HT40 channels
802.11n standard has been ratified on September, 11th 2009 according to the IEEE website : http://standards.ieee.org/announcements/ieee802.11n_2009amendment_ratified.html. It is now time to go into the details. Today we are looking at the HT40 definition.
First of all, 802.11n standard introduced several new concept and we will only look at the HT40 channel definition. In 802.11abg, channels were 20 MHz bandwith at maximum. 802.11-2007 standard, which is the latest freely available standard (at http://standards.ieee.org/getieee802/802.11.html), and which incorporates 802.11a, 802.11b and 802.11g amongst others, defines 20 MHz, 10 MHz and 5 MHz bandwith channels.
A channel is defined by its center frequency and its bandwith. As specified in 17.3.8.3.1 for 5 GHz, the channel numbering is :
- Channel center frequency = Channel starting frequency + 5 × nch (MHz).
A typical value for the channel starting frequency is 5 GHz. Here are few examples for Europe :
- Channel 36 : 5180 MHz
- Channel 100 : 5500 MHz
- Channel 140 : 5700 MHz
As specified in 18.4.6.2 for 2.4 GHz, the channel numbering is defined for all channels in range 1..14 :
- Channel 1 : 2412 MHz
- Channel 2 : 2417 MHz
- …
- Channel 12 : 2467 MHz
- Channel 13 : 2472 MHz
- Channel 14 : 2484 MHz (special case)
Channel numbering for 40MHz is defined in 20.3.15.3 by specifying 2 channels : a primary channel and a secondary channel. For instance : (36,40) or (36,+1), the last digit indicating : “secondary channel is above/below the primary channel”. This information is present in beacons as well in the HT IE and is properly decoded by Wireshark. From now on, it’s clear that “channel 36 ht40+” means : primary channel is 36 and secondary channel is 40. Since channel 32 is not authorized in Europe, we cannot have “channel 36 ht40-“. Let’s examine all channels available in Europe now. Annex J defines behavior limits set for all channels and behavior limits set (defined in Annex I) can be interpreted as follows : 13 means ht40+, 14 means ht40-.
- Channel 1 (2412 MHz) ht40+
- Channel 2 (2417 MHz) ht40+
- Channel 3 (2422 MHz) ht40+
- Channel 4 (2427 MHz) ht40+
- Channel 5 (2432 MHz) ht40+, ht40-
- Channel 6 (2437 MHz) ht40+, ht40-
- Channel 7 (2442 MHz) ht40+, ht40-
- Channel 8 (2447 MHz) ht40-
- Channel 9 (2452 MHz) ht40-
- Channel 10 (2457 MHz) ht40-
- Channel 11 (2462 MHz) ht40-
- Channel 12 (2467 MHz) ?
- Channel 13 (2472 MHz) ?
- Channel 14 (2484 MHz) ?
- Channel 36 (5180 MHz) ht40+
- Channel 40 (5200 MHz) ht40-
- Channel 44 (5220 MHz) ht40+
- Channel 48 (5240 MHz) ht40-
- Channel 52 (5260 MHz) ht40+
- Channel 56 (5280 MHz) ht40-
- Channel 60 (5300 MHz) ht40+
- Channel 64 (5320 MHz) ht40-
- Channel 100 (5500 MHz) ht40+
- Channel 104 (5520 MHz) ht40-
- Channel 108 (5540 MHz) ht40+
- Channel 112 (5560 MHz) ht40-
- Channel 116 (5580 MHz) ht40+
- Channel 120 (5600 MHz) ht40-
- Channel 124 (5620 MHz) ht40+
- Channel 128 (5640 MHz) ht40-
- Channel 132 (5660 MHz) ht40+
- Channel 136 (5680 MHz) ht40-
- Channel 140 (5700 MHz) ?
- Channel 149 (5745 MHz) ht40+
- Channel 153 (5765 MHz) ht40-
- Channel 157 (5785 MHz) ht40+
- Channel 161 (5805 MHz) ht40-