# 2.4 GHz Channel Reuse and 5 GHz Channels
###### tags: `elearning` `Multimedia Wireless Network`
###### Note Page : [:link:](https://hackmd.io/@YTL0307/HJEb6jWCo)

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## **2.4 GHz Channel Reuse**
- To overcome the limitation of the availability of only three optimum channels, 1, 6, and 11 in a Wi-Fi network, there is a need to reuse channels effectively.
- When you put two access points on the same channel, it will cause those access points (and their clients) to take turns in communicating.
- You must try to avoid this type of interference, which is called "co-channel interference".
- You can avoid such co-channel interference in your network by performing channel planning.
- Channel planning helps you to avoid placing two access points (and thus, two coverage cells) on the same channel within hearing range of each other.
- By alternating between channels 1, 6, and 11, you are effectively reducing your same-channel coverage cells to keep them as far away from each other as possible.
- This helps in effective channel reuse.
- This is a visual representation of all the access points in a building.
- In this scenario, the coverage cells of any channel, such as channel 11 should be placed alternately to avoid co-channel interference during transmission.
- Otherwise, the access points on channel 11 and all the client devices connected to them have to take turns to communicate with one another.
- As the range in the 2.4 gigahertz band is very high, the interference can be minimized, but it is almost impossible to create a perfect channel plan in the 2.4 gigahertz frequency band.

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## **5 GHz Channels**
- The primary difference between the 5 gigahertz and 2.4 gigahertz bands is that the 5 gigahertz band provides more space for channels.
- There are nearly 25 channels in the 5 gigahertz band and each channel is 20 megahertz wide, just like in the 2.4 gigahertz band.
- However, the 5 gigahertz channels do not overlap, and have their own frequency space.
- Therefore, the problem of adjacent channel interference is practically eliminated.
- The 5 gigahertz band is composed of smaller bands, known as the Unlicensed National Information Infrastructure (UNII) bands.
- UNII-1 contains four channels, UNII-2 contains four channels, UNII-2 Extended contains 12 channels, and UNII-3 contains four channels.
- There's also the Industrial, Scientific and Medical (ISM) band at the very top, which has only one channel.
- Each of these bands have their own set of rules associated with them.
- The UNII-2 and the UNII-2 Extended bands consist of the Dynamic Frequency Selection (DFS) channels.
- It is possible to encounter Terminal Doplar Weather Radar (TDWR) on these DFS channels, especially in areas near airports.
- Wi-Fi cannot be operated on these channels without a prior check due to the presence of Doplar radar signals.
- An access point must listen on the DFS channels for any radar, for a certain amount of time, before it can begin operation.
- If any radar signal is detected by an operating access point, it should transmit a Channel Switch Announcement (CSA) on the channel.
- This alerts all clients that there will be a channel switch, and it should switch to one of the non-DFS channels, such as UNII-1, UNI-2, or UNII-3.
- Access points are given standard instructions to refrain from using a set of signatures that belong to the DFS channels.