Using four instead of two constellation points provides twice the physical layer bit rate, which allows addition of redundant information to provide a degree of forward error correction. When QPSK is used, after encoding into varicode, the bits of the binary data signal is subject to a rate-1/2 channel code, which means that for every information bit, two code bits are calculated and transmitted. For that, a convolutional code with constraint length 5 (i.e. the last five bits from the input are incorporated to select two output bits per input bit) is used.