1.1.9 Analogue and digital signals

Digital signals are characterised by two discrete levels, high and low (1 or 0), while analogue signals have continuous forms. Digital and analogue signals are both utilised in modern telecommunications [7] systems and computer networks. Popular digital codes include American Standard Code for Information Interchange (ASCII) and binary-coded decimal (BCD). ASCII is used in basic character symbols for computer systems, while BCD is mainly used for seven-segment displays.

### 1.1.10 Non-return to zero (NRZ)

Non-return to zero is the simplest digital encoding as shown in Figure 1, where a logic one corresponds to a positive high signal level and the logic zero is simply at ground potential or zero voltage. The NRZ encoding is inconvenient for data transmission specially when data contain a long series of zeros or ones.

#### 1.1.11 Return to zero (RZ)

Return to zero is an improved digital encoding over the NRZ encoding, where logic one signals return to zero as shown in Figure 2. The RZ encoding is inconvenient for data transmission when data contain a long series of zeros.

#### 1.1.12 Manchester encoding

To assure reliable transmission of digital data (such as Ethernet and IP), the Manchester encoding (refers to Figures 3 and 4 with clock signal) is convenient to solve the issue of sending a long series of zeros or ones through a data communication line. The Manchester encoding encodes logic 1 as a transition from level high to low signal, while a 0 is a transition from low to high. The needed bandwidth is twice as the original signal, and there is always a change in the middle of each bit.

An improved version of this encoding is called the differential Manchester encoding, where a 0 causes the signal to change at the start of the interval (refer to Figure 5). On the other hand, a 1 causes a change at the end of the interval. A 1 keeps the signal level unchanged as in the previous bit and changes to high at the middle. This is advantageous and permits interchanging the wiring of a differential pair without any issue.

Figure 3.

Figure 2.

Unipolar return to zero (RZ).

Telecommunications Protocols Fundamentals DOI: http://dx.doi.org/10.5772/intechopen.86338

Figure 4.

7

Manchester encoding example.

Manchester encoding.

Figure 1. Unipolar non-return to zero (NRZ).

Telecommunications Protocols Fundamentals DOI: http://dx.doi.org/10.5772/intechopen.86338

Figure 2.

utilised in modern telecommunications [7] systems and computer networks. Popular digital codes include American Standard Code for Information Interchange (ASCII) and binary-coded decimal (BCD). ASCII is used in basic character symbols for computer systems, while BCD is mainly used for seven-segment displays.

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

Non-return to zero is the simplest digital encoding as shown in Figure 1, where a logic one corresponds to a positive high signal level and the logic zero is simply at ground potential or zero voltage. The NRZ encoding is inconvenient for data

Return to zero is an improved digital encoding over the NRZ encoding, where

To assure reliable transmission of digital data (such as Ethernet and IP), the Manchester encoding (refers to Figures 3 and 4 with clock signal) is convenient to solve the issue of sending a long series of zeros or ones through a data communication line. The Manchester encoding encodes logic 1 as a transition from level high to low signal, while a 0 is a transition from low to high. The needed bandwidth is twice

as the original signal, and there is always a change in the middle of each bit. An improved version of this encoding is called the differential Manchester encoding, where a 0 causes the signal to change at the start of the interval (refer to Figure 5). On the other hand, a 1 causes a change at the end of the interval. A 1 keeps the signal level unchanged as in the previous bit and changes to high at the middle. This is advantageous and permits interchanging the wiring of a differential

transmission specially when data contain a long series of zeros or ones.

logic one signals return to zero as shown in Figure 2. The RZ encoding is inconvenient for data transmission when data contain a long series of zeros.

1.1.10 Non-return to zero (NRZ)

1.1.11 Return to zero (RZ)

1.1.12 Manchester encoding

pair without any issue.

Figure 1.

6

Unipolar non-return to zero (NRZ).

Unipolar return to zero (RZ).

Figure 3. Manchester encoding.

Figure 4. Manchester encoding example.

1.1.16 Multiplexing

Telecommunications Protocols Fundamentals DOI: http://dx.doi.org/10.5772/intechopen.86338

are utilised, namely:

modulations (SSB).

2. Modulation techniques

2.1 Analogue modulation

2.1.1 Amplitude modulation (AM)

m tðÞ¼ Am cos <sup>2</sup>π<sup>f</sup> <sup>m</sup><sup>t</sup> <sup>þ</sup> <sup>ϕ</sup>ð Þ<sup>t</sup> :

analogue signals.

