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! _______________________________________________ Tinyos-help mailing list Tinyos-help@Millennium.Berkeley.EDU https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help