Friday, October 7, 2016

Understanding Types of Wireless Interference

I finally got my CCNA Wireless cert from mail today and below is how it looks like. To further advance my knowledge in the wireless domain, I've been studying for CWNA and hope to get certified before 2016 ends. I'm using CWNA-106 Official Study Guide by Coleman and Westcott and Jerome Henry's CCNA Wireless Live Lessons video course for my studies. I highly recommend Jerome Henry's videos since he tackles a wide range of wireless topics including RF communications, site survey, wireless architecture and implementation, etc. This would be my 5th IT cert if I pass it this year. I might also get CWSP next year and hopefully to get back on my security track and take Palo Alto ACE since it's free.

If two APs are configured to use the same channel and are in close proximity to each other, co-channel interference results. If the APs are set to use adjacent or overlapping channels, adjacent channel interference occurs. While those types of interference can certainly be destructive, they involve devices that are based on the 802.11 standard. For example, each access point (AP) transmits on a specific channel (possibly the wrong one!), uses a standardized channel width, and always transmits frames in the 802.11 format. As well, each AP should also follow the 802.11 rules for clear channel assessment (CCA) to maintain some etiquette to share the airtime.

Rogue APs can be a significant and pesky source of interference because they usually belong to someone else. In other words, someone outside your organization is free to bring up their own APs on channels of their choosing. As long as signals from their APs can be received within your own AP cells, you might have to deal with the interference. To mitigate the problem, you must do one of the following:

* Find the rogue AP and its owner, and then convince the owner to remove the AP or change its channel.

* Move your own AP to a different channel, which may cause other nearby APs to be moved too. When 802.11 devices interfere with each other, the result is usually poor performance due to frame re-transmissions, errors, and the lack of available airtime. In other words, the 802.11 data can still be detected but not always correctly received. In contrast, non-802.11 devices do not have to obey any of the familiar 802.11 rules. When a non-802.11 device transmits, the result is completely incompatible with 802.11. APs and 802.11 clients will view the signal as unintelligible and regard it as noise.

The following sections provide an overview of some common non-802.11 devices that can interfere with a WLAN. As you work through this chapter, keep the following two terms in mind:

* Interference - 802.11 signals that come from sources other than expected APs

* Noise - Signals or radio frequency (RF) energy that reduces the signal-to-noise ratio (SNR) and disrupts transmission or reception of an 802.11 signal.


Bluetooth is a technology used to form a personal-area network (PAN), in an effort to unify telephony and computing devices. Today, Bluetooth can be found integrated into cell phones, tablets, laptops, desktops, printers, headsets, cameras, and video game consoles Bluetooth has low power consumption, making it a good choice for mobile, battery-powered devices. Bluetooth began as Versions 1.0 and 1.0b, developed by the Bluetooth Special Interest Group (SIG).

For a time, several Bluetooth versions were incorporated into the IEEE 802.15.1 standard, but that standard is no longer maintained. The Bluetooth SIG continued to develop its own standard, currently published as core Version 4.

Bluetooth devices are grouped into three classes according to their radiated power. Classes 1 and 2 are the most common and use a maximum transmit power level of 1 mW and 2.5 mW, respectively. Because Bluetooth operates as a PAN, class 1 and 2 devices use a relatively low transmit power level and have a range of only 35 feet. Less common, "industrial" Bluetooth class 3 devices can operate up to 100 mW.

Bluetooth Low Energy (BLE) was designed for the Internet of Things (IoT), allowing small devices such as smartphones and tablets to transmit up to 20 dBm over a range of 100 meters. Apple has implemented BLE as its iBeacon technology, commonly used to interface with shoppers' mobile devices in retail stores. iBeacon can gather location information from devices, detect shoppers' proximity to items, push notifications such as advertisements and coupons to devices, and collect mobile payments.

Up to eight devices can be paired or linked into a PAN, with one device takin a master role and the others operating as slaves. 


ZigBee is wireless LAN technology that is based on relatively low power consumption and low data rates (20 to 250 Kbps). As a result, it offers reliable communication. ZigBee is commonly used for energy management and home and building automatication applications.

ZigBee is defined in the IEEE 802.15.4 standard. It allocates the 2.4-GHz ISM band into 16 channels of 5 MHz each. Even through ZigBee uses the same band as 802.11 devices, it has a low duty cycle and does not utilize a channel much of the time. As well, ZigBee devices normally use a low transmit power level, which minimizes interference, but can ramp up to a maximum of 60 mW when necessary.

Cordless Phones

Cordless pones use several wireless technologies to connect remote handsets to a central base station. Phones that are advertised to use the 2.4- and 5.8-GHz band do just that - and can cause significant interference with nearby WLANs. Cordless phones can use one channel at a time, but can also change channels dynamically. As well, transmit power levels can rise up to 250 mW, overpowering an AP aat maximum power.

