ling should not or cannot be installed for aesthetic, regulatory, or safety reasons.
Other potential sites for wireless technology include warehouses, airports, and railway stations -- places where data is usually input into laptops or hand-held devices and consolidated with central databases upon completion of a task. However, vendors say that wireless technology's flexibility benefits are just not great enough to outweigh the limitations of the technology's 1-to 2-Mbps speed. "Users certainly do not want to trade in a 10- or 100-Mbps Fast Ethernet LAN for a much slower 2-Mbps one," says Roy Chen, system manager of Z-Com, a company based in Taiwan.
New Standards
But if wireless LANs offered 10-Mbps throughput, their acceptance could grow quickly. The industry is currently working on standards that will make wireless products easy to use and fast, much the same as wired-LAN products. Seven years in t
he making, the IEEE 802.11 standard was finally formalized last June, when it was approved by the IEEE committee. The specification defines a single media-access-control (MAC) layer with three different physical-layer (PHY) options: infrared plus two spread-spectrum implementations at 2.4 GHz. In addition, it defines transmission procedures, throughput requirements, range characteristics, and configuration issues.
The IEEE 802.11 specification could succeed in unifying vendors for the first time. The new wireless standard has gained wide support from a large number of companies, including AMD, Digital Equipment, Harris Semiconductor, Oki Semiconductors, and Philips Semiconductors. Moreover, the standard will enable chip houses to implement single solutions that should help bring down the price of wireless technology.
Indeed, some of that has taken place already. For frequency hoppers, NetWave and AMD announced a one-chip solution conforming to the IEEE standard the day after it was ratified. "The
standard will have a significant impact on the wireless marketplace by driving component costs down by 50 percent for single-chip solutions and making it easier to extend enterprise LANs to the growing number of mobile-computer users," says Ralph Yang, president of Taiwan-based DB Networks.
Ultimately, the specification could lead to steady growth in vertical markets and to broader adoption by the corporate world. New standards-based hardware is not expected to begin shipping in volume until the first quarter of this year. Vendors have already begun compliance and interoperability testing. But 802.11 does not encompass all the issues surrounding wireless LANs, so products from different vendors supporting this standard might not be compatible.
For devices operating at speeds higher than 2 Mbps, a project-authorization request (PAR) has been submitted to the IEEE for an extension of the 802.11 standard that will define speeds up to 10 Mbps. Another PAR is being submitted for a 20-Mbps wireless stan
dard in the 5.2-GHz range. The FCC recently approved a new spectrum of unlicensed wireless communications in the 5.2-GHz band for use in the U.S. One company, RadioLAN, promises to soon deliver products that will operate in that band.
RF Technologies
Of the three physical-layer connections defined for spread-spectrum links, infrared gets the smallest amount of support from vendors. Infrared is capable of high speeds, uses laser infrared transmitters, and supports two types of devices: dispersed and line-of-sight. However, this technology has been relegated to a niche role because of the high cost of lasers and health concerns about laser use.
As a result, spread-spectrum radio-transmission technology has moved into a dominant position in the market. As its name indicates, spread-spectrum technology spreads the transmitted power over a wide range of the available spectrum. Therefore, it avoids a concentration of power in a single narrow-frequency band.
The two radio designs now in u
se are frequency hopping (FH) and direct-sequence modulation (DSM) spread-spectrum. Each design has its supporters. DSM lets vendors avoid excessive power concentration by spreading a signal over a wide frequency band. The transmitter maps each bit of the message into a pattern of "chips." At the destination, the chips are mapped back into a bit, recreating the original data.
Hence, the transmitter and receiver must be synchronized to operate properly. The ratio of chips per bit is called the
spreading ratio
. A high spreading ratio increases the signal's resistance to interference, while a low ratio increases the bandwidth available to the user.
Lucent Technologies is an advocate of the DSM method because of its higher throughput capability. "DSM can reach 10 Mbps, while FH becomes saturated at 3 Mbps," explains Jan Haagh, senior product manager for WaveLAN at Lucent.
Most Taiwan product makers agree with Lucent. "DSM will become a leading technology this year," says DB Networks' Ya
ng. DSM might climb to even higher speeds this year, he adds.
The FH technology spreads a signal by transmitting a short burst on one frequency. It allows a data signal to "hop" from channel to channel. Because there are many possible sequences in the 2.4-GHz band, FH allows many nonoverlapping channels to be deployed. However, the source and destination of a transmission must be synchronized so that they're on the same frequency at the same time.
Hopping patterns and dwell times are restricted by most regulatory agencies. For example, the FCC stipulates that at least 75 frequencies must be used, with a dwell time of 400 milliseconds. If interference occurs on one frequency, the data is retransmitted on a subsequent hop on another frequency.
Some vendors who use the FH approach claim it offers better security and less interference than DSM. Because the signal is constantly hopping within the frequency band, the chances of a transmission's being interrupted by noise or other types of interfer
ence are less likely. Also, FH products can operate on a lower-powered battery and can be less expensive to implement.
Speed vs. Distance
The issue of speed versus distance is an important one. Most wireless LAN links average between 1- and 2-Mbps rates, and wireless LAN transmitters typically have a transmission range of just 500 to 1000 feet within a building. For most applications, the current speeds are sufficient, National Datacomm's Chiang contends. As 2-Mbps wireless LANs evolve, products yielding between 3 and 7 Mbps are expected to come to market. And 10-Mbps technology is not far behind.
Bell Labs, the R&D arm of Lucent, has developed Direct Sequence/Pulse Position Modulation (DS/PPM), a new technology that can significantly boost wireless-LAN data rates up to 10 Mbps without shortening the distance over which wireless LANs can transmit data. The new wireless-LAN nodes will be fully backward-compatible with IEEE products that operate at 2 or even 1 Mbps. The 10-Mbps technolo
gy will let more-data-intensive applications run over wireless LANs and will also make it possible for more users to go wireless.
Where to Find
DB Networks, Inc.
Taipei Hsien, Taiwan
Phone: +886 2 268 2081
Fax: +886 2 267 1859
Internet: http://www.dbtel.com.tw
Speedy Wireless Networks
