“ I see no reason why intelligence may not betransmitted instantaneously by electricity “ Samuel Morse, 1832
What is TELECOMMUNICATIONSThe IEEE Standard Dictionary definestelecommunications as:“the transmission of signals over longdistance”• such as by telephone & telegraph, (Bywires!!)• Radio or television (Wireless!!).• Another term we often hear is electricalcommunication. This is a descriptive term,but of somewhat broader scope.
The Electrical Telegraph: An Early Form of Long-Distance Communications
TELECOMMUNICATION WILL TOUCH EVERYBODYIn industrialized nations, the telephoneis accepted as a way of life. Thetelephone is connected to the PublicSwitched TelecommunicationsNetwork (PSTN) for local, national, andinternational voice communications.
Primitive Components Of a Telecommunication System
Developments in Telecommunications•The personal computer “PC” is beginning to takeon a role similar to that of the telephone.•In many situations, the PC uses the telephonenetworks to obtain some services like the Internet.•Cable television (CATV) offers another form ofconnectivity.• Then there are the radio or” WIRELESS adjuncts”to the telephone, typically:• cellular and PCS (Personal CommunicationsServices, which offer data communications(including Internet) and facsimile (fax) as well asvoice. They are considered wireless in oppositionto being wired
Note on: Transmission MEDIAThe word media(medium, singular) is used todescribewhat is transportingtelecommunication signals. Thisis termed transmission media.There are four basic types ofmedium: wire pair, coaxialcable, fiber optics, and radio (free
FREQUENCY ?To understand more advancedtelecommunication concepts,we need a firm knowledge offrequency and relatedparameters such as band andbandwidth, wavelength, period,and phase.The IEEE definesfrequency as “the numberof complete cycles ofsinusoidal variationper unit time”
Wavelength SPEED!!Wavelength is conventionallymeasured in meters and isrepresented by the symbol λ.It is defined as the distancebetween successive peaks ortroughs of a sinusoidal wave. •Radio waves travel at the velocity of light: in a vacuum it is 3 108m/sec. •If we multiply frequency “F” in hertz times the wavelength in meters, we get a constant, the velocity of light propagation: •Fλ = 3 108 m/sec
TRANSMISSION and SWITCHING OF ELECTRICAL SIGNALS•Transmission may be defined as the electrical transfer ofa signal, message, or other form of intelligence from onelocation to another. It is one of the two major disciplines oftelecommunication.•Switching is the other principal specialty, it establishes aconnection from user X to some distant user Y.•In the old days of telephony, these disciplines were separatewith strong demarcation between one and the other. Not sotoday. The demarcation line is fast disappearing. Forexample, under normal circumstances in the PSTN, a switchprovides network timing that is vital for digital transmission.
Why CARRIER ? Why MODULATION ?•Baseband transmission can have severe distance limitations• For a voice signal transmitted from a standard telephone setover a copper wire pair may reach a distant subset earpiecesome 30 km or less distant before losing all intelligibility.Because the signal strength is so very low that it becomesinaudible. •To overcome this distance limitation, we may turn to carrier or radio transmission.
Modulation & Demodulation •Modulation is the process of impinging the information on a high frequency wave: the carrier. •Demodulation is the recovery of that information from the carrier at the destination user.The IEEE defines modulation as “a process whereby certaincharacteristics of a wave, often called the carrier, are varied orselected in accordance with a modulating function.”The modulating function is the information baseband describedbefore. There are three generic forms of modulation:1. Amplitude modulation (AM).2. Frequency modulation (FM).3. Phase modulation (PM).
Modulation a Carrier by aSingle Frequency Signal
SAMPLING Analog SignalsTo transmit analog message signals by digital means,it must be converted into digital form.• This process is known as Analog-to-Digital Conversion: “ADC”.• The sampling process is the first process performed in this conversion in accordance with the SAMPLING THEOREM.• It converts a continuous-time signal into a discrete time signal in the form of a sequence of numbers.
SHANNON’s TheoremA band-limited signal m(t) can be uniquelydetermined from its sampled values m(nTS)taken at uniform interval TS if: Or : 1/TS = fS≥ 2fMThe sampling interval Ts =1/(2fM) is called theNyquist interval and the minimum rate:fs = 1/Ts = 2fM is known as the Nyquist rate.
PCM Pulse code modulation(PCM) is an analog-to-digital conversionrepresenting an analog signal by digitalwords in a serial bit stream.
Bandwidth of a PCM SignalThe spectrum of the (serial) binary PCMsignal depends on the bit rate: R R = n fSWhere n is the number of bits in the PCMword (M = 2n) and fS is the sampling rate(sampling frequency).