Introduction to Wireless Networks and Ad Hoc Networks, Lecture notes of Computer Networks

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Wireless Networks

Local and Ad Hoc Networks

Ivan Marsic

Department of Electrical and Computer Engineering and the CAIP Center

Rutgers University

1

Contents

1.1 x 1.1.1 x 1.1. 1.1. 1.1. 1.2 x 1.2.1 x 1.2. 1.2. 1.2. 1.3 x 1.3.1 x 1.3. 1.3. 1.3. 1.3. 1.3. 1.4 x 1.4.1 x 1.4. 1.4. 1.4. 1.4. 1.6 Summary and Bibliographical Notes

Chapter 1

Introduction

In this text I review basic results about wireless communication networks. Communication may be defined as the process of successfully transferring information between two or more parties. My emphasis is on computation rather than physics of the processes, while keeping in mind that understanding the physics is necessary to design the computation.

The recurring theme in this chapter is the capacity and its statistical properties for a communication channel. Capacity is modeled differently at different abstraction levels, but the key issue remains the same: how to increase the amount of information that can be transmitted over a channel in the presence of channel impairments. A complement of capacity is delay , and although it crops up sporadically, the treatment of delays is deferred to the next chapter.

The main difference between wired and wireless networks is that there are no wires (the air link) and mobility is thus conferred by the lack of a wired tether. This leads to both the tremendous benefits of wireless networks and the perceived drawbacks to them. Some of the key technical challenges in wireless communications come from ( i ) the hostile wireless propagation medium and ( ii ) user mobility. Most of the issues covered in this chapter arise in any data network, wired and wireless likewise. What makes the wireless different is the degree of importance inherent to the problems that often appear in both types of networks. Some examples are as follows.

• Errors due to channel noise occur both in wired and wireless networks, but in the former they play only a minor role. Conversely, in wireless networks, channel errors are a major concern. Moreover, due to dynamic changes in the transmission medium and user mobility, the error probability changes dynamically over a wide range.
• Both wired and wireless networks employ broadcast where multiple stations use the same medium for communication. However, in the wireless case “listening while speaking” is

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complicated, so coordinating the medium access or MAC (Medium Access Control) is much more complicated.

• Both types of networks employ routing to bring a packet to the destination. However, wireless “link” is a relative (or “soft”) concept. Dynamic changes in the transmission medium and user mobility often cause severance or emergence of a “link” and the associated changes in the network topology, i.e., network graph connectivity.
• The point of belonging to a network is in having relationship with other entities in the network. While a mobile device is moving, the related network entities may be moving in a different direction or not moving at all. This change of the network “landscape”—an uncommon phenomenon in the wired world—presents important challenges from the network management perspective if the network is to provide an undisrupted service to the roaming user.
• Energy expenditure is important in both types of networks (recall screen savers and coolers for wired computers), but battery energy plays a key role in wireless communication. Transmission power also determines the spatial volume within which a given radio transmission interferes with other transmissions. Radio resource management refers to the control signaling and associated protocols that negotiate the optimal usage of network radio resources (transmission signal strength and available radio channels) for communication.
• Application drivers are different (e.g., multimedia, sensors, military, etc.)

As a result, it is very hard to disentangle the concerns and responsibilities of different network layers, e.g., MAC, routing, and resource allocation, in wireless networks.

Protocol Layering

While information is represented and transmitted in the form of signal waveforms, communication between the sender and the receiver is usually viewed as taking place between different layers of abstraction, which are codified as ISO OSI reference architecture, see Figure 1-1. The key design principle is such that a protocol at layer i does not know about the protocols at layers i −1 and i +1. The aspects of abstraction include the type of service offered and the type of representation of messages exchanged between the communicating entities. The services offered by a higher abstraction layer are composed of the services offered by the lower abstraction layers. The type of information grows in abstraction from physical signals and streams of symbols to software objects.

In this chapter I consider the bottom three layers: physical , link , and networking. As will become apparent below, the separation of functions between layers is not as clean as it is with point-to- point links and wired networks in general.

The key service offered by the physical layer is a symbol stream with a transmission error rate. This is achieved by the physical transmission and reception of signals over the wireless channel, reviewed in Chapter 2. While the physical layer represents messages as discrete entities, usually called frames , the information exchanged between the communicating parties is essentially viewed as a continuous signal representing a stream of symbols (bits).

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the messages to the base station in the cell of the intended recipient. In turn, this remote base station broadcasts the message within its cell for the receiving mobile station to pick up. Cellular networks provide the information transport platform for wireless local area networks (WLANs) and wireless wide area networks (WWANs).

