's serial port or "sync" connector, or directly by using an expansion card that provided an Ethernet port.
Most modern PDAs have a popular wireless protocol for mobile devices. Bluetooth can be used to connect keyboards, headsets, GPS receiver