On Saturday 30 June 2007 09:34:39 am Pedro Almeida wrote:
> Hi, Erwing;
>
> I've taken a good look at the data sheet. I've looked at it before but I
> totally forgot the details from seeing just the TEP and the header files.
>
> according to the sheet, total PPDU frame is:
>
> PPDU = SHR (5 bytes) + PHR (length: 1 byte) + MPDU
>
> now the MPDU starts with the rest of the struct (apart from the length):
>
> typedef nx_struct cc2420_header_t {
> nxle_uint8_t length;
> nxle_uint16_t fcf;
> nxle_uint8_t dsn;
> nxle_uint16_t destpan;
> nxle_uint16_t dest;
> nxle_uint16_t src;
> nxle_uint8_t type;
> } cc2420_header_t;
>
> MPDU = FCF + DSN + DestPAN + DestAdd + SRC + FCS = 11 bytes
>
> Total so far : 11 + 6 = 17
>
> The "type" I don't know where it belongs, maybe the start of the data
> payload? It does say in here:
>
> The TinyOS 802.15.4 T-frame format is as follows:
> +-------------------+---------+------------------------------+-------------
>-+
>
> | 802.15.4 Header | AM type | data | 802.15.4 CRC |
>
> +-------------------+---------+------------------------------+-------------
>-+
>
> so should be "somewhere" between the Header and the data.
>
> Anyway, the "data" field i still dont know if its size is the exact size of
> the struct i'm using, if there's some more bytes added or if it is always
> defined by the MAX setting, regardless of being filled or not.
>
> Finally, the CRC in the scheme above, i dont know if its the same as the
> FCS (as the FCS is only from the MPDU) or if after the FCS, 2 CRC bytes
> actually are inserted (extending actual size to another 2 bytes).
You're close. It's a bit confusing because the 802.15.4 spec breaks up the
radio frame into 3 layers, and in t2 the length field in cc2420_header_t is
not part of the MPDU but is in fact the PHR. Here's how I understand t2
active messages on 802.15.4:
The physical protocol data unit (PPDU), or the actual bytes that make up the
full radio frame are:
PPDU = SHR(4B preamble + 1B SFD) + PHR (1B len) + PSDU
The phy service data unit (PSDU) contains the mac protocol data unit (MPDU),
so they are synonmous:
PSDU = MPDU = MHR + MSDU + FCS(2B)
The mac header (MHR) is variable based upon bits set in the FCF. For the t2
stack it is 8 bytes long:
MHR = FCF(2B) + DSN(1B) + PANID(2B) + DST(2B) + SRC(2B)
(note that PANID is populated by TOS_AM_GROUP)
The frame check sequence (FCS) contains the CRC and is 2 bytes long.
The mac service data unit (MSDU) contains the data payload. For active
messages, this is the AMtype field and a variably sized AMdata field. The
AMdata field contains the payload of the message_t sent to AMSend.send(),
whose length is another argument to the same AMSend.send() invocation. The
maximum size of the data field is defined by TOSH_DATA_LENGTH:
MSDU = AMType(1B) + Data(<= TOSH_DATA_LENGTH bytes)
So putting it all together, for t2 active messages on the cc2420 radio, the
radio frame looks like this:
SHR(5) + PHR/len(1) + MHR/addr(8) + AMtype(1) + AMdata(TOSH_DATA_LENGTH) +
CRC(2)
If I haven't fouled up somewhere, the maximum size of the radio frame is
TOSH_DATA_LENGTH +17 bytes, or 45 bytes using the standard definition for
TOSH_DATA_LENGTH. The actual size of a particular radio frame is len + 17
bytes, where len is the length parameter supplied to AMSend.send().
All the best,
Steve
>
> Thanks for the help!
>
> Pedro
>
> On 6/30/07, Erwing Sánchez <[EMAIL PROTECTED]> wrote:
> > Hi Pedro,
> > you may want to take look to the cc2420 datasheet. I found a lot of
> > useful information regarding IEEE 802.15.4 there!
> > www.chipcon.com/files/CC2420_Data_Sheet_1_3.pdf
> > Or you may want to try the IEEE 802.15.4 standard, where you can
> > certainly find all this information.
> > Take into consideration that, for instance, the length of the frame
> > Header varies according to the value of the fcf field (I think most of
> > the time it is not changed in TinyOS applications). Besides, the frame
> > length changes in accordance to the application because the payload
> > changes.
> > However, If you're looking for maximum values, the maximum frame
> > length is 128 bytes in 802.15.4.
> > Regards,
> > Erwing
> >
> > On 6/30/07, Pedro Almeida <[EMAIL PROTECTED]> wrote:
> > > Hello all;
> > >
> > > I would like to know the exact length of the 802.15.4 frame. The reason
> >
> > that
> >
> > > i'd like to know the exact length is because i need to do some duty
> >
> > cycles
> >
> > > numbers and one of the inputs is the frame length. It would seem to
> >
> > equal
> >
> > > the payload, but given it makes no sense to me the payload on the
> > > serial frame, i might as well doubt i can figure the length on the
> > > 802.15.4's.
> > >
> > > So according to TEP 125:
> > >
> > > The TinyOS 802.15.4 T-frame format is as follows:
> >
> > +-------------------+---------+------------------------------+-----------
> >---+
> >
> > > | 802.15.4 Header | AM type | data | 802.15.4 CRC |
> >
> > +-------------------+---------+------------------------------+-----------
> >---+
> >
> > > I presume the 802.15.4 Header, as according to CC2420.h, to be 10
> > > bytes:
> > >
> > > typedef nx_struct cc2420_header_t {
> > > nxle_uint8_t length;
> > > nxle_uint16_t fcf;
> > > nxle_uint8_t dsn;
> > > nxle_uint16_t destpan;
> > > nxle_uint16_t dest;
> > > nxle_uint16_t src;
> > >
> > > /** I-Frame 6LowPAN interoperability byte */
> > > #ifdef CC2420_IFRAME_TYPE
> > > nxle_uint8_t network;
> > > #endif
> > >
> > > nxle_uint8_t type;
> > > } cc2420_header_t;
> > >
> > > AM Type, according to previous struct, another byte before "data".
> >
> > Total:
> > > 11.
> > >
> > > I suppose CRC to be 2 extra bytes added in the end. Total: 13
> > >
> > > The "data" field intrigues me.
> > >
> > > I see this being defined in CC2420.h
> > >
> > > #ifndef TOSH_DATA_LENGTH
> > > #define TOSH_DATA_LENGTH 28
> > > #endif
> > >
> > > Does this mean it's a fixed length that gets 'populated' according to
> >
> > each
> >
> > > different situation (making a fixed total of 42 bytes), or is 28 a max,
> >
> > or
> >
> > > am i understanding this all wrong?
> > > The struct in TestNetwork is 15 bytes. Would that means the total frame
> >
> > size
> >
> > > is 13+15=28 bytes? Or is there anything extra added?
> > >
> > > Any help greatly appreciated!
> > > Thanks,
> > >
> > > Pedro
> > >
> > >
> > > _______________________________________________
> > > Tinyos-help mailing list
> > > Tinyos-help@Millennium.Berkeley.EDU
> >
> > https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
>
> !DSPAM:4686801b290009716080633!
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