Asymmetric digital subscriber line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ADSL differs from the less common symmetric digital subscriber line. In ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premises than the reverse. Providers market ADSL as a service for consumers for Internet access for downloading content from the Internet, but not serving content accessed by others. ADSL works by using the frequency spectrum above the band used by voice telephone calls. With a DSL filter called splitter, the frequency bands are isolated, permitting a single telephone line to be used for both ADSL service and telephone calls at the same time. ADSL is only installed for short distances from the telephone exchange less than 4 kilometres, but has been known to exceed 8 kilometres if the laid wire gauge allows for further distribution.
At the telephone exchange, the line terminates at a digital subscriber line access multiplexer where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL are routed over the telephone company's data network and reach a conventional Internet Protocol network. There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is to be more crosstalk from other circuits at the DSLAM end than at the customer premises, thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL; the marketing reasons for an asymmetric connection are that, most users of internet traffic will require less data to be uploaded than downloaded.
For example, in normal web browsing, a user will visit a number of web sites and will need to download the data that comprises the web pages from the site, text, sound files etc. but they will only upload a small amount of data, as the only uploaded data is that used for the purpose of verifying the receipt of the downloaded data or any data inputted by the user into forms etc. This provides a justification for internet service providers to offer a more expensive service aimed at commercial users who host websites, who therefore need a service which allows for as much data to be uploaded as downloaded. File sharing applications are an obvious exception to this situation. Secondly internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate a lot of uploads. Most ADSL communication is full-duplex. Full-duplex ADSL communication is achieved on a wire pair by either frequency-division duplex, echo-cancelling duplex, or time-division duplex.
FDD uses two separate frequency bands, referred to as the downstream bands. The upstream band is used for communication from the end user to the telephone central office; the downstream band is used for communicating from the central office to the end user. With deployed ADSL over POTS, the band from 26.075 kHz to 137.825 kHz is used for upstream communication, while 138–1104 kHz is used for downstream communication. Under the usual DMT scheme, each of these is further divided into smaller frequency channels of 4.3125 kHz. These frequency channels are sometimes termed bins. During initial training to optimize transmission quality and speed, the ADSL modem tests each of the bins to determine the signal-to-noise ratio at each bin's frequency. Distance from the telephone exchange, cable characteristics, interference from AM radio stations, local interference and electrical noise at the modem's location can adversely affect the signal-to-noise ratio at particular frequencies. Bins for frequencies exhibiting a reduced signal-to-noise ratio will be used at a lower throughput rate or not at all.
The DSL modem will make a plan on how to exploit each of the bins, sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio will be chosen to transmit signals chosen from a greater number of possible encoded values in each main clock cycle; the number of possibilities must not be so large that the receiver might incorrectly decode which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in a process called "bitswap", where bins that have become more noisy are only required to carry fewer bits and other channels will be chosen to be given a higher burden; the data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined.
Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower s
Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference, it was invented by Alexander Graham Bell. In a balanced line, the two wires carry equal and opposite signals, the destination detects the difference between the two; this is known as differential signaling. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally; the noise thus produces a common-mode signal which can be canceled at the receiver when the difference signal is taken. Differential signaling starts to fail; this problem is apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles.
Twisting the pairs counters this effect as on each half twist the wire nearest to the noise-source is exchanged. Provided the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common-mode; the twist rate makes up part of the specification for a given type of cable. When nearby pairs have equal twist rates, the same conductors of the different pairs may lie next to each other undoing the benefits of differential mode. For this reason it is specified that, at least for cables containing small numbers of pairs, the twist rates must differ. In contrast to shielded or foiled twisted pair, UTP cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is common for computer networking; the earliest telephones used open-wire single-wire earth return circuits. In the 1880s electric trams were installed in many cities. Lawsuits being unavailing, the telephone companies converted to balanced circuits, which had the incidental benefit of reducing attenuation, hence increasing range.
As electrical power distribution became more commonplace, this measure proved inadequate. Two wires, strung on either side of cross bars on utility poles, shared the route with electrical power lines. Within a few years, the growing use of electricity again brought an increase of interference, so engineers devised a method called wire transposition, to cancel out the interference. In wire transposition, the wires exchange position once every several poles. In this way, the two wires would receive similar EMI from power lines; this represented an early implementation of twisting, with a twist rate of about four twists per kilometre, or six per mile. Such open-wire balanced lines with periodic transpositions still survive today in some rural areas. Twisted-pair cabling was invented by Alexander Graham Bell in 1881. By 1900, the entire American telephone line network was either twisted pair or open wire with transposition to guard against interference. Today, most of the millions of kilometres of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, only handled or seen by telephone workers.
