two computers. 2- Providing a channel between them for transmitting and receiving the signals through it.

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1 1. Introduction: Communication is the process of transmitting the messages that carrying information, where the two computers can be communicated with each other if the two conditions are available: 1- Translating the data into the signals can be transmitted between the two computers. 2- Providing a channel between them for transmitting and receiving the signals through it. 2. Fundamental Limits in Digital Transmission: Using electric current to send bits and using electric voltage to encode bits. The following figure illustrates how positive and negative voltage can be used to transmit bits across a wire. In this example, the sender applies a negative voltage to send a 1 bit or a positive voltage to send a 0 bits. Figure (1): use the positive and negative voltage to transmit bits across a wire Each transmission system has a limited bandwidth (frequency range), which is the maximum rate that the hardware can change a signal. If a sender attempts to transmit changes faster than the bandwidth, the hardware will not be able to keep up because it will not have sufficient time to complete one change before the sender attempts to make another. Thus, some of the changes will be lost. The nature imposes two fundamental limits on all channels, and these determine their bandwidth. These limits are: Nyquist limit which deals with noiseless channels. 1

2 Shannon limit for noisy channels. Nyquist proved that if an arbitrary signal has been run through a low pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H sample per second. Sampling the line faster than 2H times per second is pointless because the higher frequency components that such sampling could recover have already been filtered out. If the signal consists of V discrete levels, Nyquist s theorem states: Maximum data rate = 2H log 2 V bits/seconds For the noisy channels, the amount of the thermal noise present is measured by the ratio of the signal power to the noise power called the signal-to-noise ratio. If denote the signal power by S and the noise power by N, the signal-tonoise ratio is S/N. the quantity (10 log 10 S/N) is given. These unit are called decibels (db). SNR = AverageSignalPower AverageNoi sepower Note: In equation above, SNR is not in decibel (db) SNR (db) = 10 log 10 SNR Shannon s major results is that the maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N is given by: Maximum number of bits/second = H log 2 (1 + S/N) Example: For a channel of 3000 Hz bandwidth, and a signal-to-noise (SNR) of 30 db, the Shannon s channel capacity is SNR = 10 (SNR(dB)/10) = 10 (30/10) = 10 3 = 1000 C = W*log 2 (1+SNR) bits/seconds = 3000* log 2 (1+1000) = 3000* (log )/(log 10 2) = 3000* (3.0004)/( ) = bits/seconds C is less than 30 K bits/seconds 3. Signal transmission in channel: 2

3 When a signal is injected into a channel, it is typically altered in form, by a number of influences. Firstly, it is delayed: the time taken for a signal to pass through a channel is called the propagation delay. Secondly, loss of energy to the medium causes it to lose amplitude, a process called attenuation. If all frequency components were attenuated and delayed by the same amount, the signal would loss height, but its shape would not change. Finally, all channels are subject to noise, random fluctuations in the medium that superpose onto the signal and change its shape. these influences are: 3-1. Attenuation: Attenuation is the change in the height of any waveform caused by loss of energy in the channel. In some channels, a signal can actually draw energy from the channel and have its size increased. This negative attenuation is called amplification. Attenuation and amplification are measured using a logarithmic ratio called the decibel ratio. If a signal of maximum amplitude, A is input to a channel, and a signal of maximum amplitude A ' is output, the decibel ratio (dbr) is: dbr = 20 log A A' decibels (db) Applying this to some examples: i) A ' = 0.1 A ii) iii) A ' = 0.5 A A ' = 10 A dbr = - 20 db dbr = db dbr = 20 db Note that amplification yields a positive dbr while attenuation ratios are negative. Decibel ratios are purely ratios of two values Distortion: As we have seen, the sinusoidal components of a general signal are attenuated and phase shifted (delayed) by different amount, depending on frequency. This causes the shape of the signal to change, this known as linear distortion. 3

