Wireless and Mobile Data Networks - Rilegato

Ahmad, Aftab

Rilegato

ISBN 10: 0471670758 ISBN 13: 9780471670759

Casa editrice: Wiley-Interscience, 2005

Riassunto

Wireless and Mobile Data Networks provides a single point of knowledge about wireless data technologies, including:
* Comprehensive easy-to understand resource on wireless data technologies
* Includes wireless media, data transmission via cellular networks, and network security
* Provides a single point of knowledge about wireless data
* Focuses on wireless data networks, wireless channels, wireless local networks, wide area cellular networks and wireless network security

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Informazioni sull?autore

AFTAB AHMAD received his DSc in communications from The George Washington University. Currently, he is Associate Professor of Computer Science at Norfolk State University. He has published a number of research papers and a book, Data Communication Principles: For Fixed and Wireless Networks. His current research involves ad hoc network routing, wireless network planning, Quality of Service (QoS)–based call control in IP radio access networks (IP-RAN), and QoS routing with IP.

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Connecting to the Future of Wireless Data Technologies

Presenting complex subjects without getting into advanced mathematics, Wireless and Mobile Data Networks gives students and professionals a comprehensive overview of major wireless network architectures and standards.

The opening chapters introduce wireless data network types, architectures, and wireless local area network (WLAN) components. The author follows with physical and medium access control layers of WLANs, mobile IP, and session initiation protocol (SIP); then expands on other relevant topics, such as wide area wireless data networks, security in wireless data networks, routing in ad hoc network, and wireless personal area networks (WPAN); followed by the wireless metropolitan area networks (MANs).

This up-to-the-minute and in-depth coverage of how key standards and protocols work includes:

• Wireless LAN architectures, such as IEEE 802.11
• Wireless PAN architectures, such as Bluetooth (IEEE 802.15.1), IEEE 802.15.3/3a (high data rate and UWB-based PANs), and IEEE 802.15.4 (low power, low data rate PANs)
• Wireless broadband access, such as IEEE 802.16/16a
• Comparisons between architectures, such as IEEE Wireless MANs (IEEE 802.16 and 802.16a) compared to the European Union approaches (HIPERACCESS and HIPERMAN)
• 3G cellular standards, including CDMA2000® and W-CDMA
• Ad hoc networks
• Security in wireless data networks

Wireless and Mobile Data Networks is both an up-to-date reference for IT professionals and a comprehensive textbook for advanced communications and computer science students.

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Wireless and Mobile Data Networks

By Aftab Ahmad

John Wiley & Sons

Copyright © 2005 Aftab Ahmad
All right reserved.
ISBN: 978-0-471-67075-9

Chapter One

WIRELESS DATA-INTRODUCTION

In studying the principles of data communications, the wireless spectrum is generally treated as part of communications media only. This may give the impression that the remaining components of a wireless data network were the same as those of a fixed, wired network. The reality, however, is quite different, thanks to a number of factors with varying degree of roles in wireless and fixed, wired networks. There are network components that exist in only one network type and not the other. There are also network components existing in both types, but playing a less significant role in one or the other. There are many sub-systems, such as antenna radiation and mobility management that do not surface in the fixed, wired networks. Wall connectors are not usually part of transmission systems in wireless networks. There are systems that do make an essential part of both network types, but with much less significance in one than the other. Examples of such systems are power consumption systems, data security, and privacy, by containing signal, signal-detection techniques and error-control techniques. Lastly, there are certainly many components that play equally important roles in both types of networks, such as switching and routing techniques, flow and congestion control mechanisms and call-control procedures. Thus a study of wireless data networks has its own scope, different from networking systems in general.

Wireless, however, does not imply mobility. There are wireless networks in which both ends of communications are fixed, such as in wireless local loops. In satellite communication systems, even though the satellite is always mobile, the mobility profile of the satellite is designed so as to provide a constant signal level to the connected terminals, thus emulating a fixed end. Wireless networks with mobility, however, provide the biggest challenge to the network designer.

