Guide to Wireless LAN Technologies

W H I T E P A P E R Guide to Wireless LAN Technologies Table of Contents Introduction ...............................................................
Author: Warren Stone
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W H I T E

P A P E R

Guide to Wireless LAN Technologies

Table of Contents Introduction ................................................................................................................................................... 2 Wireless LAN Topologies .............................................................................................................................. 3 Wireless LAN Technologies .......................................................................................................................... 4 Infrared (IR) .............................................................................................................................................. 5 UHF (Narrowband) .................................................................................................................................. 5 Unlicensed Radio Frequency Bands ........................................................................................................ 8 900 MHz ............................................................................................................................................... 8 2.4 GHz ................................................................................................................................................ 8 5.0 GHz ................................................................................................................................................ 9 Spread Spectrum Technologies................................................................................................................ 9 Direct Sequence ................................................................................................................................... 9 Frequency Hopping ........................................................................................................................... 10 Strengths and Weaknesses of DSSS and FHSS Radio Technologies .................................................... 10 Wireless LAN Standards ......................................................................................................................... 12 Considerations for Selecting a Wireless LAN Solution .............................................................................. 13 Business Considerations ......................................................................................................................... 13 Technical Considerations ....................................................................................................................... 15 Wireless LAN Solutions from Intermec Technologies ............................................................................... 18

Introduction Customers are confronted today with a wide variety of wireless technologies, systems, and vendors to address needs for wireless data collection. However, most customers will find that no single wireless solution is suitable for all applications. For example, low-volume messaging can be served by the many available options for two-way paging and narrowband PCS (Personal Communication System). For higher data volumes, Wireless LANs (WLANs) offer an excellent solution for a local area. For wireless communications across a city, state, or country, wireless metropolitan or wide-area network options are possible solutions. Vendors such as Analog Cellular, Bell South (formerly RAM), Ardis, GSM, GPRS, TETRA, DECT, Digital Cellular, or PCS may offer solutions for integrated voice/data across a wide area. Finally, GEO (Geostationary Earth Orbit) and LEO (Low Earth Orbit) satellite solutions are increasingly available on a global scale. While customers may choose from many wireless data solutions, the majority of customers select a WLAN system to meet data requirements. WLANs provide high-speed, reliable data communications in a building or campus environment as well as coverage in rural areas. Wireless LANs are simple to install and do not incur monthly user fees or data transmission charges.

WLAN technologies emerged in the 1980s as a viable alternative to and extension of wired LANs for various business applications. Some of the earliest uses were associated with bar code data collection activity; Radio Frequency Data Collection (RFDC) has proliferated in warehousing and retail markets. More recently, manufacturers have deployed WLANs for process and quality control applications. Retail applications have expanded to include wireless Point of Sale (POS). Healthcare and education are also fast growing markets for WLANs. Horizontal applications such as e-mail and scheduling, intranet and Internet access are also expected to accelerate overall market growth. In addition, WLANs offer cost-effective networking solutions for hard-to-wire and historical buildings, outdoor events, and on-site training. Users of WLAN systems often see paybacks in six to nine months and return on investments of 200 to 300 percent. This guide reviews and explains WLAN technology alternatives, and provides guidance on selecting a suitable WLAN system.

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Guide to Wireless LAN Technologies

Wireless LAN Topologies PEER TO PEER CONFIGURATION

Wireless LANs can be built with either of two topologies: peer-to-peer or access point-based. In a peer-to-peer topology, client devices within the wireless cell communicate directly to each other as depicted in Figure 1. An access point is a bridge that connects a wireless client device to the wired network. An access point-based topology uses access points to bridge traffic onto a wired (Ethernet or Token Ring) or wireless backbone as shown in Figure 2. The access point enables a wireless client device to communicate with any other wired or wireless device on the network. The access point topology is more commonly used, demonstrating that WLANs do not replace wired LANs, they extend connectivity to mobile devices.

WIRELESS “CELL”

WIRELESS CLIENTS

Figure 1. In a peer-to-peer topology, wireless devices create a LAN by communicating directly with each other.

Figure 2. In a topology based on access points, wireless devices can connect to the wired LAN backbone for communication with both wired and wireless nodes.

ACCESS POINT CONFIGURATION

LAN BACKBONE

ACCESS POINT

ACCESS POINT

WIRELESS CLIENTS

WIRELESS CLIENTS

ACCESS POINT

WIRELESS REPEATER WIRELESS CLIENTS

Guide to Wireless LAN Technologies

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WIRELESS “CELL”

Another popular wireless network topology is the point-to-point or point-to-multipoint bridge. A bridge is defined as a node (or pair of nodes) with a transceiver client device that connects two networks using similar protocols. Wireless bridges connect a LAN in one building to a LAN in another, even if the buildings are many miles apart (see Figures 3 and 4). These connections require a clear line-of-sight (i.e., no obstacles, such as buildings, hills or trees) between the buildings. The line-of-sight range varies based on the type of wireless bridge and antenna used as well as environmental conditions. WLAN clients are available in a number of form factors for use in any of these network topologies. Personal computers (PCs) can connect to a WLAN using ISA and PC adapter cards. Wireless modems can attach to parallel ports, RS232, 10BaseT, IRDA, or other popular physical interfaces on a PC or other device. In this configuration, the client device communicates via the physical interface (e.g. ISA or PC Adapter, RS232, etc.) to the radio device, which in turn provides the physical interface to the WLAN. For portable applications, the most common configurations are PCMCIA adapter cards for laptop computers and integrated LAN modules for application-specific, hand-held terminals.

POINT TO MULTIPOINT WIRELESS BRIDGE

MANUFACTURING PLANT

HEADQUARTERS

RESEARCH CENTER

Figure 3. A point-to-multipoint bridge topology is useful for a campus or nearby buildings.

WIRELESS BRIDGING

Wireless LAN Technologies

LAN BACKBONE

The technologies available for use in WLANs include infrared, UHF (narrowband) radios, and spread spectrum radios. Two spread spectrum techniques are currently prevalent: frequency hopping and direct sequence. In the United States, the radio bandwidth used for spread spectrum communications falls in three bands (900 MHz, 2.4 GHz, and 5.7 GHz), which the Federal Communications Commission (FCC) approved for local area commercial communications in the late 1980s. In Europe, ETSI, the European Telecommunications Standards Institute, introduced regulations for 2.4 GHz in 1994, and Hiperlan is a family of standards in the 5.15-5.7 GHz and 19.3 GHz frequency bands.

