Wednesday, December 5, 2007
Telegraphy (from the Greek words tele (τηλε) = far and graphein (γραφειν) = write) is the long-distance transmission of written messages without physical transport of letters, originally by changing something that could be observed from a distance (optical telegraphy). Radiotelegraphy or wireless telegraphy transmits messages using radio. Telegraphy includes recent forms of data transmission such as fax, email, and computer networks in general. (A telegraph is a machine for transmitting and receiving messages over long distances, i.e. for telegraphy. The word telegraph alone generally refers to an electrical telegraph). Wireless telegraphy is also known as CW, for continuous wave (a carrier modulated by on-off keying, as opposed to the earlier radio technique using a spark gap).
Telegraphy messages sent by the telegraph operators using Morse code were known as telegrams or cablegrams, often shortened to a cable or a wire message. Later, telegrams sent by the Telex network, a switched network of teleprinters similar to the telephone network, were known as telex messages. Before long distance telephone services were readily available or affordable, telegram services were very popular. Telegrams were often used to confirm business dealings and, unlike e-mail, telegrams were commonly used to create binding legal documents for business dealings.
Wire picture or wire photo was a newspaper picture that was sent from a remote location by a facsimile telegraph.
Nikola Tesla and other scientists and inventors showed the usefulness of wireless telegraphy, radiotelegraphy, or radio, beginning in the 1890s. Alexander Stepanovich Popov demonstrated to the public his receiver of wireless signals, also used as a lightning detector, on 7 May 1895. It is considered that Guglielmo Marconi sent and received his first radio signal in Italy up to 6 kilometres in 1896. Around the turn of the century, it is reported that he broadcast signals across the English Channel and in 1901, Marconi radiotelegraphed the letter "S" across the Atlantic Ocean from his station in Poldhu, Cornwall to St. John's, Newfoundland.
In 1898 Popov accomplished successful experiments of wireless communication between a naval base and a battle ship. In 1900 the crew of the Russian coast defence ship General-Admiral Graf Apraksin as well as stranded Finnish fishermen were saved in the Gulf of Finland because of exchange of distress telegrams between two radiostations, located at Gogland island and inside a Russian naval base in Kotka. Both stations of wireless telegraphy were built under Popov's instructions.
Radiotelegraph proved effective in communication for rescue work when a sea disaster occurred. Effective communication was able to exist between ships and from ship to shore.
A continuing goal in telegraphy has been to reduce the cost per message by reducing hand-work, or increasing the sending rate. There were many experiments with moving pointers, and various electrical encodings. However, most systems were too complicated and unreliable. A successful expedient to increase the sending rate was the development of telegraphese.
Other field of research was about multiplexing connections. Once a copper wire was deployed, it was interesting to be able to pass several simultaneous connections through it. Several technologies were developed like Frequency-division multiplexing. Long submarine communications cables became possible in segments with vacuum amplifiers between them.
With the invention of the teletypewriter, telegraphic encoding became fully automated. Early teletypewriters used Baudot code, a 5-bit code. This yielded only thirty two codes, so it was over-defined into two "shifts," "letters" and "figures". An explicit, unshared shift code prefaced each set of letters and figures.
The airline industry remains one of the last users of Teletype and in a few situations still sends messages over the SITA or AFTN networks. For example, The British Airways operations computer system (FICO) as of 2004 still used teletype to communicate with other airline computer systems. The same goes for PARS (Programmable Airline Reservation System) and IPARS that used a similar shifted 6-bit Teletype code, because it requires only 8 bits per character, saving bandwidth and money. A teletype message is often much smaller than the equivalent EDIFACT or XML message. In recent years as airlines have had access to improved bandwidth in remote locations, IATA standard XML is replacing Teletype as well as (EDI).
A standard timing system developed for telecommunications. The "mark" state was defined as the powered state of the wire. In this way, it was immediately apparent when the line itself failed. The characters were sent by first sending a "start bit" that pulled the line to the unpowered "space" state. The start bit triggered a wheeled commutator run by a motor with a precise speed (later, digital electronics). The commutator distributed the bits from the line to a series of relays that would "capture" the bits. A "stop bit" was then sent at the powered "mark state" to assure that the commutator would have time to stop, and be ready for the next character. The stop bit triggered the printing mechanism. Often, two stop bits were sent to give the mechanism time to finish and stop vibrating.
