what is the mean of this funtion: rlc_decode_li?

In UMTS, lub_decoder,MAC to RLC,there is a cipher/decipher process. I have to find this process.But I can't find code in wireshark,just a function:rlc_decode_li,maybe this a cipher?? follow is the code in wireshark.

pos = fpinf->cur_tb;
if (rlcinf->ciphered[pos] == TRUE && rlcinf->deciphered[pos] == FALSE) {
    proto_tree_add_text(tree, tvb, 0, -1,
        "Cannot dissect RLC frame because it is ciphered");
    return;
}  
num_li = rlc_decode_li(RLC_AM, tvb, pinfo, tree, li, MAX_LI);
if (num_li == -1) return; /* something went wrong */
offs += num_li;