BUILDING #1

BRIDGE

BRIDGE

BUILDING #2 LAN BACKBONE

POINT-TO-POINT WIRELESS BRIDGE Figure 4. A wireless bridge connects the LAN backbones in separate buildings or locations.

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Guide to Wireless LAN Technologies

Infrared (IR) Infrared is an invisible band of radiation that exists at the lower end of the visible electromagnetic spectrum. This type of transmission is most effective when a clear line-of-sight exists between the transmitter and the receiver. Two types of infrared WLAN solutions are available: diffused-beam and direct-beam (or line-of-sight). Currently, direct-beam WLANs offer a faster data rate than diffused-beam networks, but is more directional since diffused-beam technology uses reflected rays to transmit/receive a data signal, it achieves lower data rates in the 1-2 Mbps range. Infrared optical signals are often used in remote control device applications. Users who can benefit from infrared include professionals who continuously set up temporary offices, such as auditors, salespeople, consultants, and managers who visit customers or branch offices. These users connect to the local wired network via an infrared device for retrieving information or using fax and print functions on a server. A group of users may also set up a peer-topeer infrared network while on location to share printer, fax, or other server facilities within their own LAN environment. The education and medical industries commonly use this configuration to easily move networks. As noted in Table 1 below, infrared is a shortrange technology. When used indoors, it can be limited by solid objects such as doors, walls, merchandise, or racking. In addition, the lighting environment can affect signal quality. For

example, loss of communications may occur because of the large amount of sunlight or background light in an environment. Fluorescent lights also may contain large amounts of infrared. This problem may be solved by using high signal power and an optical bandwidth filter, which lessens the infrared signals coming from outside sources. In an outdoor environment, snow, ice, and fog may affect the operation of an infraredbased system. Because of its many limitations, infrared is not a very popular technology for WLANs. According to Frost & Sullivan, infrared has less than 14 percent of the in-building WLAN market, and this market share is expected to drop in the future.

UHF (Narrowband) UHF wireless data communication systems have been available since the early 1980s. These systems normally transmit in the 430 to 470 MHz frequency range, with rare systems using segments of the 800 MHz range. The lower portion of this band 430-450 MHz is often referenced as unprotected (unlicensed) and 450-470 MHz is referred to as the protected (licensed) band. In the unprotected band, RF licenses are not granted for specific frequencies and anyone is allowed to use any frequency in the band. In the protected band, RF licenses are granted for specific frequencies, giving customers some assurance that they will have complete use of that frequency. Other terms for UHF include narrowband and 400 MHz RF. Because independent narrowband RF systems cannot coexist on the same frequency, government agencies allocate specific radio frequencies to users through RF site licenses. A limited amount of unlicensed spectrum is also available in some countries. In order to have many frequencies that can be allocated to users, the bandwidth given to a specific user is very small. The term “narrowband” is used to describe this technology because the RF signal is sent in a very narrow bandwidth, typically 12.5 kHz or 25 kHz. Power levels range from 1 to 2 watts for narrowband RF data systems. This narrow bandwidth combined with high power results in larger

Table 1. Considerations for Choosing Infrared Technology Infrared Advantages

No government regulations controlling use Immunity to electro-magnetic (EMI) and RF interference

Disadvantages

Generally a short-range technology (30-50 ft. radius under ideal conditions) Signals cannot penetrate solid objects Signal affected by light, snow, ice, fog. Dirt can interfere with infrared

Guide to Wireless LAN Technologies

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Synthesized UHF-based solutions provide the ability to install equipment without the complexity of hardware crystals. Common equipment can be purchased and the specific UHF frequency used for each device can be tuned based upon specific location requirements. Additionally, synthesized UHF radios do not exhibit the frequency drift problem experienced in crystal-controlled UHF radios, a feature that eliminates tuning problems after installations have been running for a period of time.

transmission distances than are available from 900 MHz or 2.4 GHz spread spectrum systems, which have lower power levels and wider bandwidths. According to Venture Development Corporations (VDC) 1996 Report on the Data Collection Terminal Market, UHF radio shipments in the ADC (Automated Data Collection) RF Portable Data Collection Market accounted for 23 percent of wireless ADC device revenue and 28 percent of wireless ADC device units in 1995. The growth rate for the UHF radio ADC devices was 9.1 percent (1994 to 1995), which compared to a 19.5 percent growth in spread spectrum ADC devices during the same time period. VDC made no projections regarding UHF sales into the future. Bob Egon of the Gartner Group estimates that 80 percent of new clients inquire about and select spread spectrum-enabled devices for their wireless data collection needs. The other 20 percent select UHF-enabled devices.

Multiple Frequency Operation Modern UHF systems allow access points to be individually configured for operation on one of several pre-programmed frequencies. Terminals are programmed with a list of all frequencies used in the installed access points, allowing them to change frequencies when roaming. To increase throughput, access points may be installed with overlapping coverage but using different frequencies.

Synthesized Radio Technology

Many modern UHF systems use synthesized radio technology, which refers to the way Table 2. Considerations for choosing UHF technology channel frequencies are generated in the radio. The crystal-controlled products in legacy UHF UHF Radio products require factory installation of unique Advantages Longest Range crystals for each possible channel frequency. Low cost solution for large sites with Synthesized technology uses a single, standard low to medium data throughput crystal frequency and derives the required channel requirements frequency by dividing the crystal frequency down Disadvantages Low throughput to a small value, then multiplying it up to the No multi-vendor interoperability desired channel frequency. The division and Interference potential multiplication factors are unique for each desired channel frequency, and are programmed into RF Site License required for protected bands digital memory in the radio at the time of Large radio’s and antenna’s increase manufacture. wireless client size

Table 3. Wireless Range Comparison by RF Technology: Typical Warehouse and Outdoors Wireless Technologies

Indoor Range

400 MHz UHF

300-400 Ft Range 300-500K Sq. Ft.

900 MHz Spread Spectrum

220-350 Ft Range 150-200K Sq. Ft.

