Re: 802.11 + radio headers question...

[Guy Harris <guy () alum mit edu>]

On Apr 12, 2009, at 12:06 AM, Eddie Harari wrote:

>   802.11 headers there is data field, what it this data field ?

According to IEEE Std 802.11-2007, section 7.1.2 "General frame  
format", an 802.11 frame has:

        a 2-byte frame control field;

        a 2-byte duration/ID field;

        up to 4 6-byte MAC address fields;

        a 2-byte sequence control field;

        an optional 2-byte QoS field;

        a frame body;

        a 4-byte FCS field.

I don't see any field in the 802.11 headers called "data".  Are you  
referring to the frame body?

>   does it begin with IP headers or something preceds the IP headers ?

For data frames, the frame body begins with an IEEE 802.2 header,  
possibly followed by a SNAP header, followed by the payload for the  
protocol being carried over 802.11, such as IP.

>   in ethernet  environment there is a protocol field in the headers  
> that
> tells which protocol is in the body of the packet
>   (ip / arp / rarp etc...) ,

Ethernet frames can either contain a protocol type field or a length  
field; if it's a length field, the header is either followed by an  
802.2 header possibly followed by a SNAP header or by a raw Novell IPX  
packet.

> is there something which is the same on 802.11
> headers ?

There is no type field in an 802.11 header similar to the Ethernet  
type field.  For protocols that have an Ethernet type value, for  
802.11 (and Token Ring and FDDI and some other protocols), the 802.2  
header following the 802.11 header will have source and destination  
SAP values of hex AA, meaning that there's a SNAP header following the  
802.2 header, and the SNAP header will have an OUI value of hex 000000  
and a protocol ID value that is an Ethernet type (an OUI of 000000 in  
a SNAP header means that the protocol ID value in the SNAP header is  
an Ethernet type).

See:

        IEEE Std 802.11-2007:

                http://standards.ieee.org/getieee802/download/802.11-2007.pdf

        ANSI/IEEE Std 802.2, 1998 Edition:

                http://standards.ieee.org/getieee802/download/802.2-1998.pdf

        IEEE Std 802-2001, section 10 (for SNAP):

                http://standards.ieee.org/getieee802/download/802-2001.pdf

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