9

Multiplexing occurs when data are collected from different sources and are transmitted into one common communication channel. Three types of multiplexing

1. Frequency-division multiplexing (FDM). This type of multiplexing employs

2. Time-division multiplexing (TDM). This type of multiplexing employs time

3. Quadrature multiplexing (QM). This type of multiplexing employs quadrature carriers to transmit different message signals. This type of multiplexing can be distinguished from FDM by the fact that they have overlapped frequency spectra. QM represents double-sideband (DSB) and single-sideband

In the past, digital networks were connected through telephone networks via the modem (modulation/demodulation). Modern telecommunications systems utilise optical fibres that carry many digital channels, which can be translated into voice signals in a telephone by using a codec (coder/decoder). This involves digital-toanalogue (D/A) and analogue-to-digital (A/D) conversions. When a signal

m tðÞ¼ Am cos <sup>2</sup>π<sup>f</sup> <sup>m</sup><sup>t</sup> <sup>þ</sup> <sup>ϕ</sup>mð Þ<sup>t</sup> is transmitted, it is normally modulated using a carrier c tðÞ¼ Ac sin <sup>2</sup>π<sup>f</sup> <sup>c</sup><sup>t</sup> <sup>þ</sup> <sup>ϕ</sup>cð Þ<sup>t</sup> signal, which can be changed or modulated in amplitude (Ac), phase shift (ϕc) or frequency (fc) [9]. The carrier signal can be

To transmit analogue signals over long distances, analogue modulation techniques are used by changing either the amplitude, phase or frequency of

Amplitude modulation (AM) takes place when Acð Þt is linearly related to the modulating signals (message). In this modulation technique, the carrier frequency is kept constant, and its amplitude is varied according to the amplitude of the transmitted analogue signal as shown in Figure 6. An AM signal y tð Þ is the result of multiplying the message m tð Þ and carrier c tð Þ functions. Assuming a sinusoidal carrier signal defined as c tðÞ¼ Ac sin <sup>2</sup>πfct is used to modulate the message signal

In the above equation, m is the modulation index, which is the ratio of the amplitude of the message signal Am to the amplitude Ac of the carrier signal.

y tðÞ¼½<sup>1</sup> <sup>þ</sup> <sup>m</sup> cosð2π<sup>f</sup> <sup>m</sup><sup>t</sup> <sup>þ</sup> <sup>ϕ</sup>Þ�Ac sin <sup>2</sup>π<sup>f</sup> <sup>c</sup><sup>t</sup> (3)

y tðÞ¼ 1 þ m tð Þ=Ac ½ �c tð Þ

subcarriers to transmit different message signals.

slots to transmit different message signals.

generalised as c tðÞ¼ Acð Þ<sup>t</sup> sin <sup>2</sup>πfct <sup>þ</sup> <sup>ϕ</sup>cð Þ<sup>t</sup> .

Figure 5.

Manchester differential encoding example.

#### 1.1.13 Shannon's theory

Shannon studied noisy channels, and his theory is based upon the fact that a signal has to have high signal-to-noise (S/N) ratio in order to be successfully distinguished. This influences the maximum bit rate that can be used as follows:

$$\text{Data rate in bps} = \text{bandwidth} \times \log\_2(\mathbf{1} + \mathbf{S}/\mathbf{N}) \tag{1}$$

To increase the data rate, a channel with high S/N should be used. Other means that can increase the bit rate is data compression.

#### 1.1.14 Sampling theory

To convert a continuous signal x(t) into a digital form [8], it is first sampled at equal intervals of time. To be able to reconstruct a sampled signal, xδð Þt is defined as

$$\varkappa\_{\delta}(t) = \sum\_{n = -\infty}^{\infty} \varkappa(nT\_s)\delta(t - nT\_s) \tag{2}$$

The sampling interval Ts is 1/fs, where the sampling frequency fs should be at least twice the highest frequency component fmax of the original signal x(t). The frequency 2fmax is called the Nyquist frequency.

#### 1.1.15 Analogue-to-digital (A/D) conversion

An analogue signal with a given frequency f<sup>1</sup> can be converted into a digital form by sampling it at a constant frequency fs, where f <sup>1</sup> , fs . A sampled signal has the form of pulses with different amplitudes called pulse amplitude modulation (PAM). The PAM signal is then quantised, and every level is given a binary code number. This process is called pulse-code modulation (PCM). The sampling frequency fs has to be at least twice as much as the signal frequency being sampled f <sup>1</sup> in order to produce a good approximation of the original signal that can be reproduced and converted back to analogue form. In telephony systems the 8-kHz frequency is used to sample voice that is encoded using 8-bit code. The bit rate in this case is 8000 � 8 ¼ 64 kbps. In compact disc (CD) technology, the audio is sampled at 44:1 kHz.

### Telecommunications Protocols Fundamentals DOI: http://dx.doi.org/10.5772/intechopen.86338

## 1.1.16 Multiplexing

Multiplexing occurs when data are collected from different sources and are transmitted into one common communication channel. Three types of multiplexing are utilised, namely:

