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Subject: Wireless Fidelity (Wi-Fi)
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| Sana | 08.04.2020 | | Hajmi | 26,5 Kb. | | #10229 |
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MEMORANDUM
Subject: Wireless Fidelity (Wi-Fi)
Date: December 2002
Wi-Fi is the remote wireless linking of a mobile or fixed device to an interface which would then link the device to a LAN (Local Area Network) or internet access cable (telephone line, DSL, cable modem, T-1, etc.) Current interface technology allows the integrated use of wired LANs and wireless operation simultaneously. Lucent is the main chip set manufacturer for the interface. There are only four or five chip set manufacturers, and all systems use one of those four or five chip sets. Although the various chips sets are more or less compatible with each other, each of them has unique features. Laptop computers use PCM-CIA cards for their wireless hookup.
Present costs are around 150 for the access pointinterface, and approximately 60 per computer.
Security is both by password, using names and channels, and 128-bit encryption keys, so that theoretically any number of systems could operate in the same area and not interfere with each other. Although a given interface would see the other operating systems within its range, approximately 300 feet, it would not be able to read those systems unless it had the encryption key and the passwords. Security, however, remains a major issue that will become more sophisticated as the systems mature.
Currently, there are two wi-fi systems, designated 802.11(b) and 802.11(a). Counterintuitively, (a) is the newer system. Each has its own characteristics and frequencies.
The 802.11(b) operates as an FCC Part 15 device, in the 2.4 GHz band. Its bandwidth limitations have a maximum of 11megabits per second (Mb or Mbps) operating speed and, in fact, with the systems on the market, an 11(b) system operates between 4 and 11 Mb, with the average in the range of 6 Mb.
The 802.11(a) operates in the 5 GHz band as a Part 15 device and, due essentially to its larger bandwidth, can transmit up to 54Mb per second, with the majority in the range of 20 to 25 Mb. To put wi-fi in perspective with other data movement systems, the following is a list of the speeds (in Mb per second) of the various common systems:
Telephone modem (56 Kb) .056 Mb
DSLCable modem .3 – 2.0 Mb Avg. 1.5 Mb
T-1 1.5 Mb
Wi-Fi 802.11(b) 4 – 11 Mb Avg. 6 Mb
Wi-Fi 802.11(a) 54 Mb Avg. 20-25 Mb
Wired LAN 100 Mb
Some simple conclusions can be drawn from the above table. First, in order to “hook together” fixed computers in an office setting, nothing is faster than the 100 Mb LAN systems currently in use. Second, it must be remembered that the connection between two devices is no faster than the slowest component, so that while 802.11(b) devices provide a much slower bit rate than the 802.11(a) devices, if the interface is connected to a telephone line, for instance, the internet access will be no faster than the telephone line. Accordingly, it would appear that even with the interface connected to a T- 1 line, the 802.11(b) system would be adequate.
The problems with current wi-fi systems are related to the frequencies on which they operate. As a very general rule, the higher the frequency, the poorer the propagation. General (lot. generalis - umumiy, bosh) - qurolli kuchlardagi harbiy unvon (daraja). Dastlab, 16-a.da Fransiyada joriy qilingan. Rossiyada 17-a.ning 2-yarmidan maʼlum. Oʻzbekiston qurolli kuchlarida G. As an example, the signal of an AM radio station is generally described as following the curvature of the earth (technically, the ground wave), which can go on for great distances, while FM and television are described as line-of-sight. We know, however, that FM signals, and to a lesser extent television signals, can extend for quite long distances, and are not really line-of-sight. Still, the higher the frequency, the greater the problems, or, to put it another way, the higher the frequency, the more line-of-sight the signal becomes. The upper ranges of the current television band are around 700 MHz. 802.11(b) wi-fi systems operate at 2.4 GHz, and 802.11(a) wi-fi systems operate at 5 GHz.
Another phenomenon creeps in having to do with the frequency, and that is the effect of extraneous interference. At low frequencies, such as in the AM range, electrical noise is a serious problem. Electrical noise is generated by electric motors, power transmission lines, including home wiring, automobile ignition systems, and so forth. As you move up in scale, FM and television signals are greatly improved because the effect of that electrical noise decreases to the point that it is no longer a significant factor for FM and television. However, moving further up the scale, say from television up into the 2.4 and 5 GHz ranges, results in a host of other problems, such as rainwater, snow, the leaves on trees, metal structures of buildings, affecting the quality of the transmitted signal.
The range of a wi-fi device is generally published at 100 yards, or 300 feet. But any of the above factors degrades the signal, and as you approach the limits of the wi-fi device’s effective range, the signal becomes even further degraded. The effect of the degradation is to slow down the transfer rate until it reaches a point at which no matter how slow it goes, it is unable to capture all of the data bits.
The FCC has recently (December 11, 2002) opened an inquiry into allowing these unlicensed devices to operate in different parts of the spectrum, including operation in the TV broadcast spectrum and in the 3650-3700 MHz band (current “C” band satellite). At the Commission’s current pace, authorization of those operations is at least one year – and probably two years – away. Some people with experience in wi-fi question whether the bandwidth at lower frequencies is sufficient to support wi-fi operations. If it is, lower-frequency operation will solve many of the propagation issues now affecting the 802.11(b) operations.
Wi-fi is now in its infancy. In as little as five years, it could be a mature – and profitable – business.
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