2.4 GHz Spread Spectrum

Ratio*

Outdoor Range 1 - 2 Miles

2:1

3000-5000 Ft. Range

2.5 to 6:1

100 mW U.S. and Asia Pacific/Latin America

150-200 Ft Range 70-125K Sq. Ft.

4:1

1200 Ft. Range

500 mW U.S. and Asia Pacific/Latin America

200-250 Ft Range 125-200K Sq. Ft.

2.5:1

1800 Ft. Range

100 mW Europe

125-150 Ft Range 50-70K Sq. Ft.

6:1

600 Ft. Range

* Number of radios required to provide coverage equivalent to one UHF radio.

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Guide to Wireless LAN Technologies

Narrowband Wireless Advantages

When selecting a wireless system, customers should consider future growth and support for Table 2 lists the advantages and disadvantages of additional devices and applications. For many UHF technologies. The chief advantage of UHFUHF customers, future growth can only be supbased systems is range. These systems are optiported with a clear migration path to 2.4 GHz or mum solutions for large sites with low-to-medium with hybrid systems. For new installations and data throughput requirements. Table 3 summarizes to support future growth, customers will typically the wireless range for UHF compared to other RF choose directly to install spread spectrum APs. technologies in a typical warehouse environment and outdoors in open air. Interoperability Typically, two 900 MHz access points or more Because UHF systems are proprietary, they comare required to cover the same area as one UHF municate only with devices manufactured by the based access point. With 2.4 GHz access points, system vendor. This limitation does not give a the ratio of UHF access points is 2.5:1 or more in customer the freedom to choose devices from the US and Asia Pacific/Latin America and 6:1 in Europe at a minimum. The higher ratio for Europe multiple companies. 900 MHz wireless systems is caused by regulations on radiated output power, also have this disadvantage. If interoperability is required, open standard 2.4 GHz wireless LAN which limits spread spectrum radios to 100mW systems are the better choice. 2.4 GHz wireless Equivalent Isotropic Radiated Power (EIRP). LAN standards available today include the WLI European regulations have effectively eliminated Forum OpenAir Standard for 2.4 GHz frequency any high gain antenna technologies that increase hopping and the IEEE 802.11 Standards for the radiated power above the maximum limit of intrared, 2.4 GHz frequency hopping, and 100mW EIRP. In addition, European companies 2.4 GHz direct sequence. can not deploy 900 MHz radio systems because this spectrum is used in Global System for Mobile Interference Communications (GSM) cellular telephone Any interference from outside sources within a systems. UHF systems offer lower infrastructure customer’s bandwidth will disrupt wireless data costs because of their 6:1 radio ratio, a major factor in the continued use of UHF radios in Europe. communications. Interference in unprotected (unlicensed) bands can be caused by unintentional Narrowband Wireless Disadvantages radiators such as other industrial users, or another wireless system or radio using the same RF freThroughput quencies. But even in protected (licensed) bands While UHF wireless systems can cover larger interference can also be caused by unintentional areas with each radio transmitter than 900 MHz or radiators such as factory equipment, welders, and 2.4 GHz spread spectrum systems, they also supforklifts. When a UHF system encounters interferport very low data rates. UHF systems offer data ence, data communications are degraded, however speeds ranging from 4,800 to 19,200 bits per secnarrowband equipment easily handles interference ond (bps); see Table 4 for a comparison to other that is not on its operational channel. technologies. The speed disadvantage of UHF systems results in fewer transactions per second RF Site License Required or support for fewer overall devices. Because the UHF radio spectrum is limited and independent UHF systems operating on the Table 4. Data Rates for WLAN Technologies same frequency can not coexist in the same area, radio authorities in each country must regulate Wireless Technology Data Rates and license specific frequencies to customers. 400 MHz UHF 4.8 - 19.2 Kbps Obtaining RF site licenses is time consuming and 900 MHz Spread Spectrum 100 - 400 Kbps potentially costly. Additionally, many cities have 2.4 GHz Spread Spectrum 1 - 2 Mbps run out of available frequencies, which means 2.4 GHz Future More than 10 Mbps obtaining a license is very difficult or impossible. 5.7 GHz Future More than 20 Mbps In comparison, spread spectrum systems communicate in the unlicensed ISM (Industrial, Scientific, and Medical) band. As those systems are designed to coexist with others, no license is required for installation or operation. Guide to Wireless LAN Technologies

7

Large Radios and Antennas

Not available for use in Europe or Asia because this band is reserved for GSM cellular telephone systems

Because UHF radios are typically larger than spread spectrum radios, UHF end devices are larger than spread spectrum devices. In today’s data collection environment, smaller devices have better market potential. External UHF antennas are also longer, as the RF wavelengths for 450 MHz signals are longer than those for 900 MHz or 2.4 GHz RF signals. Users of portable devices prefer integrated antennas that do not get in the way of the devices operation or the users activity.

Companies with sites around the globe cannot standardize on 900 MHz based solutions for all locations

Unlicensed Radio Frequency Bands above 900 MHz

Only a few manufacturers offer 900 MHz systems

In 1985, the FCC authorized the use of spread spectrum radio technology in the 902-928 MHz, 2.400-2.4835 GHz, and 5.725-5.850 GHz frequency bands. FCC Part 15 rules allow unlicensed use of spread spectrum data communications in these bands. Commonly referred to as 900 MHz, 2.4 GHz, and 5.7 GHz, these frequencies are also known as ISM bands. Tables 5 and 6 list the advantages and disadvantages of 900 MHz and 2.4 GHz technologies.

Table 5. Considerations for Choosing 900 MHz Technology 900 MHz Advantages

Good balance of range and data rate Data rates of 100,000 to 450,000 bps are sufficient for many WLAN applications Typically larger coverage areas than similar 2.4 GHz systems

Disadvantages

No Interoperability; each vendor employs proprietary radio protocols 900 MHz band is more crowded than other frequency bands. In the U.S., this band is used for cordless telephones, devices to extend in-home TV signals, vehicle locating systems, and other non-spread spectrum applications. Internationally, the band is widely used for cellular or military communications

900 MHz

Table 6. Considerations for Choosing 2.4 GHz Technology

The 900 MHz band has been the most popular for early WLAN applications, based on the availability of products introduced in the early 1990s. However, the market has been slowly shifting to more recently developed 2.4 GHz products. Very few products use the 5.7 GHz band today, primarily because of its high cost, limited availability of parts, and technical limitations. In spite of these limitations, thousands of 900 MHz WLANs have been implemented, supporting hundreds of thousands of devices successfully for critical business applications.

2.4 GHz Advantages

Worldwide acceptance for installation and use in most countries The industry is moving towards standards-based systems, meaning multivendor support for common WLAN infrastructures Data rates supported are 800,000 to 2,000,000 bps with higher rates in the future

Disadvantages

Limited range. Because the systems transmit less power (typically 1/10th of a watt) and 2.4 GHz has poorer propagation characteristics, the coverage area for a given radio and access points can be significantly less than that of 900 MHz or UHF systems

2.4 GHz The 2.4 GHz band is three times larger than the 900 MHz band, making it feasible to support faster data rates and higher overall system capacities. This band, while more limited in range than 900 MHz, has an advantage in aggregate speed. Transmission on the 2.4 GHz band is “cleaner,” because it carries less competing traffic. A 2.4 GHz system is a better choice for markets outside the U.S., particularly Europe and Japan. In addition, the new Wireless LAN standards such as IEEE 802.11 and WLIF OpenAir are based upon the 2.4 GHz frequency band. Together these fac-

Decreased range can lead to significant increases in system infrastructure costs for large buildings, multi-story buildings, or campus environments Outdoor installations may be difficult or impossible to cover

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Guide to Wireless LAN Technologies

Direct Sequence tors have made 2.4 GHz the primary band of choice for customers today. Microwave ovens represent the primary interference in the 2.4 GHz band. However, spread spectrum techniques and good installation practices can minimize this interference. New developments will overcome many of the current limitations of 2.4 GHz technology, such as increasing data rates to 10 Mbps or higher. New radios will increase the range of 2.4 GHz wireless LANs and reduce the cost differential between 2.4 GHz and 900 MHz systems. Wireless access points can make outdoor installations possible where no network cables are accessible, when authorized for use.

Direct Sequence Spread Spectrum (DSSS) system implementations can vary. To illustrate the basic technology, this discussion presents the 2.4 GHz approach defined by the IEEE 802.11 Wireless LAN standard (see Figure 5). Currently, DSSS provides a 1 or 2 Mbps data rate over the air, with 11 Mbps and higher data rates coming in the near future. A DSSS system uses a relatively wide 22 MHz channel. In the U.S. and much of Europe, three non-overlapping and two overlapping channels are available in the 2.4 GHz band. In Japan, only one 22 MHz band is available and regulations in other countries allow for 1, 2, or 3 channels. Typically, all access points in a given WLAN network are on the same 22 MHz channel. If one channel is subject to more interference, the system can move to another. Direct sequence works by converting each bit/symbol into an 11-bit “chip” sequence with a logic ‘0’ sequence being the compliment of a ‘1’ sequence. The chip rate is eleven times the original bit / symbol rate. A correlator is used with the spreading code that determines how many of the received chips match the ‘1’ sequence and the ‘ø’ sequence. The greater of the two matches determines if the received data is a ‘1’ or a ‘0’. Immunity to interference and system range can be improved by increasing the number of chips per bit/symbol, although the data rate will decrease if the bandwidth remains constant (or the number of channels will decrease if the data rate is constant).

5 GHz Products based on the highest of the available ISM bands, 5 GHz, are still largely experimental. It will be the year 2000 or later before WLAN products in this band become viable for commercial applications. In the U.S., development of 5.7 GHz technology will be closely tied to the National Information Infrastructure (NII), a federal program for new communications technologies. Although the 5.7 GHz frequency band will support higher data rates than current systems, it must overcome challenges for range and power consumption.

Spread Spectrum Technologies During World War II, the U.S. military developed spread spectrum techniques for secure voice communications. By operating across a broad range of radio frequencies, a spread spectrum device could communicate clearly despite interference from other devices using the same spectrum in the same physical location. In addition to its relative immunity to interference, spread spectrum makes eavesdropping and jamming inherently difficult. To decode the signal from a spread spectrum device, a receiver must know the specific spreading pattern of the transmitter. In commercial applications, spread spectrum techniques currently offer data rates up to 2 Mbps. Because the FCC does not require site licensing for the bands used by spread spectrum systems, this technology has become the standard for high-speed RF data transmission. Two modulation schemes are commonly used to encode spread spectrum signals: direct sequence and frequency hopping. Guide to Wireless LAN Technologies

DIRECT SEQUENCE SPREAD SPECTRUM SIGNAL SYMBOL SPREAD WITH A SEQUENCE • EACH SYMBOL IS DIVIDED INTO 11 CHIPS. A SIMPLE MAJORITY NEEDS TO GET THROUGH

WIDER BANDWIDTH / LOW POWER DENSITY

POWER

START FREQUENCY

Figure 5.

9

CHANNELS

STOP FREQUENCY

and over again, but the devices must be synchronized. This is accomplished by all units knowing the pattern of hopping, the duration on each hop, and the current time of the hop sequence. Because the signal moves around the band rapidly, FHSS systems can avoid interference much of the time. The FHSS system sends packets of data, checks them for errors, and if an error in a packet occurs, retransmits the packet, often in another hop. As interference increases, the FHSS system performance degrades gradually. Up to 1 Mbps over-the-air data rates in FHSS systems are possible using two-level Frequency Shift Keying (FSK) modulation in the 802.11 standard. Higher rates are possible using multi-level modulation. For example, 2 Mbps data rates are possible using four-level FSK as defined by the 802.11 standard. However, the RF range is reduced when multi-level modulation is used. Current four-level FSK radios will result in about one-half the range of two-level FSK. This means a well-designed WLAN device operating at 2 Mbps will typically have half the range of 1 Mbps operation. A poorly designed system can have substantially less range at 2 Mbps.

Because DSSS radio designs are complex and the required components are more costly, DSSS technology is typically more costly than frequency hopping technology. However, total system costs for DSSS may be less if fewer access points are needed than would be required for a comparable implementation based on frequency hopping technology.

Frequency Hopping Although Frequency Hopping Spread Spectrum (FHSS) systems allow for a variety of implementations, this discussion presents the IEEE 802.11 implementation. In frequency hopping, the signal hops among a variety of frequencies, with the exact sequence of changes (the hopping sequence) known only to the stations participating in the communication. At any instant in time, the signal is being broadcast on only one frequency, and the transmission remains on each frequency for only a short period of time (up to 0.4 second) before moving to the next frequency. Thus, interference on a single frequency, or even several frequencies, is not sufficient to disrupt the communication. A 2.4 GHz FHSS system uses 79 channels, each 1 MHz wide (most countries). The signal “hops” the entire band using a pseudo-random sequence (see figure 6). All units in a given cell must hop at the same time. Each device hops using the same pseudo-random sequence over

Strengths and Weaknesses of DSSS and FHSS Radio Technologies Wireless LAN vendors have debated the merits of the two spread spectrum techniques for a number of years. Typically, the arguments have been slanted to favor the technology each vendor manufactures. Table 7 summerizes key differences from a customer’s perspective. Most WLAN applications used today in warehousing, retail, factory and healthcare applications require substantially less than 1 Mbps data

FREQUENCY HOPPING SPREAD SPECTRUM HOPS FROM ONE FREQUENCY TO ANOTHER AT A SPECIFIC HOPPING RATE IN A SET SEQUENCE • 79 — 1 MHz WIDE CHANNELS

Table 7. Frequency Hopping versus Direct Sequence Technologies

• MULTI-USE OF BAND AND INTERFERENCE IMMUNITY BY CONSTANTLY MOVING AROUND FROM FREQUENCY TO FREQUENCY

Technology Frequency Hopping Economic Solutions CHANNELS

Lower cost radio’s Provides a per-node bandwidth about 50% of a DSSS system Can scale above 10 Mbps by adding more access points

Direct Sequence High Performance START FREQUENCY

Key Considerations

TIME

Increased range: Best suited for large coverage areas Higher data rates: Migration to 11 Mbps solutions of the future

STOP FREQUENCY

Fewer access points required: Lower total system costs Figure 6.

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Guide to Wireless LAN Technologies

For FHSS, the IEEE 802.11 standard defines 79 possible hopping sequences. Some vendors have claimed that 25 Mbps of capacity is possible through the co-location of many access points, but these claims are not based on sound engineering or analysis. The real answer to scalability requires a thorough analysis of the impact of colocating FHSS access points. The IEEE 802.11 hopping sequences are designed so that if any two hopping sequences are used, a collision on the same channel will occur only once during the time that two access points would hop through the entire sequence. In reality, collisions occur more frequently. Since the FHSS system is packing 1 Mbps of data in a 1 MHz wide channel, the 802.11 specification allows for a significant amount of the transmitted interfering RF to spill over into adjacent channels (see Figure 7). As shown in figure 7, on each hop the FHSS system occupies up to five channels. It may be possible to limit the interference to the outer channels, but the Intermec model indicates that most WLAN implementations will experience this type of interference. Where the hopping sequences are designed to collide only once in theory during a run through of all hops, in practice each hop of an access point will collide with five out of the 79 available channels.

rates, making FHSS more cost-effective. As wireless requirements for multimedia and other data intensive applications emerge, DSSS technology is likely to be favored for its higher bandwidth.

Data Rates The “over-the-air” data rates quoted by wireless system vendors are often deceiving because they describe potential speeds, not the actual performance a user will experience. A more relevant consideration is throughput and response time. The system architecture of a WLAN has a much more significant impact on throughput and response time than the chosen radio technology. The over-the-air data rate will favor DSSS by a factor of two-to-one at a given range and given similar implementations in terms of transmit power, antenna system, and other radio parameters of DSSS and FHSS systems. A 2 Mbps DSSS system offers comparable range to 1 Mbps FHSS technology. With DSSS, virtually all of the air time can be used for data throughput, while FHSS systems cannot send data while units hop from one channel to another.

Range The range of each spread spectrum technology depends on a number of implementation details. These details include transmit power, antenna system (including antenna gain, cable loss, and whether diversity/multiple antennas are used) and radio sensitivity. Because DSSS processing assists range, this technology could theoretically have a more consistent range than FHSS at a given data rate in a multipath environment. A well designed DSSS system will offer more range than a FHSS system, lowering wireless LAN infrastructure costs. However, a well-designed FHSS system will offer more range than a poorly implemented DSSS system.

WIRELESS CAPACITY PER CELL 1 Mbps FH vs. 2 Mbps DS 9 8 DS

7

Mbps CAPACITY

Scalability FHSS proponents claim that frequency hopping is more scalable than DSSS. This claim is based on the notion of using multiple access points in each cell to increase the overall data capacity available to users in any given cell. A DSSS system has an absolute limit of three non-overlapping channels; by stacking three access points in a cell the total capacity available is limited to 6 Mbps. With FHSS, the upper limit is much more difficult to determine.

6 5 4

2 1 0 1

2

3

4

5

6

7

8

9

10

11

NUMBER OF ACCESS POINTS

Figure 7.

Guide to Wireless LAN Technologies

FH

3

11

12

13

14

15

16

In a DSSS system, three access points reach the capacity limit of 6 Mbps. With FHSS, up to 16 access points can be added to achieve a little more than 8 Mbps of capacity. With more than 16 access points, the cell capacity declines because the additional access points add more interference than data capacity. As this analysis indicates, FHSS is more scalable, but at a significant cost in terms of the number of access points required to gain additional capacity (see Figure 8).

mit power, DSSS radios are typically more expensive than FHSS radios. For systems where a 1 Mbps data rate is sufficient, FHSS is more costeffective to deploy. When data rate requirements exceed 1 Mbps, DSSS is more attractive from a cost standpoint. The reason for this cost difference is that to equal the capacity of DSSS at 2 Mbps, at least two FHSS access points are required. Even a 2 Mbps FHSS wireless LAN requires twice the number of access points over DSSS because the coverage of FHSS for 2 Mbps is about half that of DSSS at 2 Mbps. DSSS systems typically support lowercost wireless repeating with one radio; FHSS systems typically require two radio access points to support repeating to reduce overhead and delay latencies with one radio.

Cost Today, DSSS technology is more costly than FHSS technology. With all things equal in terms of specifications such as frequency band and trans-

Wireless LAN Standards

Table 8: Wireless LAN Standards Standard

Several current and emerging standards are important to understand when considering wireless LAN technology. Table 8 summarizes the key standards and their scope. Figure 9 shows the relative state of each of the standards. For more information on these standards, visit Intermec’s web site at http://www.intermec.com.

Scope

WLI Forum OpenAir

2.4 GHz frequency hopping spread spectrum

IEEE 802.11

Defines wireless LAN interoperability among multi-vendor products, infrared, 2.4 GHz frequency hopping, and 2.4 GHz direct sequence spread spectrum. Future 2.4 GHz and 5 GHz high speed standards

Home Radio Frequency Working Group ‘HomeRF’

Shared Wireless Access Protocol (SWAP) for wireless networking within a home

BlueTooth Consortium

Short-range radio links using 2.4 GHz frequency hopping spread spectrum

COMPARISON OF CELL CAPACITY USING MULTIPLE APs

WIRELESS LAN STANDARD UNIVERSE

802.11 DS

802.11 HS

HiperLAN 1 OpenAir

INTERO DSSS

FHSS

FHSS

Bluetooth

Figure 8.

8 APs = 6 Mpbs

16 APs = 8 Mpbs

Y

802.11 FH

METRIC

HomeRF

3 APs = 6 Mpbs

PERABILIT

S TA N

S AGREED TEST LA B E S TA B L I S H E D PUBLIS H E D S TA N D A R D

DARD-BO

802.11 5 GHz

DY FORM ED

Figure 9.

12

Guide to Wireless LAN Technologies

Considerations for Selecting a Wireless LAN Solution

Wireless Experience

The choice of radio technology may be less important than the wireless networking software, which can also have a substantial impact on system performance and throughput. In addition to these technical factors, business factors also should be considered when selecting a WLAN vendor (see Table 9). Not all vendors in the industry today are able to support all of the capabilities described in this guide. To properly evaluate a WLAN vendor, it is important to make a weighted list of requirements, including current and anticipated future needs. Matching these requirements against the offerings of each WLAN vendor will allow you to shorten the list of prospective WLAN solutions considerably. It is possible that no single vendor’s offerings will match all of your current and future requirements. In this case, discuss future product plans with vendors to determine which offers the development and migration plans closest to your needs.

The installation of WLAN systems is sometimes more of an art than a science. The coverage patterns of WLAN radios inside buildings are affected by walls, ceilings, racks, and merchandise — even the composition of the building’s construction. Installation must consider the multitude of antenna options including omnidirectional, flat panel, directional, and yagi antennas. Splitters, couplers, and lightning arrestors are also commonly used in WLAN installations. Working with a vendor experienced in installing WLAN systems in comparable facilities can provide a less costly, trouble-free installation. An experienced WLAN solution provider can complete an RF site survey, provide a detailed installation plan, and complete the installation to provide full wireless coverage where it is needed.

Full System and Solution Provider Wireless LAN systems can become very complex as they grow with organizations. The system infrastructure includes access points, PC Cards, ISA cards, wireless client devices, and WLAN enabled software. While it is possible to purchase these products from many companies, few are truly full system and solution vendors. By working with a full system and solution provider, customers can select and purchase most system components from a single vendor, including installation and wireless client devices. In the future, a single vendor will protect mission-critical applications by providing a single point of contact for resolving system problems.

Business Considerations Cost Calculating the total cost of a WLAN system involves several elements, including client devices, access points, and ongoing maintenance. The cost of access points can vary depending on the range and throughput of the selected technology. Support costs include equipment service, problem diagnosis, and software upgrades. If a customer has installed WLAN systems in multiple locations, the ability to remotely access those systems and upgrade the access point and client devices can offer substantial savings for support costs. Another cost consideration may be whether a technology migration path will re-use the customers existing equipment. As new capabilities such as faster data rates are offered by the system vendor, they should be backward compatible with older technologies. Other cost factors include product quality, reliability, ease-of-use, and availability. All of these factors must be compared to determine the true cost of competitive product offerings.

Guide to Wireless LAN Technologies

Table 9. Business Factors for Selecting a WLAN Vendor Factor

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Key Considerations

Cost

Total system cost: equipment, support, future upgrades

Wireless Experience

Vendor experience in installing complete WLAN systems

Full Provider

Vendor provides full system, installation, and support solution

Business Stability

Company strength and commitment to wireless market

Wireless Clients

Wide selection of client devices

System Integration

Software and support to integrate devices into a complete system

Service and Support

On-site service options

Global Presence

Country approvals and local support programs

Business Stability of the Wireless LAN Vendor

A vendor that provides a wide selection of wireless client devices enables customers to select the most appropriate device for each application. WLAN client devices include hand-held, stationary, vehicle mount, and pen-based terminals, wireless modems, and PC/ISA adapter cards.

Selection of a WLAN vendor should consider the vendor’s strength for supporting future expansion and new capabilities in the system. The risks of selecting a vendor that will not be around in a few years is significant. Many WLAN vendors are small, start-up companies without the critical customer base that will sustain the continued investment required to survive. The industry has already seen a number of pioneers retrench and become very specialized niche players, or go out of business entirely. Even large companies with substantial financial and technical resources may exit the business when the WLAN segment fails to reach the company’s internal expectations. A good example is provided by Motorola, one of the WLAN industry pioneers, which withdrew from the business in 1995. Selecting a vendor that may not be around in the future makes it likely that long-term system costs will increase. The best way to evaluate the stability of a vendor, large or small, is to assess the company’s success in the WLAN market. A company with a substantial WLAN market share and a long roster of major customers is more likely to thrive and be a reliable supplier. Another criterion is whether WLAN products are a key business segment for the vendor.

System Integration Wireless LAN systems include much more than APs and client devices. A complete system includes software and services that integrate client devices with the host and the supporting software required for the application. A vendor that offers system integration services enables successful installation of a WLAN application. In turn, this application gives users a trouble-free solution that provides immediate benefits from the use of wireless technologies. The installation should include the hardware and software integration necessary to get the application running quickly.

Service and Support Once a WLAN system is installed and operational, service and support become critical factors for the continued success of the system. Changes in system requirements, the physical environment, applications, and system components are best handled by a vendor that provides on-site service and support.

Wide Selection of Wireless Clients

Global Presence

No single client device meets the requirements of all WLAN applications in use today. While PC and ISA adapter cards can be installed standalone in computing devices, most customers demand client devices with integrated radios. Integration provide secure mounting of the radio for everyday use, tuned antennas for the wireless client, battery and power management, and software driver support.