Ad hoc networks have no pre-existing (fixed) infrastructure and the network architecture is configurable. They are formed by wireless stations which may be mobile and they route paths for each other. Every station in an ad hoc network can be set up as, and play the role of, a base station where it can directly transmit and receive from other stations in the network. Packets may need to traverse multiple links to reach a destination. Due to the mobility, the routes between stations may change dynamically. Ad hoc network can co-exist and co-operate, i.e., exchange data packets, with an infrastructure-based network.

Clearly, real world is not as rigid as the above codification would imply. Another useful concept is that of wireless mesh networks. Wireless mesh network is a multihop wireless network with mostly static routing nodes. It is a stable ad hoc network—a hybrid between the extreme cases shown in Figure 1-2, where one AP connects multiple (mostly stationary) nodes to the Internet. Mobility may or may not be a problem, and since static nodes can be powered by the electric grid, battery power is not necessarily a problem. Examples of wireless mesh networks include: an intra-home or inter-home wireless network; an enterprise-wide backbone wireless network of APs; and, cellular mesh.

The characteristics of a mesh are:

• “Grows” organically
• Does not necessarily require any “infrastructure”
• Increases overall network capacity
• Robust and resilient to faults
• Self-managing and self-administering
• Identity and security is a challenge

Naming and Addressing

Names and addresses play an important role in all computer systems as well as any other symbolic systems. They are labels assigned to entities such as physical objects or abstract concepts, so those entities can be referred to in a symbolic language. Since computation is specified in and communication uses symbolic language, the importance of names should be clear. The main issues about naming include:

• Names must be unique so that different entities are not confused with each other
• Names must be resolved with the entities they refer to, to determine the object of computation or communication

The difference between names and addresses, as commonly understood in computing and communications, is as follows. Names are usually human-understandable, therefore variable

Chapter 1 • Introduction 5

length (potentially rather long) and may not follow a strict format. Addresses , for efficiency reasons, have fixed lengths and follow strict formatting rules. For example, you could name your computers: “My office computer for development-related work” and “My office computer for business correspondence.” The addresses of those computers could be: 128.6.236.10 and 128.6.237.188, respectively. Separating names and addresses is useful for another reason: this separation allows to keep the same name for a computer that needs to be labeled differently when it moves to a different physical place. For example, a telephone may retain its name when moved to a region with a different area code. Of course, the name/address separation implies that there should be a mechanism for name-to-address translation, and vice versa. Two most important address types in contemporary networking are:

• Link address of a device, also known as MAC address , which is a physical address for a given network interface card (NIC), also known as network adaptor. These addresses are standardized by the IEEE group in charge of a particular physical-layer communication standard

(a) (b)

Internet/Web Internet/Web

Mobile Node AccessPoint MobileNode

Internet/WebInternet/Web

Mobile Node AccessPoint MobileNode

Mobile Node

Mobile Node

Mobile Node Mobile Node

Mobile Node

Mobile Node

Mobile Node Mobile Node

(c) (d)

Figure 1-2: Examples illustrating different architectures for wireless networks: (a) infrastructure “cellular,” and (b) infrastructure-less ad hoc systems, both based on IEEE 802.11 wireless LAN, also called Wi-Fi. The respective network topologies with nodes and links are hubs-and-spokes (c), and mesh (d).

Chapter 1 • Introduction 7

• More localized algorithms
• More testing with real equipment
• New technologies, e.g., ultra-wideband, software radio
• Sensor network issues, such as reliability
• Building working networks out of current fundamentals

This text covers many different topics aspects of computer networks and wireless communications. At many places the knowledge networking topics is assumed and no explanations are given. The reader should check a networking book; perhaps the two most regarded networking books currently are [Kurose & Ross 2005] and [Peterson & Davie 2003].

Protocol layering and OSI reference architecture is probably best covered by [Tanenbaum 2003]. This chapter considers only the bottom three layers: physical, link including MAC, and network layers. Most of the problems are common for both wired and wireless networks, but there is a shift in emphasis. A key issue with wireless is the interference—it causes a decrease in channel capacity due to relatively high- and dynamically varying noise. Another issue is link instability, which results in changes of network topology.

An interesting generalization of layered communication is available in [Benkler 2000].

The material presented in this chapter requires basic understanding of probability and random processes. [Yates & Goodman 2004; Bertsekas & Tsitsiklis 2002] provide excellent introduction and [Papoulis & Pillai 2001] is a more advanced and comprehensive text.

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amount of energy. A property of a periodic wave that describes how many wave patterns or cycles pass by in a period of time is called frequency , f. Frequency is measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second. Another property of a

wave is its wavelength , λ, which is the distance between adjacent peaks (crests). Since the wave

travels the distance, λ, in a time for a complete vibration of the source, 1/ f , its speed, v , is given

by the expression: v = λ f. [Wave velocity λ is also known as the phase velocity.]

