External Sources of Interference

External Sources of Interference
Interference can be categorized as having two sources: external and internal. External sources are not
related to the 802.11 network and are often categorized as garage door openers, some cordless phones,
baby monitors, and so on. Internal sources originate in the 802.11 network. Table 5-1 summarizes the
potential external sources of interference in 802.11 networks and their solutions.
Table 5-1: Potential External Sources of Interference to 802.11 Networks and Their Solutions
Source of
Interference Discounting Factor or Solution
Garage door opener It is on the wrong frequency.
Microwave oven Commercial microwaves may have the power to generate enough
interference to interfere with a WLAN; residential microwaves do not
have the power to generate enough interference to be a factor beyond
the subscriber's premises.
Cordless phone Cordless phones are considered to be a nonissue in the industry. They
have too little power to interfere beyond the immediate residence or
office. If a subscriber's cordless phone is interfering with his or her
service, then the subscriber should replace the 2.4 GHz phone with a
900 MHz cordless phone. Why would a residence with a cell phone and
voice over IP (VoIP) 802.11 service still use a PSTN-connected cordless
phone?
Debunking External Interference Myths Garage door openers are purported to provide interference to
802.11 LANs. This is a myth. Garage door openers operate in the 286 to 390 MHz band; therefore, they
don't interfere with 802.11. Many cordless phones operate (900 MHz) in the 802 to 829 MHz ISM band
and don't interfere with 802.11 either. However, 2.4 GHz cordless phones do operate on the same band
as 802.11 and can cause interference. So how do you deal with interference from other applications of
the 2.4 and 5.8 GHz bands since FCC Part 15 users are granted use on a noninterference basis?
The FCC licenses 802.11 wireless APs to operate under Class B, §15.247 of the FCC regulations in the
2.4 GHz ISM band. The regulations state that any device licensed to operate under Part 15 may not
interfere with or otherwise disrupt the operation of licensed devices coexisting in the same spectrum. In
other words, unlicensed Part 15 devices are the lowest priority. They come after the federal government's
FCC-licensed services; Part 18 devices (ISM transmit-only devices) such as telemetry, radiolocation, and
radio frequency (RF) heating and lighting; and Part 97 Amateur Radio Service. Also, other unlicensed
Part 15 devices under the wrong conditions cause interference, such as 2.4 GHz cordless phones,
Bluetooth applications, microwave ovens, and 2.4 GHz baby monitors.
Engineering Wireless LANs (WLANs) to Minimize External Interference To minimize external sources of
interference, network planners must control the following five parameters:
l The channel/band used
l The distance to the interference (further is better)/the distance to intended signal (closer is better)
l Power levels of interference (lower is better)
l Antenna beam widths
l The protocol used
Changing Channels Sometimes the easiest approach is to change the channel to an unused or lower
congested channel. The specifications for both 802.11a and 802.11 stipulate multiple channels or
frequencies. If interference is being encountered on one frequency, then it is merely a matter of switching
frequencies to a channel that is not being interfered with. 802.11 provides 11 overlapping channels (for
North America), which are 22 MHz wide and centered at 5 MHz intervals (beginning at 2.412 GHz and
ending at 2.462 GHz). This means that only 3 channels do not overlap (channels 1, 6, and 11). Table 5-2
lists the 11 channels of 802.11 and their frequencies.
