Wireless Channel Management In WLAN

Wireless Channel Management In WLAN

this is a brief article on Wireless Channel Management In WLAN. Wireless LAN devices have transmitters and receivers tuned to specific frequencies of radio waves to communicate.
It is usual to assign frequencies as ranges. These ranges are then divided into smaller ranges called "channels."
This article is series of following previous article you can go through these for better understanding:

• Wireless 802.11 Frame Structure 
• CSMA / CA & Wireless Network 

Wireless Frequency Channel Saturation

If the demand for a specific channel is too high, it is likely that that channel will be oversaturated . The saturation of a wireless medium deteriorates the quality of communication. Over the years, a number of techniques were created to improve wireless communication and relieve saturation. The techniques listed below mitigate channel saturation by using channels more efficiently:

Direct sequence spread spectrum (DSSS)

This is a spread spectrum modulation technique. The spread spectrum is designed to spread the signal across a wider frequency band, which makes it more resistant to interference.

With DSSS, the signal is multiplied by a "manufactured noise" known as " extension code ". Because the receiver knows about the extension code and when it was added, you can remove it mathematically and reconstruct the original signal.
In effect, this creates redundancy in the transmitted signal in an effort to counteract the loss of quality in the wireless medium. The 802.11b standard uses DSSS. Cordless phones that operate in the 900 MHz, 2.4 GHz and 5.8 GHz bands, mobile phone networks with CDMA and GPS networks also use it. (Image 13).

Frequency jump spread spectrum (FHSS)

This also depends on the spread spectrum methods to communicate.
It is similar to DSSS, but transmits radio signals by quickly switching a carrier signal between many frequency channels.

With FHSS, the sender and receiver must be synchronized to "know" which channel to skip. This process of skipping channels allows more efficient use of the channels, which decreases the congestion of the channel. Walkie-talkies and 900 MHz cordless phones also use FHSS, and Bluetooth uses a variant of FHSS. The original 802.11 standard also uses FHSS. (Image 14).

Orthogonal Frequency Division Multiplexing (OFDM)

It is a subset of frequency division multiplexing in which a single channel uses several subchannels at adjacent frequencies.
The subchannels in an OFDM system are precisely orthogonal to each other, which allows the subchannels to overlap without interfering. As a result, OFDM systems can maximize spectral efficiency without causing interference in adjacent channels.

In effect, this makes it easier for the receiving station to "hear" the signal. Because OFDM uses subchannels, the use of channels is very effective. A number of communication systems, including 802.11a / g / n / ac standards, use OFDM. (Image 15).

Wireless Channel Selection

The IEEE 802.11b / g / n standards operate at the microwave frequencies of the radio spectrum. The IEEE 802.11b / g / n standards operate in the 2.4 GHZ at 2.5 GHz spectrum, while the 802.11a / n / ac standards operate in the 5 GHz band, which is regulated to a greater extent.

In above figure, it is highlighted which 802.11 standard operates in the 2.4 GHz, 5 GHz and 60 GHz bands. Each spectrum is subdivided into channels with a central frequency and bandwidth, similar to the way in which subdivide the radio bands.
The 2.4 GHz band is subdivided into several channels. The combined overall bandwidth is 22 MHz, and each channel is separated by 5 MHz. The 802.11b standard identifies 11 channels for North America .

• The 22 MHz bandwidth, in combination with the 5 MHz separation between the frequencies, produces an overlap between the successive channels, as shown in above figure.

• The interference occurs when an unwanted signal is superimposed on a reserved channel for a desired signal, causing possible distortion. The solution to interference is to use channels that do not overlap. Specifically, channels 1, 6 and 11 are non-overlapping 802.11b channels, as shown in following image.

For WLANs that require multiple APs, it is recommended to use non-overlapping channels. If there are three adjacent APs, use channels 1, 6 and 11. If there are only two, select those two that are separated by five channels, such as channels 5 and 10.
As enterprise WLANs migrate to 802.11n, they can use channels in a larger and less populated 5 GHz band, which reduces accidental "denial of service (DoS)." For example, the 802.11n standard uses OFDM and can support four non-overlapping channels.
The 802.11n standard can also use channel linking, which combines two 20 MHz channels into 40 MHz channels. Channel linking increases performance, since it uses two channels at the same time to deliver data.
The most modern APs can automatically adjust the channels to avoid interference.

Planning a WLAN implementation

Implementing a WLAN that makes the best use of resources and delivers the best service may require careful planning.

WLANs can range from relatively simple installations to intricate and very complex designs. Before a wireless network can be implemented, a well-documented plan must exist.

• The number of users that a WLAN can support is not a simple calculation. The number of users depends on the geographical layout of the installation, including the number of people and devices that can fit in a space, the data speeds that users expect, the use of non-overlapping channels by several APs in an ESS and transmission power settings.

You can review floor plan in above figure. When planning the location of the APs, the administrator cannot simply draw circles from the coverage area and throw them on a plane. The approximate circular coverage area is important, but there are some additional recommendations:

• If APs must use existing wiring or if there are locations where APs cannot be placed, mark these locations on the map.
• Position the APs above the obstructions.
• If possible, position the APs vertically, near the ceiling, in the center of each coverage area.
• Place APs in locations where users are expected to be. For example, conference rooms are, in general, a better location for APs than a hallway.

Wireless Coverage area

Once these points are addressed, calculate the expected coverage area of ​​an AP . This value varies according to the WLAN standard or the combination of standards that are implemented, the nature of the installation, the transmission power that is configured in the AP, and so on. Always check the specifications for the AP when planning coverage areas.
BSAs represent the coverage area provided by a single channel. In an ESS, there should be a 10% to 15% overlap between BSAs. With a 15% overlap between BSAs, an SSID and non-overlapping channels (that is, one cell in channel 1 and the other in channel 6), mobile capacity can be created.

In above figure, an example of how BSAs could be superimposed is provided.

Other factors include site surveys, which are detailed analyzes of where to locate the different PAs.