Which of the following wireless transmission types requires a clear line of sight to function?

7.1.1 History of Wireless Networks

The first wireless networks were developed in the preindustrial age. These systems transmitted information over line-of-sight distances (later extended by telescopes) using smoke signals, torch signaling, flashing mirrors, signal flares, and semaphore flags. An elaborate set of signal combinations was developed to convey complex messages with these rudimentary signals. Observation stations were built on hilltops and along roads to relay these messages over large distances. These early communication networks were replaced first by the telegraph network (invented by Samuel Morse in 1838) and later by the telephone. In 1895, twenty years after the telephone was invented, Marconi demonstrated the first radio transmission from the Isle of Wight to a tugboat 18 miles away, and radio communications was born. Radio technology advanced rapidly to enable transmissions over larger distances with better quality, less power, and smaller, cheaper devices, thereby enabling public and private radio communications, television, and wireless networking.

Early radio systems transmitted analog signals. Today most radio systems transmit digital signals composed of binary bits, where the bits are obtained directly from a data signal or by digitizing an analog voice or music signal. A digital radio can transmit a continuous bit stream or it can 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 developed at the University of Hawaii in 1971. This network enabled computer sites at seven campuses spread out over four islands to communicate with a central computer on Oahu via radio transmission. The network architecture used a star topology with the central computer at its hub. Any two computers could establish a bidirectional communications link between them by going through the central hub.

Alohanet incorporated the first set of protocols for channel access and routing in packet radio systems, and principles underlying these protocols are still in use today. Activity in packet radio, promoted by DARPA, peaked in the mid 1980s, but the resulting networks fell far short of expectations in terms of speed and performance. Packet radio networks today are mostly used by commercial providers of wide area wireless data services. These services, first introduced in the early 1990s, enable wireless data access (including e-mail, file transfer, and Web browsing) at fairly low speeds, on the order of 20 Kbps. The market for these data services has not grown significantly.

In the 1970s Ethernet technology steered companies away from radio-based networking. Ethernet's 10 Mbps data rate far exceeded anything available using radio. In 1985 the Federal Communications Commission (FCC) enabled the commercial development of wireless LANs by authorizing the public use of the Industrial, Scientific, and Medical (ISM) frequency bands for wireless LAN products. The ISM band was very attractive to wireless LAN vendors since they did not need to obtain an FCC license to operate in this band. However, the wireless LAN systems could not interfere with the primary ISM band users, which forced them to use a low power profile and an inefficient signaling scheme. Moreover, the interference from primary users within this frequency band was quite high. As a result these initial LAN systems had very poor performance in terms of data rates and coverage.

The poor performance, coupled with concerns about security, lack of standardization, and high cost (the first network adaptors listed for $1,400 as compared with a few hundred dollars for a wired Ethernet card) resulted in weak sales for the initial wireless LAN systems. The current generation of wireless LANs, based on the IEEE 802.11 standard, has better performance, although the data rates are still low (on the order of 2 Mbps) and the coverage area is still small (around 500 feet). Ethernets today offer data rates of 100 Mbps, and the performance gap between wired and wireless LANs is likely to increase over time without additional spectrum allocation. Thus, it is not clear if wireless LANs will be competitive except where users sacrifice performance for mobility or when a wired infrastructure is not available.

By far the most successful application of wireless networking has been the cellular telephone system. Cellular telephones have approximately 200 million subscribers worldwide, and their growth continues at an exponential pace. The convergence of radio and telephony began in 1915, when wireless voice transmission between New York and San Francisco was first established. In 1946 public mobile telephone service was introduced in 25 cities across the United States. These initial systems used a central transmitter to cover an entire metropolitan area. This inefficient use of the radio spectrum coupled with the state of radio technology at that time severely limited the system capacity: thirty years after the introduction of mobile telephone service the New York system could only support 543 users. A solution to this capacity problem emerged during the fifties and sixties when researchers at AT&T Bell Laboratories developed the cellular concept.