Table 5-2: The 11 Channels of
802.11
Channel Frequency
(GHz)
1 2.412
2 2.417
3 2.422
4 2.427
5 2.432
6 2.437
7 2.442
8 2.447
9 2.452
10 2.457
11 2.462
802.11a provides 12 channels, which are each 20 MHz wide and centered at 20 MHz intervals (beginning
at 5.180 GHz and ending at 5.320 GHz for the lower and middle Unlicensed National Information
Infrastructure [U-NII] bands, and beginning at 5.745 GHz and ending at 5.805 GHz for the upper U-NII
band). It is important to note that none of these channels overlap.[2]
The 5 GHz U-NII band is far less congested, and Wi-Fi has a greater amount of spectrum in which to
operate. It also permits more channels. The standards bodies are working on protocols that enable
multiple APs to negotiate among themselves automatically for the proper frequency allocation. The
802.11g standard uses orthogonal frequency division multiplexing (OFDM) on 2.4 GHz, which is less
susceptible to interference and provides more channels. However, the operational range of both 802.11g
and 802.11a may be an issue in larger environments.[3]
Once a source of interference has been identified, a common practice among wireless Internet service
providers (WISPs) is to negotiate and decide which broadcasters (the WISPs) will transmit on what
frequency. If such an arrangement cannot be achieved, an WISP can switch to multiple channels in order
to avoid interference.
Dealing with Distance A signal on the same frequency as the 802.11 WLAN, for example, will not cause
interference if the source is too distant. That is, the interfering signal becomes too weak to present
interference. In addition, if the distance between the AP and the subscriber device is greater than optimal,
the signal becomes weak over the distance and becomes susceptible to interference as the interfering
signal is greater than the desired signal.
If 802.11 is used as a last-mile solution providing access to a residence or small business, the potential
sources of interference must be considered. If the sources of interference (cordless phones or microwave
ovens) can be eliminated within the residence or small enterprise, then the second possible source of
interference would come from neighboring residences. The potential for those sources of interference are
limited by the distance to the subscriber's network and the power level of that interference. Household
appliances such as microwave ovens and cordless phones generate too little power to offer interference
beyond the building in which they are located unless the device is defective. For example, the door seal
may need to be replaced. In this case, the defective microwave oven is a hazard in itself.
Engineering with Power The power levels of the primary and interfering signals must also be taken into
account. If the power level of the interfering signal gets close to the power level of the intended 802.11 or
other WLAN signal, then interference will occur. The simplest solution is to increase the power level of
the WLAN signal in order to overcome the interfering signal. The limitation here is that the service
provider must not interfere with licensed spectrum operators on similar (unlikely) spectrum. The other
solution is to reduce the power level of the interfering signal. However, it is important to understand that
increasing the power can cause interference for other users of the band and that FCC regulations set
legal power output limits.[4]
Antenna Beam Widths Another way to eliminate interference is to use antennas to shape where the
transmitter's signal goes and where the receiver listens. A narrow beam width antenna can increase the
effective power toward the receiver and also increase the signal strength of the received signal.
Steerable Antennas Another engineering approach to overcome QoS issues is to use smart antenna
technology. Steering the antenna can do the following for the wireless network: Improve the signal-tonoise
ratio (SNR) with beam forming, reduce interference due to channel reuse, and mitigate intersymbol
interference in multipath environments. Much of this technology falls under the heading of multiple in,
multiple out (MIMO).
San Francisco-based Vivato is now marketing their Wi-Fi, the first of their kind. Wi-Fi switches deliver the
power of network switching with phased-array radio antennas. These Wi-Fi switches use phased-array
radio antennas to create highly directed, narrow beams of Wi-Fi transmissions. The Wi-Fi beams are
created on a packet-by-packet basis. Vivato calls this technology PacketSteering™.
Unlike current WLAN broadcasting, Vivato's switched beam is focused in a controlled pattern and pointed
precisely at the desired client device. These narrow beams of Wi-Fi enable simultaneous Wi-Fi
transmissions to many devices in different directions, thus providing parallel operations to many users-the
essence of Wi-Fi switching. These narrow beams also reduce co-channel interference, since they are
powered only when needed.[5]
Protocol By using 802.11g instead of 802.11, you gain the advantage of OFDM, which is less susceptible
to interference and multipath.
Other Approaches Other approaches can be taken to control interference. The environment can be
controlled so that the interference is limited by controlling the devices used. For example, many airports
are requiring all RF applications to be run through their frequency coordinators. The correct band can be
chosen for each application. A WISP may consider using 802.11a so that its wireless wide area network
(WWAN) does not interfere with a possible WLAN solution.