802.11 Channels
2.4 G
| Channel | Frequency (MHz) | North America [5] | Japan[5] | Most of world [5][6][7][8][9][10][11] |
|---|---|---|---|---|
| 1 | 2412 | Yes | Yes | Yes |
| 2 | 2417 | Yes | Yes | Yes |
| 3 | 2422 | Yes | Yes | Yes |
| 4 | 2427 | Yes | Yes | Yes |
| 5 | 2432 | Yes | Yes | Yes |
| 6 | 2437 | Yes | Yes | Yes |
| 7 | 2442 | Yes | Yes | Yes |
| 8 | 2447 | Yes | Yes | Yes |
| 9 | 2452 | Yes | Yes | Yes |
| 10 | 2457 | Yes | Yes | Yes |
| 11 | 2462 | Yes | Yes | Yes |
| 12 | 2467 | NoB | Yes | Yes |
| 13 | 2472 | NoB | Yes | Yes |
| 14 | 2484 | No | 11b onlyC | No |
In the 2.4G band, each channel is separated from 5 MHz with a channel bandwidth of 22 MHz for legacy 802.11b and 20 MHz for 802.11g and 802.11n.
There is only 3 non-overlapped legacy channels in 20 MHz wide.
The implementation of 40 MHz channels in 2.4 GHz is a very bad idea (especially in enterprise networks) as this result in interference to adjacent channels.
40 MHz channels are defined with one legacy channel number designated as the primary channel, and the other the secondary channel.
It's not possible to build 2 overlapping networks with 40 MHz.
802.11n includes the ability to use 40 M Hz channels in the 2.4G band, but only with coexistence mechanism to reduce the interference between networks using 20 MHz and 40 MHz channels.
The coexistence and protection mechanisms is provided in 802.11n D7.0:
- Coexistence Conditions
- Legacy 802.11 overlapping BSS (OBSS) on a channel that partially overlaps the 40 MHz channel
- Legacy 802.11 Overlapping BSS on channel that is completely overlapping with 40 MHz primary channel
- Non-802.11 devices
- Coexistence and Protection Mechanisms
- Overlapping BSS scanning The AP (or some of its associated HT STAs) is required to scan all of the channels of the current regulatory domain in order to ascertain the operating channels of any existing 20 MHz BSSs and 20/40 MHz BSSs.
- An AP must perform an Overlapping BSS scan prior to establishing a BSS.
- An AP can not establish a 20/40 MHz BSS if there is an OBSS on a partially overlapping channel.
- Complete overlap of a 20 MHz BSS with the primary channel of the 20/40 MHz BSS is permitted.
- 40 MHz Intolerant bit in 2.4 GHz 40 MHz Intolerant bit allows ANY device to indicate to an AP that it may not operate in 40 MHz anywhere in the 2.4 GHz band. A station may also broadcast the 40 MHz Intolerant bit to overlapping BSS to force them to stop 40 MHz operation.
- Active 20/40 MHz HT stations are required to periodically perform periodic Overlapping BSS scans to determine that no overlapping BSSs exist.
- Scan for beacons, frames with Forty MHz Intolerant field set to 1
- If an overlapping BSS is detected on a partially overlapping channel, the station reports this to the AP. Upon report, the AP must immediately switch the BSS to 20 MHz operation
- Mixed environments of legacy 802.11b or 802.11g require protected transmissions
When in 40M Hz, the specification calls for requiring one primary 20 MHz channel as well as a secondary adjacent channel spaced ±20 MHz away. The primary channel is used for communications with clients incapable of 40 MHz mode. The center frequency is actually the mean of the primary and secondary channels.
40 MHz wide channels are shown by listing the primary(control) channel followed by and indicator of where the secondary(extension) is located and its channel number. For example a 40 MHz channel consuming channels 36 and 40 is referred to 36+40 (secondary above) or 40-36 (secondary below).
5 G
802.11n:
HT Capability
HT Operation
802.11ac: A Survival Guide
An important component of the 802.11ac standard is the way that a BSSID can switch channel bandwidth dynamically on a frame by frame basis.
To help with dividing up airtime between channels, 802.11ac introduces the terminology of primary and secondary (or, more formally, non-primary) channels.
