Wireless LANs

Wireless LANs

Wireless LANs provide high-speed data within a small region, e.g. a campus or small building, as users
move from place to place. Wireless devices that access these LANs are typically stationary or moving at
pedestrian speeds. Nearly all wireless LANs in the United States use one of the ISM frequency bands.
The appeal of these frequency bands, located at 900 MHz, 2.4 GHz, and 5.8 GHz, is that an FCC license
is not required to operate in these bands. However, this advantage is a double-edged sword, since many
other systems operate in these bands for the same reason, causing a great deal of interference between
systems. The FCC mitigates this interference problem by setting a limit on the power per unit bandwidth
for ISM-band systems. Wireless LANs can have either a star architecture, with wireless access points or
hubs placed throughout the coverage region, or a peer-to-peer architecture, where the wireless terminals
self-configure into a network.
Dozens of wireless LAN companies and products appeared in the early 1990’s to capitalize on the
“pent-up demand” for high-speed wireless data. These first generation wireless LANs were based on
proprietary and incompatible protocols, although most operated in the 900 MHz ISM band using direct
sequence spread spectrum with data rates on the order of 1-2 Mbps. Both star and peer-to-peer architec-
tures were used. The lack of standardization for these products led to high development costs, low-volume
production, and small markets for each individual product. Of these original products only a handful
were even mildly successful. Only one of the first generation wireless LANs, Motorola’s Altair, operated
outside the 900 MHz ISM band. This system, operating in the licensed 18 GHz band, had data rates on
the order of 6 Mbps. However, performance of Altair was hampered by the high cost of components and
the increased path loss at 18 GHz, and Altair was discontinued within a few years of its release.
The second generation of wireless LANs in the United States operate with 80 MHz of spectrum in
the 2.4 GHz ISM band. A wireless LAN standard for this frequency band, the IEEE 802.11b standard,
was developed to avoid some of the problems with the proprietary first generation systems. The standard
specifies frequency hopped spread spectrum with data rates of around 1.6 Mbps (raw data rates of 11
Mbps) and a range of approximately 500 ft. The network architecture can be either star or peer-to-
peer. Many companies have developed products based on the 802.11b standard, and these products are
constantly evolving to provide higher data rates and better coverage at very low cost. The market for 802.11b wireless LANs is growing, and most computer manufacturers integrate 802.11b wireless LAN
cards directly into their laptops. Many companies and universities have installed 802.11b base stations
throughout their locations, and even local coffee houses are installing these base stations to offer wireless
access to customers. After fairly slow growth initially, 802.11b has experienced much higher growth in
the last few years.
In addition to 802.11b, there are two additional wireless LAN standards that have recently been
deployed in the marketplace. The IEEE 802.11a wireless LAN standard operates in 300 MHz of spectrum
the 5 GHz unlicensed band, which does not have interference from ISM primary users as in the 2.4
GHz band. The 802.11a standard is based on OFDM modulation and provides 20-70 Mbps data rates.
Since 802.11a has much more bandwidth and consequently many more channels than 802.11b, it can
support more users at higher data rates. There was some initial concern that 802.11a systems would be
significantly more expensive than 802.11b systems, but in fact they are becoming quite competitive in
price. The other standard, 802.11g, also uses OFDM and can be used in either the 2.4 GHz and 5 GHz
bands with speeds of up to 54 Mbps. Many new laptops and base stations have wireless LAN cards that
support all three standards to avoid incompatibilities.
In Europe wireless LAN development revolves around the HIPERLAN (high performance radio LAN)
standards. The first HIPERLAN standard, HIPERLAN Type 1, is similar to the IEEE 802.11a wireless
LAN standard and promises data rates of 20 Mbps at a range of 50 meters (150 feet). This system
operates in the 5 GHz band. Its network architecture is peer-to-peer, and the channel access mechanism
uses a variation of ALOHA with prioritization based on the lifetime of packets. The next generation of
HIPERLAN, HIPERLAN Type 2, is still under development, but the goal is to provide data rates on the
order of 54 Mbps with a similar range, and also to support access to cellular, ATM, and IP networks.
HIPERLAN Type 2 is also supposed to include support for Quality-of-Service (QoS), however it is not
yet clear how and to what extent this will be done.