t-38 fax requirement question

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Aug 3rd, 2010
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good day,

We are about to embark on a VM fax server that requires t38 mode. my question is, is there any special router hardware required for our 3845 router? and is anyone using this today in a heavy fax enviroment?

thank you

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Overall Rating: 3.5 (4 ratings)
Jonathan Schulenberg Wed, 08/04/2010 - 05:37
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    2017 IP Telephony

No special hardware is required, the DSP supports it assuming you have the correct microcode loaded on it. Use the IOS software advisor to ensure you have T.38 support in your IOS image. You do need to enable support for it under voice service voip using the fax protocol command though. Also be aware of bugs, there have been a few, especially on MGCP-controlled gateways.

I will limit my 'seen it work' answer to a specific product: Sagem-Interstar's XMediusFAX. As long as the VM is not oversubscribed you should be good to go. T.38 buys the VM a small amount of forgiveness but if it is starving for I/O you will still have problems.

Steven Holl Wed, 08/04/2010 - 11:06
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Yeah, like Jonathan mentioned, you should be fine.

Just keep in mind that the fax server will require protocol based switchover.  As a result, you won't be able to do T.38 fax switchovers to any SCCP-controlled devices, since SCCP devices are NSE only for switchovers.  And if you have any ATA186/8 or 6608 kicking around, those don't support T.38 either.

-Steve Holl

safety2008 Thu, 08/05/2010 - 03:45
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Thank you both for info,

The vendor has sent me a 600 page configuration manual from dialogic. is there a preferred setup on this type of app??? we are ccm 6.0 and, excuse my ignorance, I really am very new to the t 38 world. and they are really offering no guidance in the setup.

Thanks again!

Michael Hanes Thu, 08/05/2010 - 08:51
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With CUCM version 6.0, you can support standards-based T.38 fax to your 3845 using H.323, SIP, or MGCP as the call control. My personal preference would be to stick with H.323 or SIP. If your MGCP voice gateway is configured by CUCM, then there seem to be recurrent problems with the T.38 fax config getting overwritten upon reset. I think that these have been mostly cleared up in the latest CUCM and IOS versions but this can still be a pain in previous versions.

You must use H.323 or SIP to connect the fax server to CUCM. I would then recommend using H.323 or SIP to connect the 3845 to CUCM as well. Be aware that the call controls do not have to match as CUCM will handle any conversions. Basically, you have this from a call control perspective -

Fax Server--------------------------CUCM-------------------------3845

                    H.323 or SIP               H.323 or SIP

For the Fax Server to CUCM connection using H.323, add the fax server to CUCM as an H.323 Gateway. Disregard the Dialogic documents advice to add the fax server in CUCM as an Intercluster trunk (ICT). For SIP, you build a SIP trunk in CUCM that points to the fax server. The Dialogic guide should show you the proper configuration details.

For the CUCM to 3845 configuration, repeat what you did for the fax server - in CUCM add the 3845 as an H.323 Gateway or build a SIP trunk in CUCM pointing to the 3845. On the 3845 voice gateway itself, you will need to build H.323 or SIP dial-peers pointing to CUCM. The critical statements for the dial peers to have from a T.38 fax perspective are the following:

codec g711ulaw

fax protocol t38

All of this should be covered pretty well in the Dialogic guide. Your only decision would be to decide between H.323 and SIP for the fax server to CUCM connection and the CUCM to 3845 connection. You should use the call control that you are the most comfortable with. Hopefully this helps and gives you a little more guidance on the overall setup.


safety2008 Thu, 08/05/2010 - 10:42
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Thanks very much for this info, it is more than my vendor can provide and more than I will know in the timeframe I have. i have included a snapshot of my GW'S today..is this, hopefully, a stare and compare of my existing 323 fax GW, which uses t1'to connect?  using just a spare port i have in the 3845.? if you can let me know, and thank you again!!!!

