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Wireless LANs (WLANs): Focus on 802.11bWireless LANs (WLANs) are LANs (Local Area Networks) that don't involve wires. No wires turns out to be a big advantage, as the time and expense associated with wiring and re-wiring is avoided. That is a real benefit in a dynamic environment where there is frequent reconfiguration of the workplace. WLANs also make a great deal of sense in temporary quarters, where costly cable and wire systems soon would have to be abandoned, and in older buildings where wires are difficult or impossible to run. WLANs also may offer the advantage of portability, meaning that you may be able to take your connectivity with you, much as you might take a modem with you wherever you go. No wires doesn't mean no problems, however. There are some issues of signal quality and security, for instance, and they are not trivial. The Physical Layer: RF versus Ir There are two underlying transmission technologies that support WLANs: Radio Frequency (RF) and Infrared (Ir). The most common approach is that of RF, which involves fitting each client workstation with a low-power transmit/receive radio antenna, which generally is in the form of a PCMCIA (Personal Computer Memory Card International Association) card, also known as a PC card. Frequency assignments for commercial applications are in the 900 MHz, 2.4 GHz, and 5 GHz bands. A hub antenna is housed in a wireless access point that ideally is centrally located where line-of-sight connectivity can be established with the various terminal antennas. While line-of-sight is not strictly required, it is desirable in consideration of signal quality. This is particularly so at the higher frequencies, which suffer greater attenuation (i.e., loss of signal strength). The wireless access point then connects to the servers, peripherals and other hosts via cabled connections, although wireless connectivity is possible. Multiple hub antennas can be interconnected by wires for communications between rooms, floors, buildings, and so on. This scenario involving access points is known as infrastructure mode, and is typical. In ad hoc mode, or peer-to-peer mode, the wireless devices (e.g., laptop clients) communicate directly with each other. In order to serve multiple workstations simultaneously, spread spectrum radio technology commonly is employed to maximize the effective use of the limited bandwidth supported by the narrow frequency ranges available to WLANs. Frequency Hopping Spread Spectrum (FHSS) involves scattering packets of a data stream across a range of frequencies in a carefully choreographed hop sequence, rather than using a single transmission frequency. A side benefit of spread spectrum is that of increased security, since the signal is more difficult to intercept. While the raw aggregate bandwidth of an RF LAN generally is described as falling into a range (e.g., 1-11 Mbps) sensitive to link quality at any given time, the effective throughput generally is considerably less due to issues including overhead and error control mechanisms. Some wireless LANs also use Direct Sequence Spread Spectrum (DSSS) transmission, which calls for the signal to be transmitted simultaneously over several frequencies, thereby increasing the likelihood that the signal will get through to the receiving antenna. Regardless of the frequency range employed, buildings are full of metal and other sources of interference that combine to reduce the effectiveness of RF-based wireless LANs. Lead paint, metal studs, nails, foil-backed insulation and even glass windows with metal content all can cause interference. For that matter, any dense physical matter (e.g., walls, floors, ceilings, your four-year-old son, and the neighbor's cat) will cause some amount of attenuation, as it absorbs, reflects and scatters some signal energy to various degrees. The more dense the physical matter, the worse the effect. Some WLANs use frequencies (e.g., 902 MHz, 2.4 GHz, and 5.7 GHz) in the unlicensed ISM (Industrial/Scientific/Medical) bands. This approach avoids expensive and lengthy licensing processes through regulatory authorities such as the FCC (U.S.), but involves significant potential for interference from other such systems in proximity. A wide variety of other devices (garage door openers, bar code scanners and industrial microwave ovens) also use the same frequencies. As these LANs (and other devices in the ISM band) operate at fairly low power levels, the actual risk of interference is relatively slight, but it does exist. As the popularity of such LANs has increased, situations have developed in which such interference has, indeed, become an issue. Although it is somewhat unusual, infrared light (Ir) also can serve as the transmission medium. An Ir-based WLAN system generally requires line-of-sight between the light source and receiver, although you often can bounce a signal off of a wall or two without affecting connectivity too significantly. Whether based on RF or Ir at the physical layer, wireless LANs are a relatively immature technology that is a long way from being ubiquitous, but that is gaining acceptance rapidly. While acquisition costs aren't necessarily all that low compared to wired LANs, configuration and reconfiguration costs are virtually zero since there are few, if any, cables and wires to consider. Definitely on the positive side of the equation, wireless offers the advantage of portability, particularly in the case of the unlicensed frequencies. On the negative side, bandwidth is limited since the frequency range is limited and throughput is limited and also because link quality can be poor and retransmissions can be necessary. Further, security is a real concern affecting any RF-based transmission system. 802.11b (Wi-Fi) The most common WLANs currently are those conforming to the IEEE 802.11b specification. Not only are they increasingly deployed in private enterprise applications, but also in public applications such as airports and coffee shops. Dubbed Wi-Fi (Wireless Fidelity) by the Wireless Ethernet Compatibility Alliance (WECA), 802.11b includes three transmission options, one of which is Ir-based and two of which are RF-based. 802.11b employs DSSS modulation using the Barker code chipping sequence. Each bit is encoded into an 11-bit Barker code (e.g., 10110111000), with each resulting data object forming a chip. The chip is put on a carrier frequency (i.e., a small frequency range that carries the signal) in the 2.4 GHz range, and the waveform is modulated using one of several techniques. 802.11b, the most commonly used specification in WLANs, uses frequencies in the 2.4GHz band and employs direct sequence spread spectrum technology.
