September 16, 2010

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This weeks issue of Knowledge Base is written by Dialogic.
Author: Bud Walder, Dialogic


Fax, Session Border Control and SIP Trunking


Often overlooked in an enterprise transition to VoIP services and solutions, is the fax machine and the fax server. Wait - do enterprises still use fax machines regularly? Dialogic, as the company that owns the venerable Brooktrout fax server technology brand on which the greatest percentage of fax servers are deployed worldwide, can assure you that fax is not only alive and well in 2010, it actually remains a communications cornerstone for core business processes in many industries. In contract work alone, it remains the leading document transmission technology as legal proof of signatory approval.

Unfortunately, fax is often overlooked during a TDM to IP migration, or expected just to work across VoIP infrastructure. In short, fax does not work well if treated as a voice call over IP networks. The highly reliable PSTN fax protocol known as T.30 becomes quite unreliable when exposed to the VoIP vagrancies of jitter, latency and packet loss. In fact, research shows that each individual fax page sent has only an 80% chance of success, even when T.30 fax aware pass-through techniques are employed. And this failure rate is cumulative to the point where a 50 page fax document has about a zero percent chance of being successfully transmitted in T.30 pass-through mode.

Enter the T.38 protocol. This ITU protocol was designed specifically for fax over IP or FoIP. Its reliability rivals traditional T.30 over PSTN networks, and also consumes much less bandwidth on the IP network versus T.30 pass-through techniques. As it applies to SIP trunking and enterprise SBCs, the ‘own-ness’ is really on the SIP trunk service provider to deliver end-to-end T.38 where ever the fax rides on their IP network. In truth, this is not something that many ITSPs can promise or deliver today. At Dialogic, the interoperability team for our FoIP software platform, Dialogic® Brooktrout® SR140 Fax Software, has validated a few ITSPs for reliable FoIP T.38 support, but generally it is a capability that the network service providers are still developing. The SIP Forum and i3 Forum have made a joint announcement recently to establish an international test program for FoIP. This is good news for developing more reliable fax over SIP trunking should help accelerate the adoption and deployment of T.38 within the ITSP networks.

As for fax and the session border controller, T.38 does use SIP signaling, and the benefits of the SBC at the network edge apply to FoIP as well, but only to enable it to pass securely from the enterprise network to the ITSP network. It does not contribute otherwise to the successful transmission of the fax content. It does however apply to an SBC edge device with an integrated gateway subsystem such as the newly announced Dialogic® BorderNet™ 500 Gateways. If routing fax over SIP trunking service provider is a priority for an enterprise, the gateway subsystem can be equipped to support T.30 to T.38 conversion, even running at the highest fax transmission rate known as v.34. This FoIP gateway functionality allows legacy fax servers and fax machines to utilize the SIP trunk service to transmit faxes, and get the SIP trunk benefits for their fax transmissions as well.


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