[SIPForum-discussion] Failed called dial-up modem and fax

Joel Gerber Joel.Gerber at corp.eastlink.ca
Mon Aug 17 12:03:51 UTC 2015

Hello Rafael,

I’ve encountered many Fax/Data related problems on VoIP myself and there are a multitude of possible underlying causes. The first and most important thing to consider is whether the media gateways on either end of the call properly recognize the call as a data call quickly, and disable echo cancellation. Data calls rely on echo (as funny as that sounds) in order to reliably work. If you keep echo cancellation enabled on data calls, data transmissions will suffer. The other thing that needs to occur is possible CODEC re-negotiation. You do not want to use a lossy CODEC on data calls. Either the G.711 (for pure data) or T.38 (for Fax, this can also be G.711) CODECs should be used on data calls. If voice calls typically used a compressed CODEC, the media gateways need to quickly re-negotiate to G.711/T.38 before actual data transmission beings.

Another big item to consider is if there is any jitter on your network. Jitter is when packets arrive at the other end of your network with differing delays. This will kill data calls unless either end of the network employs properly configured jitter buffers on data calls in order to hide the jitter from the elements at either end. Jitter buffers typically get enabled at the same time that echo cancellation and CODEC re-negotiation occurs on a media gateway.

Another thing to consider is the complete end to end latency on your network. The conversion of POTS into VoIP and back again adds additional delay to a call. Sometimes this added delay could exceed the delay budget that a certain data application is able to handle. In that case, you might have to consider reducing the packetization rate, removing some hops on your IP network, or finding an alternative path with less delay to route this traffic through.

I hope this information helps. In my experience, fax issues are typically caused by misconfiguration of a media gateway, with the second most likely culprit being the IP data network.

Joel Gerber
Network Operations Specialist - Telephone
Joel.Gerber at corp.eastlink.ca<mailto:%20Joel.Gerber at corp.eastlink.ca>    T: 519.786.1241

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Rafael Diego Farias de Oliveira
Sent: August-15-15 5:19 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Failed called dial-up modem and fax


I would like to share with you some of my esperiência on the subject addressed.
Daily realize voice network migration, tdm to sip and encounter many problems in relation to fax and dial-up modem
In some cases resolve by adjusting the speed of the equipment, but there are cases that can not solve.
I have a question on how to perform a more effective troubleshooting, for I perceive that this is a problem of SIP technology, for these attributes.
To adjust the codecs, but sometimes it does not.
Working with Cisco and HP routers and no matter what type of router the problem is always the same.
I wonder if anyone has experience on this subject.
What in the sip could impact on the problem seems addressed attributes.
I appreciate the opportunity to be posting on this site.

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