The Digital Enhanced Cordless Telecommunication (DECT) standard was developed by the European Telecommunications Standards Institute (ETSI) and uses the upper portion of the 1.8 GHz band in Europe, Asia, Australia, and South America. In the United States, cordless phones are based on DECT 6.0, which uses the 1.9-GHz band.

Because DECT and DECT 6.0 phones do not use the 2.4-GHz ISM band, they should not interfere with 802.11 WLANs. However, some similar "DECT-like" phones may operate in the 2.4- and 5.8-GHz bands and interfere.

Microwave Ovens

You might not think of a microwave oven as a communication device. After all, microwave ovens are designed to cook food - not to transmit data. To heat food and liquids and make popcorn, a microwave oven transmits RF energy into a sealed cavity. The energy is meant to stay inside the oven where it can penetrate food items.

In practice, the RF shielding around microwave oven doors is not ideal, allowing some amount of energy to leak out into the surrounding area. Like many other consumer devices, microwave ovens are free to use the 2.4-GHz ISM band. In fact, most microwave produce a signal that spreads over a large portion of the band. The signal is simply crudely transmitted energy that does not need to follow any standard or frame format. Microwaves are commonly rated to generate around 700 W of power inside the oven. Leaked energy often interferes with nearby APs and 802.11 devices.

To mitigate interference coming from microwave ovens, you can move the oven farther away from WLAN coverage areas. Even better, suggest that the oven be swapped out for a commercial model that has higher-quality RF shielding around the oven and its door.


Worldwide Interoperability for Microwave Access (WiMAX) is a wireless technology to provide "last mile" broadband access to consumers within a geographic area. WiMAX does not require line of sight with a base station, so it can offer connectivity to many fixed and mobile users within a 3 to 10-km radius.

WiMAX is defined by the WiMAX Forum and published as the IEEE 802.16 standard. Although WiMAX, with its central base station and a shared wireless medium, sounds similar to 802.11 WLANs, the two are incompatible. WiMAX operates in several bands between 2 and 11 GHz from 10 to 66 GHz.

Depending on the frequency being used, WiMAX can possibly interfere with 802.11 devices, but such interference is highly unlikely. No widely deployed solutions use the ISM bands; the systems that are advertised for ISM are not supported by any major WiMAX players.

Other Devices

You may encounter other types of non-802.11 devices in and around a WLAN. The following devices can cause varying degrees of interference:

* Canopy - A fixed wireless broadband technology developed by Motorola for Internet service providers; uses the 900-MHz, 2.4-GHz, 5.2-GHz, 5.4-GHz, and 5.7-GHz bands.

* Continuous transmitter - A device that transmits a continous generic waveform that causes steady interference.

* Jammer - A device that is designed to disrupt radio signals so that channels or bands become completely unusable.

* SuperAGS - A proprietary set of extensions developed by Atheros to make Wi-Fi transmissions more efficient. SuperG (802.11g) and SuperAG (both 802.11a and 802.11g) define schemes to compress frames, send bursts of frames, and bond channels for improved throughput. However, they are incompatible with the 802.11 standard and can cause interference.

* Video camera - Wireless security cameras that transmit on the 900-MHz, 2.4-GHz, and 5.8-GHz bands with analog or non-802.11 signals.

* Wi-Fi invalid channel - Wireless devices that use a nonstandard channel or one that is slightly offset from the familiar channel numbers and center frequencies in the 2.4- and 5-GHz bands. These devices are proprietary and can be difficult to detect because they sit on unexpected frequencies. They can overlap normal 802.11 channels and cause interference.

* Wi-Fi inverted - Devices that invert the components of an RF signal from what is normally expected. The inverted signals appear as noise to 802.11 devices. However, two inverted devices can correctly receive and use each other's signal and operate as an undetectable wireless bridge.

* Xbox - a video game console developed by Microsoft. Its wireless controller uses a frequncy-hopping technique that can interfere with 802.11 devices in the 2.4-GHz band.

Tip: You might encounter other technologies such as Wi-Fi Direct and Near Field Communication (NFC) in your environment. Neither one is likely to interfere with 802.11. NFC is used for very close range (10 cm) communication between devices. It uses non-802.11 frequencies. Wi-Fi direct is a means to allow convenient peer-to-peer communication without an AP, while remaining compatible with 802.11 and any nearby APs. Because it is based on 802.11, Wi-Fi Direct does not interfere with Wi-Fi, but it can impact it. Wi-Fi Direct increases the channel utilization; as more devices use it simultaneously, the 802.11 channels may become unusable.


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  2. Thanks for the kind words and I'm glad it became useful to you!