Cellular systems exploit the fact that the power of a transmitted signal falls off with distance, so the same frequency channel can be allocated to users at spatially separate locations with minimal interference. A cellular system divides a geographical area into adjacent, nonoverlapping “cells.” Cells assigned the same channel set are spaced apart so that interference between them is small. Each cell has a centralized transmitter and receiver (called a base station) that communicates with the mobile units in that cell, both for control purposes and as a call relay. All base stations have high-bandwidth connections to a mobile telephone switching office (MTSO), which is itself connected to the public-switched telephone network (PSTN). The handoff of mobile units crossing cell boundaries is typically handled by the MTSO, although in current systems some of this functionality is handled by the base stations and/or mobile units.

The original cellular system design was finalized in the late 1960s and deployed in the early 1980s. The large and unexpected growth led to the development of digital cellular technology to increase capacity and improve performance.

The current generation of cellular systems are all digital. In addition to voice communication, these systems provide e-mail, voice mail, and paging services. Unfortunately, the great market potential for cellular phones led to a proliferation of digital cellular standards. Today there are three different digital cellular phone standards in the U.S. alone, and other standards in Europe and Japan, none of which are compatible. The incompatible standards make roaming throughout the U.S. using one digital cellular phone impossible. Most cellular phones today are dual-mode: they incorporate one of the digital standards along with the old analog standard that provides coverage throughout the U.S.

Radio paging systems represent another example of a successful wireless data network, with 50 million subscribers in the U.S. Their popularity is starting to wane with the widespread penetration and competitive cost of cellular telephone systems. Paging systems allow coverage over very wide areas by simultaneously broadcasting the pager message at high power from multiple base stations or satellites. Early radio paging systems were analog 1-bit messages signaling a user that someone was trying to reach him or her. These systems required callback over the regular telephone system to obtain the phone number of the paging party.

Paging systems now allow a short digital message, including a phone number and brief text, to be sent to the pagee. In paging systems most of the complexity is built into the transmitters, so that pager receivers are small, lightweight, and have a long battery life. The network protocols are also very simple since broadcasting a message over all base stations requires no routing or handoff. The spectral inefficiency of these simultaneous broadcasts is compensated by limiting each message to be very short. Paging systems continue to evolve to expand their capabilities beyond very low-rate one-way communication. Current systems are attempting to implement two-way, “answer-back” capability. This requires a major change in pager design, since it must now transmit signals in addition to receiving them, and the transmission distances can be quite large.

Commercial satellite communication systems form another major component of the wireless communications infrastructure. They provide broadcast services over very wide areas and help fill the coverage gap between high-density user locations. Satellite mobile communication systems follow the same basic principle as cellular systems, except that the cell base stations are now satellites orbiting the earth. Satellite systems are typically characterized by the height of the satellite orbit, low-earth orbit (LEO), medium-earth orbit (MEO), or geosynchronous orbit (GEO).

The idea of using geosynchronous satellites for communications was first suggested by the science fiction writer Arthur C. Clarke in 1945. However, the first deployed satellites, the Soviet Union's Sputnik in 1957 and the NASA-Bell Laboratories Echo-1 in 1960, were not geosynchronous due to the difficulty of lifting a satellite into such a high orbit. The first GEO satellite was launched by Hughes and NASA in 1963, and from then until very recently GEOs dominated both commercial and government satellite systems. The current trend is to use lower orbits so that lightweight handheld devices can communicate with the satellite. Services provided by satellite systems include voice, paging, and messaging services, all at fairly low data rates.

6.11 Conclusions

In this chapter, the physical and technological basis of wireless network is presented. The wireless technologies are mature and wireless networks from any application (medical, emergency warning, educational, etc.) can be interfaced with traditional network using standard interfaces and protocols and extensively used in remote area, monitoring systems, aviation industry, etc.

