Electromagnetic Radiation and Human Health
In 1985, the concerns of the paranoid among the security community were confirmed. Wim van Eck released a paper confirming that a well-resourced attacker can read the output of a cathode-ray tube (CRT) computer monitor by measuring the electromagnetic radiation (EMR) produced by the device. This isn't particularly easy to do, but it is by no means impossible. Wim's paper can be found here
Although wireless networking began to penetrate the market in the 1990s, the technology has actually been around since the 1800s. A musician and astronomer, Sir William Her-schel (1738 to 1822) made a discovery that infrared light existed and was beyond the visibility of the human eye. The discovery of infrared light led the way to the electromagnetic wave theory, which was explored in-depth by a man named James Maxwell (1831 to 1879). Much of his discoveries related to electromagnetism were based on research done by Michael Faraday (1791 to 1867) and Andre-Marie Ampere (1775 to 1836), who were researchers that came before him. Heinrich Hertz (1857 to 1894) built on the discoveries of Maxwell by proving that electromagnetic waves travel at the speed of light and that electricity can be carried on these waves.
The waveform starts as an AC signal that is generated by a transmitter inside an access point (AP) and is then sent to the antenna, where it is radiated as a sine wave. During this process, current changes the electromagnetic field around the antenna, so it transmits electric and magnetic signals. Because the term frequency is thrown around quite a bit in wireless networking, you need to have a clear understanding of it. Frequency, as discussed in Chapter 1, determines how often the signal is seen. It is the rate at which something occurs or is repeated over a particular period or in a given sample or period. It is insufficient to say that frequency is how often a signal is seen. If you are going to measure frequency, you need a period of time to look at it. Frequency, which is usually measured in seconds, is the rate at which a vibration occurs that constitutes a wave this can be either in some form of material, as in sound waves, or it can be in an electromagnetic field, as you...
FDDI uses optical fiber as the primary transmission medium, but it also can run over copper cabling. As mentioned earlier, FDDI over copper is referred to as Copper-Distributed Data Interface (CDDI). Optical fiber has several advantages over copper media. In particular, security, reliability, and performance all are enhanced with optical fiber media because fiber does not emit electrical signals. A physical medium that does emit electrical signals (copper) can be tapped and therefore would permit unauthorized access to the data that is transiting the medium. In addition, fiber is immune to electrical interference from radio frequency interference (RFI) and electromagnetic interference (EMI). Fiber historically has supported much higher bandwidth (throughput potential) than copper, although recent technological advances have made copper capable of transmitting at 100 Mbps. Finally, FDDI allows two kilometers between stations using multi-mode fiber, and even longer distances using a...
The term radio frequency defines a relatively small portion of the known electromagnetic spectrum. Figure 3-3 shows a small portion of the electromagnetic spectrum. The whole of the electromagnetic spectrum is significantly more wide-ranging in terms of frequencies than what is shown in the figure. Smaller still is the portion of the spectrum specifically associated with RF (5 MHz to 1 GHz). Figure 3-3 Partial Electromagnetic Spectrum Figure 3-3 Partial Electromagnetic Spectrum
A fiber optic is a glass or plastic conductor that transmits information using light. A fiber-optic cable is one or more optical fibers enclosed together in a sheath or jacket. Because it is made of glass, fiber-optic cable is not affected by electromagnetic interference or radio frequency interference. Fiber-optic cable can reach distances of several miles or kilometers before the signal needs to be regenerated. However, fiber-optic cabling is usually more expensive to use than copper cabling. The connectors are more costly and harder to assemble. And all signals have to be converted to light pulses to enter the cable and back into electrical signals when they leave it.
You have rea d about the wireless system, the electromagnetic spectrum, and the common frequency bands you will likely see in a networking environment. It is now time to look at the theory behind radio communications. Electromagnetic Energy To understand how wireless communications work, you must understand how electromagnetic energy is generated and propagated through free space. When current passes through a piece of wire, an electric field is created, which in turn creates a magnetic field. When the current is alternating, as opposed to direct, the electromagnetic field is created and collapses at the same rate as the frequency of the alternating current. The magnetic field also builds and collapses at the same rate as the electrical field. This action generates an electromagnetic wave, which is radiated from the wire, which at this point can be called an antenna.
Figure 1-1 shows the entire electromagnetic spectrum. Notice that the frequency ranges used in CB radio, FM radio, and TV broadcasts are only a fraction of the entire spectrum. Most of the spectrum is governed by folks like the FCC. This means that you cannot use the same frequencies that FM radio uses in your wireless networks. As Figure 1-1 illustrates, the electromagnetic spectrum spans from Extremely Low Frequency (ELF) at 3 to 30 Hz to Extremely High Frequency (EHF) at 30 GHz to 300 GHz. The data you send is not done so in either of these ranges. In fact, the data you send using WLANs is either in the 900-MHz, 2.4-GHz, or 5-GHz frequency ranges. This places you in the Ultra High Frequency (UHF) or Super High Frequency (SHF) ranges. Again, this is just a fraction of the available spectrum, but remember that the FCC controls it. You are locked into the frequency ranges you can use. Table 1-2 lists the ranges that can be used in the United States, along with the frequency ranges...
