Correlate Access Code - Tag Stream
Examine input for specified access code, one bit at a time.
input: stream of floats (generally, soft decisions)
output: stream of bits, 2 bits per output byte (data in LSB, flag in next higher bit)
Each output byte contains two valid bits, the data bit, and the flag bit. The LSB (bit 0) is the data bit, and is the original input data, delayed 64 bits. Bit 1 is the flag bit and is 1 if the corresponding data bit is the first data bit following the access code. Otherwise the flag bit is 0.
This block searches for the given access code by slicing the soft decision symbol inputs. Once found, it expects the following 32 samples to contain a header that includes the frame length (16 bits for the length, repeated). It decodes the header to get the frame length in order to set up the the tagged stream key information.
The output of this block is appropriate for use with tagged stream blocks.
- Access Code
- is represented with 1 byte per bit, e.g.,
- It is important to choose an access code that is not "cyclical". A cyclical code contains a repetition of itself within the length of the code. This can cause false access code detections, and will cause byte boundaries (and hence valid packet length and payload recovery) to be recovered in error, with bizarre results!
- For example, an access code of
11111111is a poor choice because it is cyclical. Let's look at why this is.
- The first eight
1bits in a row from the block's input stream will correctly detect the first access code. The first packet (in standard form
<access code> <length1> <length2> <payload>) will be recovered nominally.
- However, when the correlate access code block's state machine goes back into scanning/detection mode after the first packet, the first bit of the NEXT access code (which forms the
11111111preamble of the next packet) will cause an immediate access code detection. This is because the access code detection routine within the block always shifts the last detected access code one bit to the left, and ORs in the current input bit.
- In our example, the valid bits
11111111from the first access code detected will get shifted left one bit (to give
11111110), and then the next input bit (the first bit after the end of the first packet) will be ORed into the value (
11111110 OR 1, giving
11111111) resulting in an erroneous detection of the next access code, because
11111111will of course match the access code that the block is searching for.
- This will falsely detect the start of the next packet (alignment off by 7 bits) and your flowgraph will explode with subsequent bad headers and invalid data. Not a pretty site.
- So, when choosing an access code, select one that is not cyclical!
- A possible (and historically interesting) non-cyclical 32-bit access code might be 0xe15ae893:
- which was the "unique word" used on several early communications satellites.
- maximum number of bits that may be wrong
- Tag Name
- key of the tag inserted into the tag stream
This flowgraph is taken from the Packet_Communications tutorial.
- C++ files
- Header files
- Public header files
- Block definition