TypesofInformation. Types of information:

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1 TypesofInformation 1 Modern digital networks transmit digital information transparently, i.e. the network does not necessarily need to know what kind of information the data contain. Types of information: Speech (telephony, fixed, or cellular) Moving images (television or video) Printed pages or still picture (facsimile or multimedia messaging) Text (electronic mail or short text messaging) Music All types of computer information such as program files. 80

2 DifferentRequirements 2 Communication Requirements of Different Applications 81

3 Simplex, Half-Duplex, Full-Duplex 3 These are operational terms. Simplex is one way operation; there is no reply channel provided. Radio and television broadcasting are simplex. Certain types of data circuits might be based on simplex operation. Half-duplex is a two-way operation. It is defined as transmission over a circuit capable of transmitting in either direction, but only used in one single direction at a time. Full -duplex (or just duplex)defines simultaneous two-way independent transmission on a circuit in both directions. All PSTN-type circuits generally use full-duplex operation. 82

4 Frequencyand Bandwidth 4 The transmitted signal (the alteration of voltage or current) consists of multiple frequencies. The range of frequencies is called the bandwidth of the signal. The frequency refers to the number of cycles through with the wave (cosine or sine) oscillates in a second. Cosine wave and frequency 83

5 NaturalVoice Bandwidth Frequency components of the human voice extend down to some 20 Hz and as high as 32,000 Hz. The lower frequency components carry the voice energyand the higher frequency components carry emotion. 5 It is not the intent of the PSTN operator to provide high fidelity communications between telephone users, only intelligible connectionsand that the speaker can be recognized. In telephony, the range of frequencies that is needed for a good enough quality of voice was defined to be the range from 300 to 3,400 Hz. This means that the bandwidth of the telephone channel through the network is 3, Hz = 3.1 khz. 84

6 TelephoneSignalBandwidth 6 The bandwidth is normally measured from the points where the signal power drops to half from its maximum power. Attenuation or loss of channel is given as a logarithmic measure called a decibel (db), and half power points correspond to a 3-dB loss. Bandwidth, together with noise, is the major factor that determines the informationcarrying capacity of a telecommunications channel. Bandwidth of the telephone speech channel 85

7 Analogand DigitalSignals 7 Examples of analog and digital signals 86

8 AdvantagesofDigitalTechnology 8 Some main advantages of digital signals (over analog signals): can be processed by digital circuit components on a single chip through the use of Very Large Scale Integration (VLSI) technology, which allows lower cost, better reliability, reduced space, and lower power consumption; can be stored, transmitted, and reproduced exactly (however, with high noise or weak reception, a digital signal degrades much less gracefully than an analog signal); are fairly immune to the imperfections of real electronic systems; codes can be used to protect information transmitted in the communication channel from disturbances ( channel coding ); codes can be used to compress source information and reduce occupied bandwidth ( source coding ); encryption can be used to achieve security, so that only the intended receiver can decode the signal; different services, with low and large bandwidth, can be easily and efficiently integrated into a single binary flux; digital networks offer greater flexibility than analogue networks in several types of processing. 87

9 ExamplesofInformation Sources 9 Analog signal on analog network Digital signal on analog network Digital signal on digital network 88