1. Problem description
Topology:
PRI——espace U1980——IAD——Fax
IAD: SC-2200wau(SunComm)
Problem description: fax receiving successful, but sending failed.
2. Problem analysis
Fax sending flow and package analysis.
This is the fax sending working flow previously.
This is the 200 OK with SDP from IAD, no attribute line “a=silenceSupp:off”.
According to the IETF sip(fax) standard, for pass through mode, there should have the attribute as below:
m=audio 156 RTP/AVP 8 0
a=silenceSupp:off
a=ecan:fb on
You can refer to:
http://tools.ietf.org/html/draft-ietf-sipping-realtimefax-01
4.4 <#section-4.4> Internet telephony gateways and fax pass-through mode
This section deals real-time facsimile communications over IP using fax pass-through instead of T.38 fax relay. In this mode, the facsimile communication is handled as a PCM audio call (PCMA/PCMU as specified in ITU-T recommendation G.711).
The fax pass-through mode is important to prevent call failures, for example in cases when one of the SIP communication peers does not support T.38. For Internet telephony gateways with support for PCM/G.711 audio but no support for T.38 real-time fax, it is recommended to switch the session to fax pass-through mode.
Internet telephony gateways SHOULD handle the fall back mode to fax pass-through by recognizing the SDP T.38 connection and proposing to switch to a new audio connection. The new audio connection SHOULD have the following characteristics: at a minimum, specify PCM G.711 codec, silence suppression OFF. The telephony gateway that originated the failed T.38 re-INVITE SHOULD initiate the subsequent re-INVITE to fax pass-through mode.
Refer to section 6.2 <#section-6.2> for a sample call flow illustrating this scenario.
3. Conclusion and solution
Conclusion: Since Huawei PBX follow the IETF standard, but the Suncomm IAD don’t contain the attribute as the IETF standard required, so the media negotiation failed cause fax sending failed.
4. Test result:
After apply the patch , Huawei engineer test with Pak Syaiful, on the same IAD 1 interface register to VOIPNOW and the other 1 register to Huawei PBX.
Try 8 times on VOIPNOW ,4 times success and 4 times failed;
Try 15 times on Huawei PBX ,10 times success and 5 times failed.
And after analysis the failed log , some response code 480,which means temporarily unavailable, the worse network condition or RTP interruption may cause this.
5. Appendix: Fax mode introduction
Transparent transmission: The transparent fax encapsulates fax signals in G.711 packets. It features simple implementation and low delay; however, it requires high bandwidth (fixed to be 64 kbit/s) and is easily affected by network fluctuations. Therefore, a transparent fax is not applicable to networks of poor quality (for example, networks in which jitter and packet loss occur frequently), whereas in networks of good quality, it is highly recommended. Both high-speed and low-speed faxes can adopt the transparent transmission mode.
T.38: The T.38 fax converts T.30 signals to T.38 packets that are applicable to the packet switched network. It has the following features: low bandwidth, highly reliability (by adopting redundancy frame and forward check), and immune to network jitter and delay; however, its implementation is complex. If a fax is to be transmitted over two IP networks and then a TDM network, the T.38 mode is recommended, since a large delay occurs in such a transmission. Only low-speed faxes adopt the T.38 mode.
VBD: This mode complies with T-REC-V.152. It adopts the G.711 codec. Compared with the transparent mode, it supports fax SDP negotiation either before or after session SDP negotiation. Therefore, its operation and application are more flexible. Both high-speed and low-speed faxes can adopt the VBD mode. This mode is seldom used in the existing network.
