geography_notes

Jun 28th, 2015
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Koh-e-Safed,Waziristan, Sulaiman, Kirther, rivers, Streams, Koh-e-Safed, Glaciers, Thar, Thal, Kharan, Kalachitta, Margala hills .Soan, Khewra, Makrachi ,Kallar kahar , Khabeki, Uchchali, barani farming, economic sources, oil refinery, HMC Taxila, Rock Salt, Barani farming, wheat, pulses, oil seeds, Zhob & Loralai.

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The History of Data Communications SignalsPerhaps the earliest example of digital signals being used to transfer information over long distances dates back to the Greek and Roman signal fires. This was a purely binary system, signaling the occurrence or non-occurrence of predefined events. We have all heard of the use of jungle drums by African tribes and smoke signals used by North American Indians. These were also digital systems, essentially. They conveyed information by changing the length of time between beats or puffs of smoke. In each of these cases the basic requirements of a communications system are met. Senders, receivers and medium agree on methods of encoding and coding the information.Telegraphy was the earliest method of serial data communications using electrical methods. Again, the system included senders, receivers and a medium, as well as an encoding system called the Morse code (shown on one of the next slides). The terms mark (signifying a logic1) and space (signifying a logic 0) were coined as a result of early attempts to automate the telegraph. This system drew lines on a strip of paper moving under the armature attached to the receiving coil.The teletypewriter was a later development that further automated the transmission and reception of data using electromechanical systems operated by synchronous motors. The arrival of the telephone, designed for voice communication, prompted the development of techniques to transmit and recei

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