Faxing with VG248 and CM 4.1.3

Posted on 2010-08-18
Last Modified: 2012-05-10
Hello we have a CM 4.1.3 cluster and a VG248. For years we have had

a horrible time with faxes. Just recently we have a new need for successfuly faxing. We have about 30 fax machines hanging off the VG248.

Problem is half the time faxes go thru, other half we get random fails, redials, and some times it will even say successful but never appear on the other side.

NOTE alot of our faxing is internal, i.e. dialing internal extensions on the fax machine.

What can I do to help fix this problem? I recently updated VG248 code to latest version but it did not help.

Question by:wreed420
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Accepted Solution

Alex Bahar earned 500 total points
ID: 33473626
Faxing is a very sensitive topic. If the faxes going through PRIs between sites, you need to make sure there are no clock slips on the E1/T1 controllers. If you have FXO interfaces then you may need to tweak attenuation settings.
The fax machines are connected to analog FXS ports. The media is an analog signal subject to electromagnetic interference.
To avoid interference make sure the cables do not cross near powerful electric motors. You need to make sure your VG248 devices are connected to ground (earth).
If you have only IP between VG248s, then you need to make sure QoS LLQ is enabled between the VG248s. There are no packet drops between two VG248s. Your service provider (SP) must have QoS implemented and should be honoring your TOS/DSCP markings. Your TOS/DSCP markings should be compatible with what your SP is expecting. I.e. if the SP is expecting DSCP=EF for all voice packets, then you need to make sure you're marking your voice/fax with EF. You need to have bandwidth guarantee between your sites. If you have contention on the WAN, this may cause random  packet drops which may fail fax.
Manually force G711 for your fax calls on VG248. Try to disable Fax relay (T38 and Cisco fax relay) and configure your VG248 ports with fixed G711 codec. Disable echo suppression. Disable VAD (voice activity detection). Increase the jitter buffer to 300ms or longer. Apply the above settings for all VG248 ports.
Just to make sure you're using G711 between Fax devices, you might want to create dedicated ANALOG regions on every site. Place all VG248 in those regions. And configure G711 between those regions and all other regions.
I know there are a lot to check here. You may not need do all of these to get your fax calls working. I am pointing you all possible fixes/workarounds to get fax working.
Please do not forget to select the most complete answer. Thanks!

Expert Comment

by:Alex Bahar
ID: 33473647
BTW CCM version 4 is end of life soon. You may want to upgrade it before it is too late.

Author Comment

ID: 33477401
This is occuring when faxing between machines on a single VG248. I am running Basic call mode with no fax relay on all ports. I have all machines forced to 14.4 and no ECM.  Problem is still occuring.

What to try next?
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Expert Comment

by:Alex Bahar
ID: 33481670
This is really unexpected. I have used VG248 in the past for faxing. It is one of the most reliable device for fax calls. Do you have the same issue when two fax machines are directly attached to the VG248 within a couple of meters? I am just trying to eliminate the cause of electromagnetic interference. How do you provide ground to VG248? I.e. where did you connect the VG248 ground?

Author Comment

ID: 33481681
It is not grounded. And now that I think about it the machines that are close work all the time. Machines that are on other floors far away from VG248 have issues.

The ground is really that important?! How should I ground?

Expert Comment

by:Alex Bahar
ID: 33482289
At the moment I do not have access to a VG248 to check if has a supplementary earth/ground connection. If there is not any, usually the ground connector on the power plug supplies the ground (if the power sockets/building are really properly grounded).
From your description, I guess that the VG248 grouding is fine because closer fax machines are working well. For distant fax machines, I suggest checking if the cables are crossing near powerful electric motors like air conditioners, lifts etc. They might be picking up noise from them. In some cases the noise/crosstalk is so noticable, you can hear it when you make a phone call from those lines using an analog phone.
It is quite difficult to fix fax issues because of analog circuits' performance can get affected by the distance, poor joints, corrosion, old and thin wiring, and the noise they pick from the environments and from each other. I suggest you to prepare a matrix for those fax machines, and try to figure out which circuits are failing by making calls between them. If you can narrow down the issue to a certain location, then you can check the MDF or joints or machinery around to find the root cause.

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