The primary channel is the channel used to transmit something at its native bandwidth.
One of the reasons for the notion of primary and secondary channels is that it helps multiple networks to share the same frequency space. Two 160 MHz networks, they may both transmit 80 MHz frames at the same time if their primary 80 MHz channels are different.
The ability to share wider channels depends on the ability of an 802.11ac device to detect transmissions not only on its primary channel but also on any secondary channels in use.
VHT Capabilities Information Element
VHT Capabilities information element is placed in Probe Request and Probe Response frames to enable client devices to match their capabilities to those offered by a wireless network.
The Element ID for the VHT Capabilities element is 191.
It has a simple structure, consisting of two fields :
- VHT Capabilities Info the protocol features supported by the transmitter.
- Supported VHT MCS and NSS Set the speeds that the transmitter is capable of using.
- Supported Channel Width set (2 bits)
- 00 if there is no 160 MHz support
- 01 if the transmitter supports 160 MHz contiguous operation only
- 10 if it supports both 160 MHz contiguous operation and 80+80 MHz operation
- 11 The value 3 is reserved.
- Supported VHT MCS and Number of Spatial Stream Set
The VHT Operation Information Element
All 802.11 physical layers have an information element (IE) that describes their operation.
The VHT Operation IE describes the channel information and the basic rates supported by the transmitter.
The Element ID for the VHT Capabilities element is 192.
- VHT Operation Information
- Channel Width (1 byte)
- 0 For either 20 MHz or 40 MHz operation\
- 1 80 MHz operation
- 2 160 MHz channel
- 3 80+80 MHz channel
- Channel Center Frequency 0 (1 byte) This fields are used only with 80 and 160 MHz operation. In 80+80 MHz operation, it is the center channel frequency of the lower frequency segment.
- Channel Center Frequency 1 (1 byte) This field is used only with 80+80 MHz operation, and is used to transmit the center channel frequency of the second segment.
- Basic VHT MCS and Spatial Stream Set
For the 80MHz usage,
36/80 (0xe02a)
149/80 (0xe09b)
40/80 (0xe12a)
153/80 (0xe19b)
44/80 (0xe22a)
157/80 (0xe29b)
48/80 (0xe32a)
161/80 (0xe39b)
You only need to configure the primary 20 MHz channels: 36, 40, 44, 48, 149, 153, 157 and 161.
As the center frequency, you can check the information element (element ID=192) which is provided in the Beacon frame.
The 11AC standard described it in "8.4.2.161 VHT Operation element":
The structure of the VHT Operation Information field:
Channel Width :
Set to 1 for 80 MHz operating channel width.
Channel Center Frequency Segment 0:
For 80 MHz or 160 MHz operating channel width, indicates the channel center frequency index for the 80 MHz or 160 MHz channel on which the VHT BSS operates.
For ex., if you configure it to be 153/80.
The value of "Channel Center Frequency Segment 0" could be 155.
802.11ac Channel Planning
The forthcoming 802.11ac Gigabit Wi-Fi amendment will bring with it support for larger channels at 80 MHz and 160 MHz widths. This is one of the primary drivers behind the increased peak performance and bandwidth of wireless APs and clients. Therefore, careful consideration of channel widths allowed on APs and the channel plan for WLAN deployments must be made prior to an enterprise deployment.
Channel Numbering
First, let's tackle how channels are numbered and referenced in 802.11ac. The standard method to denote 5 GHz channels has been to always use the 20 MHz center channel frequencies for both 20 MHz and 40 MHz wide channels. Starting with 802.11n, 40 MHz channels were referenced as the primary 20 MHz channel plus an extension channel either above or below the primary channel. An example would be a 40 MHz channel consisting of channel 36 (primary) + 40 (extension above).
802.11ac changes how we reference larger channel widths. Instead of continuing to reference the 20 MHz extension channel(s), we will now reference the center channel frequency for the entire 20, 40, 80 or 160 MHz wide channel.
