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Contributor I
Posts: 34
Registered: ‎12-05-2011

What's the use of DFS?

Hi everyone:

I'm reading the DG_HighDensity_VRD1,when refering to the DFS.I'm wondering why there is no DFS in china.

And what's the use of DFS?Is there any situation I should refer to DFS?

Any help or suggestion will be appreciated.

No Pain No Gain~
Moderator
Posts: 53
Registered: ‎04-09-2007

Re: What's the use of DFS?

zAke, 

 

Not sure why their is no DFS in China, but in the US it is required due to radar intereference that an AP tune off a DFS designated channel if it detects radar and stay off that channel for a period of time before it can use it again (if clear of radar for that period of time).  It is required by the FCC that all AP vendors operating in these DFS specified channels that we follow these rules as not to interfere with radar.  Hope that helps.

 

Regards, 

 

Austin

Contributor I
Posts: 34
Registered: ‎12-05-2011

Re: What's the use of DFS?

yeah~ so why I should use this feature in HD wlan? what's the relationship to HD?

No Pain No Gain~
Moderator
Posts: 53
Registered: ‎04-09-2007

Re: What's the use of DFS?

zAke, 

 

More channels = More capacity/performance on a WLAN.  15 of the available 24 (20Mhz) 5Ghz channels in the US are subject to DFS rules.  They are disabled by default on the Aruba controllers because an AP must tune to a non-DFS channel if it detects radar.  If an AP tunes to one of the 9 non-DFS channels when it detects radar AND it then overlaps with some APs in the same area that are already using one of the 9 non-DFS channels, then you have overlap and no extra performance/capacity from that AP.  The problem is exxagerated when you consider 40Mhz bonded channels.  With nonDFS, this will leave 4 non-overlapping bonded channels, if you want more capacity in a single area than that can provide, you can consider the DFS channels.  One must be careful when attempting to use these channels to ensure they either know they will not be impacted by radar and/or know the side effects if you do.

 

Regards, 


Austin

Contributor I
Posts: 34
Registered: ‎12-05-2011

Re: What's the use of DFS?


Allowed Channels for Country Code "HK" Country "Hong Kong"
----------------------------------------------------------
PHY Type Allowed Channels
-------- ----------------
802.11g (indoor) 1 2 3 4 5 6 7 8 9 10 11 12 13
802.11a (indoor) 36 40 44 48 52 56 60 64 100 104 108 112 116 120 124 128 132 136 140 149 153 157 161 165
802.11g (outdoor) 1 2 3 4 5 6 7 8 9 10 11 12 13
802.11a (outdoor) 100 104 108 112 116 120 124 128 132 136 140 149 153 157 161 165
802.11g 40MHz (indoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (indoor) 36-40 44-48 52-56 60-64 100-104 108-112 116-120 124-128 132-136 149-153 157-161
802.11g 40MHz (outdoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (outdoor) 100-104 108-112 116-120 124-128 132-136 149-153 157-161
802.11a (DFS) 52 56 60 64 100 104 108 112 116 120 124 128 132 136 140

 

As it showned,can I comprehend it as this when refer to HD wlan,in 5GHz there are more channels than 2.4GHz.

Without the DFS channels,there are 36 40 44 48 149 153 157 161 165 channels which can be used.

And in HD wlan,cause in 5GHz there some DFS channel.

If i don't want to reuse the channel in HD wlan and 5GHz band, i should use the DFS channel,cause it can support more channels.

So in normal situation,when ap use 5GHz channel,it won't use the DFS channel until I enabled it?

Is that right?

No Pain No Gain~
Moderator
Posts: 53
Registered: ‎04-09-2007

Re: What's the use of DFS?

[ Edited ]

zAke, 

 

In a default configuration, the Aruba contoller will NOT select the DFS channels as "allowed" for ARM to assign to the APs due to the concerns I mentioned, so your last statement is, in general, correct, if you select 5Ghz, the DFS channels will not be used until you enable them.  You can see this by looking at the regulatory domain profile in use on the controller for that region ad checking the allowed channels.

 

A couple other things to note about 2.4Ghz vs. 5Ghz (including 5Ghz DFS channels):

 

- 20Mhz channels you list overlap each other on 2.4Ghz and cannot all be used at the same time in the same area (or should not anyway), typically only 1, 6, and 11 are non-overlapping 20Mhz channels that can be leveraged on 2.4Ghz.  This reduces even further the number of available channels on 2.4Ghz for HD design

- 20Mhz channels you list DO NOT overlap each other for 5Ghz and all can be used for HD design (must consider DFS nuances I mention)

- By default, Aruba will channel bond the 5Ghz channels and NOT the 2.4Ghz channels in an 11n deployment.  This will reduce the number of available channel on non-DFS 5Ghz to 4 instead of 9.

- You will notice in the HD VRD that we make mention about ability to possibly get more performance/capacity in an HD design by disabling channel bonding on 5Ghz and using all 9 x 20Mhz channels in a single area as opposed to 4 x 40Mhz channels.

 

Hope that helps.  

 

Regards, 


Austin

Contributor I
Posts: 34
Registered: ‎12-05-2011

Re: What's the use of DFS?

Excellent~ Thanks for your prompt reply.

No Pain No Gain~
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