What is channel width? How to setup/understand 20MHz or 40MHz?

What WLAN Channels are available?

There are 14 channels designated for wireless networks in the 2.4-GHz frequency band and 42 channels in the 5-GHz frequency band.

The 14 channels in the 2.4-GHz band are spaced 5 MHz apart. The protocol requires 25 MHz of channel separation, meaning that it is possible for adjacent channels to overlap and then interfere with each other. For this reason, only channels 1, 6, 11 are typically used in the US to avoid interference. In the rest of the world, the four channels 1, 5, 9, 13 are typically recommended. The 2.4-GHz frequency band is heavily used because most devices can operate on that band.

The 5-GHz band is actually four frequency bands: 5.1 GHz, 5.3 GHz, 5.4 GHz, and 5.8 GHz. The 5-GHz band has a total of 24 channels with 20- MHz bandwidth available. Unlike the 2.4-GHz band, the channels are non-overlapping, therefore all channels have the potential to be used in a single wireless system. Because only 802.11a devices formerly used this band (occasionally 802.11n uses it also) this band is less crowded and targeted for increased use for new 802.11 technologies under development.

What Channels are not available?

Because each country has different regulatory requirements, the country code determines which channels you can configure on the radios. When you specify the country of operation for an access point, the radios are restricted to using the valid channels for that country.

The FCC (United States) requires that devices operating the 5-GHz band must employ dynamic frequency selection (DFS) and transmit power control (TPC) capabilities. This is to avoid interference with weather-radar and military applications. Additional channels in the 5-GHz band are restricted to avoid interference with Terminal Doppler Weather Radar (TDWR) systems. This eliminates the use of channels 120, 124, and 128. Channels 116 and 132 can be used if they are separated by more than 30 MHz (center-to-center) from a TDWR located within 35 km of the device.

Table 1: Channels a Device Can Use

Device Capability	Band and Channel Width used:	Typical Channel Use
802.11b	2.4-GHz band, 20-MHz channel width	1, 6, 11 (US) or 1, 5, 9, 13
802.11g	2.4-GHz band, 20-MHz channel width	1, 6, 11 (US) or 1, 5, 9, 13
802.11n	2.4-GHz band, 20-MHz channel width 5-GHz band, 40-MHz channel width	1, 6, 11 (US) or 1, 5, 9, 13 (36,1) (40,-1) (44,1) (48,-1) (52, 1) (56,-1) (60,1) (64,-1) (100,1) (104,-1) (108,1) (112,-1) (116,1) (120,-1) (124,1) (128,-1) (132,1) (136,1) (149,1) (153,-1) (157,1) (161,-1)
802.11a	5-GHz band, 40-MHz channel width	3, 11

802.11n Channels Can Be Wider and Work on Both Bands

802.11n devices work on the 5-GHz radio band as well as the more-populated 2.4-GHz radio band. On the 5-GHz band, 802.11n channels can be either 40 MHz or 20 MHz wide as shown in Table 1. This is one reason that 802.11n devices can be faster.

Note: 40 MHz channels work only with 802.11n on the 5-GHz band—40 MHz channels cannot not be configured on a 2.4-GHz radio.

802.11n radios configured for the 5-GHz band have a primary channel and a secondary channel. The primary channel is listed using the channel number, and the secondary channel adds another 20 MHz to make the channel 40 MHz. Therefore, the secondary channel is either the channel above (1) or the channel below (-1) the primary channel. This notation keeps you from configuring non-contiguous channels. For example, if the primary channel is 36 and the secondary channel is 40, the combination would be (36,1). If the primary channel is 44 and the secondary channel is 40, the notation would be (44,-1).

The following channels, listed as (primary channel, secondary channel) are supported for 802.11n at 5 GHz with 40 MHz bandwidth: (36,1) (40,-1) (44,1) (48,-1) (52, 1) (56,-1) (60,1) (64,-1) (100,1) (104,-1) (108,1) (112,-1) (116,1) (120,-1) (124,1) (128,-1) (132,1) (136,1) (149,1) (153,-1) (157,1) (161,-1)

What is WiFi Channel width?