The valid channel numbers for various channel widths are:
This results in channel numbers that may look unfamiliar to most WLAN administrators. Simply remember that channel numbers increment by one for every 5 MHz increase in frequency. This will probably be easier to reference through a graphic for most people. In the graphic below, identify the center of each 80 MHz and 160 MHz channel block, follow it up to the 20 MHz IEEE channel numbers, then split the difference between the two 20 MHz channel numbers that it falls between. For example, the 80 MHz channel block in UNII-1 is centered between channels 40 and 44; splitting the difference gives us channel 42.
Non-Overlapping Channels
As I previously detailed in my post on the impact of 802.11ac on enterprise networks, these wide channel widths may not be realistic to use in an enterprise environment where multiple access points are deployed on non-overlapping channels and co-channel interference must be minimized. To recap:
I'm purposely going to skip the 160 MHz wide channels that are possible using 80+80 MHz discontiguous channels for simplicity at this point.
It's clear that 80 MHz channels will be hard to implement in an enterprise setting that requires high capacity due to issues with channel re-use and minimizing co-channel interference. Even when DFS is used, only 4 or 5 non-overlapping channels will be available. And forget about using 160 MHz channels in the enterprise... leave those for home use where only one AP will be deployed (and hopefully you're neighbors don't live too close to cause interference)!
However, it's not quite as dire a situation as that. There is a saving grace that will allow enterprises to take advantage of these wider channels on a "best-effort" basis. Let's step back for a moment - with 802.11n, 40 MHz channels were an all-or-nothing proposition. The APs channel width was statically set at 20 or 40 MHz. On the other hand, 802.11ac allows per-frame channel width and bandwidth signaling. Practically, this means that WLAN administrators can allow the use of wider channels by APs and clients when all of the constituent smaller channels are clear. If a portion of the large channel is busy at the point in time when a frame needs to be transmitted, for instance a neighboring AP or WLAN is actively using a 20 or 40 MHz portion, then the AP or client can simply back down and use the primary 20 or 40 MHz portion of the larger channel that is clear. For the next frame transmission, if the entire 80/160 MHz channel is clear then the AP or client can ramp back up and use the full channel width.
Critical to this dynamic per-frame channel width procedure is the notion of the primary and secondary channels. The WLAN administrator must designate which 20 MHz segment within a 40, 80, or 160 MHz wide channel is the primary 20 MHz channel. This channel forms the core frequency segment that the BSS (basic service set) or AP radio operates on. Based on the channel blocks depicted in the table above, the BSS will then automatically designate the primary 40 MHz and primary 80 MHz channels by extending the primary 20 MHz channel (moving downward through the table). Only the 40 MHz and 80 MHz channels pictured in the table are allowed. For example primary 20 MHz channel 56 can only be expanded into 40 MHz channel 54 (combining channel 56 and 52); combination with channel 60 is not allowed. For easy reference, just use the channels as depicted :)
For example, consider a 160 MHz channel in UNII-1 / UNII-2 where the WLAN admin has selected channel 60 as the primary 20 MHz channel. The primary 40 MHz channel will be 62, and the primary 80 MHz channel will be 58. If any portion of the secondary 80 MHz channel (ch42) is busy then the frame can use the primary 80 MHz channel (ch58). If any portion of the secondary 40 MHz channel (ch54) is busy then the frame can use the primary 40 MHz channel (ch62). And if any portion of the secondary 20 MHz channel (ch64) is busy then the frame can use the primary 20 MHz channel (ch60). This allows an AP or client to dynamically fallback to narrower channel widths in the presence of co-channel interference or noise that only affects a portion of the larger channel.
This has some interesting implications for channel planning!
Developing a Channel Plan
Since eliminating co-channel interference is one of the main objectives in designing a WLAN channel plan, you'll want to carefully consider how you select the operating channel width and primary 20 MHz channels for APs in order to avoid co-channel interference.
First, determine if DFS channels can be used in your environment based on proximity to radar systems and client device support. DFS client support with 802.11a/n has been spotty at best. Hopefully 802.11ac clients will support DFS channels since 5 GHz support is mandated by the amendment and the FCC has eased DFS band adoption by clients since they no longer have to implement radar detection if they passively scan (listen-only) for an AP before transmitting any frames. In essence, they are relying on the AP to perform the radar detection and begin operating on a DFS channel if allowed. But until the time comes when a majority of clients support DFS channels, administrators must verify what channels the client devices on their network support so they don't cause coverage holes by having APs operate on channels that clients can't use.
