Planning and Implementing Wireless LANS By Peter Rysavy Types of Wireless LANs It might seem obvious that the key differentiating factor between wireless LANs and wireless WANs is that they operate in a local area, but local operation has many significant and not necessarily obvious consequences. First and foremost, wireless LANs operate at much higher speeds, ranging from 1 Mbps to 20 Mbps compared to wireless WANs, which today range from 4 Kbps to 30 Kbps. Higher speeds are possible because that band of the spectrum is shared by a much smaller number of users. Whereas a cellular base station can serve a radius of over 10 kilometers (six miles), a wireless LAN access point typically serves a maximum radius of about a hundred meters. Due to the shorter distances involved in wireless WANs, radio signals experience less interference and distortion from the environment, thus reducing the amount of error control re quired. Users are also stationary or moving at walking speeds, while wide area networks support users moving at highway speeds where signals are subject to a form of interference known as Rayleigh fading. Another factor is that smaller distances result in much better signal-to-noise ratios. All these factors in combination allow much higher throughputs. The higher throughput of wireless LANs has the virtue of allowing you to use existing network operating systems and applications (e.g., file and printer sharing, database access) compared to the modem-like applications for wireless WANs. And unlike wireless WANs, which are mostly operated by public carriers with usage fees, you get to buy and operate your own network. This gives you control of the whole network, but leaves you responsible for its proper installation and functioning. Fortunately, wireless LAN technology is well past its infancy and is ready to meld into your organization as a reliable subsystem. And the radio bands used by nearly all wireless LANs let you deploy networks without obtaining a license. In this section, we delve into the different topologies available: spread spectrum, which is the most common RF technology used today (the two types of spread spectrum include direct sequence and frequency hopping); a low-power, narrowband approach that enables higher speeds; HiperLAN, which is a European standard; and infrared approaches. Topologies Spread Spectrum Low-Power Narrowband HiperLAN Infrared LANs Infrared Data Association (IRDA) Unlicensed PCS Topologies The term wireless is actually somewhat misleading, since most wireless LANs interconnect with wired networks. The bulk of the distance between a wireless node and another node may well be over wires or fiber. Nevertheless, it is possible to build a network that is completely wireless. In such an instance, the physical size of the network is determined by the maximum reliable propagation range of the radio signals. Networks such as these are referred to as ad hoc networks, and are well suited for temporary situations such as meetings, conferences and sporting events. It is more likely that you will install what is called an infrastructure network, where your wireless LAN connects to an existing wired LAN. In this instance you will need an access point that effectively bridges wireless LAN traffic onto your LAN. This function may be handled by software in a workstation that houses both a wireless card and a wired (e.g., Ethernet) card. But most wireless LAN vendors recommend dedicated hardware called an access point for this function. The access point can also act as a repeater for wireless nodes, effectively doubling the maximum possible distance between nodes. Spread Spectrum Most wireless LANs today use spread spectrum technology, not because spread spectrum is the best radio technology for wireless LANs but more as a result of FCC rules (Federal Code of Regulations 15.247) that allow for unlicensed operation in a number of radio bands, including 902 to 928 MHz, 2.400 to 2.483 GHz and 5.725 to 5.85 GHz. These are the industrial, scientific and medical (ISM) bands where unlicensed users are "secondary users" of the band and must not interfere with licensed primary users. Fortunately such interference has not been an issue because wireless nodes are restricted to 1 watt of power for transmissions and because the nature of spread spectrum is that it appears as noise to all but intended receivers. Nevertheless, as a user of wireless LAN technology you need to be aware that primary users of the spectrum are not restricted to 1 W of transmission and could potentially interfere with your network. Moreover, companies are finding more an d more use for the ISM bands, including wireless speakers and cordless telephones. The Metricom Ricochet network for instance, uses the 900-MHz ISM band. Will you experience interference problems using spread spectrum? Probably not, but you may want to think twice before using wireless LANs for mission-critical or life-and-death applications. In today's market, the 900-MHz ISM band best serves consumer products, while the 2.4-GHz band best serves midrange performing wireless LANs (1 to 3 Mbps) and the 5.7-GHz band best serves higher-performance wireless LANs (5 to 10 Mbps). The 2.4-GHz band has the advantage of being available