April 12, 2001, 1:40 PM — ISPs and private companies are rapidly installing IEEE
802.11-based wireless point-to-multipoint networks to provide fixed wireless
connectivity to businesses and homes in areas that are not served well by DSL,
ISDN, or cable modems. An issue, however, is that wireless ISPs and private
wireless metropolitan solutions operating in the same area often interfere with
each other. This degrades throughput and causes occasional disruptions in service,
which aggravates wireless ISP owners and their customers.
What's the problem?
The primary cause of RF interference between wireless MANs is that most implementations
utilize 802.11 products operating in the same 2.4 GHz frequency band. Because
the FCC designates the 2.4 GHz band as license-free (at least in the United
States), competing users of the technology are not required to coordinate transmissions.
For example, an ISP using frequency hopping spread spectrum (FHSS)
can operate with any of the 802.11-defined hopping sequences, and ISPs using
direct sequence spread spectrum (DSSS)
can select any of the authorized DSSS channels. As a result, the lack of coordination
among competitors leads to simultaneous use of the frequency band and damaging
DSSS is the basis of the popular
802.11b standard, and most wireless ISPs install systems with 802.11b-compliant
products. DSSS doesn't operate well, however, in the presence of FHSS implementations.
A FHSS system transmits signals uniformly over the entire 2.4 GHz band (83.5
MHz) by rapidly hopping over different frequencies. On the other hand, DSSS
continuously transmits over approximately one-third of the band (22 MHz). This
means that DSSS will overlap with roughly one-third of an FHSS transmission,
but FHSS overlaps with the entire DSSS signal. This causes an FHSS system to
impart greater damage in terms of RF interference to one based on DSSS. If a
competitor starts blasting signals from a nearby FHSS network, then a DSSS (i.e.,
802.11b) system will generally fall to its knees.
How can wireless ISPs live happily together? In some areas, such as Anguilla
in the West Indies, companies can obtain special licenses from local governments
to have exclusive use of the 2.4 GHz band. This is rare in other areas such
as the United States and Europe, however, and most wireless ISPs must be prepared
to deal with possible consequences of the uncontrolled frequency space.