Second, determine the channel width that you want to attempt to "guarantee" to client devices, which needs to be free of co-channel interference as much as possible. This needs to be based on the density of your AP deployment as well as client device capabilities. Remember - 802.11ac certified clients must support 80 MHz channel width, while 160 MHz channel width is optional. The key is to ensure that every AP has fewer neighboring APs within radio range than non-overlapping channels available.
Third, make a list of the acceptable primary 20 MHz channels that results in non-overlapping channels at the "guaranteed" channel width. This will maximize the likelihood of transmitting at the wider channel width without causing interference with other APs. For example, if attempting to guarantee non-overlapping 80 MHz channels, limit the allowed subset of primary 20 MHz channels to 36, 52, 100, 116, 132, and 149. The specific 20 MHz channels could be different than I have listed in this example, but the key point is that only ONE primary 20 MHz channel be allowed within each "guaranteed" wider channel width.
Finally, channels will be configured by WLAN administrators in two steps:
802.11ac offers exciting prospects for "Gigabit" Wi-Fi. However, most of the benefit of the first wave of products centers around the use of ever-wider channels. Two barriers to the use of these wider channels exist for enterprise WLANs:
From an implementation perspective, most enterprises should plan around non-overlapping 40 MHz channels, or even 20 MHz channels in high-density areas. If the FCC frees up an additional 195 MHz of shared spectrum in late 2014 or early 2015 then designing around non-overlapping 80 MHz channels (or possibly even 160 MHz channels) in the U.S. will become much more practical.
The big appeal of 802.11ac is higher bandwidth, which will be accomplished in first generation 11ac products primarily through the use of wider 80 MHz channels. Channel planning in 802.11ac also involves assigning primary channels to allow for dynamic per-frame channel width adjustments to reduce co-channel interference (CCI). In my last post, I recommended that you select only one primary 20 MHz channel at the channel width you can likely "guarantee" is free from CCI. This means that most enterprises should design around 20 MHz or 40 MHz channels since they provide more non-overlapping channels to reduce CCI.
Why shouldn't you plan around 80 MHz channels? There likely aren't enough non-overlapping channels at 80 MHz to reduce co-channel interference, so you are better off planning around non-overlapping 40 MHz channels.
But many organizations will still want to benefit from the higher peak performance gains that 802.11ac can provide. That's the big appeal, right?! The answer is to take advantage of the per-frame channel width capabilities of 802.11ac to dynamically allow wider channel use when the entire 80 MHz channel is clear (not busy).
Let's demonstrate by using two examples...
Example 1 - Planning around 40 MHz channels
You design your enterprise WLAN around non-overlapping 40 MHz channels because there are a sufficient number of channels for you to safely re-use channels across your environment without creating co-channel interference. You also enable 80 MHz channel width on your WLAN, which will be used on a "best-effort" basis if the entire 80 MHz channel is clear.
Note - One big assumption with this example is that DFS channels are supported by your clients. If not, then you're still best off planning around non-overlapping 20 MHz channels and using both 40 MHz and 80 MHz on a best-effort basis.
You designate primary 20 MHz channels so that it results in non-overlapping 40 MHz channels. If you're in the U.S. you can't use 40 MHz channels 118 and 126 (due to TDWR restrictions), so this results in 10 non-overlapping channels. If you're in the UK/EU you can't use 40 MHz channels 151 and 159 (due to Band C licensing), so this also results in 10 non-overlapping channels.
Remember that administrators only configure the primary 20 MHz channel, and the primary 40 MHz and 80 MHz channels are dynamically assigned by the AP. I provide a deeper explanation in my post on 802.11ac Channel Planning. In this graphic, primary channels at various channel widths are denoted with gray and dotted-gray shading.
You'll also need to consider AP channel assignment based on physical AP locations in order to maximize the likelihood that 80 MHz channel widths can be used without co-channel interference. You can accomplish this by skipping one primary 20 MHz channel when assigning channels to neighboring APs. For example, if AP1 and AP2 are neighbors, assign AP1 primary channel 36 and AP2 primary channel 52, skipping channel 44. In this manner, neighboring APs result with different 80 MHz channels which are less likely to interfere with one another.
