The primary IEEE 802.11 standards in use today are 802.11a and
802.11b,
which both use radio waves for transferring information wirelessly over a network.
Few people realize, however, that the 802.11 standard also includes the 802.11
Infrared (IR) Physical Layer. 802.11 IR defines 1Mbps and 2Mbps operation by
bouncing light off ceilings and walls to provide connectivity within a room
or small office. This infrared version of the standard has been available since
the initial release of the 802.11 standard in 1997.
The reason that 802.11 IR is unheard of is that there are no known vendors
that sell products compliant with 802.11 IR. Some offer infrared-based wireless
LANs that come close to the standard. For example Spectrix, once the chair of the 802.11
IR group, offers wireless LAN products that implement diffused optical technologies
very similar to 802.11 IR.
There have not been any updates to the 802.11 IR standard in order to successfully
compete with the higher performing 802.11a and 802.11b. With only 2Mbps data
rates, most IT organizations opt for radio-based solutions instead. As a result,
802.11 is known primarily as a radio-based technology.
The primary difference between infrared and radio wireless LANs is the frequency
of the transmitted signal. 802.11a and 802.11b operate in the 2.4 and 5GHz bands
respectively, but infrared systems use frequencies in the terahertz range. This
places infrared signals in the light region invisible to the human eye and beyond
the control of the Federal Communications Commission (FCC).
Because of the extremely high frequency, infrared light is highly reflective,
which makes it behave like an incandescent light bulb. Each client station (e.g.,
laptop or PC) is equipped with an infrared transducer that can both transmit
and receive light signals. The transducer diffuses the light, which makes the
signal available from anywhere within a typical room.
Infrared-based wireless LANs pulse a light on and off to represent the transmission
of data bits with a carrier sensing mechanisms that are similar to what the
802.11 Medium Access
Control (MAC) Layer provides. Stations take turns sending light signals
addressed to another station that’s capable of receiving the light.
Attributes Worth Considering
Before assuming that a radio-based solution is best for meeting wireless LAN
requirements, think about the following attributes that infrared wireless
LANs provide:
- Higher security. Infrared light doesn’t go through walls, so it’s
nearly impossible for an unauthorized person from outside the physical controlled
area of the facility to eavesdrop on the system. This also makes denial of
service attacks difficult to stage. - No radio frequency (RF) interference. Because infrared wireless
LANs operate in the light frequency range, there is no possibility of either
inward or outward interference with RF systems. In fact, an infrared wireless
LAN can satisfy specific mobile applications (possibly ones requiring very
high security) in the midst of a radio-based wireless LAN without any consideration
of RF interference. - Limited range. Infrared light doesn’t propagate through walls, which
demands an access point in each room when providing coverage throughout a
facility. This could make the system more expensive than deploying 802.11b
access points that can each cover several rooms. The limitation of range to
a single room, however, enables a higher concentration of access points, which
can provide relatively high capacity despite the somewhat lower data rates
provided by infrared access points. - Proprietary products. One of the biggest issues with infrared wireless
LANs is the lack of conformance with standards. As a result the solutions
are proprietary, meaning that the transducers and access points must be from
the same vendor. This limits the use of infrared wireless LANs to scenarios
where you have total management control over the client devices, which is
not the case in public hotspots (e.g., airports and convention centers) and
larger corporations.
Even though the traditional technology for wireless LANs is radio waves, at
least ponder the idea of using infrared as an alternative. In almost all cases,
the use of radio waves will prevail, but you may have a unique application that
an infrared wireless LAN will satisfy the best.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
and offers workshops
on deploying wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