802.11b systems running at 1 Mbps use Barker code and BPSK (Binary Phase Shift Keying) modulation, and those running at 2 Mbps use Barker code and QPSK (Quaternary Phase Shift Keying) modulation. Systems running at 5.5 Mbps and 11 Mbps use CCK (Complementary Code Keying) and QPSK modulation. CCK involves 64 unique code sequences, each of which supports 6 bits per code word. The CCK code word is then modulated onto the RF carrier using QPSK, which allows another two bits to be encoded for each 6-bit symbol. Therefore, each 6-bit symbol contains 8 bits (i.e., 1 byte). The FCC limits the power output of the 802.11b system to 1 watt EIRP (Equivalent Isotropically Radiated Power). At this low power level the physical distance between the transmitting devices becomes an issue due to signal attenuation, with error performance suffering as the distance increases. (Note: 100 meters is a pretty good rule of thumb for an 802.11b WLAN with clear line-of-sight.) Any dense physical obstructions between transmitter and receiver add considerably to the problem. Therefore, the devices adapt to longer distances, physical obstructions and other factors that impact signal strength by using a less complex encoding technique, and a resulting lower signalling speed, which translates into a lower data rate. For example, a system running at 11 Mbps using CCK and QPSK, might throttle back to 5.5 Mbps by halving the signalling rate as the distances increase, doors and walls get in the way, and error performance drops. The situation gets worse when you move your laptop out to the deck to work on a sunny summer afternoon, so the system might throttle back to 2 Mbps using only QPSK, and 1 Mbps using BPSK. This process is much the same as that used by conventional fallback modems that might initiate a call at 56 Kbps (actually 53.3 Kbps), and fall back to rates of perhaps 28.8 Kbps or 14.4 Kbps as the quality of the dial-up PSTN connection degrades. Note: As I mentioned earlier, the actual throughput of an 802.11b system is much less than the raw bandwidth. Physical layer overhead consumes 30%-50% of the bandwidth. An 802.11b system running at the full rate of 11 Mbps, therefore provides throughput of only 6 Mbps or so, assuming overhead in the range of 40%. If there are a lot of errors in transmission, throughput drops precipitously, as the receiving station must advise the transmitting station of the errored frames and then wait for retransmissions. If, for example, the error rate is 50%, the actual throughput drops to about 2 Mbps. This scenario is a blend of a best case 11 Mbps and a worst case error rate. In actuality, such an error rate would cause the system to fall back to a lower transmission rate of perhaps 2 Mbps, at which rate Barker code and QPSK would be used and the error rate would drop. The 802.11b specification divides the assigned RF spectrum into 14 channels. The FCC allows the use of 11 channels. Since the U.S. 2.4 GHz band is only 83 MHz wide and the 802.11b channels are 25 MHz wide, however, only three (3) channels can be used simultaneously. While other regulators in other jurisdictions allow the use of more or fewer channels (e.g., Japan allows the use of only one), none allows the use of all 14, at least not as far as I know. So, not only is the amount of spectrum highly limited to begin with, but not even all of that is used. There also is overlap between adjacent channels (e.g., channels two and three), which affects performance and which, therefore, requires that any given system maintain maximum channel separation from other systems in proximity. 802.11b specifies two security mechanisms. The most basic is the Electronic System ID (ESSID, or SSID), which is in the form of a identifier code used for authentication. The SSID is established by the system administrator for each device set up to gain access through each access point. SSID doesn't provide much security at all. The next level is WEP (Wired Equivalent Privacy), which uses a 40- or 128-bit encryption key to protect data in transit. WEP doesn't provide great protection, either, as it has been shown to be easily compromised. Any real inherent security will have to wait for another standards-based solution. In the meantime, user organizations have to overlay their own higher-strength security mechanisms, generally in the form of a VPN (Virtual Private Network). To b Or Not To b?: Could That Be The Question? 802.11b has enjoyed great success in the last year or so, but its big brother has finally made an appearance. 802.11a is now available, running at speeds up to 54 Mbps ... but not without some problems. We'll examine this emerging WLAN solution in the next column. It's been great sitting out here on the deck, composing this column and surfing the Internet over my 802.11b link. I'm running at only 2 Mbps, but poor link quality is a small price to pay for the pleasure of working out here in the sunshine, and my ADSL connection only runs at 1.544 Mbps anyway. |
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