Channel width basically controls how broad the signal is for transferring data. Think of it like a highway. The wider the road, the more traffic (data) can pass through. On the other hand, the more cars (routers) you have on the road, the more congested the traffic becomes.

By increasing the channel width, we can increase the speed and throughput of a wireless broadcast. By default, the 2.4 GHz frequency uses a 20 MHz channel width. A 20MHz channel width is wide enough to span one channel. A 40 MHz channel width bonds two 20 MHz channels together, forming a 40 MHz channel width; therefore, it allows for greater speed and faster transfer rates.

Obviously, two channels are better than one, right? In theory, yes. But not if those channels are crowded with noise and interference. In crowded areas with a lot of frequency noise and interference, a single 20MHz channel will be more stable. 40MHz channel width allows for greater speed and faster transfer rates but it doesn’t perform as well in crowded areas.

However, noise and interference is not always the issue. Sometimes it’s the distance. If greater distance is the primary objective, my preference is the 2.4GHz band.

Case Study:

 802.11a/b/g 

Previous 802.11b/g and 802.11a radios used a transmission channel that was 22 MHz wide. Conventionally, these were referred to as "20 MHz Channels." When the quality of a channel (based on signal-to-noise ratio and interference) was excellent and the received signal strength was high enough a connection rate of 54 Mbps could be achieved. With 802.11a/b/g technology roughly half of the connection rate was available for TCP/IP data transmission. Hence, a maximum throughput of roughly 27 Mbps was possible (although the practical maximum is seen to be closer to 22 Mbps because of typical environmental factors).

The 2.4 GHz 802.11b/g Channel Space

Shown below is a frequency map of the 2.4 GHz ISM (Industrial, Scientific and Medical) band used by 802.11b/g equipment and available for use by 802.11n equipment. There are 14 channels defined (spaced 5 MHz apart) and channels 1 through 11 are available for use in North America. The end result of this arrangement is that a maximum of three channels can be configured for use (channels 1, 6 and 11) without overlapping between adjacent channels. All best-practices 802.11b/g design use only these three channels. As you study the channel layout you should also note that a 40 MHz 802.11n channel would be comprised of eight 802.11b/g channels. Remember that these channels overlap. Their center frequencies are 5 MHz apart but each channel is 22 MHz wide. To get a contiguous 40 MHz wide channel requires that the frequencys allocated to 8 separate channels be sacrificed.

The Capacity of 802.11n 40 MHz Channels

802.11n allows the configuration of 40 MHz wide channels. Because adjacent channels need a slight gap between them (to separate them in the frequency band) a single 40 MHz channel has slightly more than twice the bandwidth of two adjacent 20 MHz channels (because the inter-channel frequency gap is now part of the actual channel space). Hence, a 40 MHz 802.11n channel provides just a little better than twice the throughput capacity of a single 20 MHz 802.11n channel. If an 802.11n transmitter is operating in a 20 MHz channel and can establish a 72.2 Mbps connection then a 40 MHz channel would provide for a 150 Mbps connection. When you double the channel width you double (plus about 4%) the capacity of the resultant double-wide 802.11n channel.

A Fundamental Challenge to the Use of 40 MHz Channels

When a wireless LAN uses 802.11n exclusively it's referred to as an "802.11n Greenfield Mode" implementation. Most wireless LANs will be required to provide support for 802.11b/g clients in the 2.4 GHz ISM band. Some wireless LANS will be required to support 802.11a clients (or 802.11a backhaul point-to-point connectivity) in the 5.8 GHz ISM band. 802.11a/b/g operates using conventional 20 MHz channels. If a device were to transmit in a 20 MHz channel which was half of a double-wide, bonded 40 Mhz channel there would be a problem. In mixed-mode networks (802.11n in the presence of 802.11b/g or 802.11a) the only 40 MHz channels that can be successfully used are those where both of the 20 MHz channels are free of legacy 802.11b/g or 802.11a transmitters. This prerequisite can typically be met without a problem in the 5.8 GHz ISM band (where there are upwards of 12 20 MHz channels to choose from) but is almost impossible to meet in the 2.4 GHz band (where 802.11b/g clients are common). A notable exception to this is the case where a homeowner sets up a residential Wi-Fi network using 802.11n in the 2.4GHz ISM band with a 40 MHz channel setting. There may not be any nearby 802.11b/g devices to cause interference or, at least, to cause only minimal interference.

