Multimedia Communications and Networking

Mario Marques da Silva

• Publisher: CRC Press
• Year: 2016
• Print ISBN: 9781439874844
• eBook ISBN: 9781466559202
• Topic: Computer Science
• Subtopic: Electrical Engineering & Telecommunications
• Index: Computer Science

The result of decades of research and international project experience, Multimedia Communications and Networking provides authoritative insight into recent developments in multimedia, digital communications, and networking services and technologies. Supplying you with the required foundation in these areas, it illustrates the means that will allow

1

AN INTRODUCTION TO MULTIMEDIA COMMUNICATIONS AND NETWORKING

1.1 Fundamentals of Communications

Communication systems are used to enable the exchange of data between two or more entities (humans or machines). As can be seen from Figure 1.1, data consists of a representation of an information source, whose transformation is performed by a source encoder. An example of a source encoder is a thermometer, which converts temperatures (information source) into voltages (data). A telephone can also be viewed as a source encoder, which converts the analog voice (information source) into a voltage (data) before being transmitted along the telephone network (transmission medium). In case the information source is analog and the transmission medium is digital, a CODEC (coder and decoder) is employed to perform the digitization. A VOCODER (voice coder) is a codec specific for voice, whose functionality consists of converting analog voice into digital at the transmitter side and the reciprocal at the receiver side.

The emitter of data consists of an entity responsible for the insertion of data into the communication system and for the conversion of data into signals. Note that signals are transmitted, not data. Signals consist of an adaptation^* of data, such that their transmission is facilitated in accordance with the used transmission medium. Similarly, the receiver is responsible for converting the received signals into data.

The received signals correspond to the transmitted signals subject to ­attenuation and distortion and added with noise and interferences. These channel impairments originate that the received signal differs from that transmitted. In the case of ­analog signals, the resulting signal levels do not exactly translate the original information source. In the case of digital signals, the channel impairments originate corrupted bits. In both cases, the referred channel impairments originate a degradation of the signal-to-noise plus interference ratio (SNIR).^† A common performance indicator of digital communication systems is the bit error rate (BER). This corresponds to the number of corrupted bits divided by the total number of transmitted bits over a certain time period.

Figure 1.1 A generic block diagram of a communication system.

A common definition associated with information is knowledge. It consists of a person’s ability to have access to the right information at the right time. The conversion between information and knowledge can be automatically performed using information systems, whereas the information can be captured by sensors and distributed using communication systems.

1.1.1 Analog and Digital Signals

Analog signals present a continuous amplitude variation over time. An example of an analog signal is the voice. Contrarily, digital signals present time discontinuity (e.g., voltages or light pulses). The bits^* generated in a computer are examples of digital data. The text is another example of digital data. Examples of analog and digital signals are depicted in Figure 1.2.

Digital signals present several advantages (relating to analog). The following advantages can be listed:

• Error control is possible in digital signals: corrupted bits can be detected and/or corrected.

• Since they present only two discrete values, the consequences of channel impairments can be more easily detected and avoided (as compared to analog signals).

• Digital signals can be regenerated, almost eliminating the effects of channel impairments. Contrarily, the amplification process of analog signals results in the amplification of signals, noise, and interferences, keeping the SNR relationship unchanged.^†

• The digital components are normally less expensive than the analog components.

• Digital signals facilitate cryptography and multiplexing.

• Digital signals can be used to transport different sources of information (voice, data, multimedia, etc.) in a transparent manner.

Figure 1.2 Example of (a) analog and (b) digital signals.

However, digital signals present an important disadvantage:

• For the same information source, the bandwidth required to accommodate a digital signal is typically higher than the analog counterpart.^* This results in a higher level of attenuation and distortion.

1.1.2 Modulator and Demodulator

As can be seen from Figure 1.3, when the source (e.g., a computer) generates a digital stream of data and the transmission medium is analog, a modem (modulator and demodulator) is used to perform the required conversion. The modulator converts digital data into analog signals, whereas the demodulator (at the receiver) converts analog signals into digital data. An example of an analog transmission medium is radio transmission, whose signals consist of electromagnetic waves (presenting a continuous variation in time).

A modem (e.g., asynchronous digital subscriber line [ADSL] or cable modem) is responsible for modulating a carrier wave with bits, using a certain modulation scheme.^† The reverse of this operation is performed at the receiver side. Moreover, a modem allows sending a signal modulated around a certain carrier frequency, which can be another reason for using such device.

In case the data is digital and the transmission medium is also digital, a modem is normally not employed, as the conversion between digital and analog does not need to be performed. In this case, a line encoder/decoder (sometimes also referred to as digital modem, nevertheless not accurately) is used. This device adapts the original digital data to the digital transmission medium^‡ and adapts parameters such as levels, pulse duration, and so on. Note that in using such digital encoders, the signals are transmitted in the baseband.^§

Figure 1.3 A generic communication system incorporating a modem.