Here you can see that the greater number of 40 MHz channels reduces CCI when compared to 80 MHz channels. 80 MHz channel width can still be on a best-effort basis, if enabled, but remember that two adjacent 40 MHz channels will still use the same 80 MHz channel width. In this example, channels 38 and 46 would share the same 80 MHz channel 42. We have staggered them in our RF design to decrease the signal strength between the two and maximize the possibility of 80 MHz use, even though we can't guarantee it.
This happens because when you plan around the larger 80 MHz channel width, the primary channels that are assigned at the smaller channel widths are more likely to result in co-channel interference as well. Therefore, if co-channel interference does occur, the neighboring APs will be unable to back-down to smaller channel widths to avoid the interference.
It would be better to allow them to back-down to non-overlapping 40 MHz channels, breaking apart their collision domains so they can both transmit at the same time and avoid co-channel interference. This is exactly what happens when you plan around smaller channel widths instead!
Final Thoughts
With 802.11ac it may be tempting to use 80 MHz channel widths for peak performance. However, in order to reduce co-channel interference it is recommended that you derive your channel plan using non-overlapping 20 MHz or 40 MHz channels instead, allowing 80 MHz channel width on a best-effort basis. This allows APs to back-down to smaller channel widths that are non-overlapping when 80 MHz CCI is present using per-frame channel width capabilities available with 802.11ac. This allows APs to use the higher peak performance when possible, while maintaining separate collision domains at smaller channel widths when 80 MHz transmissions are not possible.
Modulation and Coding Set (MCS)
MCS number is a value that describes the number of spatial streams, modulation(BPSK, QPSK, 16-QAM, or 64-QAM), and error correction code used for a transmission.
Channel Numbering
First, let's tackle how channels are numbered and referenced in 802.11ac. The standard method to denote 5 GHz channels has been to always use the 20 MHz center channel frequencies for both 20 MHz and 40 MHz wide channels. Starting with 802.11n, 40 MHz channels were referenced as the primary 20 MHz channel plus an extension channel either above or below the primary channel. An example would be a 40 MHz channel consisting of channel 36 (primary) + 40 (extension above).
802.11ac changes how we reference larger channel widths. Instead of continuing to reference the 20 MHz extension channel(s), we will now reference the center channel frequency for the entire 20, 40, 80 or 160 MHz wide channel.
The valid channel numbers for various channel widths are:
| Channel Width | Valid Channel Numbers |
| 20 MHz | 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 144, 149, 153, 161, 165, 169 |
| 40 MHz | 38, 46, 54, 62, 102, 110, 118, 126, 134, 142, 151, 159 |
| 80 MHz | 42, 58, 106, 122, 138, 155 |
| 160 MHz | 50, 114 |
This results in channel numbers that may look unfamiliar to most WLAN administrators. Simply remember that channel numbers increment by one for every 5 MHz increase in frequency. This will probably be easier to reference through a graphic for most people. In the graphic below, identify the center of each 80 MHz and 160 MHz channel block, follow it up to the 20 MHz IEEE channel numbers, then split the difference between the two 20 MHz channel numbers that it falls between. For example, the 80 MHz channel block in UNII-1 is centered between channels 40 and 44; splitting the difference gives us channel 42.
 |
| 5 GHz Channels, with DFS and TDWR Restrictions |
As I previously detailed in my post on the impact of 802.11ac on enterprise networks, these wide channel widths may not be realistic to use in an enterprise environment where multiple access points are deployed on non-overlapping channels and co-channel interference must be minimized. To recap:
- 80 MHz wide channels allow for five (5) non-overlapping channels in the U.S. and five (5) in the UK/EU (channels 149 and higher require light licensing for outdoor use only) when DFS is used, but only two (2) channels in the U.S. and one (1) in UK/EU without DFS.
- 160 MHz wide channels allow for one (1) non-overlapping channel in the U.S. and two (2) in the UK/EU, with DFS being mandatory for their use in all circumstances.
