Re: [Discuss-gnuradio] lost bytes and reed-solomon

[Rick Parrish]

Brett L. Trotter wrote:
> Got a few good replies on my question about lost bytes. I appreciate the
> input, that sounds like it'll work great in conjunction with the error
> correction then. I'm hoping to have that in place within the next couple
> of hours and start testing.
>   
Two words ... preambles and frame sync.

One thing to be aware of ... it is possible with most software PLLs ... 
under crummy signal conditions ... to drop single bits (but not whole 
bytes). If the PLL is completely out of phase from where the mid 
point(s) of the symbol stream are located ... it can correct by either 
speeding up or slowing down. Depending upon the previous state and 
symbol history up to that point ... it can have the effect of inserting 
an erroneous bit or dropping a bit from the stream.

Any frames of data where this occurs are very likely trashed and not 
recoverable. You'll want a mechanism to reliably detect the start of 
each frame. For fixed sized frames of "N" bits ... bits added or missing 
distory the bit count for one frame. If you assume the next "N" bits 
belong to the next frame ... the bits in the next frame will be shifted 
by one bit ... effectively trashing the follow-on frames too.

Most protocols use a fixed bit pattern ... a frame sync ... to mark the 
beginning of each frame. You'll lose some bandwidth ... but it makes the 
job of recovering from a lost frame much easier.

If your protocol calls for short bursts of data ... you may want to pad 
the start of each transmission with an alternating symbol pattern. This 
gives the software PLL time to synchronize at the symbol level and 
accept the real data that follows.

-rick