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In Brazil, we would say you are "brushing bits" . Originaly, Mica2 change messages in HDLC-like framing, based on RFC1662 reference model. The first Byte - 7e - is the starting flag. The last Byte - 7e - is the ending flag. If you want to undestand the 802.15.4 message format you have to read IEEE802.15.4 Reference Model. You can take it free in IEEE web page. I think you have reported the Frame of MAC Layer. The first and the second bytes are "Frame Control": 01 08 The third byte is sequence: 01 FF FF is destination PAN ID FF FF is short address of destination 16 11 0b 21 05 is the message payload, as you said 6d 3c is probably the FCS, to confirm you have to calculate it over the rest of the message. In IEEE802.15.4 Reference Model, the FCS is the end of the message, then probably the last FF is a flag. The length of the Mac Frame is specified in PHY Header (the header of the physical layer), in a 7 bit number. Then you will not need to specify the length of the message in the MAC Frame. ***************** João Giacomin Prof-DCC-UFLA ***************** ---------- Original Message ----------- From: [UTF-8?]"ä»å·" <[EMAIL PROTECTED]> To: "tinyos-help" <[email protected]> Sent: Mon, 3 Nov 2008 12:08:07 +0800 Subject: [Tinyos-help] length filed missing > Hi, > > definition of TOS_MSG in tinyos1.x is as below: > > typedef struct TOS_Msg > { > /* The following fields are transmitted/received on the radio. */ > uint16_t addr; > uint8_t type; > uint8_t group; > uint8_t length; > int8_t data[TOSH_DATA_LENGTH]; > uint16_t crc; > > /* The following fields are not actually transmitted or received > * on the radio! They are used for internal accounting only. > * The reason they are in this structure is that the AM interface > * requires them to be part of the TOS_Msg that is passed to > * send/receive operations. > */ > uint16_t strength; > uint8_t ack; > uint16_t time; > uint8_t sendSecurityMode; > uint8_t receiveSecurityMode; > } TOS_Msg; > > there is a length filed before data filed indicate payload length.When i > read from serial port ,the data is something like: > > 7E 42 FF FF 16 11 03 0B 21 05 B1 9F 7E > > 03 is the lenght field and indicate lenght of following data and 0B 21 05 is > data format we defined in our application. > > but when i directly receive data with 802.15.4,the data is like: > > 01 08 01 FF FF FF FF 16 11 0B 21 05 6D 3C FF > > 01 08 01 FF FF FF FF is the Mac header of 802.15.4,and the length filed is > missing! > > > so my question is: is the data lenght filed really in the 802.15.4 frame?if > not ,what's it used for? > > any help would be appreciated! > > > > 2008-11-03 ----------------------------------------------------------------------- > Oden ------- End of Original Message -------
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