Check with the vendor to ensure that equipment is approved for use in all countries where a WLAN solution will be required today or in the future. In addition, verify that the vendors support and service programs in each country are adequate for the needs of the local installations.

“WLAN selection should be based on end-user bandwidth requirements and system scalability.Who supplies the solution is based upon assessments of standards compatibility, integration and manageability” Gartner Group, 1998

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Guide to Wireless LAN Technologies

Technical Considerations

Performance (Throughput)

The technical considerations for selecting a WLAN product should not be based on a theoretical argument between one technology or another. Instead, the decision should be based on how the product meets business needs for capabilities, features, and performance (see Table 10). Because each customer’s needs will differ, the factors discussed in this section should be weighed in their importance to these needs.

WLAN products typically specify the “over-the-air” data rates provided (i.e., 1 or 2 Mbps). However, what matters most to the user is the actual throughput of the system in a specific application and environment. Performance tests can determine actual throughput rates in differing conditions. Among the variables to consider in a throughput test are the range of the device from the access point, the system load (number and data traffic of clients using an access point), the typical packet size on the network, and the Network Operating System (NOS). Additionally, the networking software used in each implementation — including how that system supports roaming, the use of dynamic load balancing, and a number of other factors —will affect the performance of that system. Each WLAN product will provide substantially different results that may have little correlation with the over-the-air data rates specified by the product vendor.

Seamless Integration into the Corporate Network A critical aspect of a WLAN system is seamless integration into the corporate network. WLANs are not a replacement for, but instead are an extension of the wired LAN. The WLAN should seamlessly integrate into the wired network whether based on Ethernet or Token Ring. As the network topology changes, the WLAN system should support new networking requirements. The WLAN should also support a variety of topologies from wired access points, wireless access points (repeaters), and wireless bridging to offer flexibility for implementation regardless of the physical requirements of the facility. A WLAN system should support roaming across the enterprise, regardless of where access points are physically attached to the network infrastructure. When a wireless client user roams from an access point attached to one subnet on the network to an access point attached to another subnet, the WLAN system should allow seamless communications for the client device. The system should not produce a lost connection or worse, a lost connection plus re-initialization of the router or intelligent bridge. By selecting a WLAN system that has built-in support for roaming across subnets, wireless applications work seamlessly wherever users roam in the facility.

Response Time and Delay Another consideration for determining system performance is the response time of a wireless client transaction. Response time includes host and network delay when delivering individual packets for a given system. Again, the wireless networking software will have the most substantial impact on this performance attribute. Software factors which determine this impact include the technique used to support roaming, use of dynamic load balancing, and the access point forwarding of buffered packets during roaming. Also important is the reliability of the WLAN device transmission at various ranges, which determines the number of packet errors and retransmission required. Different WLAN products may have wide variations in their delay performance. Table 10. Technical Factors for Selecting a WLAN Solution

Cost and Coverage

Factor

A number of factors will effect the range of a WLAN device, including receiver sensitivity, transmit power output, multipath immunity, and antenna system performance (including the proper use of antenna diversity). The greater the range of WLAN devices, the fewer number of access points that will be required to cover a given building or installation. Device range may ultimately become the key factor in determining total system cost.

Guide to Wireless LAN Technologies

15

Key Considerations

Seamless Integration

Integration with wired LANs; roaming support

Cost and Coverage

Device range for total system cost

Performance

Data rates in the target environment

Response Time

Delay-management techniques in the WLAN software

Size and Convenience

Use and placement of access points and client devices

Interfaces

Availability of hardware and software options

Interoperability

Communication with other WLAN systems and client devices

Power Management

Minimized battery changes for client devices

Security

Encryption of data transmissions

Size and Convenience The attributes of size and convenience are closely related for determining user satisfaction with a wireless system. Small access points that can be mounted in or on ceilings, attached to walls, and placed on desks or cubicle walls are much more convenient to implement than access points with the form factor of a desktop PC.

For client devices, a single-piece, PCMCIA Type II card form factor will usually be more convenient than a PCMCIA interface card with a separate, cable-attached radio. Additionally, for specialized hand-held terminals or Personal Digital Assistants (PDAs), a built-in WLAN device will usually be better than using a PCMCIA slot with an external wireless device. With a true PCMCIA form factor, the potential availability of specialized hand-helds and other wireless devices will be greatly increased. It is likely that once all WLAN client devices reach a true PCMCIA Type II form factor and all access points are small, consideration of equipment size will become less important for choosing a system vendor.

Table 11. Access Point Options Factor

Key Considerations

Topologies Supported

Access Point, Wireless Access Point (Repeater), Wireless Bridge

Networks Supported

Ethernet, Token Ring

Connections

10 BaseT, 10Base2, AUI

Wireless Technologies Supported

900 MHz , 2.4 GHz OpenAir, IEEE 802.11 Frequency Hopping, IEEE 802.11 Direct Sequence, future high-speed wireless technologies

Availability of Hardware and Software Interfaces

Table 12. Access Point Capabilities Factor Basic

A number of different hardware or software options may or may not be supported by a WLAN system. The importance of these options depend on a user’s short- and long-term requirements. Tables 11 and 12 describe the most important options.

Key Considerations Basic support for required IEEE 802.11 protocols Access point filtering of packets (multicast, broadcast, etc.) Access point filtering of traffic to wireless devices not associated with an access point

Advanced

Interoperability

Roaming across subnet boundaries

Until a few years ago, interoperability among WLAN systems was not possible. A particular WLAN system could communicate only with the client devices offered by the same vendor. With the introduction of the WLI Forum 2.4 GHz OpenAir Specification in 1995, customers gained the ability to purchase WLAN systems and client devices from a variety of vendors. In the future, the IEEE 802.11, Bluetooth, and HomeRF standards and specifications will further interoperability. For more information or these standards, visit Intermec’s web site at www.Intermec.com. WLAN systems that support interoperability give customers the freedom to choose equipment from a variety of vendors. Interoperability increases competition, a result that historically has yielded lower costs, increased features, and new product selections.

