data: a string (values between 0 and 255) that contains byte numbers
startByteIndex: the zero-based index from which on data should be unpacked (from data[startByteIndex]). Can be omitted in which case 0 is used.
byteCount: the amount of bytes that should be unpacked. Can be omitted in which case 0 is used (which indicates that the maximum of bytes should be unpacked from the indicated startByteIndex).