User's Manual

Extended Range Amplified WLAN System Guide
Version 07/18/02
APPENDIX B: WIRELESS OVERVIEW
IEEE 802.11b Overview
Since the early 1970’s, the basic technology has been in place for LANs to blossom in both the public and
private sectors. Standard LAN protocols, such as Ethernet, operate at relatively high speeds using
inexpensive connection hardware to bring digital networking to almost any computer. Until recently
however, LANs were limited to the physical, hard-wired infrastructure of the building. Even with phone
dial-ups, network nodes were limited to access through wired, landline connections. The major
motivation for and benefit of wireless LANs is increased mobility. Simply stated, the architecture
employed uses fixed network Access Points (APs), which are capable of communicating with mobile
nodes. The network APs are then connected via landlines to widen the LAN's capability by bridging
wireless nodes to other, wired nodes. By overlapping the service areas, handoffs can be made to occur.
This structure is very similar to the present day cellular networks around the world.
The Institute of Electrical and Electronic Engineers (IEEE) is an international body that defines standards
for electrical devices. IEEE 802.11 is the proposed standard for wireless LANs, with provisions for data
rates of either 1 Mbps or 2 Mbps. The standard encompasses Infrared (IR) Pulse Position Modulation,
Frequency Hopping Spread-Spectrum (FHSS), and Direct Sequence Spread-Spectrum (DSSS)
technologies. For the two spread-spectrum technologies, IEEE 802.11 calls for operation in the 2.4 -
2.4835 GHz frequency range – an unlicensed band that the Federal Communications Commission (FCC)
has authorized for industrial, scientific and medical (ISM) applications.
While the FHSS technology is limited to a throughput of 1 - 2 Mbps under the 802.11 standard, the DSSS
technology operates under an enhanced version of the standard (IEEE 802.11b), which enables operation
with a variable throughput capability of 1, 2, 5.5 or 11 Mbps. The result of the much higher bandwidth
afforded by employing IEEE 802.11b DSSS wireless communications is the ability to transfer much more
data than is possible using IEEE 802.11 FHSS or the many other radio frequency (RF) data
communications media operating in the 400, 450, 800, and 900 MHz frequency bands. This greater
bandwidth finally permits the implementation of dynamic, highly mobile, wireless LANs capable of data
throughput and performance characteristics comparable to that found in typical wired networks.
It must be noted, however, that there are significant limitations associated with the IEEE 802.11b
technology. First, the higher operating frequencies result in an inherently shorter communication range
for a given RF power output. Second, as a radio technology, it is primarily a half-duplex device –
meaning that it will not transmit and receive simultaneously. The advertised throughput speeds are raw
data rates. Due to overhead and the half-duplex nature of the device, the effective throughput speeds are
generally less than half of their advertised rates. Finally, as with most low power RF devices, optimal
data throughput speeds will be achieved only when clear line-of-sight (LOS) communications can be
maintained between the devices. The FCC has limited the power output of IEEE 802.11b radio
transmissions to a maximum of 1-watt peak power or not more than +36dB signal strength with any given
omni-directional antenna. Most commercial-off-the-shelf (COTS) IEEE 802.11b equipment operates at
power levels well below the FCC specification. Teletronics International has designed an Extended
Range Amplified WLAN System that takes advantage of the FCC power limits by installing an amplifier
within the system to achieve maximum range and quality of communications between the mobile units