We will devote this chapter to various types of wireless data networks that network engineers have to design and deal with. We will start the discussion with wireless voice communication, as the bulk of data in cellular networks is still voice. Also, most of the telecommunications developments have been in telephony.

1.1. WIRELESS VOICE

Before beginning a discussion on wireless data networks, a few words about the voice signal might be advisable. Even though the wireless data systems were the precursor of all electronic communications systems, most of the progress in telecommunications is a result of voice networks. In fact, most of the developments in cellular systems to date owe their existence to the voice signal. Wireless voice poses somewhat relaxed requirements to system designers, which make it easier to make engineering decisions. Here are some examples of the characteristics of wireless voice.

1.1.1. Fixed Minimum Bandwidth

The voice signal has most of its energy within 300 Hz to 3400Hz, giving a bandwidth of 3.1kHz, such as shown in Figure 1-1. For typical digital transmissions, a nominal value of 4 kHz is assumed. Consequently, all channels with a bandwidth of 4 kHz or higher could ideally provide the same quality of transmitted voice if all other factors are kept constant. Digital speech is transmitted in one of the several standard coding forms, such as ITU G.711, G.721, G.722, G.723, G.728 and G.729. These standards are based on different mechanisms of speech digitization and compression, and produce a digital bit stream of either fixed (G.711, 721,) or variable but a known average rate (G.728, 729). PSTN uses G.711, which is based on 8-bit per sample PCM transceiver using one of the two quantization techniques (A-Law in Europe and m-Law in North America and Japan), both resulting in a 64 kbps encoded voice bit stream. PCM is a waveform coding technique that deals directly with the speech signal for digitization and transmission purposes.

Other standards use model-based coding, which extracts certain parameters from the speech signals and transmits these parameters instead of the speech signal. These later systems, called Vocoders, result in bit streams anywhere from 16 kbps to less than 4kbps. However, due to the inflexible nature of the PSTN, the 64 kbps standard is the one most used for voice transmission. For bandwidth-constrained systems, such as wireless networks, lower bit rate coding techniques have been considered as better alternatives. For example, the European GSM systems typically employ regular pulse excited hybrid voice coding (RPE), resulting in 13 kbps bit stream and the U.S. Department of Defense (DoD) uses 4.8 kbps code excited linear predictive (CELP) technique in federal standard FS 1016. In either case, once a network is designed to support a certain type of voice coding, the required minimum bandwidth is fixed. Such is not the case in data communications. Numerical, textual, or graphical data could be transmitted using any bandwidth without impairing its quality, as long as error-control mechanisms are employed to remove errors or retransmit lost packets and packets with errors. The channel bandwidth can only limit the speed of data transmission.

1.1.2. Vague Definition of Service Quality

A second characteristic of voice signal is a lack of a strict scientific definition of the quality of transmitted speech. The quality of voice transmitted is perception-driven and can't be adequately measured. Even though the International Telecommunications Union (ITU) standards based on scientific definition of quality perception allow for an automated measurement of voice quality, the most commonly used metric is still the mean opinion score (MOS), a subjective quality-determining mechanism in which listeners allocate a number between 1 and 5, where 5 is for excellent quality. The procedure for MOS is defined in ITU recommendation ITU-TP.800. A standard introduced for automated quality assessment was introduced in the early part of 2001. Called Perceptual Evaluation of Speech Quality (PESQ), it takes into account factors such as packet loss, delay and jitter. PESQ is defined in the ITU standard ITU-T862. Though its usability for Internet is agreeable, its validation, too, is done by comparing it to MOS.

In circuit-switched wireline networks, a fixed voice coding mechanism is employed, usually based on waveform coding. However, in connectionless Internet, neither fixed coding scheme must be employed, nor do the network characteristics remain constant. In wireless networks, the wireless channel is highly unstable, enhancing the vagueness of quality. In fact, despite strides in speech coding mechanisms, there is a discernable degradation in the quality of transmitted speech in cellular networks as compared with PSTN voice quality.

