Thanks again to all those that participated in the Webinar titled "Fax - SIP+T.38 to the Rescue" we held a few weeks ago. As noted in a previous blog post, it was a very busy webinar. You can listen to the on-demand recording of the webinar at: http://www.tmcnet.com/webinar/audiocodes5/
As noted on the webinar, we had run out of time and were unable to get to all the great questions posed by the attendees. Find below some of the Questions and Answers that we didn't have time to address:
Q: (from Scott) is T.37 being employed and deployed?
A: T.37 is used in some messaging applications, but it is quite different than T.38. T.37 uses email technology to transport documents and thus is not real-time. It's great for store and forward applications, but not widely used or supported by carriers. For more on T.37, visit wikipedia
Q: (from Scott) who offers T.38 SDK's for fax API's?
A: AudioCodes and a number of other manufacturers offer a T.38 SDKs. AudioCodes has certified our gateways with a number of other other third-party manufacturer's T.38 software.
Q: (from Tim) I think T.38 was specified to use "UDPTL" or TCP - with RTP only added to the spec recently. What do people use in practice?
A: We are using UDPTL, TCP adds addional timing issues into the equation.
Q: (from Jayaprakash) Does Gateways always do the Fax tone detection? or it is Fax Application server responsibility?
A: Our gateways can detect the CNG fax tone, which makes fax detection much easier for application developers. Detection can be done by the application servers, but it is much harder and less accurate.
Q: (from Oliver) Which SIP trunking carriers support T.38?
A: AT&T and Quest are two carriers which have been certified to work very well. Others are in lab testing.
Q: (from Edward) How much bandwidth required for SIP+T.38 normally ?
A: T.38 with triple redundancy and operating with V.17 speeds would use roughly 48 kbps. Less redundency on good networks would use even less. In comparison, a fax over G.711 would require close to 80 kbps.
Q: (from Prashanth) Can T.38 fax be done using other codecs than G.711?
A: No, transporting fax via T.38 is done instead of using a voice coder (like G.711). Other voice coders compress the audio and are even less reliable than G.711.
Q: (from Yossman) Is your media gateway compatible with other brand devices that also supports SIP+T.38 ?
A: Yes, AudioCodes media gateways are many times deployed in mixed network environments and have excellent compatibility and reliability.
Q: (from Shawn) do you need to have a t.38 media server on both sides (sending and receiving)?
A: For T.38 to work, both ends of the IP connection need to support T.38
Q: (from Anil) Does these Media Gateways also have similar issues like Denial of service and other attacks and how do we solve this?
A: (While not the topic of this thread, this is a good question) Security is always a big concern and media gateways do need to be hardened to protect against attack. AudioCodes gateways have been hardened and passed stringent DoD certifications for deployment in DoD networks.
There are many more questions and I'll post more in the next few days....