The Use of 40 MHz Channels in the 2.4GHz Band is Not Reasonable for Commercial Networks

The 2.4 GHz band is often heavily congested, particularly in dormitories, apartments and condominiums, and multi-story office buildings. This alone precludes effective use of 40 Mhz channels. A home user, or a commercial user requiring only a single access point may obtain improved throughput (relative to 802.11g) using a single 40 MHz channel. Interior space larger than 3000 square feet will probably need more than one 802.11n access point to provide maximum bit-rate connectivity to 100% of the area. When adding a second access point to an 802.11n network is considered in the 2.4 GHz band the problem with 2.4GHz 40 MHz channels becomes evident.

Only One 40 Mhz 802.11n Channel Is Available in the 2.4 GHz ISM Band

Considering the channel map shown above, assume that a 40 MHz 802.11n channel were to be created with Channel 1 as the low side. That means that the new channel would span from 2400 MHz to 2440 MHz (the yellow channel rectangle on the left). If another 40 MHz channel were imagined in the band it would have to span from 2440 MHz to 2480 MHz (actually 2483 MHz if you include the required inter-channel gap between the two channels). 2483 MHz is outside the 2.4GHz ISM band. Two 40 MHz channels can not co-exist in the 2.4 GHz band - they would overlap! You can only configure a single 40 MHz channel when you're using 2.4 GHz 802.11n and that's not a reasonable or useable option for most commercial wireless LAN implementations. Only the 5.8 GHz band has enough channels to make 40 MHz commercial implementation feasible.

The 5.8 GHz Channel Space

Channel definitions in the 2.4 GHz band are part of the Industrial, Scientific and Medical (ISM) specifications. These definitions allow for the fact that the 2.4 GHz channels overlap. The channel space defined for unlicensed use in the 5 GHz range are defined under the Unlicensed National Information Infrastructure (U-NII, pronounced "you-nee") radio specifications. U-NII channels are exactly 20 MHz wide but they're spaced 20 MHz apart. There are 20 channels divided into four bands called "U-NII-1", "U-NII-2", "U-NII-2 Extended" and "U-NII-3".

The U-NII power restrictions in the United States are regulated under FCC Part 15 rules. Part 15 Subpart B is best known for its regulations regarding unintentional radiators (devices that produce electromagnetic noise) and Subpart C regulations for small transmitters (like FM microphones and "walkie-talkies"). Subpart E regulates unlicensed U-NII transmitters which include 802.11a and 5 GHz 802.11n access points. U-NII-1 is designated for indoor use and limits transmit power to 40 mW. U-NII-2 can be used for indoor or outdoor applications with a power limit of 200 mW. U-NII-3 is designated for outdoor use with an 800 mW power limit. Each channel shown in the diagram is 20 MHz wide.

802.11n 40 MHz Channels As They Relate to Dynamic Frequency Selection (DFS) and Radar Avoidance

Military radar and some weather radar can operate in the U-NII-2 band. To avoid conflict between unlicensed transmitters (like Wi-Fi access points) operating in the U-NII-2 band the FCC requires [Rule #15.407(h)(2)] that all unlicensed transmitters operating in U-NII-2 implement Dynamic Frequency Selection (DFS). DFS allows the radio to detect the presence of radar signals and to dynamically and automatically change to a different transmit frequency if radar is discovered. Because this requirement demands that a manufacturer of a Wi-Fi access point implement additional, specialized detection mechanisms some manufacturers have simply decided to not provide support for U-NII-2 operation. While almost all major manufacturers do support U-NII-2 it's important to check for U-NII-2 capability when selecting a particular brand of equipment.