Unshielded twisted pair cables are found in many Ethernet networks and telephone systems. For indoor telephone applications, UTP is grouped into sets of 25 pairs according to a standard 25-pair color code developed by AT&T Corporation. A typical subset of these colors shows up in most UTP cables; the cables are made with copper wires measured at 22 or 24 American Wire Gauge, with the colored insulation made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is divided into small but identical bundles; each bundle consists of twisted pairs. The bundles are in turn twisted together to make up the cable. Pairs having the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are selected to minimize crosstalk within a large cable. UTP cable is the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, can use UTP cables.
Twisted pair cabling is used in data networks for short and medium length connections because of its lower costs compared to optical fiber and coaxial cable. UTP is finding increasing use in video applications in security cameras. Many cameras include a UTP output with screw terminals; as UTP is a balanced transmission line, a balun is needed to connect to unbalanced equipment, for example any using BNC connectors and designed for coaxial cable. Twisted pair cables incorporate shielding in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield; such shielding can be applied to individual quads. Individual pairs are foil shielded, while an overall cable may use any of braided screen or foi
A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such in computer networking, telecommunication equipment, audio headsets. Modular connectors were developed for use on specific Bell System telephone sets in the 1960s, similar types found use for simple interconnection of customer-provided telephone subscriber premises equipment to the telephone network; the Federal Communication Commission mandated in 1976 an interface registration system, in which they became known as registered jacks. The convenience of prior existence for designers and ease of use led to proliferation of modular connectors for many other applications. Many applications that used a bulkier, more expensive connector have converted to modular connectors; the best known applications of modular connectors are for telephone and Ethernet. Accordingly, various electronic interface specifications exist for applications using modular connectors, which prescribe physical characteristics and assign electrical signals to their contacts.
Modular connectors are referred to as modular phone jack and plug, RJ connector, Western jack and plug. The term modular connector arose from its original use in a novel system of cabling designed to make telephone equipment more modular; this includes 4P4C used for handset connectors. Registered jack designations describe the signals and wiring used for voice and data communication in modular and other connectors, it is common to use a registered jack number to refer to the physical connector itself. A common use of 8P8C connectors is Ethernet over twisted pair; the 4P4C connector is sometimes erroneously called RJ9 or RJ22 and various six-position modular connectors may be incorrectly called RJ11. The first types of small modular telephone connectors were created by AT&T in the mid-1960s for the plug-in handset and line cords of the Trimline telephone. Driven by demand for multiple sets in residences with various lengths of cords, the Bell System introduced customer-connectable part kits and telephones, sold through PhoneCenter stores in the early 1970s.
For this purpose, Illinois Bell started installing modular telephone sets on a limited scale in June 1972. The patents by Edwin C. Hardesty and coworkers, US 3699498 and US 3860316, followed by other improvements, were the basis for the modular molded-plastic connectors that became commonplace for telephone cords by the 1980s. In 1976, these connectors were standardized nationally in the United States by the Registration Interface program of the Federal Communication Commission, which designated a series of Registered Jack specifications for interconnection of customer-premises equipment to the public switched telephone network. Modular connectors have gender: plugs are considered to be male, while jacks or sockets are considered to be female. Plugs are used to terminate cables and cords, while jacks are used for fixed locations on surfaces of walls and equipment. Other than telephone extension cables, cables with a modular plug on one end and a jack on the other are rare. Instead, cables are connected using a female-to-female adapter, having two jacks wired back-to-back.
Modular connectors are designed to latch together. As a plug is inserted into a jack, a plastic tab on the plug locks so that the plug cannot be pulled out. To remove the plug, the latching tab must be depressed against the plug to clear the locking edge; the standard orientation for installing a jack in a wall or panel is with the tab down. The latching tab may snag on other cables and break off resulting in loss of the secure latching feature. To prevent this, tabs are protected with a boot over the plug, or a special tab design, on snagless cords. Most protective boots must be installed onto a cable; this means. However, protective boots or rigid protective ramp adapters are available which can be snapped over an installed unprotected modular plug. Modular connectors are designated using two numbers that represent the maximum number of contact positions and number of installed contacts, with each number followed by P and C, respectively. For example, 6P2C is a connector having two installed contacts.
