5 Replies Latest reply on Apr 2, 2012 2:47 PM by unified

    Fax problems

    mcdeeiis New Member

      What is the correct configuration for a fax to work. One of my client was trying to fax this morning and after several attempts I got one fax. Also, most of the times my fax starts and it looks like I am about to get a fax but then after seconds nothing happens. The one I received had out of order packets 4294965810 and discard packets 1847.

      Any idea on how can I improve my faxing capabilities.

       

      Total Access 908

      Firmware : A4.10.00.E

      Codec : G711


      I had to use G711 for all my audio calls because of fax. Is there way to use G711 only for fax and G729 for all audio calls. I have tried this setting but it does not work for incoming fax. Incoming fax tries to use G729 which results in failure.


      Thanks,


        • Re: Fax problems
          david Employee

          Thanks for posting in the ADTRAN Support Community!

           

          You will first want to verify that there are no errors on any interface.  Next, below is what I would consider the recommended configuration for an FXS port used only for faxing.

           

          voice codec-list 711

            codec g711ulaw

          !

          voice user 1001

            connect fxs 0/1

            modem-passthrough

            codec-group 711

          !

           

          You can also use the global command "voice codec-priority user" to ensure that the codec-group applied to the voice user is enforced.  However, this is a global command, so all users will be affected.  It is likely better to leave that command defaulted since even without this command, once fax tones are detected by "modem-passthrough", the unit will reINVITE to setup the call as G.711.  The far end should support similar functionality.

           

          Regarding increasing your fax reliability, there are several variables that come into play when faxing over an IP network.  The most important variable is the performance, or lack of loss, delay and jitter, of the network.  Most faxing problems can be traced back to a lack of QoS in the network or a device on which QoS is misconfigured.  Below are a couple of examples of very simple, yet effective, QoS maps that might be applied to a WAN interface.

           

          # PPP Example

          qos map VoIP 10

            match dscp 46

            priority unlimited

          !

          interface ppp 1

            qos-policy out VoIP

           

          # Ethernet WAN Example

          qos map VoIP 10

            match dscp 46

            priority unlimited

          !

          interface eth 0/1

            traffic-shape rate 1544000

            qos-policy out VoIP

           

          Once we have verified QoS is setup properly on the ADTRAN unit, we will want to verify that all devices in between the RTP endpoints are setup for QoS.  We can then check the network performance with "show voice quality-stats".  It's important to note that some statistics, particularly the lost packets, can be improperly reported if the RTP sequence number is not consistent from the far end for the duration of the call.  Below is an example of a voice call followed by a fax call.

           

                      Start                                                                 Lost  Discard   Delay

          ID       Time          From       To        Duration Codec Pkts  Pkts            Avg    Max

          --------------------------------------------------------------------------------

          1964     8:38 AM   9001      2125       0:30     G729        0         0            80    100

          1965     8:38 AM   9001      2126       2:29     G711u      0         0             50    50

           

          For the fax call, you should see the average and maximum delay at 50ms.  This is because a call in which fax tones are detected will switch to a fixed 50ms jitter buffer.  However, the normal phone calls can give us clues about network performance.  In a network with low delay and jitter, they normal phone calls will stay with the 50ms jitter buffer.  However, as shown in the example above, increasing delay indicates that the adaptive jitter buffer increased due to excessive jitter.  In general, faxing becomes unreliable when network jitter forces the jitter buffer to enlarge to greater than 70ms for non-fax calls.  Our Voice Quality Monitoring feature (VQM) can also be used to gain insight on network performance.

           

          Sometimes it is necessary to adjust the FXS port gains or impedance to improve fax reliability.  Below are the default settings that can be adjusted.

           

          interface fxs 0/1

            impedance 600c

            rx-gain -3.0

            tx-gain -6.0

           

          This is often a trial and error process, but generally the transmit and receive gain will not need to be adjusted more than 3db.  One common change that results in better fax reliability is to set the impedance to "600r".

           

          Lastly, the fax machine settings can often increase reliability.  Common changes would include lowering the default baud rate and disabling error correction.  We have seen both of these settings improve performance.

          • Re: Fax problems
            david Employee

            Mcdeeiis,

             

            If none of the above tips and best practices solves the faxing issues, you will need to create a normal Technical Support ticket by calling 888-423-8726.  We can then begin getting debug, DSP, and packet captures for your specific installation.  If we find a solution that will help other members of the Support Community, I will post that information on this thread.

             

            Thanks again,

            David

              • Re: Fax problems
                mcdeeiis New Member

                I have opened a ticket #1306546

                  • Re: Fax problems
                    david Employee

                    I went ahead and marked this post as "assumed answered". Feel free to mark any correct or helpful answers from this post. If you still need assistance with this issue I would be more than happy to help, just let me know in a reply.

                     

                    In your ticket created on the regular support queue, it appears that your T-1 clock settings were changed to reduce or eliminate interface errors.  Also, the jitter buffer was increased to 70ms to allow for more jitter in the network.  Did these recommendations solve the issue?

                     

                    Thanks,

                    David

                • Re: Fax problems
                  unified Past_Featured_Member

                  If your provider supports T38 I found the following to work well:

                  (config)#voice trunk t02

                  (config-T02)#modem-passthrough

                  (config-T02)#t38

                  (config-T02)#t38 redundancy high-speed 3

                  (config-T02)#t38 redundancy low-speed 2

                  (config-T02)#t38 fallback-mode g711

                  (config-T02)#t38 max-rate 9600

                  (config-T02)#exit