Figure 2-1 shows the full range of frequencies (electromagnetic spectrum) and its uses for communication. Frequencies below 1 GHz are usually referred to as radio frequencies, and higher frequencies are referred to as microwave. Radio frequencies are further divided at 30 MHz. Below 30 MHz, long-distance propagation is possible by reflection from the ionosphere. Above 30 MHz, the ionosphere is transparent to electromagnetic waves and propagation is on line-of- sight paths. However, not all materials behave the same way to both light frequencies and radio frequencies, and radio frequencies can penetrate different materials to a different extent. In a sense, the waves with millimeter wavelengths are quasi-optical. For example cardboard is transparent to radio waves and is opaque to (blocks) light waves. Dielectrics such as cardboard, paper, clear glass, Teflon, some plastics, pure water and many building materials have low attenuation coefficients and radio waves reflect from them as well as pass through them. The prime portion of radio spectrum consists of the frequencies running from 30 to 3,000 MHz. These frequencies can penetrate buildings (which is why radios and cell phones work indoors) and transmit over long distances at low power (which lets batteries run longer and limits potentially harmful radiation). At higher frequencies a signal that is generated outdoors does not penetrate into buildings, and the signal generated indoors stays confined to a room. In all countries the

Freq.

Freq.

902 MHz 928 MHz 2.4 GHz 2.4835 GHz 5.725 GHz 5.785 GHz Freq.

(AM radio) MF

(SW radio) HF

(FM radio - TV) VHF

(TV – Cell.) LF UHF SHF 30 KHz 300 KHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz

Infrared VisibleUV^ X rays

Gamma rays 1 KHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz

IEEE 802.11a HiperLAN II

IEEE 802.11b, g Bluetooth Microwave ovens

Cordless phones Baby monitors (old) Wireless LANs

ISM UNII

Figure 2-1: The electromagnetic spectrum allocation. ISM = Industry, Science & Medicine, UNII = Unlicensed National Information Infrastructure.

Chapter 2 • The Radio Channel 3

government regulates the spectrum licensing and the usage differs from country to country. Wireless networks operate in both licensed and unlicensed parts of the spectrum. For example, wireless telephone networks operate in the bands licensed to the telecommunications networking companies.

Of particular interest are the unlicensed bands, which are free to anyone to transmit without the need of a license or specific spectrum allocation from the spectrum regulatory body. The government mandates the maximum transmission power and sometimes the form of transmission. For example, to minimize interference between the uncoordinated devices, the devices in the unlicensed bands may be required to use wideband digital modulation (spread spectrum) technologies. Similar rules apply in different countries. Two important unlicensed bands for public wireless communications are ISM (Industrial, Scientific, Medical) and UNII (Unlicensed National Information Infrastructure).

The location of the ISM bands varies somewhat from country to country. Table 2-1 shows their location in the United States. Garage door openers, cordless phones, radio-controlled toys, wireless mice, and numerous other wireless household devices use the ISM bands. UNII bands are created to provide high-speed wireless networks that work over a short range. Typical uses are transfer of data, voice, graphics, teleconferencing, videoconferencing, and other multimedia services.

Table 2-1: Unlicensed spectrum location in the United States.

Unlicensed Bands Spectrum Typical Applications ISM: Industry, Science and Medicine 902-928 MHz, 2.4-2.4835 GHz & 5.725-5. GHz

234.5 MHz Cordless phones, (old) Wireless LANs (WLAN) and Wireless PBXs (WPBX) UPCS: Unlicensed PCS Asynchronous: 1910-1920, 2390-2400 MHz Isochronous: 1920-1930 MHz

20 MHz 10 MHz

WLAN (IEEE 802.11b, Bluetooth) WPBX (Microwave ovens) UNII: Unlicensed National Information Infrastructure UNII (5.15-5.25 GHz) UNII (5.25-5.35 GHz) UNII (5.525-5.825 GHz)

100 MHz 100 MHz 100 MHz

IEEE 802.11a, HiperLAN II

Indoor applications: WLAN, WPBX Short outdoor links, campus applications Long outdoor links, Point-To-Point links Millimeter Wave (59-64 GHz) 5 GHz Home networking applications

Radio spectrum is considered a scarce resource. Wired media provide us with an easy means to increase capacity—if affordable, we can lay more wires where required. With the wireless medium, we are restricted to a limited available band of operation, and we cannot obtain new bands or easily duplicate the medium to accommodate new users. Most of the prime spectrum is already licensed for different uses. However, there are indications that the majority of the licensed spectrum goes unused most of the time, thus creating an artificial shortage of spectrum, and there are efforts to free this spectrum for wireless networks, see e.g., [Woolley 2002]. Even if these efforts succeed, it will not happen in the near future and if it eventually does happen, the amount of freed spectrum will greatly vary from country to country. Thus, the designers of mobile applications must continue working under the assumption that the spectrum is a scarce resource.

