/*! \brief Send frame to media channel (rtp) */
static int sip_write(struct ast_channel *ast, struct ast_frame *frame)
{
	struct sip_pvt *p = ast_channel_tech_pvt(ast);
	int res = 0;

	switch (frame->frametype) {
	case AST_FRAME_VOICE:
		if (ast_format_cap_iscompatible_format(ast_channel_nativeformats(ast), frame->subclass.format) == AST_FORMAT_CMP_NOT_EQUAL) {
			struct ast_str *codec_buf = ast_str_alloca(64);
			ast_log(LOG_WARNING, "Asked to transmit frame type %s, while native formats is %s read/write = %s/%s\n",
				ast_format_get_name(frame->subclass.format),
				ast_format_cap_get_names(ast_channel_nativeformats(ast), &codec_buf),
				ast_format_get_name(ast_channel_readformat(ast)),
				ast_format_get_name(ast_channel_writeformat(ast)));
			return 0;
		}
		if (p) {
			sip_pvt_lock(p);
			if (p->t38.state == T38_ENABLED) {
				/* drop frame, can't sent VOICE frames while in T.38 mode */
				sip_pvt_unlock(p);
				break;
			} else if (p->rtp) {
				/* If channel is not up, activate early media session */
				if ((ast_channel_state(ast) != AST_STATE_UP) &&
				    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
				    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
					ast_rtp_instance_update_source(p->rtp);
					if (!global_prematuremediafilter) {
						p->invitestate = INV_EARLY_MEDIA;
						transmit_provisional_response(p, "183 Session Progress", &p->initreq, TRUE);
						ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);
					}
				}
				if (p->invitestate > INV_EARLY_MEDIA || (p->invitestate == INV_EARLY_MEDIA &&
									 ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT))) {
					p->lastrtptx = time(NULL);
					res = ast_rtp_instance_write(p->rtp, frame);
				}
			}
			sip_pvt_unlock(p);
		}
		break;
	case AST_FRAME_VIDEO:
		if (p) {
			sip_pvt_lock(p);
			if (p->vrtp) {
				/* Activate video early media */
				if ((ast_channel_state(ast) != AST_STATE_UP) &&
				    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
				    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
					p->invitestate = INV_EARLY_MEDIA;
					transmit_provisional_response(p, "183 Session Progress", &p->initreq, TRUE);
					ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);
				}
				if (p->invitestate > INV_EARLY_MEDIA || (p->invitestate == INV_EARLY_MEDIA &&
									 ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT))) {
					p->lastrtptx = time(NULL);
					res = ast_rtp_instance_write(p->vrtp, frame);
				}
			}
			sip_pvt_unlock(p);
		}
		break;
	case AST_FRAME_TEXT:
		if (p) {
			sip_pvt_lock(p);
			if (p->red) {
				ast_rtp_red_buffer(p->trtp, frame);
			} else {
				if (p->trtp) {
					/* Activate text early media */
					if ((ast_channel_state(ast) != AST_STATE_UP) &&
					    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
					    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
						p->invitestate = INV_EARLY_MEDIA;
						transmit_provisional_response(p, "183 Session Progress", &p->initreq, TRUE);
						ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);
					}
					if (p->invitestate > INV_EARLY_MEDIA || (p->invitestate == INV_EARLY_MEDIA &&
										 ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT))) {
						p->lastrtptx = time(NULL);
						res = ast_rtp_instance_write(p->trtp, frame);
					}
				}
			}
			sip_pvt_unlock(p);
		}
		break;
	case AST_FRAME_IMAGE:
		return 0;
		break;
	case AST_FRAME_MODEM:
		if (p) {
			sip_pvt_lock(p);
			/* UDPTL requires two-way communication, so early media is not needed here.
				we simply forget the frames if we get modem frames before the bridge is up.
				Fax will re-transmit.
			*/
			if ((ast_channel_state(ast) == AST_STATE_UP) &&
			    p->udptl &&
			    (p->t38.state == T38_ENABLED)) {
				res = ast_udptl_write(p->udptl, frame);
			}
			sip_pvt_unlock(p);
		}
		break;
	default:
		//ast_log(LOG_WARNING, "Can't send %u type frames with SIP write\n", frame->frametype);	// AURIS 2015/02/20
		ast_log(LOG_WARNING, "Can't send %u type frames with SIP write: Channel Name (%s)\n", frame->frametype, ast_channel_name(ast));	// AURIS 2015/02/20
		return 0;
	}

	return res;
}