Alternate designations omit the letters while separating the position and contact quantities with either an x or a slash. When not installed, contacts are omitted from the outer positions inward, such that the number of contacts is always even; the connector body positions with omitted or unconnected contacts are unused for the electrical connection, but ensure that the plug fits correctly. For instance, RJ11 cables have connectors with six positions and four contacts, to which are attached just two wires; the contact positions are numbered sequentially starting from 1. When viewed head-on with the retention mechanism on the bottom, jacks will have contact position number 1 on the left and plugs will have it on the right. Contacts are numbered by the contact position. For example, on a six-position, two-contact plug, where the outermost four positions do not have contacts, the innermost two contacts are numbered 3 and 4. Modular connectors are manufactured in four sizes, with 4-, 6-, 8-, 10-positions.
The insulating plastic bodies of 4P and 6P connectors have different widths, whereas 8P or 10P connectors share an larger body width. Internally, the contacts on the plugs have sharp prongs that, when crimped, pierc
The telex network was a public switched network of teleprinters similar to a telephone network, for the purposes of sending text-based messages. Telex was a major method of sending written messages electronically between businesses in the post-World War II period, its usage went into decline. The "telex" term refers to the network, not the teleprinters. Teleprinters evolved from telegraph systems, like the telegraph, they used binary signals, which means that symbols were represented by the presence or absence of a pre-defined level of electric current; this is different from the analog telephone system, which used varying voltages to represent sound. For this reason, telex exchanges were separate from the telephone system, with their own signalling standards and system of "telex numbers". Telex provided the first common medium for international record communications using standard signalling techniques and operating criteria as specified by the International Telecommunication Union. Customers on any telex exchange could deliver messages around the world.
To lower line usage, telex messages were first encoded onto paper tape and read into the line as as possible. The system delivered information at 50 baud or 66 words per minute, encoded using the International Telegraph Alphabet No. 2. In the last days of the telex networks, end-user equipment was replaced by modems and phone lines, reducing the telex network to what was a directory service running on the phone network. Telex began in Germany as a research and development program in 1926 that became an operational teleprinter service in 1933; the service, operated by the Reichspost had a speed of 50 baud — 66 words per minute. Telex service spread around the world. By 1978, West Germany, including West Berlin, had 123,298 telex connections. Long before automatic telephony became available, most countries in central Africa and Asia, had at least a few high-frequency telex links. Government postal and telegraph services initiated these radio links; the most common radio standard, CCITT R.44 had error-corrected retransmitting time-division multiplexing of radio channels.
Most impoverished PTTs operated their telex-on-radio channels non-stop, to get the maximum value from them. The cost of TOR equipment has continued to fall. Although the system required specialised equipment, as of 2016 many amateur radio operators operate TOR with special software and inexpensive hardware to connect computer sound cards to short-wave radios. Modern cablegrams or telegrams operate over dedicated telex networks, using TOR whenever required. Telex served as the forerunner of modern fax and text messaging — both technically and stylistically. Abbreviated English as used in texting originated with telex operators exchanging informal messages in real time — they became the first "texters" long before the introduction of mobile phones. Telex users could send the same message to several places around the world at the same time, like email today, using the Western Union InfoMaster Computer; this involved transmitting the message via paper tape to the InfoMaster Computer and specifying the destination addresses for the single text.
In this way, a single message could be sent to multiple distant Telex and TWX machines as well as delivering the same message to non-Telex and non-TWX subscribers via Western Union Mailgram. Telex messages are routed by addressing them to a telex address, e.g. "14910 ERIC S", where 14910 is the subscriber number, ERIC is an abbreviation for the subscriber's name and S is the country code. Solutions exist for the automatic routing of messages to different telex terminals within a subscriber organization, by using different terminal identities, e.g. "+T148". A major advantage of telex is that the receipt of the message by the recipient could be confirmed with a high degree of certainty by the "answerback". At the beginning of the message, the sender would transmit a WRU code, the recipient machine would automatically initiate a response, encoded in a rotating drum with pegs, much like a music box; the position of the pegs sent an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient.