By 1935, message routing was the last great barrier to full automation. Large telegraphy providers began to develop systems that used telephone-like rotary dialing to connect teletypes. These machines were called "telex". Telex machines first performed rotary-telephone-style pulse dialing, and then sent baudot code. This "type A" telex routing functionally automated message routing.
The first wide-coverage telex network was implemented in Germany during the 1930s. The network was used to communicate within the government.
At the then-blinding rate of 45.5 bits per second, up to 25 telex channels could share a single long-distance telephone channel by using "voice frequency telegraphy" multiplexing, making telex the least expensive method of reliable long-distance communication.
In 1970, Cuba and Pakistan were still running 45.5 baud type A Telex. Telex is still widely used in some developing countries' bureaucracies, probably because of its low costs and reliability. The UN asserts that more political entities are reliably available by Telex than by any other single method.
Around 1960[?], some nations began to use the "figures" baudot codes to perform "Type B" telex routing.
Telex grew around the world very rapidly. Long before automatic telephony was available, most countries, even in central Africa and Asia, had at least a few high-frequency (shortwave) telex links. Often these radio links were the first established by government postal and telegraph services (PTTs). 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 (TOR) channels non-stop, to get the maximum value from them.
The cost of Telex on radio (TOR) equipment has continued to fall. Although initially specialised equipment was required, many amateur radio operators now operate TOR (also known as RTTY) with special software and inexpensive adapters from computer sound cards to shortwave radios.
Modern "cablegrams" or "telegrams" actually operate over dedicated Telex networks, using TOR whenever required.
In Germany alone, more than 400,000 telex lines remain in daily operation. Over most of the world, more than three million telex lines remain in use.
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 (in this case Telefonaktiebolaget L M Ericsson in Sweden) and S is the country code. Solutions also 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 was (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 (who are you) code, and the recipient machine would automatically initiate a response which was usually 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 was sure that he was connected to the correct recipient. The WRU code would also 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 other unreliable forms of communications such as telephone and fax.
The usual method of operation was that the message would be prepared off-line, using paper tape. All common Telex machines incorporated a 5-hole paper tape reader and paper tape punch. Once the paper tape had been prepared, the message could be transmitted in minimum time. Telex billing was always by connected duration, so minimising the connect time saved money. However, it was also possible to connect in "real time", where the sender and the recipient could both type on the keyboard and these characters would be immediately printed on the distant machine.
Telex could also be used as a rudimentary but functional carrier of information from one IT system to another, in effect a primitive fore-runner of Electronic Data Interchange. The sending IT system would create an output (e.g. an inventory list) on paper tape using a mutually agreed format. The tape would be sent by Telex and collected on a corresponding paper tape by the receiver and this tape could then be read into the receiving IT system.
Almost in parallel with Germany's telex system, AT&T in the 1930s decided to go telex one better, and began developing a similar service (with pulse dialing among other features) called "Teletype Wide-area eXchange" (TWX). AT&T, also known as the Bell system, acquired the Teletype Corporation in 1930 and used its teleprinters for TWX.
TWX originally ran 75 bits per second, sending Baudot code and dial selection. However, Bell later developed a second generation of "four row" modems called the "Bell 101 dataset", which is the direct ancestor of the Bell 103 modem that launched computer time-sharing. The 101 was revolutionary because it ran on ordinary subscriber lines that could (at the office) be routed to special exchanges called "wide-area data service". Because it was using the public switched telephone network, TWX had special area codes: 510, 610, 710, 810 and 910. With the demise of TWX service, these codes were re-provisioned as standard geographic NPAs in the 1990s.
Bell's original consent agreement limited it to international dial telephony. Western Union Telegraph Company had given up its international telegraphic operation in a 1939 bid to monopolize U.S. telegraphy by taking over ITT's PTT business. The result was deemphasis on telex in the U.S. and a cat's cradle of small U.S. international telex and telegraphy companies. These were known by regulatory agencies as "International Record Carriers".
Bell telex users had to select which IRC to use, and then append the necessary routing digits. The IRCs converted between TWX and Western Union Telegraph Co. standards.
Western Union Telegraph Company developed a spinoff called "Cable System". Cable system later became Western Union International.
ITT's "World Communications" was amalgamated from many smaller companies: "Federal Telegraph", "All American Cables and Radio", "Globe Wireless", and a common carrier division of Mackay Marine.