A digital radio can either transmit a continuous bit stream or group the bits into packets. The latter type of radio is called a packet radio and is characterized by bursty transmissions: the radio is idle except when it transmits a packet. The first packet radio network, ALOHANET, was

Chapter 2 • The Radio Channel 5

2.2 Channel Implementation

A channel is the path, or link, that interconnects transmitter and receiver (see Figure 2-3). Ideally, communication channel should accept messages at the input and deliver them unaltered at the output. Unfortunately, every real channel suffers from noise, to a different degree. Noise is probably the most important problem of communication. The problem with noise is that it is unpredictable—it is a random process. Its effect on signal at the receiver is that one symbol could be mistaken for another.

Signals found in communication systems are complex waveforms. Any signal can be represented through Fourier transform as a sum of sine waves of different frequencies. Consider a signal with sinusoidal component frequencies extending from near zero to some upper limit, B. Such a signal is commonly called a baseband signal , and examples are audio signal from a microphone or a video signal from a camera.

2.2.1 Transmission Rate

For digital information, the data rate is the rate, in bits per second, at which data are communicated. The maximum rate at which data can be transmitted over a given communication channel, under given conditions, is referred to as the channel capacity. Under ideal conditions—a noiseless channel with no impairments to the signal—the channel capacity is limited only by the bandwidth of the transmitted signal^2. The Nyquist’s sampling theorem states that a signal of bandwidth B Hz can have 2 B independent and successive amplitudes each second. In other words, a waveform low-pass limited to frequencies at most B Hz is completely determined by its values

(^2) The bandwidth of a signal is the range of frequencies that must be used to represent it.

Information Source

Information Source TransmitterTransmitter^ ReceiverReceiver

Noise Source

Noise Source

DestinationDestination Communication Channel

0 1 0 0 1 1 0 0 1 0 Input Signal:

Source Data:

Noise: Output Signal: Sampling Times:

SourceData: 0 1 0 0 1 1 0 0 1 0

DataReceived: 1 1 0 0 1 1 1 0 1 0

Bits in error

S

N S+N Decision threshold

Figure 2-3: Shannon’s diagram of the communication process.

Ivan Marsic • Rutgers University 6

at each 1/(2 B ) seconds. Conversely, if it is required that a signal waveform have successively 2 B independent amplitudes in a second, it must have a bandwidth of B Hz.

Higher transmission frequencies ideally offer greater channel capacity since it is easier to assign a greater bandwidth to the communication channel. This can be seen from the equation that links

frequency and wavelength, f = c / λ. By taking a difference (discrete derivative) with respect to λ,

we obtain

λ λ^2

f c =− Δ

By taking the absolute value, we have Δ f = ( c × Δλ)/ λ^2. Thus, given the width of a wavelength

band, Δλ, we can compute the corresponding frequency band, Δ f , and from that the data rate the

band can produce. The wider the band, the higher the data rate. However, it is much more difficult to design a transmission system with high signal frequency and propagation characteristics of these frequencies unfavorable, as mentioned above.

Note that the Nyquist theorem specifies the signal rate. If the signals to be transmitted are binary (two voltage levels), then B Hz signal supports the maximum data rate, i.e., capacity, C = 2 B bps. However, signals with more than two levels are used, where each signal element can represent more than one bit. With multilevel signaling, the Nyquist limit is:

C = 2 B ⋅ log 2 M (2.2)

where M is the number of discrete signal (e.g., voltage) levels. Although increasing the number of different signal elements increases the data rate, it also increases the complexity of the receiver, which now must distinguish one of M possible signals.

2.2.2 Symbols To Signals

The communication process can be summarized as in Figure 2-4. The techniques commonly employed in the physical layer of wireless networks are modulation and error-control encoding of source signals. Modulation is the transformation of messages into signals suitable for transmission over the medium. Encoding is an accurate representation of one thing by another. As will be seen below, modulation involves encoding but does not include redundancy as a means of error-control. The latter is the task of channel encoding. Both modulation and error-control encoding are employed at the transmitter and inverse processes, demodulation and decoding are employed at the receiver.

Notice that some communication systems do not employ modulation. For example, digital logic circuits communicate in the baseband, both within the VLSI chip and between the chips in the computer. Also, many wired networks communicate in the baseband, e.g., most Ethernet (IEEE 802.3) networks. In such cases, the rectangular pulse waveform is placed directly on the medium.

2.2.3 Modulation

Modulation is the general name for the process by which a signal is transformed into a new form, with bandwidth translated and increased or decreased, while preserving the information content in a retrievable form. It is needed since at the source the messages are rarely produced in a form