Wireless technologies had been suggested as early as 1890s, since the days of Marconi (1897) and Tesla (1901) but evolved slowly compared to the metallic line-based telephone systems suggested by Bell (1876) and actively deployed through 1990s. In the Bell System environment, wireless communication had been studied in detail. The signal degradation due to noise in all aspects of communications had been analyzed. In the landline applications, the signal recovery was targeted before being completely buried in noise. However, with the advent and deployment of satellite communications systems, wireless technologies received particular interest in developing new coding algorithms and noise control in wireless communication systems. The era of NGMN (discussed in the next chapter) had started to accommodate dependable mobile network architecture and their configurations.

2.4 Why Wireless Networks?

Wireless networks are getting more and more common because of their comfort of use. Consumer/user is no more rely on cables, so it is so easy to move from one place to another and enjoy being connected to the network. One of the great characteristics of wireless networks that make them attractive and different between the traditional wired networks is movability. With this characteristic the consumer has the ability to move without limits, while connecting to the network. Wireless networks are relatively easy to install compared with wired network. While using wireless networks, there is no need to worry about pulling the cables/wires in wall and ceilings. Wireless networks can be set up according to the need of the consumers. These can extend from small number of consumers to large networks that the number of users is in thousands. Wireless networks are very useful especially for areas where the wire cannot be installed like hilly areas.

Wireless Network Scanner

Wireless network scanners are sometimes referred to as “war-driving” tools or wireless protocol analyzers. These tools are good for detecting open wireless networks in your facility. If you have a policy that prohibits wireless networks, you may want to walk around the facility with a wireless network scanner to see if you detect any unauthorized Wi-Fi networks. Popular wireless network scanners are available at the following URLs:

Netstumbler, an open source tool (www.netstumbler.com)

WiFiScanner, an open source tool (http://wifiscanner.sourceforge.net)

CommView for WiFi by Tamosoft (www.tamos.com)

iStumbler for Max OSX wireless network discovery (www.istumbler.net/)

Sniffer® Wireless Intelligence by Network General (www.networkgeneral.com)

Wireless Recon by Helium Networks (www.heliumnetworks.com)

AiroPeek SE by WildPackets (www.wildpackets.com)

StumbVerter is an open source tool for mapping the results of a wireless network scan and is available at www.sonar-security.com/sv.html.

Wireless network scanners

Wireless network scanners are sometimes referred to as “war-driving” tools or wireless protocol analyzers. These tools are good for detecting open wireless networks in your facility. If you have a policy that prohibits wireless networks, you may want to walk around the facility with a wireless network scanner to see if you detect any unauthorized WiFi networks. Popular wireless network scanners are available at the following URLs:

WiFiScanner, an open source tool (http://www.wifiscanner.sourceforge.net)

CommView for WiFi by Tamosoft (http://www.tamos.com)

OmniPeek by WildPackets (http://www.wildpackets.com)

iStumbler for Max OSX wireless network discovery (http://www.istumbler.net/)

WifiInfoView by NirSoft Freeware (http://www.nirsoft.net)

WIFi Locator by TCPEeye (http://www.tcpmonitor.altervista.org)

WiFi Scanner (for Mac OS X) by WLANBook (http://www.wlanbook.com)

WiFi Channel Scanner by Wifichannelscanner (http://www.wifichannelscanner.com)

Abstract

Wireless Network-on-Chip (WiNoC) has emerged as an enabling technology to design low power and high bandwidth massive multi-core chips. The performance advantages mainly stem from using the wireless links as long-range shortcuts between far apart cores. This performance gain can be enhanced further if the characteristics of the wireline links and the processing cores of the WiNoC are optimized according to the traffic patterns and workloads. This chapter demonstrates that by incorporating both processor- and network-level dynamic voltage and frequency scaling (DVFS) in a WiNoC, the power and thermal profiles can be enhanced without a significant impact on the overall execution time.