I'm purposely going to skip the 160 MHz wide channels that are possible using 80+80 MHz discontiguous channels for simplicity at this point.
It's clear that 80 MHz channels will be hard to implement in an enterprise setting that requires high capacity due to issues with channel re-use and minimizing co-channel interference. Even when DFS is used, only 4 or 5 non-overlapping channels will be available. And forget about using 160 MHz channels in the enterprise... leave those for home use where only one AP will be deployed (and hopefully you're neighbors don't live too close to cause interference)!
However, it's not quite as dire a situation as that. There is a saving grace that will allow enterprises to take advantage of these wider channels on a "best-effort" basis. Let's step back for a moment - with 802.11n, 40 MHz channels were an all-or-nothing proposition. The APs channel width was statically set at 20 or 40 MHz. On the other hand, 802.11ac allows per-frame channel width and bandwidth signaling. Practically, this means that WLAN administrators can allow the use of wider channels by APs and clients when all of the constituent smaller channels are clear. If a portion of the large channel is busy at the point in time when a frame needs to be transmitted, for instance a neighboring AP or WLAN is actively using a 20 or 40 MHz portion, then the AP or client can simply back down and use the primary 20 or 40 MHz portion of the larger channel that is clear. For the next frame transmission, if the entire 80/160 MHz channel is clear then the AP or client can ramp back up and use the full channel width.
Critical to this dynamic per-frame channel width procedure is the notion of the primary and secondary channels. The WLAN administrator must designate which 20 MHz segment within a 40, 80, or 160 MHz wide channel is the primary 20 MHz channel. This channel forms the core frequency segment that the BSS (basic service set) or AP radio operates on. Based on the channel blocks depicted in the table above, the BSS will then automatically designate the primary 40 MHz and primary 80 MHz channels by extending the primary 20 MHz channel (moving downward through the table). Only the 40 MHz and 80 MHz channels pictured in the table are allowed. For example primary 20 MHz channel 56 can only be expanded into 40 MHz channel 54 (combining channel 56 and 52); combination with channel 60 is not allowed. For easy reference, just use the channels as depicted :)
 |
| 802.11ac Primary and Secondary Channels (Image from 802.11ac: A Survival Guide) |
For example, consider a 160 MHz channel in UNII-1 / UNII-2 where the WLAN admin has selected channel 60 as the primary 20 MHz channel. The primary 40 MHz channel will be 62, and the primary 80 MHz channel will be 58. If any portion of the secondary 80 MHz channel (ch42) is busy then the frame can use the primary 80 MHz channel (ch58). If any portion of the secondary 40 MHz channel (ch54) is busy then the frame can use the primary 40 MHz channel (ch62). And if any portion of the secondary 20 MHz channel (ch64) is busy then the frame can use the primary 20 MHz channel (ch60). This allows an AP or client to dynamically fallback to narrower channel widths in the presence of co-channel interference or noise that only affects a portion of the larger channel.
This has some interesting implications for channel planning!
Developing a Channel Plan
Since eliminating co-channel interference is one of the main objectives in designing a WLAN channel plan, you'll want to carefully consider how you select the operating channel width and primary 20 MHz channels for APs in order to avoid co-channel interference.
First, determine if DFS channels can be used in your environment based on proximity to radar systems and client device support. DFS client support with 802.11a/n has been spotty at best. Hopefully 802.11ac clients will support DFS channels since 5 GHz support is mandated by the amendment and the FCC has eased DFS band adoption by clients since they no longer have to implement radar detection if they passively scan (listen-only) for an AP before transmitting any frames. In essence, they are relying on the AP to perform the radar detection and begin operating on a DFS channel if allowed. But until the time comes when a majority of clients support DFS channels, administrators must verify what channels the client devices on their network support so they don't cause coverage holes by having APs operate on channels that clients can't use.
Second, determine the channel width that you want to attempt to "guarantee" to client devices, which needs to be free of co-channel interference as much as possible. This needs to be based on the density of your AP deployment as well as client device capabilities. Remember - 802.11ac certified clients must support 80 MHz channel width, while 160 MHz channel width is optional. The key is to ensure that every AP has fewer neighboring APs within radio range than non-overlapping channels available.