RCA communications had specialised in crossing the Pacific. It later joined with Western Union International to become MCI.
Before World War I, Tropical Radiotelegraph put radio telegraphs on ships for its owner, The United Fruit Company, in order to deliver bananas to the best-paying markets. Communications expanded to UFC's plantations, and were eventually provided to local governments. TRT Telecommunications (as it is now known) eventually became the national PTT of many small Central American nations.
The French Telegraph Cable Company (owned by French investors) had always been in the U.S. It laid cable from the U.S. to France. It was formed by "Monsieur Puyer-Quartier". This is how it got its telegraphic routing ID "PQ".
Firestone Rubber developed its own IRC, the "Trans-Liberia Radiotelegraph Company". It operated shortwave from Akron, OH to the rubber plantations in Liberia. TL is still based in Akron. TWX
As of 2007, most telegraphic messages are carried by the Internet in the form of e-mail.
Around 1965, DARPA commissioned a study of decentralized switching systems. Some of the ideas developed in this study provided inspiration for the development of the ARPANET packet switching research network, which later grew to become the public Internet.
As the PSTN became a digital network, T-carrier "synchronous" networks became commonplace in the U.S. A T-1 line has a "frame" of 193 bits that repeats 8000 times per second. The first bit, called the "sync" bit, alternates between 1 and 0 to identify the start of the frames. The rest of the frame provides 8 bits for each of 24 separate voice or data channels. Customarily, a T-1 link is sent over a balanced twisted pair, isolated with transformers to prevent current flow. Europeans adopted a similar system (E-1) of 32 channels (with one channel for frame synchronisation).
Later, SONET and SDH (the synchronous digital hierarchy) were adapted to combine carrier channels into groups that could be sent over optic fiber. The capacity of an optic fiber is often extended with wavelength division multiplexing, rather than rerigging new fibre. Rigging several fibres in the same structures as the first fibre is usually easy and inexpensive, and many fibre installations include unused spare "dark fibre", "dark wavelengths", and unused parts of the SONET frame, so-called "virtual channels."
Currently (2006), the fastest well-defined communication channel used for telegraphy is the SONET standard OC-768, which sends about 40 gigabits per second.
The theoretical maximum capacity of an optic fiber is more than 10^12 bits (one terabit or one trillion bits) per second. No current (2006) encoding system approaches this theoretical limit, even with wavelength division multiplexing.
Since the Internet operates over any digital transmission medium, further evolution of telegraphic technology will be effectively concealed from users.
Arrival of the Internet
E-mail was first invented for Multics in the late 1960s. At first, e-mail was only possible between different accounts on the same computer. UUCP allowed different computers to be connected to allow e-mails to be relayed from computer to computer. With the growth of the Internet, E-mail began to be possible between any two computers with access to the Internet.
Various private networks (UUNET, the Well, GEnie, DECNET) had e-mail from the 1970s, but subscriptions were quite expensive for an individual, $25 to $50 a month, just for E-mail. Internet use was then largely limited to government, academia and other government contractors until the net was opened to commercial use in the 1980s.
By the early 1990s, modems on affordable personal computers with graphical user interfaces made e-mail a viable alternative to telex systems in a business environment. But individual e-mail accounts were not widely available until local internet service providers were in place, although demand grew rapidly, as e-mail was seen as the Internet's killer app. The broad user base created by the demand for e-mail smoothed the way for the rapid acceptance of the World Wide Web in the mid-1990s.
E-mail displaces telegraphy
International Telex remains available via e-mail ports. It is one's e-mail address with numeric or alpha prefixes specifying one's International Record Carrier and account. Telex has always had a feature called "answerback" that asks a remote machine to send its address. If one is using telex via e-mail, this address is what a remote telex user will want in order to contact an e-mail user.
Western Union announced the discontinuation of all of its telegram services effective from the 31 January 2006. Only 20,000 telegrams were sent in 2005, compared with 20 million in 1929. According to Western Union, which still offers money transfer services, its last telegram was sent Friday, 27 January 2006
Telegraphy as a legacy system
Clacks, a fictional, semaphore-based, communications system used in Terry Pratchett's Discworld
K9YA Telegraph, an e-Zine containing articles on telegraphy in amateur radio.
Signal Corps in the American Civil War
Posted by gigihong07 at 8:10 AM