Reply
New Member
jeremyha
Posts: 28
Registered: ‎10-17-2011
Posts: 28
Kudos: 1
Registered: 10-17-2011

Rule of thumb for HT20 and HT40?

Hello,

I'm wondering about HT-20 and HT-40 modes - I've done some brief internet searches (and searched this forum) but not really found any substantial information about the difference between these modes.

I understand HT20 refers to a 20Mhz spectrum and HT-40 refers to 40Mhz, and that the slimmer spectrum (HT-20) should be used in congensted 2.4Ghz locations..

Is anyone able to provide some 'rules of thumb' for when it would be appropriate to use either mode, and what the implications would be?

Cheers
Jeremy
Ancient Member
WHT
Posts: 28,950
Registered: ‎03-17-2008
Posts: 28950
Kudos: 5147
Solutions: 93
Registered: 03-17-2008

Re: Rule of thumb for HT20 and HT40?


I understand HT20 refers to a 20Mhz spectrum and HT-40 refers to 40Mhz

Not really. It's 20 MHz channel width, not spectrum per se. Though it's often called spectral or spectral emission width.


the slimmer spectrum (HT-20) should be used in congensted 2.4Ghz locations..

The wide the channel width (or a wider slice of spectrum used) allows greater payload throughput. A 40 MHz wide channel allows for 40 Mbps speeds (in one direction or 20 Mbps in both directions under full symmetrical load). Most WISPs don't offer that fast of a subscriber tier, so you don't need that big of chunk of spectrum.

10 MHz is popular because it allows a symmetrical 5 Mbps, which is adequate for most speed tiers, otherwise you can use 20 MHz. Now when it comes to the second chain in a MIMO radio, each 10 MHz chain is aggregated to get you 20 Mbps.

The smaller the channel, the less interference from adjacent channels and a higher signal level output. That is an advantage when lots of other channels are in use. It does not solve same channel congestion.
WHT = Short Form Acronym for "You couldn't handle me even if I came with instructions!"
Well engineered projects are indistinguishable from crazy ideas.

Speed, distance, reliability, cost...Pick three.
...World's First Ubiquiti AirMax WISP....
Member
ZuoBohKing
Posts: 184
Registered: ‎05-07-2013
Posts: 184
Kudos: 7
Registered: 05-07-2013

Re: Rule of thumb for HT20 and HT40?

I found a table from http://mcsindex.com/

 

It seems like HT40 seems to carry larger bandwidth. Then why is there a need for HT20?

 

If I have at least 30 active wireless devices communicating to a single UAP, in order to have maximum bandwidth for each users, what shall be the preferred option? What about 1-5 users environment per AP? What would the AP do if we choose HT20/HT40 option?

 

Come to think of it, the backbone of UAP is only 100Mbps, if 30 users really runs against the single AP concurrently, theoritically each device can only get 3.3Mbps assuming there is no overhead......

 

How can a single UAP achieve 300Mbps per mobile device when the backbond is only 100Mbps?

Member
ZuoBohKing
Posts: 184
Registered: ‎05-07-2013
Posts: 184
Kudos: 7
Registered: 05-07-2013

Re: Rule of thumb for HT20 and HT40?

New Member
jkoopmann
Posts: 15
Registered: ‎10-23-2013
Posts: 15
Registered: 10-23-2013

Re: Rule of thumb for HT20 and HT40?

... only that the answer does not seem to exist any more does it? 

Member
PaulN
Posts: 204
Registered: ‎07-11-2013
Posts: 204
Kudos: 34
Solutions: 5
Registered: 07-11-2013

Re: Rule of thumb for HT20 and HT40?


ZuoBohKing wrote:

It seems like HT40 seems to carry larger bandwidth. Then why is there a need for HT20?

Come to think of it, the backbone of UAP is only 100Mbps, if 30 users really runs against the single AP concurrently, theoritically each device can only get 3.3Mbps assuming there is no overhead......

How can a single UAP achieve 300Mbps per mobile device when the backbond is only 100Mbps?


Not all devices support HT40 and in addition it eats more spectrum which you don't always want, especially when you need to place many AP's to serve a bigger number of clients.

Dividing bandwidth by number of clients is only correct if all clients continue use the bandwidth (like when downloading, streaming video, ...)... but normal internet usage is not like that.  While you read this post, your bandwidth usage is close to 0 bytes/second.

300 Mbps is a combination of up and down, so 150 theoretical... an ancient 54 Mbps AP neither did 54 Mbps but around 30 Mbps at best.

Reply