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Praphul Chandra James F. Ransome John Rittinghouse
What is it that makes a wireless medium so unique? What are the problems of operating in the wireless medium and how are they overcome? What are the different types of wireless networks in use today? How does each one of them work and how do they differ from each other? The aim of this chapter is to answer these questions so as to establish a context in which wireless security can be studied in the following chapters.
The first successful commercial use of wireless telecommunication was the deployment of cellular phones (mobile phones). In this book, we refer to these networks as traditional wireless networks (TWNs). These networks were designed with the aim of extending the existing wired public switched telephone network (PSTN) to include a large number of mobile nodes. The deployment of TWNs allowed users to be mobile and still make voice calls to any (fixed or mobile) phone in the world. In other words, TWNs were designed as a wide area network (WAN) technology enabling voice communication. These networks have evolved over time to support both voice and data communication but the underlying feature of the TWNs being a WAN technology is still true.
For a long time, TWNs were the predominant example of wireless telecommunication. In the late 1990s, another wireless technology emerged: wireless local area networks (WLANs). Unlike TWNs, WLANs were designed primarily with the aim of enabling data communication in a limited geographical area (local area network). Though this aim may seem counterintuitive at first (why limit the geographical coverage of a network?), this principle becomes easier to understand when we think of WLANs as a wireless Ethernet technology. Just as Ethernet (IEEE 802.3) provides the backbone of wired local area networks (LANs) today, IEEE 802.11 provides the backbone of wireless LANs.
Just as TWNs were initially designed for voice and over time evolved to support data, WLANs were initially designed for data and are now evolving to support voice.
Probably the most prominent difference between the two standards is that the former is a WAN technology and the latter is a LAN technology. TWNs and WLANs are today the two most dominant wireless telecommunication technologies in the world. Analysts are predicting the convergence (and co-existence) of the two networks in the near future.
Finally, we are today seeing the emergence of wireless mobile ad-hoc networks (MANETs). Even though this technology is still in an early phase, it promises to have significant commercial applications. As the name suggests, MANETs are designed with the aim of providing ad hoc communication. Such networks are characterized by the absence of any infrastructure and are formed on an as-needed (ad hoc) basis when wireless nodes come together within each others' radio transmission range.
We begin by looking at some of the challenges of the wireless medium.
1.1 The Wireless Medium
1.1.1 Radio Propagation Effects
The wireless medium is a harsh medium for signal propagation. Signals undergo a variety of alterations as they traverse the wireless medium. Some of these changes are due to the distance between the transmitter and the receiver, others are due to the physical environment of the propagation path and yet others are due to the relative movement between the transmitter and the receiver. We look at some of the most important effects in this section.
Attenuation refers to the drop in signal strength as the signal propagates in any medium. All electromagnetic waves suffer from attenuation. For radio waves, if r is the distance of the receiver from the transmitter, the signal attenuation is typically modeled as 1/r2 at short distances and 1/r4 at longer distances; in other words, the strength of the signal decreases as the square of the distance from the transmitter when the receiver is "near" the transmitter and as the fourth power when the receiver is "far away" from the transmitter. The threshold value of r where distances go from being "near" to being "far away" is referred to as the reference distance. It is important to emphasize that this is radio modeling we are talking about. Such models are used for simulation and analysis. Real-life radio propagation is much harsher and the signal strength and quality at any given point depends on a lot of other factors too.
Attenuation of signal strength predicts the average signal strength at a given distance from the transmitter. However, the instantaneous signal strength at a given distance has to take into account many other effects. One of the most important considerations that determine the instantaneous signal strength is, not surprisingly, the operating environment. For example, rural areas with smooth and uniform terrain are much more conductive to radio waves than the much more uneven (think tall buildings) and varying (moving automobiles, people and so on) urban environment. The effect of the operating environment on radio propagation is referred to as shadow fading (slow fading). The term refers to changes in the signal strength occurring due to changes in the operating environment. As an example, consider a receiver operating in an urban environment. The path from the transmitter to the sender may change drastically as the receiver moves over a range of tens of meters. This can happen if, for example, the receiver's movement resulted in the removal (or introduction) of an obstruction (a tall building perhaps) in the path between the transmitter and the receiver. Shadow fading causes the instantaneous received signal strength to be lesser than (or greater than) the average received signal strength.