Forwarding of buffered packets between access points when roaming Dynamic load balancing of client devices between access points Fault-tolerant setup possible Capability for self-configuration and firmware upgrades via BOOTP does not support firmware upgrades by itself Remote diagnostics, configuration, software upgrades and management via Telnet, FTP and/or SNMP WEP (Wired Equivalent Privacy) encryption support Support for the optional IEEE 802.11 specified PCF (Point Coordination Function), which enables realtime multimedia Environmental Considerations

Heating, cooling, sealing

Client Device Connections

PCMCIA card and socket services, ISA Cards, RS232, 10BaseT, 10Base2, AUI, RS422, Parallel port

Client Device/ Capabilities

Drivers: NDIS2, NDIS3, NDIS4, ODI, Operating Systems: DOS, Windows, Windows 95, Windows NT, OS 2, MAC, Unix Advanced: Advanced packet power management, serial communications support, software upgradable via flash memory

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Guide to Wireless LAN Technologies

Customers may assume that when a standard is in place and interoperability is supported, products from different vendors will work and perform identically. However, even with standardscompliant products, substantial performance differences may exist between products offered by different vendors. The reason for such variations is typically that although the communications protocol is specified by the standard, the functionality of access points and client devices is determined by each vendor.

ping patterns of the frequency hopping technique were designed to avoid casual decoding. Spread spectrum radio technologies offer a great deal of protection because of these techniques. However, because interoperable WLAN solutions use the same spread spectrum patterns, this security protection is diminished. If a customer requires more security, the IEEE 802.11 Wireless LAN standard specifies use of WEP (Wired Equivalent Privacy) encryption. This specification utilizes the RSA Data Security, Inc. RC4 encryption algorithm to encrypt over-the-air data transmissions. Encryption operates on top of the security provided by spread spectrum techniques. With WEP encryption, a users wireless transmission is meant to be as secure as an encrypted transmission over a wired LAN. An important consideration is that the IEEE 802.11 standard secures only over-the-air transmissions. An access point will send information over the Ethernet or Token Ring network without encryption. For higher-level security requirements, customers can use an end-to-end encryption technique such as that specified by the IEEE 802.10 standard. With end-to-end encryption layered on top of the security measures in the wireless system, user data should be totally secure. The choice of security should be based on individual application requirements, and consider the trade-off of cost, performance, and complexity.

Power Management Most WLAN client devices are battery-operated, with a limited battery life. The use of radios for communications can significantly affect battery life. Most users would not be satisfied if they had to change batteries frequently. Wireless client devices should offer advanced power management support to maximize battery life and minimize battery changes (optimally once per work shift).

Security Considerations Security has always been a concern for wireless communications, as evidenced by the issues around interception of cellular telephone transmissions. Radios utilizing a broadcast-mode transmission scheme allow the possibility of interception by unintended receivers. Much of today’s technology for spread spectrum radio was developed with exactly this problem in mind. The unique spreading patterns in DSSS transmissions were meant to be difficult to decode for a receiver that didn’t know the specific pattern. Similarly, the pseudo-random hop-

Guide to Wireless LAN Technologies

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Wireless LAN Solutions from Intermec Technologies Intermec’s INCA (Integrated Network Communications Architecture) is the industry’s leading architecture for wireless LANs. Based on the principles of openness and flexibility, INCA gives businesses freedom of choice for meeting their networking needs today and in the future. The

INCA architecture blends network connectivity and wireless LANs into a complete, plug-and-play system. It even allows roaming across router boundaries and the integration of different radio frequencies into a single-system solution (see Figure 10).

INCA

Intermec’s Network Communications Architecture NETWORK INDEPENDENT ™

UNIX

IBM

TOKEN RING

NT

ETHERNET DATA COLLECTION SERVER

RADIO INDEPENDENT ™ TOKEN RING ACCESS POINT

WLIF

IEE 80 E Frequ2.11 Hopp ency ing

DUAL RADIO ACCESS POINT

IE 80 EE D 2.11 Seq irec uenc t e

re tu ss Fu irele ards W nd Sta

UH

F

0 90 z MHead Spr ctrum Spe

INDUSTRY’S LARGEST SELECTION OF END DEVICES

HAND-HELD

VEHICLE-MOUNT

PEN-BASED

STATIONARY

Figure 10.

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Guide to Wireless LAN Technologies

Intermec and Wireless Networking

The INCA Radio Independent™ feature gives customers freedom of choice for radio technologies. This design supports systems based on 400 MHz UHF, 900 MHz spread spectrum, 2.4 GHz OpenAir, IEEE 802.11 Direct Sequence, IEEE 802.11 Frequency Hopping, and future wireless radio technologies. The dual-radio feature enables customers to mix different types of wireless radios to meet different range and throughput requirements within a single environment. The INCA Network Independent design supports 10BaseT and 100BaseT Ethernet, Token Ring, Coax, Twinax, and SDLC networks. Wireless networks built upon the INCA foundation are compliant with industry-standard networking protocols such as SNA, TCP/IP, and SPX/IPX. The Application Independent design of the INCA architecture seamlessly supports a wide range of current and future business applications. Systems based on the INCA architecture have the ability to migrate seamlessly from today’s solution to tomorrow’s technology, keeping pace with the needs of the enterprise. This migration strategy enables customers to enjoy all the key INCA advantages in the future, including freedom of choice. The INCA architecture enables businesses to easily adopt new RF technologies, create new applications, and expand network coverage. This migration path also ensures protection of current investments in wireless systems and network designs.

Guide to Wireless LAN Technologies

Intermec is a full system and solution provider for WLANs. Intermec’s wireless networking products are based on more than 30 years of industry leadership in data collection and wireless technologies. Intermec Technologies Corporation developed the first wireless data collection network and has more than 250,000 wireless and 500,000 wired terminals installed to date. Other achievements include the first spread spectrum RF data collection technology approved for sale by the FCC, the first 2.4 GHz data collection technology, and the first multi-radio access point. Intermec equipment is certified and approved by more enterprise resource planning (ERP) and warehouse management system (WMS) providers than the products of any other data collection vendor.

To Learn More For more information on the INCA architecture and Intermec wireless LAN solutions, contact Intermec Technologies Corporation at 1-800-347-2636 or visit Interme’s web site at http://www.intermec.com.

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Where Information Gets Down To Business



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Copyright © 1998 Intermec Technologies Corporation. All rights reserved. All trademarks are the property of their respective owners. Contents subject to change. Printed in the U.S.A. #608582 10/98