Frequency Channel Saturation
Wireless LAN devices have transmitters and receivers tuned to specific frequencies of radio waves to communicate. A common practice is for frequencies to be allocated as ranges. Such ranges are then split into smaller ranges called channels.
If the demand for a specific channel is too high, that channel is likely to become oversaturated. The saturation of the wireless medium degrades the quality of the communication. Over the years, a number of techniques have been created to improve wireless communication and alleviate saturation. These techniques mitigate channel saturation by using the channels in a more efficient way.
Direct-Sequence Spread Spectrum (DSSS) – This is a modulation technique designed to spread a signal over a larger frequency band. Spread spectrum techniques were developed during war time to make it more difficult for enemies to intercept or jam a communication signal. It does this by spreading the signal over a wider frequency which effectively hides the discernable peak of the signal, as shown in the figure. A properly configured receiver can reverse the DSSS modulation and re-construct the original signal. DSSS is used by 802.11b devices to avoid interference from other devices using the same 2.4 GHz frequency.
Frequency-Hopping Spread Spectrum (FHSS) – This relies on spread spectrum methods to communicate. It transmits radio signals by rapidly switching a carrier signal among many frequency channels. With the FHSS, the sender and receiver must be synchronized to “know” which channel to jump to. This channel hopping process allows for a more efficient usage of the channels, decreasing channel congestion. FHSS was used by the original 802.11 standard. Walkie-talkies and 900 MHz cordless phones also use FHSS, and Bluetooth uses a variation of FHSS.
Orthogonal Frequency-Division Multiplexing (OFDM) – This is a subset of frequency division multiplexing in which a single channel uses multiple sub-channels on adjacent frequencies. Sub-channels in an OFDM system are precisely orthogonal to one another which allow the sub-channels to overlap without interfering. OFDM is used by a number of communication systems including 802.11a/g/n/ac. The new 802.11ax uses a variation of OFDM called Orthogonal frequency-division multiaccess (OFDMA).
A best practice for WLANs requiring multiple APs is to use non-overlapping channels. For example, the 802.11b/g/n standards operate in the 2.4 GHz to 2.5 GHz spectrum. The 2.4 GHz band is subdivided into multiple channels. Each channel is allotted 22 MHz bandwidth and is separated from the next channel by 5 MHz. The 802.11b standard identifies 11 channels for North America, as shown in the figure (13 in Europe and 14 in Japan).
Note: Search the internet for 2.4 GHz channels to learn more about the variations for different countries.
The figure shows 11 channels that are 22MHz wide and 5MHz between each. The spectrum is between 2.2GHz and 2.5GHz.
2.4 GHz Overlapping Channels in North America
Interference occurs when one signal overlaps a channel reserved for another signal, causing possible distortion. The best practice for 2.4 GHz WLANs that require multiple APs is to use non-overlapping channels, although most modern APs will do this automatically. If there are three adjacent APs, use channels 1, 6, and 11, as shown in the figure.
The figure shows three APs using channels 1, 6, and 11.
2.4 GHz Non-Overlapping Channels for 802.11b/g/n
For the 5 GHz standards 802.11a/n/ac, there are 24 channels. The 5 GHz band is divided into three sections. Each channel is separated from the next channel by 20 MHz. The figure shows all 24 Unlicensed National Information Infrastructure (U-NNI) 24 channels for the 5 GHz band. Although there is a slight overlap at the tails of each channel’s frequency, the channels do not interfere with one another. 5 GHz wireless can provide faster data transmission for wireless clients in heavily populated wireless networks because of the large amount of non-overlapping wireless channels.
Note: Search the internet for 5 GHz channels to learn more about the variations for different countries.
The figure shows 8 channels that have 20MHz between each. The spectrum is between 5150 MHz and 5350 MHz.
5 GHz First Eight Non-Interfering Channels
As with 2.4 GHz WLANs, choose non-interfering channels when configuring multiple 5 GHz APs that are adjacent to each other, as shown in the figure.
The figure shows three APs using channels 36, 48, and 60.
the figure shows a map of a venue with different areas, entrances, and exits. There are circles in different areas to limit the area of coverage.
5 GHz Non-Interfering Channels for 802.11a/n/ac
Plan a WLAN Deployment
The number of users supported by a WLAN depends on the geographical layout of the facility, including the number of bodies and devices that can fit in a space, the data rates users expect, the use of non-overlapping channels by multiple APs in an ESS, and transmit power settings.
When planning the location of APs, the approximate circular coverage area is important (as shown in the figure), but there are some additional recommendations:
- If APs are to use existing wiring or if there are locations where APs cannot be placed, note these locations on the map.
- Note all potential sources of interference which can include microwave ovens, wireless video cameras, fluorescent lights, motion detectors, or any other device that uses the 2.4 GHz range.
- Position APs above obstructions.
- Position APs vertically near the ceiling in the center of each coverage area, if possible.
- Position APs in locations where users are expected to be. For example, conference rooms are typically a better location for APs than a hallway.
- If an IEEE 802.11 network has been configured for mixed mode, the wireless clients may experience slower than normal speeds in order to support the older wireless standards.
When estimating the expected coverage area of an AP, realize that this value varies depending on the WLAN standard or mix of standards that are deployed, the nature of the facility, and the transmit power that the AP is configured for. Always consult the specifications for the AP when planning for coverage areas.