for unlicensed use in some European countries and Japan, and is the band where most new wireless LAN products operate today. As to coverage, spread spectrum usually operates over a typical range of about 100 meters and coverage areas ranging from 5,000 to 25,000 square meters (50,000 to 250,000 square feet). Spread spectrum was developed by the U.S. military as a robust radio technology that is both difficult to jam and to eavesdrop on. It works by spreading a signal that would normally occupy a certain amount of spectrum over a much broader amount of spectrum. There are two forms of spread spectrum: frequency hopping and direct sequence. Both are allowed by FCC rules. In frequency hopping, the signal dwells momentarily on one frequency, then hops to another, then another in a pseudorandom sequence that eventually repeats itself. A receiver must hop at exactly the same time to exactly the right frequency to be able to receive the signal. FCC rules require that the band be divided into a certain number of frequencies and that the hopper must use a certain number of these frequencies. Direct sequence is very different. Each "one" in the binary data is converted to a sequence of predetermined ones and zeroes and each "zero" is converted to the inverted sequence. The binary data in the sequences are referred to as chips, and the ratio of chips to original bits is referred to as the spreading ratio, or gain, of the system. FCC rules require a minimum spreading ratio or gain. Some wireless LANs are based on frequency hopping, some on direct sequence. Direct sequence allows higher throughputs, although such designs may cost more and use more power. There is almost a holy war about which type of spread spectrum is better, though mobile designs today tend to use frequency hopping. You should choose your network based on features and price, and not on which spread spectrum technology it uses. Low-Power Narrowband An alternative approach to spread spectrum that some wireless LAN vendors are using is to transmit narrowband signals at low-power levels, a method allowed by FCC CFR 15.249 rules. By transmitting at low-power levels, vendors do not have to use spread spectrum, which gives them the ability operate at higher data rates. RadioLAN's product uses this approach and operates at 10 Mbps in the 5.8-GHz band with 50 milliwatts (mW) of peak transmission power. The price of this higher performance is a reduced transmission range of about 30 meters (100 feet) in an office environment. HiperLAN HiperLAN, an abbreviation for Higher Performance Radio LAN, is a wireless technology standard developed by the European Telecommunications Standards Institute. It boasts very impressive capabilities, including a data rate of about 24 Mbps using a channel width of 23.5 MHz. In Europe, spectrum is available in the 5.15 to 5.3 GHz range, allowing for five separate channels. This type of throughput readily supports multimedia applications. Unfortunately, no commercial products are yet available. But the technology is under consideration for new spectrum in the United States in the 5-GHz band as part of the U.S. Unlicensed National Information Infrastructure band. Infrared LANs An alternative approach to radio-based wireless L ANs is infrared communications. Infrared networking uses electromagnetic radiation with wavelengths of 820 to 890 nanometers, corresponding to a frequency of about 350,000 GHz. The advantages of IR include no need for licenses, no safety issues, huge potential capacity and good control of interference. IR does not penetrate walls, so infrared LANs must be contained in a room. Note that IR LANs generally do not operate in outdoor areas where there is sunlight. IR transmitters and receivers can be designed either for directional use or for diffuse use, where signals bounce off walls and other objects to reach the receiver. In fact, IR is specified as one of the physical layer options in the new IEEE 802.11 standard. Though it is a promising technology, there are relatively few IR LAN products available today. But one type of infrared technology that has been broadly deployed is the use of IR for short point-to-point connections following standards specified by the Infrared Data Association. Infrared Data Association (IRDA) The Infrared Data Association is a consortium of vendors that has defined low-cost IR communications characterized by: Directional point-to-point communications of up to one meter 115-Kbps and 4-Mbps connectivity Walk-up ad hoc connectivity for LAN access, printer access, and portable computer to portable computer communications Many laptops today include IRDA ports, though devices such as LAN access points and printers with IR capability are not yet very common. The IRDA estimates some 60 million IRDA ports in the market. Unlicensed PCS When allocating spectrums for Personal Communications Service, the FCC included some bands for what is called unlicensed PCS: 1910 to 1920 MHz and 2390 to 2400 MHz were reserved for data and 1920 to 1930 MHz for voice. Unfortunately, restrictions on the use of this spectrum have limited its usefulness for wireless data to the extent that no product offerings are yet available. Updated January 14, 1998 Print This Page E-mail this URL