Gsm is an implementation of the final draft GSM 06.10
standard for full-rate speech transcoding.
Test data for implementations of this particular document
can be bought and used to verify an implementation.
The encoded test data uses a format different from what
one would use to transmit frames with the least number
of bits.
Gsm_explode() and gsm_implode() convert between the
internal, small, 33-byte format and the 76-word format
used by the test data.
RETURN VALUE
gsm_explode() returns -1 if the passed frame is invalid, else 0.