case 0:
                if (p->rtp)
                        f = ast_rtp_read(p->rtp);       /* RTP Audio */
                else
                        f = &null_frame;
                break;
        case 1:
                if (p->rtp)
                        f = ast_rtcp_read(p->rtp);      /* RTCP Control Channel */
                else
                        f = &null_frame;
                break;
        case 2:
                if (p->vrtp)
                        f = ast_rtp_read(p->vrtp);      /* RTP Video */
                else
                        f = &null_frame;
                break;
        case 3:
                if (p->vrtp)
                        f = ast_rtcp_read(p->vrtp);     /* RTCP Control Channel for video */
                else
                        f = &null_frame;