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Quick History of Telecommunications
Earliest achievements fire signals by Greeks in 8th BC Century for sending
alarms, call for help or announcements
• The advent of telegraphy in the 1830s (by Samuel F. B. Morse) replaced
the use of light by electricity and began the era of electrical
communications. ( 10b/s, Morse code)
• The first transatlantic telegraph cable went into operation in 1866 (digital
scheme two electrical pulses of different durations; dots and dashes of the
Morse code)
• The invention of telephone in 1876; electrical signals were
transmitted in analog form.
• 1895: first radio transmission (Marconi, Isle of Wight,
18 mile distance. ”SIR JAGADISH CHANDRA BOSE !” )
• 1915: Wireless voice transmission established between San Francisco
and New York
• First co-axial cable system, put into service in 1940, was a 3-MHz system
capable of transmitting 300 voice channels or a single TV channel.
BW of such system is limited by the frequency dependent cable loss,
which increase rapidly for frequencies beyond 10MHz.
• 1945: Arthur C. Clarke(sci-fi writer) suggests geostationary satellites
• Most advanced coaxial system put into service in 1975 and operated at bit
rate of 274Mb/s with a repeater spacing of around 1km)
1946: Public mobile telephony introduced in 25 US cities
• 1947: Invention of cellular concept (AT&T)
• The first microwave system operating at carrier frequency of 4GHz was put into
service in 1948. ( 100Mb/s)
• 1957: First deployed communication satellite (Sputnik, Soviet Union)
• Invention of Laser and its demonstration in 1960.
• 1963: First deployed geostationary satellite (NASA)
• Figure of merit (BL) of 100Mb/s-km were available by 1970.
• 1971: First packet-based radio network (ALOHANET, Univ. of Hawaii)
• In 1960s, fiber loss in excess of 1000dB/km; 20dB/km by 1970 at 1000nm; In 1979,
fiber loss of 0.2dB/km was realized at 1550nm.
• 1983: First analog cellular system deployed (Chicago)
• 1985: Unlicensed frequency bands first authorized for WLAN use
• Ca. 1990: First digital cellular systems (”2G”)
In 1992, WDM technology was invented and was commercialized in 1997.
• In 1995, Optical amplifier (EDFA) was installed in trans-oceanic cable system.
• In 2000, Raman amplifier was developed.
• 2000 - : Standardization of 3rd generation mobile communication
systems, WLANs, WPANs, sensor network radios,...
• Now- : Standardization/deployment of 4G…
Digital Communication has a number of advantages:
• Privacy is preserved by using data encryption
• Noise does not accumulated from repeater to repeater
• Errors can be detected and corrected, even when there is a large
amount of noise on the received signal.
• Errors may often be corrected by the use of coding
• Relatively inexpensive digital circuits may be used
• Data from voice, video and data sources may be merged and
transmitted over a common digital transmission system
• Greater dynamic range (the difference between the largest and
smallest values) is possible
• Other: Component Tolerance, Storage facility, Multiplexing
Dr. Feroz Ahmed ECR 301 Telecommunication System I 10
Digital Communication also has disadvantages:
• Generally, more bandwidth is required than that for analog systems
• Synchronization is required
Classification of Signals:
• Deterministic Signals
• Random Signals
• Analog & Digital Signals
• Real & Complex Signals
Noise
• Noise is defined as unwanted electrical energy of random and
unpredictable nature present in the system due to any cause.
• External & Internal Noise
• External Noise:
• Atmospheric noise: Lightning discharge and natural
disturbance in the atmosphere
• Man-made or industrial noise: Automobile and aircraft
ignition, electrical motor etc.
• Extraterrestrial or space noise: Solar noise (sun),
Cosmic noise ( stars & galaxies)
• Internal Noise:
• Thermal or Johnson’s noise or white noise: due to the
random motion of electrons inside the resistor.
• Shot noise: BJT & amplifiers; due to the random motion of
electrons and holes inside the transistor.
Multiplexing
Multiplexing facilitates the simultaneous transmission of multiple messages over a
single transmission channel. It allows the maximum possible utilization of the available
bandwidth of the system.
• If all these baseband audio signals are simultaneously transmitted through a single
channel, then they will be mixed together. It is not possible to separate them.
• To avoid this difficulty, each signal can be translated to a different frequency spectrum,
such that every signal differs in its transmitted frequency. This is done through
modulation.
• At the receiver, the different signals can be easily separated because they are at different
frequencies. Therefore, multiplexing becomes possible only because of modulation.