Another propagation effect that strongly affects radio propagation is Raleigh fading (fast fading). Unlike slow fading which effects radio propagation when the distance between the transmitter and the receiver changes of the order of tens of meters, fast fading describes the changes in signal strength due to the relative motion of the order of a few centimeters. To understand how such a small change in the relative distance may affect the quality of the signal, realize that radio waves (like other waves) undergo wave phenomena like diffraction and interference. In an urban environment like the one shown in Figure 1.1a, these phenomena lead to multipath effects; in other words, a signal from the transmitter may reach the receiver from multiple paths. These multiple signals then interfere with each other at the receiver. Since this interference can be either constructive or destructive, these signals may either reinforce each other or cancel each other out. Whether the interference is constructive or destructive depends on the path length (length the signal has traveled) and a small change in the path length can change the interference from a constructive to a destructive one (or vice versa). Thus, if either of the transmitter or the receiver move even a few centimeters, relative to each other, this changes the interference pattern of the various waves arriving at the receiver from different paths. This means that a constructive interference pattern may be replaced by a destructive one (or vice versa) if the receiver moves by as much as a few centimeters. This fading is a severe challenge in the wireless medium since it implies that even when the average signal strength at the receiver is high there are instances when the signal strength may drop dramatically.
Another effect of multipath is inter-symbol interference. Since the multiple paths that the signal takes between the transmitter and the receiver have different path lengths, this means that the arrival times between the multiple signals traveling on the multiple paths can be of the order of tens of microseconds. If the path difference exceeds 1-bit (symbol) period, symbols may interfere with each other and this can result in severe distortion of the received signal.
1.1.2 Hidden Terminal Problem
Wireless is a medium that must be shared by all terminals that wish to use it in a given geographical region. Also, wireless is inherently a broadcast medium since radio transmission cannot be "contained." These two factors create what is famously known as the hidden terminal problem in the wireless medium. Figure 1.1b demonstrates this problem.
Figure 1.1b shows three wireless terminals: A, B and C. The radio transmission range of each terminal is shown by a circle around the terminal. As is clear, terminal B lies within the radio transmission range of both terminals A and C. Consider now what happens if both A and C want to communicate with B. Most media access rules for a shared medium require that before starting transmission, a terminal "senses" the medium to ensure that the medium is idle and therefore available for transmission. In our case, assume that A is already transmitting data to B. Now, C also wishes to send data to B. Before beginning transmission, it senses the medium and finds it idle since it is beyond the transmission range of A. It therefore begins transmission to B, thus leading to collision with A's transmission when the signals reach B. This problem is known as the hidden terminal problem since, in effect, A and C are hidden from each other in terms of radio detection range.
1.1.3 Exposed Terminal Problem
The exposed terminal problem is at the opposite end of the spectrum from the hidden terminal problem. To understand this problem, consider the four nodes in Figure 1.1c.
In this example, consider what happens when B wants to send data to A and C wants to send data to D. As is obvious, both communications can go on simultaneously since they do not interfere with each other. However, the carrier sensing mechanism raises a false alarm in this case. Suppose B is already sending data to A. If C wishes to start sending data to D, before beginning it senses the medium and finds it busy (due to B's ongoing transmission). Therefore C delays its transmission unnecessarily. This is the exposed terminal problem.
"The Queen is dead. Long Live the Queen."
Bandwidth is one of the most important and one of the most confusing topics in telecommunications today. If you keep up to date with the telecommunication news, you would have come across conflicting reports regarding bandwidth. There are a lot of people claiming "bandwidth is cheap" and probably as many people claiming "it is extremely important to conserve bandwidth." So, what's the deal? Do networks today have enough bandwidth or not?
The problem is there is no single correct answer to that. The answer depends on where you are in the network. Consider the core of the IP and the PSTN networks: the two most widely deployed networks today. The bandwidth available at the core of these networks is much more than required: bandwidth therefore is cheap at the core of the network. Similarly the dawn of 100 Mbps and Gigabit Ethernet has made bandwidth cheap even in the access network (the part of the network that connects the end-user to the core). The wireless medium, however, is a little different and follows a simple rule: bandwidth is always expensive. This stems from the fact that in almost all countries the wireless spectrum is controlled by the government. Only certain bands of this spectrum are allowed for commercial use, thus making bandwidth costly in the wireless world. All protocols designed for the wireless medium therefore revolve around this central constraint.
Excerpted from Wireless Security by Praphul Chandra Alan Bensky Tony Bradley Chris Hurley Steve Rackley John Rittinghouse James F. Ransome Timothy Stapko George L. Stefanek Frank Thornton Jon Wilson Copyright © 2009 by Elsevier Inc. . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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