Subject: Re: if_media and 802.11
To: None <>
From: David Young <>
List: tech-kern
Date: 02/16/2007 02:43:57
On Thu, Feb 15, 2007 at 05:08:21PM -0800, Sam Leffler wrote:
> The way if_media is currently used for 802.11 does not scale well and
> adding 11n support isn't even possible because there aren't enough free
> bits in places.  Does anything have any feelings about this stuff?  I
> think that splitting the tx rate out will make things work.  Not sure
> how to deal with backwards compatibility.

I believe we must leave the existing NetBSD media words as-is, but that
is probably not too onerous.  I am A-OK with leaving 802.11n rates out
of the media word.

I have been working w/ 802.11n for a client for the past several months,
so I have a lot of ideas.  Here is a strawman proposal for 802.11n
media settings:

Let's add some new 802.11 modes.  Today we have 7 bits to store the
802.11 mode.  Here are today's modes:

#define IFM_IEEE80211_11A       0x00010000      /* 5 GHz, OFDM mode */
#define IFM_IEEE80211_11B       0x00020000      /* Direct Sequence mode */
#define IFM_IEEE80211_11G       0x00030000      /* 2 GHz, CCK mode */
#define IFM_IEEE80211_FH        0x00040000      /* 2 GHz, GFSK mode */

Let's add/rename 802.11 modes like so.  (I am sneakily adding a new
"legacy" mode for "pure G" stations, and giving an old mode a more
suitable name.)

#define IFM_IEEE80211_11A       0x00010000      /* 5 GHz, OFDM mode */
#define IFM_IEEE80211_11B       0x00020000      /* Direct Sequence mode */
#define IFM_IEEE80211_11BG      0x00030000      /* 2 GHz, CCK/OFDM */
#define IFM_IEEE80211_FH        0x00040000      /* 2 GHz, GFSK mode */
#define IFM_IEEE80211_11G       0x00050000      /* 2 GHz, OFDM only */
#define IFM_IEEE80211_11AGN     0x00060000      /* HT plus legacy OFDM */
#define IFM_IEEE80211_11ABGN    0x00070000      /* HT plus legacy CCK/OFDM */

Let's let the determined operator "fix" a legacy rate in all modes above.

Let's add a media word for "pure" N mode.  At a glance, 0x00080000 is
unused, so I use that.  I don't see any use in letting apps set an HT
rate using the media word.  We will provide for that a little later.

#define IFM_IEEE80211_11N       0x00080000      /* Pure N mode */

Legacy modes do not make any sense in "pure" N mode.  Let's just fix
all other parameters at 

We are not quite finished.  For 802.11n, we need to specify some
additional media parameters, such as MCS ID, short/long guard interval,
channel width, and upper/lower channel extension.  I believe short/long
guard interval and eligible MCS IDs belong in one ioctl:

        #define IEEE80211_IOC_HT        TBD

        struct ieee80211_ht_req {
                char                            i_name[IFNAMSIZ];
                uint16_t                        i_type;
                struct ieee80211_ht_params      *i_param;

        struct ieee80211_ht_params {
                uint32_t        p_flags;
#define IEEE80211_HTFLAG_GI_MASK        __BITS(1,0)     /* GI mode */
#define IEEE80211_HTFLAG_GI_AUTO        __SHIFTIN(0, IEEE80211_HTFLAG_GI_MASK)
#define IEEE80211_HTFLAG_GI_LONG        __SHIFTIN(2, IEEE80211_HTFLAG_GI_MASK)
#define IEEE80211_HTFLAG_40             __BIT(2)/* ok to auto-select a 40 MHz
                                                 * channel
                uint8_t         p_mcsset[16];   /* bitmap of eligible MCS IDs */

To set the channel width and extension channel, let's use the upper bits
of the 16-bit channel word we pass to SIOCS80211CHANNEL[1]:

#define IEEE80211_CHAN_40MHZ_UPEXT      0x8000
#define IEEE80211_CHAN_40MHZ_LOEXT      0xC000

I have no doubt left media parameters out, made a bunch of typos, etc.


[1] FWIW, I have used and still like the ifconfig notation 'ch+' and
    'ch-' for upper/lower channel extension.

        ifconfig w0 chan 11-
        ifconfig w0 chan 1+

David Young             OJC Technologies      Urbana, IL * (217) 278-3933