The WRU code would be sent at the end of the message, so a correct response would confirm that the connection had remained unbroken during the message transmission. This gave telex a major advantage over group 2 fax; the usual method of operation was that the message would be prepared using paper tape. All common telex machines incorporated reader. Once the paper tape had been prepared, the message could be transmitted in minimum time. Telex billing was always by connected duration, so minimizing the connected time saved money. However, it was possible to connect in "real time", where the sender and the recipient could both type on the keyboard and these characters would be printed on the distant machine. Telex could be used as a rudimentary but functional carrier of information from one IT system to another, in effect a primitive forerunner of Electronic Data Interchange; the sending IT system would create an output on paper tape using a mutually agree
Australia the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the world's sixth-largest country by total area; the neighbouring countries are Papua New Guinea and East Timor to the north. The population of 25 million is urbanised and concentrated on the eastern seaboard. Australia's capital is Canberra, its largest city is Sydney; the country's other major metropolitan areas are Melbourne, Brisbane and Adelaide. Australia was inhabited by indigenous Australians for about 60,000 years before the first British settlement in the late 18th century, it is documented. After the European exploration of the continent by Dutch explorers in 1606, who named it New Holland, Australia's eastern half was claimed by Great Britain in 1770 and settled through penal transportation to the colony of New South Wales from 26 January 1788, a date which became Australia's national day; the population grew in subsequent decades, by the 1850s most of the continent had been explored and an additional five self-governing crown colonies established.
On 1 January 1901, the six colonies federated. Australia has since maintained a stable liberal democratic political system that functions as a federal parliamentary constitutional monarchy, comprising six states and ten territories. Being the oldest and driest inhabited continent, with the least fertile soils, Australia has a landmass of 7,617,930 square kilometres. A megadiverse country, its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east. A gold rush began in Australia in the early 1850s, its population density, 2.8 inhabitants per square kilometre, remains among the lowest in the world. Australia generates its income from various sources including mining-related exports, telecommunications and manufacturing. Indigenous Australian rock art is the oldest and richest in the world, dating as far back as 60,000 years and spread across hundreds of thousands of sites. Australia is a developed country, with the world's 14th-largest economy.
It has a high-income economy, with the world's tenth-highest per capita income. It is a regional power, has the world's 13th-highest military expenditure. Australia has the world's ninth-largest immigrant population, with immigrants accounting for 26% of the population. Having the third-highest human development index and the eighth-highest ranked democracy globally, the country ranks in quality of life, education, economic freedom, civil liberties and political rights, with all its major cities faring well in global comparative livability surveys. Australia is a member of the United Nations, G20, Commonwealth of Nations, ANZUS, Organisation for Economic Co-operation and Development, World Trade Organization, Asia-Pacific Economic Cooperation, Pacific Islands Forum and the ASEAN Plus Six mechanism; the name Australia is derived from the Latin Terra Australis, a name used for a hypothetical continent in the Southern Hemisphere since ancient times. When Europeans first began visiting and mapping Australia in the 17th century, the name Terra Australis was applied to the new territories.
Until the early 19th century, Australia was best known as "New Holland", a name first applied by the Dutch explorer Abel Tasman in 1644 and subsequently anglicised. Terra Australis still saw occasional usage, such as in scientific texts; the name Australia was popularised by the explorer Matthew Flinders, who said it was "more agreeable to the ear, an assimilation to the names of the other great portions of the earth". The first time that Australia appears to have been used was in April 1817, when Governor Lachlan Macquarie acknowledged the receipt of Flinders' charts of Australia from Lord Bathurst. In December 1817, Macquarie recommended to the Colonial Office. In 1824, the Admiralty agreed that the continent should be known by that name; the first official published use of the new name came with the publication in 1830 of The Australia Directory by the Hydrographic Office. Colloquial names for Australia include "Oz" and "the Land Down Under". Other epithets include "the Great Southern Land", "the Lucky Country", "the Sunburnt Country", "the Wide Brown Land".
The latter two both derive from Dorothea Mackellar's 1908 poem "My Country". Human habitation of the Australian continent is estimated to have begun around 65,000 to 70,000 years ago, with the migration of people by land bridges and short sea-crossings from what is now Southeast Asia; these first inhabitants were the ancestors of modern Indigenous Australians. Aboriginal Australian culture is one of the oldest continual civilisations on earth. At the time of first European contact, most Indigenous Australians were hunter-gatherers with complex economies and societies. Recent archaeological finds suggest. Indigenous Australians have an oral culture with spiritual values based on reverence for the land and a belief in the Dreamtime; the Torres Strait Islanders, ethnically Melanesian, obtained their livelihood from seasonal horticulture and the resources of their reefs and seas. The northern coasts and waters of Australia were visited s