Finally, clear explanation of TinyOS 2.x serial packet structure

TinyOS 2.x

A message sent through serial ports from motes to PC will typically look like this:

00 FF FF 00 00 08 00 09 19 77 02 07 00 13 00 00

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• The first byte 00 is the leading zero.
  
• After that, the whole packet is of the type "serial_packet_t", defined in tinyos-2.x/tos/lib/serial/Serial.h.
  
• Inside the serial_packet_t structure, the first 7 bytes are of the type "serial_header_t", which is also defined in tinyos-2.x/tos/lib/serial/Serial.h.
  
  In the case above, it would be FF FF 00 00 08 00 09.
  
  Within the "serial_header_t", the following structure takes place:
• nx_am_addr_t dest // Destination (2 bytes)
• nx_am_addr_t src // Source (2 bytes)
• nx_uint8_t length // Payload Length (1 byte)
• nx_am_group_t group // Group ID (1 byte)
• nx_am_id_t type // Active Message Handler Type (1 byte), some enum value that you can define yourself.
  
• After the 7 byte header, here comes the payload, which can be up to TOS_DATA_LENGTH (28 bytes) long. The content could be anything.
  
  In the above example, the payload is: 19 77 02 07 00 13 00 00, which is of the length (8 bytes) denoted in the "Payload Length" field.

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To note, besides the payload part, we need to fill out the destination address and payload length field by calling SerialActiveMessageC.AMSend(am_addr_t dest, message_t* msg, uint8_t len). It would be a good programming manner to fill the len with sizeof(). And we also need to specify the Active Message Handler Type by wiring the AMSend to SerialActiveMessage.AMSend[am_id_t id];

ref:
http://www.tinyos.net/tinyos-2.x/doc/html/tutorial/lesson4.html. The doc here doesn't seem to be up-to-date since it left out the source address from the header.
GREAT THANKS TO http://tristan-tech.blogspot.com/ and his blog!!!!
(I copied whole blog from him in order to have in mine blog everything I might use in future)