1.1.3. Delay Requirements

A third and perhaps the strictest characteristic of conversational speech is the stringent requirement on maximum delay. Due to its highly interactive nature, the conversational speech signal is required not to have more than a fraction of a second delay (250msec maximum recommended by ITU). The variation in delay is expected to be even smaller by at least an order of magnitude. These requirements make the flexibility of packet switching somewhat less than ideal for voice communications. Therefore, the voice networks have traditionally been circuit switched. This applies to cellular wireless networks as well. Consequently, a voice network consists of a simple circuit-switched data part and a rather complex signaling system to monitor, supervise, and audit calls and resources.

In fact, the complexity and intelligence of the modern PSTN is due to its signaling systems. The contemporary cellular networks make use of the same signaling systems by adding a mobile part to it for mobility management and interaction with fixed PSTN. Future cellular networks, (termed as beyond 3G or 4G) are expected to circumvent signaling systems altogether and use connectionless packet switching for voice and other applications. This also leads us into a debatable definition of data networks. It is the our view that by data networks we imply packet-switched networks, such as IP networks. This is perhaps because such networks are ideally suited to bursty data applications, such as file and e-mail transfers, which can use store-and-forward mechanisms.

With the increase in demand for packet-switched data, the wireless data networks have evolved into many types, such as:

Wireless LANs that provide wireless access just like the broadcast type fixed LANs provide access to fixed wide area networks. These wireless local area networks, relative latecomers as compared with their wired counterparts, are taking over the scene rather quickly. Their integration with the wide area cellular networks has become possible due to packet-switched third-generation (3G) systems.

Wide area cellular systems, predominantly designed for voice, have incorporated packet switching all over the world from 3G and above. In fact, the precursor to 3G systems (sometimes dubbed as 2.5G) started packet data transmission before 3G technologies.

Fixed wireless systems are becoming popular for broadband Internet access for ease of installation.

Personal area networks (PANs) are the latest addition for short-range, serial-line-like wireless connections with limited mobility.

Satellite-based data systems, though nothing new, are an essential part of the wireless and mobile networks.

We will look at the characteristics of some of these networks in this chapter. More detail will follow throughout the rest of the book.

1.2. WIRLESS LOCAL AREA NETWORKS (WLANS)

Protocols for wireless local area networks (WLANs) typically consist of specifications for the OSI-RM equivalent of physical and the data link control layers. The physical layer specifications deal with utilizing the indoor wireless channel for transmission and reception of wireless signal. These specifications have two types of limitations; the ones set by frequency regulation agencies, and the others set by the protocol specification agencies. Usually, the bandwidth and radiation amounts are regulated by the spectrum regulating agencies and the bandwidth utilization mechanisms (modulation, data rates) and power radiation mechanisms (direct, indirect, line-of-sight) are set by protocol agencies, according to the guidelines provided by the spectrum regulating agencies. The medium access control (MAC) specifications are set altogether by the protocol specification agencies. These specifications deal with issues such as channel access, synchronization of frames, power control, resource management for multimedia, and so forth.

The most popular WLAN standards, recommended by IEEE (we call these the IEEE 802.11 suite), use infrared and the unlicensed spectra. These spectra are allocated in many countries for research and developments in industry (I), science (S) and medicine (M)-therefore, called the ISM band. The IEEE standard PHY provides several mechanisms for the use of ISM band (and unlicensed national information infrastructure U-NII band), designed to combat interference from other sources of the same bands. This is necessary because the use of such a system does not require license from the government, which could result in numerous sources of interference. The infrared band specifies only one type of radiation, that is, indirect radiation reflected from a course surface (called diffused infrared). The medium access control mechanism specifies a distributed coordination function (DCF) for channel access, distributed referring to the fact that it is to be implemented in all participating wireless stations. It defines several device types, for example, a mobile station (STA),which is a user terminal, and an access point (AP), which relays data between two stations or a station and a terminal on a fixed LAN. This gives rise to two configurations of WLANs, as shown in Figure 1-2, the infrastructure WLANs and ad hoc WLAN.