The output of a line encoder consists of a digital signal, as it comprises discrete voltages that encode the source logic states. Consequently, it can be stated that the line encoder is used when the transmission medium is digital. On the other hand, the output of a modulator consists of an analog signal, as it modulates a carrier, which is an analog signal.

In case of high data rate, the required bandwidth necessary to accommodate such signal is also high,^* and the medium may originate high level of attenuation or distortion at limited frequency components of the signal. In such case, the use of a modem can be a good choice, which allows the modulation of the signal around a certain carrier frequency. The carrier frequency can be carefully selected such that the channel impairments in the frequencies around it (corresponding to the signal bandwidth) do not seriously degrade the SNR. The reader should refer to Chapter 6 for a detailed description of the modulation schemes used in modems and a description of digital encoding techniques.

1.1.3 Transmission Mediums

Transmission mediums can be classified as cable or wireless. Examples of cable transmission mediums include twisted pair cables, coaxial cables, multimode or single-mode optical fiber cables, and so on. In the past, local area networks (LAN) were made of coaxial cables. These cables were also used as a transmission medium for medium- and long-range analog communications. Although coaxial cables were replaced by twisted pair cables in LAN, the massification of cable television enabled their reuse.

As a result of the telephone cablings, twisted pairs are still the dominant transmission medium in houses and offices. These cablings are often reused for data. With the improvement of isolators and copper quality, as well as the development of shielding, the twisted pair became widely used for providing high-speed data communications, in addition to its initial use for analog telephony.

Currently, multimode optical fibers have been installed more and more in homes, which allows reaching throughputs of the order or several gigabits per second (Gbps). Moreover, single-mode optical fibers are the most used transmission medium in transport networks. A transport network consists of the backbone (core) network, used for transferring high amounts of data among different main nodes. These main nodes are then connected to secondary nodes, and then finally connected to customer nodes.

A radio or wireless communication system is composed of a transmitter and a receiver, using antennas to convert electrical signals into electromagnetic waves and vice versa. These electromagnetic waves are propagated over the air. Note that wireless transmission mediums can be either guided or unguided. In the former case, directional antennas are used at both the transmitter and the receiver sides, such that electromagnetic waves propagate directly from the transmitting antenna into the receiving antenna. The reader should refer to Chapter 4 for a detailed description of cable transmission mediums. Chapter 5 introduces wireless transmission mediums.

1.1.4 Synchronous and Asynchronous Communication Systems

Synchronous and asynchronous communications refer to the ability or inability to have information about the start and end of bit instants.^* Using asynchronous communications, the receiver does not achieve perfect time synchronization with the transmitter, and the communication accepts some level of fluctuation. Consequently, start and stop bits are normally included in a frame^† to periodically achieve bit synchronization of the receiver with the transmitter. Note that between the start and the stop bit, the receiver of an asynchronous communication suffers from a certain amount of time shift. The referred periodic synchronization using start and stop bits is normally included as part of the functionalities implemented by a modem when establishing a communication in asynchronous mode of operation. Normally, asynchronous communications do not accommodate high-speed data rates. They are normally used for random (not continuous) exchanges of data (at low rate).

On the other hand, synchronous communications consider a receiver that is bit-synchronized with the transmitter. This bit synchronization can be achieved using one of the following methods:

• By sending a clock signal multiplexed with the data or using a parallel dedicated circuit

• When the transmitted signal presents a high zero crossing rate, such that the receiver can extract the start and end of bit instants from the received signal

Synchronous communications are normally used in high-speed lines...

Table of contents

• Cover
• OTHER TELECOMMUNICATIONS BOOKS FROM AUERBACH
• Title Page
• Copyright
• Dedication
• Contents
• Preface
• About the Author
• Chapter 1 An Introduction to Multimedia Communications and Networking
• Chapter 2 Network Protocol Architectures
• Chapter 3 Channel Impairments
• Chapter 4 Cable Transmission Mediums
• Chapter 5 Wireless Transmission Mediums
• Chapter 6 Source Coding and Transmission Techniques
• Chapter 7 Advanced Transmission Techniques to Support Current and Emergent Multimedia Services
• Chapter 8 Cellular Communications
• Chapter 9 Transport Networks
• Chapter 10 Data Link Layer
• Chapter 11 Network Layer
• Chapter 12 Transport Layer
• Chapter 13 Services and Applications
• Chapter 14 Network Security
• Annex A
• Annex B
• Annex C
• References
• Index