4 3-3. Noise: Noise is the term for random fluctuations of the medium, which will superpose onto a message signal, corrupting it. If the noise has frequency components in the same band as the signal then it can not removed by filtering. The greater amplitude of the noise relative to that of the signal, the more destructive its effect. This is measured by signal-to-noise ratio (SNR) defined by SNR = AverageSignalPower AverageNoi sepower decibels, the larger the SNR, the better. Noise comes from various sources and is of different forms. Gaussian Noise that known as white noise. The commonest form is thermal noise due to firstly, to thermal motion of electrons, present in all electrical conductors and in all electrical equipments and, secondly, to thermal radiation from surrounding objects. The noise is the fundamental limiting factor in communication system performance. Another important type of noise is crosstalk, unwanted electrical coupling (due to capacitance and inductance) between signal paths. Two wires running in parallel will suffer most severely from this. A signal transition on one wire will tend to generate ghost transition on the other, due to inductive and capacitive coupling between them. This ghost superposes on the true signal carried by the second wire, corrupting it. Finally, impulse noise consists of irregular, large noise pulses caused by external electromagnetic disturbances due to for example, switching in other electrical or electronic equipment, electrical storms, etc. 4. Baseband transmission versus Broadband transmission: cables: There are two methods for transmitting the signals a crossing the 4.1. Baseband transmission: A network technology that uses a small part of the electromagnetic spectrum and sends one signal at a time over the underlying medium. It uses 4

5 digital signaling and a digital signal is inserted on the line as voltage pulses. The entire frequency spectrum of the medium is used to form the signal; hence, frequency-division multiplexing (FDM) cannot be used. The transmission is in bidirectional that is a signal inserted at any point on the medium propagates in both directions to the end. Most LAN use Baseband signaling (e.g., Ethernet and FDDI). There is a probability that the transmitted signal will attenuated and this causes a difficulty in identifying the content of the signal, for this reason, the Baseband network uses a signal repeaters that receive the signal and improve it and retransmitting again Broadband transmission: A network technology that uses a large part of the electromagnetic spectrum to achieve higher throughput rates. Broadband refers to the use of analog signaling, usually broadband system employ FDM to allow multiple independent communications to proceed simultaneously over a single underlying medium. The transmission in unidirectional, but we can solve this problem by using one of the two solutions: i) by using dual-cable, where each computer will connected with two wires, one for sending and the other for receiving. ii) by using one cable with dividing the bandwidth into two sections, where, will be two channels that uses different frequency. one for sending and the other for receiving. Much greater distances are possible with broadband compared to Baseband. This is because the analog signals that carry the digital data can propagate greater distance before the noise and attenuation damage the data. The broadband systems use sepecial devices for improving the signals called amplifiers. 5- Modulated transmission: It is the Process of combining an input signal m(t) and a carrier at frequency f c to produce signal V m (t) with bandwidth centered on f c. The 5

6 objective of modulation is to transform the information that available in a Baseband signal m(t) to another waveform V m (t), where, V m (t) have some required properties that not available in m(t). _ Why modulate analog signals? Higher frequency may be needed for effective transmission. Unguided transmission needs HF. Modulation permits frequency division multiplexing the typical form for carrier is c(t) = a c cos 2 f c t where the typical form for modulation (t) V m =f{m(t),c(t)}, ac is constant, f c preselected frequency, f() is a non-linear function. There are three types of analog modulation: 5.1. Amplitude Modulation (AM): The simplest form of AM is suppressed carrier AM (SCAM). In this case the amplitude of the carrier is varied with the Baseband signal m(t). the modulated signal V m (t) is: V m (t) = a m( t) cos 2 f t c c The modulation may be carried out in practice by an analog multiplier to multiply the envelop and the carrier. Demodulation requires that the receiver generate a sinusoid of the same frequency as the carrier from a local oscillator kept tightly in phase with the carrier itself by a device called a phase locked loop. This called synchronous demodulation Angle Modulation: In angle Modulation, the carrier amplitude remains constant but the argument if the sinusoidal carrier is varied as m(t) changes. This actually 6

7 introduces a new class of sinusoid-like functions with instantaneously variable frequency. There are two important forms of angle modulation: In Frequency modulated (FM) signal, modulation equipment varies the carrier frequency with the Baseband signal m(t). the instantaneous angular frequency of the modulated signal is given by: w ( t) W k m( t) i f, where W =2 f, k f is constant this can be restated in terms of angle (t) t ( t ) w t k m( x) dx f In a Phase modulated (PM) signal, m(t) is used to vary the phase angle of the carrier directly: w i d w k p m(t), where p dt k is a constant in term of angle: ( t) w t k f m( t) Figure (2): Analog Modulation 5-3. frequency division Multiplexing: By modulating several low bandwidth signals by different carrier frequencies, they may be transmitted simultaneously over a single high 7

8 bandwidth channel. This called FDM. To avoid interference, the spectra of the modulated bandpass signals must be kept separate by guard bard of un used frequencies. FDM used in the telephone system to multiplex many telephone conversation onto one high capacity line, as well as to carry many digital signals on a single cable in broadband computer networks. 8