Notice that without U-NII-2 support there are only four available channels for indoor use and four for outdoor use. That's a non-issue when 20 MHz channels are being used (i.e. for 802.11a). In mathematics, the "Four Color Theorem" states that if a plane (a floorplan or area map, for example) is divided into contiguous regions (analogous to access point coverage cell areas) then the regions can be colored using at most four colors so that no two adjacent regions have the same color. If we substitute "channel" for "color" then the Four Color Theorem tells us that if you have four channels to work with then a design can be created that avoids detrimental channel overlap (where two or more access points cover an area with the same channel resulting in system degradation).

When considering 40 MHz channels it can be seen that use of the U-NII-2 band is critical. You can't get four (or even three) 40 MHz channels for either indoor or outdoor use unless you take channel space out of the U-NII-2 band. The channel map shown below indicates how

Conclusion

The use of 40 MHz channels with 802.11n gives you just a little bit better than twice the throughput capacity of 20 MHz channel systems. Using a 40 MHz channel in the 2.4 GHz band is suitable only for residential use or single-access point designs and mixed-mode 802.11b/g with 802.11n in the 2.4 GHz band degrades 802.11n performance even more. Nonetheless, 802.11n will always offer greater maximum throughput than 802.11g.

What is wireless spectrum and frequency band and Understanding of channel selection?

What is a Wireless Spectrum?

Most wireless products use the following Wi-Fi spectrums: 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac. These Wi-Fi spectra are established by The Institute of Electrical and Electronics Engineers, a non-profit organization also known as the IEEE. This is why you’ll often see “IEEE 801.11a/b/g/n” on retail boxes for wireless devices, routers, etc.

 

 

What is a WiFi frequency band?

There are basically two frequency bands used for Wi-Fi technology, 2.4GHz and 5GHz. 2.4GHz has been around longer. It’s also an unregulated frequency. As a result, vendors can manufacture 2.4GHz devices less expensively than regulated spectrum like the 5GHz band. The downside to this “unregulated” band is that manufacturers use it for everything: cordless phones, baby monitors, microwaves, garage door openers, etc. 5GHz is a regulated frequency which costs manufacturers more to produce.

The 2.4GHz band offers better range. It handles obstacles better than 5.0GHz. However, 5GHz offers much faster throughput for maximum performance. It just doesn’t handle obstructions and channel noise quite as well. However, keep in mind that because there are more 2.4GHz wireless networks, the frequency channels can become crowded in some areas.

Interference and obstructions greatly affect the range and quality of a wireless signal. Interference often comes from noise created by other wireless routers, cordless phones, baby monitors, etc. Obstructions are physical obstacles like walls and trees or large metal objects such as a refrigerator.

Remember this when troubleshooting because it may not be just one thing causing problems. It could be that there are too many devices using the same frequency. It could also be multiple obstacles, inadequate antenna gain, or a little of everything.

Choosing 2.4 ghz vs 5 ghz Wifi

In short, you should choose 5 ghz for your 802.11 Band for simple reasons. It operates at much faster speed with the latest technology.

2.4 ghz vs 5 ghz Wireless Interference

5 ghz has less signal interference.

The idea is that more devices in your house operate at 2.4 ghz. The wifi networks owned by your neighbors have a higher probability of running on 2.4 ghz so that the wifi signals can interfere with one another. The reasons can be that neighbors still have 2.4 ghz devices that they need to support.

If you choose to use 5 ghz over 2.4 ghz , because of the shorter range on the 5 ghz wifi, your 5 ghz network has less chances of interfering with neighbor’s 5 ghz.

2.4 ghz vs 5 ghz Data Throughput

5 ghz offers faster speed since more data can be transferred via the 5 ghz band. This means you should always pick 5 ghz if your device is close to the router that you are hosting the network with.

2.4 ghz vs 5ghz Support comparison.

2.4 ghz supports more devices (backward compatible).

5 ghz is a more recent technology whereas the older devices only support 2.4 ghz. So if you are still using older devices, your real only option is to let your network support 2.4 ghz. Unless you want to bridge devices using other routers. But then again when you have mixes of 2.4 ghz and 5 ghz, you are likely to bring down the overall performance of the network.