Lisa Pierce

Lisa Pierce | May 31, 2010 |


Ask the Analyst: SIP Trunk Security, Faxing, Interop

Ask the Analyst: SIP Trunk Security, Faxing, Interop In our latest installment, see how carriers and vendors responded to Lisa's inquiries on a few key topics.

In our latest installment, see how carriers and vendors responded to Lisa's inquiries on a few key topics.

Many No Jitter readers continue to express interest in information exchanged during sessions held at this spring’s VoiceCon tradeshow. In this series of Questions and Answers, we’ll address some of the questions raised by the audience during the SIP Trunking Services panel I chaired at VoiceCon on March 22. I say "we" because many of the panelists provided useful input to these questions, so a significant amount of the information contained in this brief is a compendium of the best information provided by multiple panelists.

Nine panelists participated in the SIP Trunking services session--four service providers and five vendors. They included: AT&T Business Services--Rupesh Chokshi; Verizon Business--Thomas Dalrymple; Sprint Business--Daniel Jacobson; Global Crossing - Christopher Smith; Acme Packet - Seamus Hourihan; Empirix--Gordon Eddy; Network Equipment Technologies--Kevin Issacs; Audiocodes--Alan Percy; and Sonus- Gregory Zweig. The questions we'll address in this brief concern Fax over IP, Security, Intercarrier SIP interoperability, and SIP Calling Flexibility.

Fax over IP
We received numerous questions on this topic during the panel, and at multiple webinars. Here, I've combined similar questions/answers. They were:

Q: Do you see many issues running fax over SIP? Can you discuss how SIP Trunks support fax? I've heard mixed results--is it prudent to retain some TDM trunks for fax?

A: Sonus, Acme Packet, Audiocodes, and Network Equipment Technologies provided the following: Using SIP trunks for fax traffic is a very viable option but it does require some due diligence and preparation. Fax was designed for the challenges of sending data over an analog line. It is inherently sensitive to data sequence issues and reacts to errors by reducing transmission speeds and requesting resends. It assumes that errors are the result of poor quality analog signal. This is problematic in an IP environment where packet delivery is inherently inconsistent as a result of occasional bandwidth deprivation and latency (vs. MPLS with voice QoS). If appropriate steps are not taken to assure QoS, the fax transmissions speeds can degrade substantially. Many of the field issues we have seen with fax are related to fax transmissions using nothing but standard G.711 and limited QoS parameters; in these instances delayed packet transmission creates a cycle of resends and speed reductions by the fax machine. This is frustrating for end users as fax transmissions appear slow or may fail.

There are two different options for supporting fax over SIP trunks. The first option is through the use of the T.38 (real-time faxing over IP), which the SIP Trunk Service provider must support. The second option is to transcode the fax traffic into a voice codec (i.e. G.711) and carry the media across the SIP trunk as G.711. G.711 (fax over clear voice) works on perfect lines. The T.38 option is generally preferred due to the fact that each fax/voice transcoding instance increases the likelihood of media impairment. High quality gateways are required in order to ensure reliable fax. With T.38 and a quality gateway, retaining TDM trunks are not required for fax.

Here's my bottom line: use of T.38 is strongly preferred and recommended, but all your ducks must be lined up and tested--this is where proven vendor expertise can make all the difference. This clearly is worthwhile for medium-high volume fax requirements. If your company is dealing with a handful of faxes/month, look at maintaining a POTS line or using T.37 (store and forward fax over IP using email protocols like MIME or SMTP.

Q: What T.38 Fax over SIP Trunk capabilities (fax local home number porting to SIP Trunks) are available from each vendor and service provider? Who has successfully centralized a high volume of analog fax machines (many different models) on SIP Trunks?

A: Information provided by the above four vendors indicates they all support T.38. As for service providers, AT&T supports T.38 today (SIP Trunks to AVPN), as does Sprint. Verizon plans to support T.38 by 3Q 2010. Global Crossing provided this information:

...Global Crossing supports both T.38 for fax, as well as G.711 fallback. When customers send faxes to our network over SIP, the network core will convert it to regular inband G.711. Customers will experience the best results related to fax over IP when connecting via MPLS with voice quality of service transport. Companies using Global Crossing for fax termination include enterprise corporations sending fax as part of normal business practice, as well as large hosted IP fax application providers that send nothing other than fax traffic to enterprises as a core service...

Global Crossing's last point is interesting, because another member of the audience asked about vendor/carrier expertise with "...a high volume of analog fax machines (many different models) on SIP Trunks." Apart from that, no panelist supplied concrete examples. If faxes are a particularly significant part of your company’s life going forward, my recommendation is to use a fax over IP application provider, or invest in a good analog T.38 gateway, but it won't be free.

Q: Do you see customers combining voice and fax traffic on SIP Trunks or do you see splitting of traffic on dedicated voice vs. fax trunks?

A: The response is virtually unanimous: with some limited exception, the most common and recommended case is to combine voice and fax.


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