Publisher Summary

Wireless networks suffer from a variety of unique problems including low throughput, dead spots, and interference. However, their characteristics, such as the broadcast nature of the medium, spatial diversity, and significant data redundancy provide opportunities for new design principles to address these problems. This chapter describes advances in employing network coding to improve the throughput and reliability of wireless networks. Wireless networks have been designed using the wired network as the blueprint. The design abstracts the wireless channel as a point-to-point link and grafts wired network protocols onto the wireless environment. The wireless medium is fundamentally different. While wired networks have reliable and predictable links, wireless links have high bit error rate, and their characteristics could vary over short time scales. Transmissions in a wired network do not interfere with each other, whereas interference is a common case for the wireless medium. Network coding enables more efficient, scalable, and reliable wireless networks. These opportunities come with a need for rethinking the media access control (MAC), routing, and transport protocols. Recent years have seen significant successes in integrating network coding into wireless systems and the emergence of practical implementations with significant throughput and reliability gains.

Publisher Summary

Wireless networks is a general term to refer to various types of networks that communicate without the need of wire lines. Wireless networks can be broadly categorized into two classes based on the structures of the networks—wireless ad hoc networks and cellular networks. The main difference between these two is whether a fixed infrastructure is present. Wireless ad hoc networks do not require a fixed infrastructure; thus it is relatively easy to set up and deploy a wireless ad hoc network. Without the fixed infrastructure, the topology of a wireless ad hoc network is dynamic and changes frequently. It is not realistic to assume static or a specific topology for a wireless ad hoc network. Besides the conventional wireless ad hoc networks, there are two special types that should be mentioned—wireless sensor networks and wireless mesh networks. Wireless sensor networks are wireless ad hoc networks, most of the network nodes of which are sensors that monitor a target scene. The wireless sensors are mostly deprived devices in terms of computation power, power supply, bandwidth, and other computation resources. Wireless mesh networks are wireless networks with either a full mesh topology or a partial mesh topology in which some or all nodes are directly connected to all other nodes. The redundancy in connectivity of wireless networks provides great reliability and excellent flexibility in network packet delivery.

Passive Attacks on Wireless Networks

A passive attack occurs when someone listens to or eavesdrops on network traffic. Passive attacks on wireless networks are extremely common, almost to the point of being ubiquitous. Detecting and reporting on wireless networks has become a popular hobby for many wireless war-driving enthusiasts.

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Detecting wireless networks Utilizing new tools created for wireless networks and the existing identification and attack techniques and utilities originally designed for wired networks, attackers have many avenues into a wireless network. The first step in attacking a wireless network involves finding a network to attack. The most popular software developed to identify wireless networks is the Windows-based NetStumbler (www.netstumbler.com). This type of scan, driving around looking for wireless networks, is known as war driving.

Protecting against wireless network detection To defend against the use of NetStumbler and other programs to detect a wireless network easily, administrators should configure the wireless network as a closed system. This means that the AP will not respond to empty set SSID beacons and will consequently be “invisible” to programs such as NetStumbler, which rely on this technique to discover wireless networks.

Crunch Time

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Protecting against sniffing and eavesdropping To protect wireless users from attackers who might be sniffing is to utilize encrypted sessions wherever possible: SSL for e-mail connections, secure shell (SSH) instead of Telnet, and secure copy (SCP) instead of file transfer protocol (FTP). Additionally turn off any network identification broadcasts and, if possible, close down the network to any unauthorized users.

Disadvantages of a Wireless Network

Wireless networks also have some disadvantages as well:

Security The main disadvantage of wireless is its lack of security. If proper planning and configuration are not taken into consideration, any wireless network can be easily compromised.

Complexity and reliability Although adding wireless access points increases accessibility to the wired network, it can also increase the complexity of the network design. System administrators must be aware of the impact that the wireless devices have on the network and troubleshoot problems such as weak signals and dropped wireless connections.

Network performance Although the advertised speed of a wireless connection is 54 Mbps, the actual data throughput is often much lower, especially if many computers are using the same wireless connection. A wired network drop will almost always outperform a wireless connection.