An engine order telegraph, used to send instructions from the bridge of a ship to the engine room, fails to meet both criteria; it has a limited distance and very simple message set. There was only one ancient signalling system described that does meet these criteria. There is no definite record of the system ever being used, but there are several passages in ancient texts that some think are suggestive. CCAPs aim to replace the conventional, integrated CMTS which only provides data and Edge QAMs used for video which are separate pieces of equipment. By using digital video compression techniques, multiple standard and high-definition TV channels can be carried on one 6 or 8 MHz frequency carrier, thus increasing the channel carrying capacity of the HFC network by 10 times or more versus an all-analog network. In theory, these trains could enjoy dramatic savings through electrification, but it can be too costly to extend electrification to isolated areas, and unless an entire network is electrified, companies often find that they need to continue use of diesel trains even if sections are electrified. Optical telegraph lines were installed by governments, often for a military purpose, and reserved for official use only.

Lines of signalling relay stations can send messages to any required distance, but all these systems are limited to one extent or another in the range of messages that they can send. An optical telegraph is a telegraph consisting of a line of stations in towers or natural high points which signal to each other by means of shutters or paddles. One of the oldest examples is the signal towers of the Great Wall of China. The company finally succeeded in 1866 with an improved cable laid by SS Great Eastern, the largest ship of its day, designed by Isambard Kingdom Brunel. The Chinese signalling system extended well beyond the Great Wall. While the signalling was complex (for instance, different-coloured flags could be used to indicate enemy strength), only predetermined messages could be sent. Possible messages were fixed and predetermined, so such systems are thus not true telegraphs. According to Morse, telegraph dates only from 1832 when Pavel Schilling invented one of the earliest electrical telegraphs.

The signals were observed at a distance with the newly invented telescope. The early electrification of railways used direct current (DC) power systems, which were limited in terms of the distance they could transmit power. Having eliminated the mechanical transmission circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits. In recent years, the LIN bus (Local Interconnect Network) standard has been introduced to complement CAN for non-critical subsystems such as air-conditioning and infotainment, where data transmission speed and reliability are less critical. Later, a Telex was a message sent by a Telex network, a switched network of teleprinters similar to a telephone network. American early and middle 20th century telephone cables had load coils at intervals of a mile (1.61 km), usually in coil cases holding many. Multi-system operators (MSOs) developed methods of sending the various services over RF signals on the fiber optic and coaxial copper cables.

These continue to be called telegrams or cables regardless of the method used for transmission. Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Wireless telegraphy is transmission of messages over radio with telegraphic codes. This is to be distinguished from semaphore, which merely transmits messages. Telegrams became a popular means of sending messages once telegraph prices had fallen sufficiently. A telegraph message sent by an electrical telegraph operator or telegrapher using Morse code (or a printing telegraph operator using plain text) was known as a telegram. If more than 32 unit loads need to communicate, the network can be expanded across parallel buses using DMX splitters. The most extensive heliograph network established was in Arizona and New Mexico during the Apache Wars. Miles had previously set up the first heliograph line in the US between Fort Keogh and Fort Custer in Montana. Holzmann and Pehrson, for instance, suggest that Livy is describing its use by Philip V of Macedon in 207 BC during the First Macedonian War. However, in the event of multiple failures of redundant computers, the A320 does have a mechanical back-up system for its pitch trim and its rudder, the Airbus A340 has a purely electrical (not electronic) back-up rudder control system and beginning with the A380, all flight-control systems have back-up systems that are purely electrical through the use of a "three-axis Backup Control Module" (BCM).

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