1.2.1. Ad hoc WLAN

In an ad hoc or independent WLAN, two stations communicate directly with each other without an access point. Mobile stations for such networks may require the capability to forward a packet, thus acting as a repeater. With this relaying capability, two mobile stations could exchange data packets even if they are unable to receive signals directly from each other.

Wide area networking in ad hoc networks is possible if one or more stations are connected to a wide area network, such as an IP network. However, this connectivity is not guaranteed and there is no guaranteed communication mechanism outside the ad hoc network.

1.2.2. Infrastructure WLAN

In an infrastructure WLAN, two stations exchanging data can communicate only through an access point. Figure 1-2 shows an access point connected to the ceiling with a cable connection to the wired network. The access point performs several functions in addition to relaying packets between stations in wireless and wired networks; such as implementing a point coordinating function (PCF) to allow reservation based communications for delay-bound traffic.

The MAC sublayer of the IEEE WLAN provides access-related mechanisms. For this purpose, it employs a mechanism similar to Ethernet. The Ethernet MAC (IEEE 802.3) uses carrier sense multiple access with collision detection (CSMA/CD). However, collision detection can't be efficient in wireless media, due to the rapid attenuation of the signal with distance. Instead of collision detection, a mechanism for collision avoidance is specified. Collision avoidance is implemented by requiring certain minimum time between any two packets transmitted. This time is called the inter-frame spacing (IFS). Due to the collision avoidance mechanism, the IEEE 802.11 MAC procedure is called CSMA/CA, carrier sense multiple access with collision avoidance. The subject of WLANs is as important as the application of such networks and it will be extensively discussed throughout the text.

1.3. WIDE AREA CELLULAR NETWORKS

Voice communication has been and continues to be the main application of cellular systems. These systems use PSTN-friendly infrastructure that employs circuit switching and signaling systems. However, with the wide spread of the Internet use, packet-switched services were introduced in enhancements of digital cellular systems. These included time division multiple access (TDMA)-based systems, such as GPRS (general packet radio service) and enhancement of code division multiple access (CDMA)-based systems IS-95B. The wireless standards for the new millennium that were internationally coordinated under the name of international mobile telecommunications 2000 (IMT-2000) (known from their air interfaces, WCDMA in Europe and cdma2000 in North America) have IP capability with data rates much higher than GPRS and IS-95B. The data networks of the first digital cellular generation were a result of defining new user terminals types, network devices and signaling system above the existing voice network, as shown in Figure 1-3 for GPRS. The latest generation wide area cellular networks provide access mechanisms for circuit- and packet-switched communication. For a true packet-switched cellular network an access mechanism similar to the WLANs could provide a better transport vehicle for data applications. Work is in progress in that direction and some countries already have WLAN access using the wide area cellular backbone for auditing and admission control purposes. Such networks are expected to make broadband wireless access as ubiquitous as the Ethernet for the Internet. The next releases of the cellular networks could be a starting point for this true merger of IP and cellular networking. At this time, broadband wireless is available in the form of fixed wireless networks only.

A universal installation of cellular systems based on 3G and above has been hampered by various technical, economic, and political factors. On the technical side, the world remains divided into groups based on evolution of their current systems. Two main camps are the European, supporting Wideband CDMA and North American, supporting cdma2000 evolution. The 3G partnership projects (3GPP for WCDMA and 3GPP2 for WCDMA) are destined to help actual implementation of 3G+ systems and take steps toward harmonization of the two camps.

(Continues...)

Excerpted from Wireless and Mobile Data Networksby Aftab Ahmad Copyright © 2005 by Aftab Ahmad. Excerpted by permission.
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Dettagli dell?edizione

• Editore: Wiley-Interscience
• Data di pubblicazione: 2005
• Lingua: Inglese
• ISBN 10: 0471670758
• ISBN 13: 9780471670759
• Rilegatura: Copertina rigida
• Numero di pagine: 346
• Contatto del produttore: non disponibile