Some Questions on the Communications Toolbox Turbo Encoder/Decoder blocks

Hi, I just started using the Comm Systems toolbox and have been working with the Turbo Encoder/Decoder modules and following the example code. A couple simple questions:

1) In one example there is a line saying

 "% Convert to log-likelihood ratios for decoding
        llrData = (-2/noiseVar).*rData;"

And in a different example:

 "% Convert received signal to log-likelihood ratios for decoding
        receivedBits  = step(hTDec, (-2/(noiseVar/2))*real(receivedSignal));"

I don't really understand either of these, typically I have see an LLR being formed as a log(Prob1/Prob0) or something like that, can anyone explain where the -2 is coming from??

2) As a follow up to the first question, is it possible to change the decoding algorithm channel statistic assumptions? When I developed a turbo code, I was able to modify the conditional channel probability density funciton used in the turbo decoder so that I could specify a certain mean and variance to describe the channel noise, is this possible? If not, does anyone know what the decoder assumes?

2) Are there serially concattenated codes included in the toolbox or only parallel concattenated turbo codes?

3) I see I can use different generator polynomial for the constituent codes to change rates, is it also possible to implement puncturing for precise rate control? (Seems like all I should need to do is delete selected parity bits and reinsert them after the channel to be zeros, although I think I will need to take a close look at the constituent codes to determine what are the parity bits.)

Thanks,

Mike