Since the FCC opened up the spectrum in the 2.4-GHz band for unlicensed use, companies such as Proxim, Lucent, and Symbol Technologies have led the charge of proprietary wireless connectivity solutions into vertical markets. These include warehousing/manufacturing, government, and educational institutions. Only recently have solutions started to leak into the enterprise and consumer markets. From all indications, however, the trickle is about to become a flood as companies rally around wireless local-area-network (WLAN) standards such as 802.11b and HomeRF, and personal-area networks (PANs) like Bluetooth.
Devices based on each of these standards are in various stages of rollout. Each has its own demons to overcome in terms of cost, performance, interoperability, security, and overall implementation. In addition, as these devices proliferate, there's much concern over their ability to coexist peacefully. This concern is only heightened by proposals to widen the instantaneous bandwidth of frequency-hopping (FH) systems to allow for higher data rates. It could be argued that widening the FH bandwidth is a good thing. The sooner the 2.4-GHz band gets overcrowded, then the sooner the jump will be made to the much-anticipated 5-GHz band, which still remains tantalizingly at bay.
Though the various WLANs face many complicated problems, the possible solutions are a welcoming match of variety and elegance. All of these solutions are designed to improve the performance, lower the cost, and shrink the size of upcoming designs, while making the devices more user-friendly.
At the forefront of chip development for 802.11b is Intersil. With its Prism II chip set, the company has been one of the leading suppliers of chip solutions to OEMs such as Symbol Technologies. Symbol has incorporated the Prism II into its Spectrum24 11-Mbit/s line (see opening photo).
Originally introduced as a five-chip set, the Prism II takes advantage of higher integration and Intersil's own UHF2 biCMOS process to integrate into one chip the baseband processor and the media-access controller (MAC) (Fig. 1). Dubbed the Prism 2.5, the solution brings the chip count for a complete solution down to four. Available this summer, the Prism 2.5 weighs in with a bill-of-materials (BOM) cost of roughly $50 for a reference design.
Why DSSS?
Both Intersil and Symbol have myriad reasons for focusing on the direct-sequence, spread-spectrum (DSSS)-based standard of 802.11b. But both agree that the high data-rate capability of DS, plus the fact that it's an approved, ratified standard at this point, is a really strong selling point. At this stage, no high-rate standard exists for frequency-hopping spread-spectrum (FHSS), which remains at the original 802.11 specification of 1 to 2 Mbits/s. That goes along with FCC regulations specifying that an FH radio has to hop 79 channels and have an instantaneous bandwidth of 1 MHz.
This has been a thorn in the side of FH developers. The laws of physics and communications theory limits the possible maximum data throughput with encoding to the neighborhood of around 2 Mbits/s. While it's possible to encode more bits per symbol, the energy per bit increases, thereby violating the FCC's output-power limitation. Spread spectrum is limited to an output power of 1 W, though most radios use lower-power outputs, in the 100-mW range.
The DS modulation scheme specifies three, non-overlapping channels, with a 22-MHz bandwidth. This wide bandwidth per channel allows up to 8 bits per symbol, to give an aggregate throughput of up to 11 Mbits/s. But in reality, much like 10Base-T Ethernet, this may only reach 4 to 7 Mbits/s.
DS itself refers to taking a pseudo-random-noise (PN) code, and using it to directly spread the waveform. The 802.11 standard specifies Barker , or 11-bit, codes for 1 and 2 Mbits. For higher data rates, complementary code keying (CCK) is used—six bits in the code, then two more bits by rotating the waveform 90°, using quadrature phase-shift keying (QPSK).
Now that DS has reached data rates comparable to that of Ethernet, MIS departments are taking a closer look at WLANs as a viable augmentation to their networks. According to John Hughes, director of product management for wireless products at Symbol, this raises the interesting question, "Which users will drive the WLAN market—those who buy the LAN for home and bring it to work, or those who use it at work and bring it home?"
The issue revolves around the enterprise buying large amounts of 802.11b high-rate products. The circle will then begin whereby prices will fall and volumes will go up. "So, while there's a hodgepodge in the home now, you still want something easy and standard to implement—that someone in the office knows how to work so you can call them," says Hughes. "I believe 802.11b will win at home. The products are all interoperable."
Interoperability is a major issue with any WLAN, and 802.11b is no different. Because of this, devices developed under 802.11b are submitted to tests at the University of New Hampshire's Interoperability Lab. According to Michael Froning of the wireless lab, the devices are very close to full interoperability. But they tend to fall down in the areas of the Wired Equivalent Privacy (WEP) protocol and power save. WEP refers to how the data frames are encrypted and offers a security option. To date, failures have been restricted to select pairs, with the problem residing in the MAC.
"Another issue," says Froning, "and one that's not defined in the standard, is the handling of roaming, in terms of how an access point (AP) notifies another AP as a cell roams." There isn't a way within the 802.11 standard to define how the AP actually does that, so each vendor has their own solution. As a result, it behooves MIS departments to avoid mixing and matching APs in a network until the issue is resolved. Security and roaming will be enhanced through firmware upgrades.
Reprints