- In high-density areas, this should be 20 MHz (stadiums, large event centers, urban areas with many neighboring WLANs, etc.).
- In normal-density areas, this is likely to be 40 MHz channels (large building office space).
- In low-density areas this could be 80 MHz channels (small buildings, few neighbors, etc.).
- In single-AP areas, this could be 160 MHz channels (such as homes or very small offices).
Third, make a list of the acceptable primary 20 MHz channels that results in non-overlapping channels at the "guaranteed" channel width. This will maximize the likelihood of transmitting at the wider channel width without causing interference with other APs. For example, if attempting to guarantee non-overlapping 80 MHz channels, limit the allowed subset of primary 20 MHz channels to 36, 52, 100, 116, 132, and 149. The specific 20 MHz channels could be different than I have listed in this example, but the key point is that only ONE primary 20 MHz channel be allowed within each "guaranteed" wider channel width.
Finally, channels will be configured by WLAN administrators in two steps:
- Select a channel width (20, 40, 80, 160 MHz) for AP or WLAN operation
This should be the "guaranteed" channel width, at minimum. It could be larger than the "guaranteed" channel width if you want to allow APs and clients to achieve higher peak performance when the network is fairly idle. Since this can be done on a per-frame basis and clear-channel assessment is performed prior to transmission, allowing dynamic use of wider bandwidth shouldn't result in significantly more collisions. But it will result in more co-channel interference (sharing of bandwidth) between neighboring APs. The exact impact of per-frame channel width and co-channel interference in a multi-AP environment will require more in-depth testing once 802.11ac equipment is released. I would stick with using the "guaranteed" channel width until you are able to test in your own environment. - Assign the primary 20 MHz channel to each AP (or allow auto-assignment)
The primary 40 MHz and 80 MHz channels will be determined automatically based on the primary 20 MHz channel selected. If "Auto" channel planning is used, which is common in enterprise WLAN equipment, ensure the subset of primary 20 MHz channels allowed to be assigned to APs is limited to those in your list.
802.11ac offers exciting prospects for "Gigabit" Wi-Fi. However, most of the benefit of the first wave of products centers around the use of ever-wider channels. Two barriers to the use of these wider channels exist for enterprise WLANs:
- Limited spectrum, resulting in insufficient channels to facilitate a re-use plan that effectively allows wider channels without excessive co-channel interference.
- Greater reliance on DFS channels to provide more spectrum and channels, which many Wi-Fi clients do not support today.
From an implementation perspective, most enterprises should plan around non-overlapping 40 MHz channels, or even 20 MHz channels in high-density areas. If the FCC frees up an additional 195 MHz of shared spectrum in late 2014 or early 2015 then designing around non-overlapping 80 MHz channels (or possibly even 160 MHz channels) in the U.S. will become much more practical.
Safely Using 80 MHz Channels with 802.11ac
http://revolutionwifi.blogspot.tw/2013/03/safely-using-80-mhz-channels-with.htmlThe big appeal of 802.11ac is higher bandwidth, which will be accomplished in first generation 11ac products primarily through the use of wider 80 MHz channels. Channel planning in 802.11ac also involves assigning primary channels to allow for dynamic per-frame channel width adjustments to reduce co-channel interference (CCI). In my last post, I recommended that you select only one primary 20 MHz channel at the channel width you can likely "guarantee" is free from CCI. This means that most enterprises should design around 20 MHz or 40 MHz channels since they provide more non-overlapping channels to reduce CCI.
Why shouldn't you plan around 80 MHz channels? There likely aren't enough non-overlapping channels at 80 MHz to reduce co-channel interference, so you are better off planning around non-overlapping 40 MHz channels.
But many organizations will still want to benefit from the higher peak performance gains that 802.11ac can provide. That's the big appeal, right?! The answer is to take advantage of the per-frame channel width capabilities of 802.11ac to dynamically allow wider channel use when the entire 80 MHz channel is clear (not busy).
Let's demonstrate by using two examples...
