Here are a couple of posts I wrote at a different web site for someone wanting to know which 802.11 channel to select in a multitenant building full of ISM 2.4 Ghz channels. And some Wi-Fi terminology. I do not have the question that was asked so I am only posting both replys as two large sections.
Section One:
It is common to have a lot of 802.11 devices in a building. And it is also possible to minimize interference from these devices with a little cooperation from the neighbors. Channels 1, 6, and 11 are the best channels to use in the 2.4 GHz ISM Band. The 2.4 GHz ISM Band has a total of 14 channels. Only 11 channels can be used in the United States. Each channel is 22 MHz wide for Direct Sequence Spread Spectrum (DSSS), aka 802.11 and High Rate-Direct Sequence Spread Spectrum (HR-DSSS), aka 802.11b.
If you are using Extended Rate Physical – Orthogonal Frequency Division Multiplexing ERP-OFDM, aka 802.11g the channels are 20Mhz wide but it still requires 25Mhz separation between channels to avoid adjacent channel interference.
Each OFDM channel in the ISM 2.5 Ghz Band is separated by 5 MHz. The center of channel 1 is 4.12 GHz, it spans from 4.01 GHz to 4.23 which is 11 MHz on each side. The center of channel 2 is 4.17 GHz because of the 5 MHz separation between channels. In other to avoid co-channel interference each channels needs to be separated by 25 MHz The next channel to use from channel 1 is channel 6. The next channel that is 25 MHz away is channel 11.
If you use a channel any other channel besides 1, 6, and 11 you will cause interference on channels in-between. If channel 9 is used it will cause interference on channels 7-8 and channel 10. Co-channel interference can be seen with a Spectrum Analyzer and a Protocol Analyzer. Two signs of co-channel interference that can be seen with a Protocol Analyzer is seen information such as MAC address information and/or ACK frames from a station that is on a different channel. Another sign is seen a delay in transmissions due to medium contention and retransmissions due to corrupt frames. The corrupt frames are been cuased due to interference from and adjacent channel. The other major cause of interference and retransmissions is from other radios using the same channel, either too close or transmitting with too much power.
In an enterprise environment the channels are arranged in a honey comb pattern. In the case of 1, 6, and 11 they are located in such a way that each channel is separated by one channel. In a multi-floor setup care must be taken so that the same channel will not be used directly adjacent (To the side). In the case of two apartments per floor one person can use channel 1 and the neighbor next door can use channel 11. The person above and below can use the channel that is best accomodating to the neighboor next door. This configuration works best because most Residential Wireless Gateways come with a Dipole antenna. The Dipole antenna radiates the radio waves in a donut shape pattern. Most of the energy is spread out to the sides, if the antenna is vertically polarized as it should be. If you want your signal to go farther a dipole antenna with a higher gain is the best solution. The higher gain will cause the donut shape pattern to flatten out and spread the radiated signal more to the sides than above and below. Remember Half-Wave Dipole antennad has a radiation power of 360 Degrees. So if your neighboor is using the furthest channel possible it will minimize both of your signals from interfering with each others transmission.
If you want to eliminate interference from 2.4 GHz ISM Band devices the best option is to switch to equipment that used the 5.0 GHz UNII Band. The UNII Band has 23 non-overlapping channels. The signal of the 5GHz Band can be heard from a shorter than the 2.4 GHz but that should not be a problem. It depends on materials around the environment. The High Throughput technology aka 802.11n with Multi/Input-Multi/Output technology can make up for the shorter range. Mostly the multi/Input but it depends on what devices are been used so that requires separete discussion dealing with antenna technology and equipment compatability.
Wi-Fi is trade mark. It was created by an Alliance of companies. The Wi-Fi group was created to test for interoperability of 802.11 wireless equipment. There is no such thing as a WiFi signal and there is no such thing as a WiFi system. Equipment is WiFi certified. That means it was tested by the WiFi Allince Group to comply with their standard of interoperability. They use a list of minimum features to certify
Equipment as Wi-Fi compliant.
A security option named so and added to wireless equipment by the Wi-Fi Alliance is WPA2 (Wi-Fi Protected Access 2). WPA2 is based on the IEEE 802.11i RSN (Robust Security Network). RSN only allows only CCMP-AES and 802.1X as methods of authentication and encryption when used with 802.11n Modulation and Coding Scheme (MCS). WPA2 added the option to use TKIP (Temporal Key Integrity Protocol) for authentication and encryption. The IEEE 802.11i Security amendment does not allow the use of TKIP when using 802.11n MCS.
Section Two:
Before I said there is no such thing as a WiFi device. I said that because in an enterprise environment wireless equipment is not described as a WiFi device. The equipment is described by its function. An Access Point is sometimes called a Gateway Portal or Access Point. A Wireless LAN controller is used to be called a Wireless Switch and a wireless hub is called a Wireless Group Bridge.
The term WiFi, to describe wireless equipment is mostly used at the consumer market level. It is used as a marketing tool or to describe a device that uses 802.11 technology to transmit a radio signal using one of the UNII or ISM bands. If you look at the picture on this page
http://en.wikipedia.org/wiki/Wi-Fi you see a wireless Access Point described as a WiFi device. The problem with that description is that someone looking at this picture will believe that all wireless network devices have to be WiFi. The reality is that most wireless equipment in an enterprise environment is proprietary. When you setup a wireless network in an enterprise environment all the equipment has to be from the same manufacturer to assure everything works with everything else. There is no WiFi certification involved most of the time.
I also said there is no such thing as a WiFi signal. I said that because a radio signal is not WiFi certified. A radio signal is part of the electromagnetic spectrum. Specifically the Microwave part of it. Technically you are using a Microwave Radio. The frequencies are gathered into Band and each range of frequency inside a Band is called channel. A channel number is used as an easier way of represent a bandwidth of the carrier signal. A carrier signal is a frequency that has been modulated to carry data.
So for the sake of simplicity and marketing we call 802.11 wireless equipment a WiFi device and we call the microwave channel a WiFi signal. I guess it’s better than calling it a Microware Radio. Once again with are the victims of corporate advertisement.