Fiber has a number of benefits over traditional cable. Fiber is thin and lightweight, able to cover longer distances with virtually no loss of signal or noise, and is immune to outside sources of electromagnetic interference. Because the number of amplifiers is reduced, there is some monetary benefit associated with support and equipment costs. There is some discussion as to which is easier to handle, cable or fiber. Essentially, that discussion comes down to preference. Cable tends to be rigid and sturdy whereas fiber is thin and somewhat pliable, requiring some advanced skills and care to properly terminate.
The cable TV industry uses the RF portion of the electromagnetic spectrum. Within the cable, different frequencies are used to carry TV channels and data. At the subscriber end, equipment such as TVs, VCRs, and High Definition TV set-top boxes tune to certain frequencies that allow you to view the TV channel or, using a cable modem, to receive high-speed Internet access.
As a brief review, remember that Ethernet uses a twisted pair of wires to transmit and another twisted pair to receive, to reduce electromagnetic interference. You typically use straight-through Ethernet cables between end user devices and the switches. For the trunk links between the switches, you use crossover cables because each switch transmits on the same pair, so the crossover cable connects one device's transmit pair to the other device's receive pair. The lower part of the figure reminds you of the basic idea behind a crossover cable.
Improvements in technology enable you to renew your outlook on solutions that were previously dismissed as technically or economically unfeasible. An extreme example is the use of optical communications. Although successful many years ago in the form of torches and flares, in the last century, optical communication was disbanded because it was much easier to guide lower-frequency electromagnetic radiation using simple copper cable. With the advent of optical fiber, however, this is no longer true.
When electron streams change direction rapidly within a wire or antenna, the electrostatic and magnetic fields around the wire or antenna change at the same rapid rate. These rapidly changing fields are called electromagnetic waves. The electromagnetic waves do not simply stay near the antenna they travel away at nearly the speed of light 186,000 miles per second (300,000,000 meters per second). The changing electron flow within the antenna has been transformed into electromagnetic (wireless) waves traveling away from the antenna.
Whereas copper and coax cabling carry transmission frequencies in the megahertz range, fiber-optic cabling carries frequencies at a much higher band, approximately a million times higher. Whereas copper and coax cabling carry transmission signals in the form of electromagnetic waves, fiber-optic cabling carries signals in the form of light waves. Fiber-optic cabling can carry signals as high as 10 Gigahertz (GHz).
Fiber-optic cabling is all but immune to line problems such as electromagnetic interference (EMI) because light is used and not electricity. In secured environments, fiber-optic transport is preferred because it is difficult to tap and it is simple to detect if a tap is present. Also, there is no signal resonance as with copper wiring, so signal hijacking becomes exponentially more difficult.
Changes in the electron flow in an antenna cause the same changes in the electromagnetic fields around the antenna. Another word for electron flow in a wire is current. Without changes in the antenna current, there would be no change in the electromagnetic fields around the antenna therefore, there would be no useable wireless signal moving outward, away from the antenna. The numbe r of tiupes each second that the current in the antenna goes through one complete positive-negative-posi2ive change-cycle is the same as the frequency of the wireless waves that radiate outward from the antenna. If you drew a graph of the current flow in the antenna, the resulting graph would be a sine wave. The positive distance (above the centerline) and the negative distance (below the centerline) represent the amplitude, or strength, of the current. The greater the amplitude of the current, the stronger the radiated electromagnetic waves.
ISDN is deployed between the service provider's CO and the customer's premises. The local loop, deployed over a combination of copper and possibly optical cable, must observe the same stipulations of any digital technology. In other words, the ISDN carrier facilities are equally as susceptible to problems such as cross talk and attenuation as any analog circuit. As far as interference goes, fiber-optic trunks do not need to worry about electromagnetic interference (EMI). Optical trunks can also transport information for much greater distances before the need for signal regeneration. Service providers tend to deploy high bandwidth, fiber trunks to integrated digital loop carrier (IDLC) systems and then provide copper service from there to the customer's premises.
In the 1980s, Bellcore developed high-data-rate DSL (HDSL) technology. HDSL was developed to enhance the capabilities of T1 technology by replacing alternate mark inversion (AMI) encoding with 2 binary 1 quaternary (2B1Q) encoding. HDSL reduces the need for repeaters, and it causes less electromagnetic interference (EMI) in cable bundles. In the 1990s, technology vendors began to see an explosion in the use of the Internet. This Internet revolution increased the demand for pure data, as opposed to voice and data, access to the Internet. The increase of high-bandwidth Internet services, such as video-on-demand, audio-on-demand, e-shopping, e-learning, and telecommuting, has resulted in the demand for higher bandwidth data connections. Studies showed that the majority of the traffic that was sourced from the Internet was destined for the end user (downstream), and that only a small percentage of the traffic was actually sourced by the user (upstream). This conclusion lead to the...
Ethernet is an appropriate technology choice for customers concerned with availability and manageability. An Ethernet LAN that is accurately provisioned to meet bandwidth requirements and outfitted with high-quality components, including NICs, cables, and internetworking devices, can meet even the most stringent demands for availability. Many troubleshooting tools, including cable testers, protocol analyzers, and network-management applications, are available for isolating the occasional problems caused by cable breaks, electromagnetic interference, failed ports, or misbehaving NICs.
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