Example 1 - Planning around 40 MHz channels
You design your enterprise WLAN around non-overlapping 40 MHz channels because there are a sufficient number of channels for you to safely re-use channels across your environment without creating co-channel interference. You also enable 80 MHz channel width on your WLAN, which will be used on a "best-effort" basis if the entire 80 MHz channel is clear.
Note - One big assumption with this example is that DFS channels are supported by your clients. If not, then you're still best off planning around non-overlapping 20 MHz channels and using both 40 MHz and 80 MHz on a best-effort basis.
You designate primary 20 MHz channels so that it results in non-overlapping 40 MHz channels. If you're in the U.S. you can't use 40 MHz channels 118 and 126 (due to TDWR restrictions), so this results in 10 non-overlapping channels. If you're in the UK/EU you can't use 40 MHz channels 151 and 159 (due to Band C licensing), so this also results in 10 non-overlapping channels.
 |
| 802.11ac Non-Overlapping 40 MHz Channels |
You'll also need to consider AP channel assignment based on physical AP locations in order to maximize the likelihood that 80 MHz channel widths can be used without co-channel interference. You can accomplish this by skipping one primary 20 MHz channel when assigning channels to neighboring APs. For example, if AP1 and AP2 are neighbors, assign AP1 primary channel 36 and AP2 primary channel 52, skipping channel 44. In this manner, neighboring APs result with different 80 MHz channels which are less likely to interfere with one another.
Here you can see that the greater number of 40 MHz channels reduces CCI when compared to 80 MHz channels. 80 MHz channel width can still be on a best-effort basis, if enabled, but remember that two adjacent 40 MHz channels will still use the same 80 MHz channel width. In this example, channels 38 and 46 would share the same 80 MHz channel 42. We have staggered them in our RF design to decrease the signal strength between the two and maximize the possibility of 80 MHz use, even though we can't guarantee it.
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| 40 MHz Co-Channel Interference is Less LikelyIn this manner, we have enabled 80 MHz channel use, but have assured ourselves that we can safely fallback to 40 MHz channel width on a per-frame basis if the larger channel width is busy. This allows us to take advantage of the higher performance that 802.11ac wide channels offer without creating large collision domains and high levels of co-channel interference. You design your enterprise WLAN around non-overlapping 80 MHz channels, even though there is greater AP density than non-overlapping channels. You've decided to take a gamble and see if you can get the higher performance that wider channels bring all the time, at the risk of creating more co-channel interference.
However, you have a fairly dense AP deployment, resulting in some co-channel interference between APs. Let's say two APs, both using 80 MHz channel 42 can hear one another and sense that the air is busy. Using the per-frame channel width capabilities of 802.11ac, they attempt to back-down to smaller channel widths. However, there is a problem... since you've designed your primary channels based on an 80 MHz channel width, both APs attempt to back-down to the same primary 40 MHz channel (ch38) and primary 20 MHz channel (ch36). They can't avoid the co-channel interference! This results in both APs sharing airtime and reducing network performance and capacity.
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This happens because when you plan around the larger 80 MHz channel width, the primary channels that are assigned at the smaller channel widths are more likely to result in co-channel interference as well. Therefore, if co-channel interference does occur, the neighboring APs will be unable to back-down to smaller channel widths to avoid the interference.
It would be better to allow them to back-down to non-overlapping 40 MHz channels, breaking apart their collision domains so they can both transmit at the same time and avoid co-channel interference. This is exactly what happens when you plan around smaller channel widths instead!
Final Thoughts
With 802.11ac it may be tempting to use 80 MHz channel widths for peak performance. However, in order to reduce co-channel interference it is recommended that you derive your channel plan using non-overlapping 20 MHz or 40 MHz channels instead, allowing 80 MHz channel width on a best-effort basis. This allows APs to back-down to smaller channel widths that are non-overlapping when 80 MHz CCI is present using per-frame channel width capabilities available with 802.11ac. This allows APs to use the higher peak performance when possible, while maintaining separate collision domains at smaller channel widths when 80 MHz transmissions are not possible.
Modulation and Coding Set (MCS)
MCS number is a value that describes the number of spatial streams, modulation(BPSK, QPSK, 16-QAM, or 64-QAM), and error correction code used for a transmission.
http://mcsindex.com/
MCS : Index
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