Add standardized BERT mode.

docs/data_link_layer.rst

@@ -1,6 +1,6 @@
 Data Link Layer
 ===============
-The Data Link layer is split into two modes:
+The Data Link layer is split into three modes:
 
 * Packet mode
    Data are sent in small bursts, on the order of 100s to 1000s of bytes
@@ -11,6 +11,10 @@ The Data Link layer is split into two modes:
    with no break in physical layer output, until the stream ends. e.g. voice data,
    bulk data transfers, etc.
 
+* BERT mode
+   PRBS9 is used to fill frames with a deterministic bit sequence.  Frames are sent
+   in a continuous sequence.
+
 When the physical layer is idle (no RF being transmitted or received),
 the data link defaults to packet mode. 
 
@@ -41,10 +45,11 @@ All frames are preceded by a 16-bit *synchronization burst*.
     * Link setup frames shall be preceded with 0x55F7.
     * Stream frames shall be preceeded with 0xFF5D.
     * Packet frames shall be preceeded with 0x75FF.
+    * BERT frames shall be preceeded with 0xDF55.
 
 All syncwords are type 4 bits.
 
-These sync words are based on `Barker codes`_.  The sequence 0xDF55 (symbols -3 +3 -3 -3 +3 +3 +3 +3) is reserved.
+These sync words are based on `Barker codes`_.
 
 .. _`Barker codes`: https://en.wikipedia.org/wiki/Barker_code
 
@@ -415,3 +420,81 @@ equate to a slot time of 4 and a P-persistence value of 63.
 
 The benefit of this method is that it imposes no penalty on uncontested networks.
 
+BERT Mode
+---------
+
+BERT mode is a standardized, interoperable mode for bit error rate testing.  The preamble is 
+sent, followed by an indefinite sequence of BERT frames.  Notably, a link setup frame must not
+be sent in BERT mode.
+
+BERT Frame
+~~~~~~~~~~
+
+Each BERT frame is preceeded by the BERT sync word, 0xDF55.
+
+The BERT frame consists of 197 bits from a `PRBS9 <https://en.wikipedia.org/wiki/Pseudorandom_binary_sequence>`_ 
+generator.  This is 24 bytes and 5 bits of data.  The next frame starts with the 198th bit from the PRBS9
+generator.  The same generator is used for each subsequent frame without being reset.  The number of bits
+pulled from the generator, 197, is a prime number.  This will produce a reasonably large number of unique
+frames with even a small PRBS generator.
+
+The PRBS9 uses the ITU standard polynomial :math:`x^{9}+x^{5}+1`
+
+The PRBS9 is initialized with a state of 1.  (Any non-zero value will work.)  The pseudorandom sequence
+is generated by an LFSR as if fed with a stream of 0's.
+
+.. list-table:: Bit fields of BERT frame
+   :header-rows: 1
+
+   * - Bits
+     - Meaning
+   * - 0-196
+     - BERT PRBS9 payload
+   * - 4
+     - Flush bits for convolutional coder
+
+
+The 201 bits are convolutionally encoded to 402 type 2 bits.
+
+The 402 bits are punctured using the P2 puncture matrix to get 368 type 3 bits.
+
+The 368 punctured bits are interleaved and decorrelated to get the type 4 bits to be transmitted.
+
+.. list-table:: BERT frame
+   :header-rows: 1
+
+   * - Bits
+     - Meaning
+   * - 16 bits
+     - Sync word 0xDF55
+   * - 368 bits
+     - Payload
+
+BERT Receiver
+~~~~~~~~~~~~~
+
+PRBS generators are self-syncronizing. The receiver detects the frame is a BERT frame based on the
+sync word received.  If the PRBS9 generator is reset at this point, the sender and receiver should
+be synchonized at the start.  This, however, is not required.
+
+Syncronization
+==============
+
+The receiver will feed the bits from a decoded frame into the LFSR used for the PRBS.  Errors will
+come out of the PRBS as a burst of three (3) 1's.  Once a sequence of twenty (20) 0's are recovered,
+the stream is considered syncronized.  The receiver counts 20 bits received with no errors.  It then
+runs the LFSR as if it were a free-running PRBS generator and compares the bit differences.  Each
+difference increments the error counter.  This method, comparing the ideal output of a PRBS rather
+than feeding the received bitstream into the LFSR, avoids triple-counting bit errors.
+
+Resynchronization
+=================
+
+The receiver must keep track of the number of bit errors over a period of 128 bits.  If more than
+25 bit errors occur, the synchronization process starts anew.  This is necessary in the case of
+missed frames or other serious synchronization issues.
+
+References
+~~~~~~~~~~
+
+http://www.itu.int/rec/T-REC-O.150-199210-S