Telegraphy - Telegraphic Improvements

Telegraphic Improvements

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 research focused on the multiplexing of telegraph connections. By passing several simultaneous connections through an existing copper wire, capacity could be upgraded without the laying of new cable, a process which remained very costly. Several technologies were developed like Frequency-division multiplexing. Long submarine communications cables became possible in segments with vacuum tube amplifiers between them.

With the invention of the teletypewriter, telegraphic encoding became fully automated. Early teletypewriters used the ITA-1 Baudot code, a five-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 teletypewriters 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 uses teletypewriters to communicate with other airline computer systems. The same goes for Programmed Airline Reservation System (PARS) and IPARS that use a similar shifted six-bit Teletype code, because it requires only eight bits per character, saving bandwidth and money. A teletypewriter 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 Teletypewriter data as well as EDI.

The first electrical telegraph developed a standard signalling system 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 moving pointer telegraphs started the pointer's motion with a "start bit" that pulled the line to the unpowered "space" state. In early Telex machines, 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. Stop bits initially lasted 1.42 baud times (later extended to two as signalling rates increased), in order to give the mechanism time to finish and stop vibrating. Hence an ITA-2 Murray code symbol took 1 start, 5 data, and 1.42 stop (total 7.42) baud times to transmit.