802.11n Primary and Secondary Channels

802.11n Primary and Secondary Channels

By CWNP On 08/17/2007 - 55 Comments

The 802.11n draft gives us 20 and 40 MHz wide channels.  This is no secret and has been widely publicized for months.  What hasn't been publicized is how 40 MHz channels work.  The draft explains the concepts of "primary" and "secondary" channels - each 20 MHz wide using OFDM modulation.  A Secondary Channel is defined as a 20 MHz channel associated with a primary channel used by HT stations for the purpose of creating a 40 MHz channel.


The Secondary Channel Offset element is used by an AP in a BSS or a STA in an IBSS as part of the Channel Switch Announcement element when changing to a new 40 MHz channel.  This offset element is constructed as follows:

Element ID (1)
Length (1)
Secondary Channel Offset (1)

...where the numbers are length in octes.  The Secondary Channel Offset element may also be included in Beacons and Probe Responses.  The Secondary Channel Offset element is present when switching to a 40 MHz channel.  It may be present when switching to a 20 MHz channel (in which case the secondary channel offset is set to 0). 

The Secondary Channel Offset field represents the position of the secondary channel relative to the primary channel.  The Secondary Channel Offset field is set to the value 1 to indicate that the secondary channel is above the primary channel; the value 3 indicates that the secondary channel is below the primary channel; the value 0 indicates that no secondary channel is present, and the value 2 is reserved.  An HT AP that indicated a value of 0 in its most recently transmitted Secondary Channel Offset field (in a beacon) is not allowed to transmit a 40 MHz mask PPDU.  In the case where the station is moving to a 40 MHz channel (i.e., a pair of 20 MHz channels), the channel number refers to the primary channel.

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08/24/2007 at 14:36pm
The HT Capabilities Element allows the 11n device to advertise its' support for 20 or 20/40 MHz reception.

The HT Information Element is used in Beacon, Probe Responses and Association Responses to advertise the parameters of the BSS. It advertises the primary (aka control) channel and the secondary (aka extension) channel offset as defined in the article above.

The 'Notify Channel Width' HT action frame is used to indicate a change (either AP or STA) of it's ability to receive 20 or 20/40MHz transmissions according to the capabilities of the HT Capabilities Element.

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