Michael Hanes Thu, 08/05/2010 - 11:08
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Looking at your screenshot, I see two H.323 voice gateways, BOS3845-VG1 and BOS3845-VG2. You can use T1 ports on either of these voice gateways for the T.38 faxing. You just need to configure the H.323 dial peers that point to CUCM correctly for T.38 as mentioned previously and that is it. Does that answer your question? If not, let me know and I will try and provide further clarification.



safety2008 Thu, 08/05/2010 - 11:40
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very grateful for info..and excuse my ignorance on this. this fax project came out of nowhere. and i hate to say i would hate to do something foolish to a working GW. Yes bos vg1 and vg2 are t1's going to our existing fax servers,  mostly for redundancy, which is one thing i believe we lose going with this VM solution. they, vg1 + 2 are each using one port off of the 3845. i have a spare port on vg1. but i think when i create it, it will conflict with the device name which is already there, i also have to delete that spare port, 0/2/1, which was used at one time off the 3845..as stated i just need to move slooowly so i dont trash existing GW. ya the dial peer's i think also might be a horror show as i have, IF THE T38 TRIAL WORKS, about 500 numbers that would change from the vg1+2 over to the new port..sorry for the very long message, just the guidance you have gave has made it a wee bit clearer.....and for my last stupid ? for this day, on bosvg1+2 i connect a t1 cable to our fax server from GW, What is connecting to the IPFAX server if anything, the vendor says nothing, which leads me to think why direct dial peers to a port that "goes nowhere"

i think just for reading a beer and lunch is in order!!


Michael Hanes Fri, 08/06/2010 - 06:41
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OK, this is getting a bit more complicated.  ;-)

Just to make sure I understand your current setup, I am picturing something like this on your BOS-VG1 gateway for your current fax traffic flow:

Fax machine--------PSTN-------------BOS-VG1--------------Fax Server

                                        T1                         T1

I would guess that BOS-VG2 would be similar? Fax calls coming in from the PSTN on BOS-VG1 can use a TDM hairpin to be routed to the T1 connected to existing fax server OR they can be routed over IP to BOS-VG2 for redundancy? Is this correct?

It sounds like what you are looking to do is to switch from the fax server being a direct T1 connection to the gateway and instead the fax server is connected directly to the IP network. There will be some configuration changes needed to make this work but it should not be too bad hopefully. As I diagrammed above, your connection to the PSTN is through the 3845s, right? Or are you using a SIP trunk solution to a provider? If you are using a PSTN connection through the 3845s, then you are simply going to unplug the T1s from the existing fax server. Fax calls coming in from the PSTN to the 3845 will now be pointed to CUCM (or even directly to the fax server if you want to bypass CUCM) which will route them to the new fax server that has an IP address and is connected to your IP network. A simplified topology would look like the following:

Fax machine---------PSTN-----------3845-------------CUCM-------------Fax Server

                                        T1               IP                    IP

Does this make sense? You are obviously correct in that you cannot keep dial peers pointing to "nothing" when the T1 fax servers are removed. Calls will have to be routed over IP to CUCM or directly to the IP fax server. If you can clarify or even trace though your existing fax call flows then I may be able to add some more insight.

Beer sounds good!  ;-)



safety2008 Fri, 08/06/2010 - 07:58
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by the time this is done we may be up to a case..anyway and again, i must just be missing something simple, or im having brain blockage. i view what we have now as this

pstn>>>>>cucm/3845>>>>>>fax server

          t1                       t1

Ok so you cleared my port ? traffic will flow via ip as opposed to be pointed by route list and by DP config..and you are correct in that we will be replacing the t1 fax server with the FOIP server down the road...but the blockage i am having, and gateway/routers admitedly i need more work on,..is just creating a new h323 gateway, that i thought would need a port defined 0/2/0 from the 3845...as you can see in the snap port 0/3/1 is boken out of the 3845 for the h323 fax server of today, i assume i need something identical, kindof, but when i created the new biscom h323 gw, i conflict with the ip already in use..so how do i associate the new gw with the 3845 and i also assume that instead of being port ie 0/3/1 driven i will be enetring the ip of the NEW fax sever in the gw config screen..this may be more than you want, i was just trying to get this figured out myself without calling in our router vendor for assistance..i am probably making this bigger than it had to be.

anyway im in Boston and you already have at least 2 beers your way

thanks again David

Tracy Larson Mon, 08/09/2010 - 08:05
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Just a note, I am connected to a biscom fax server with CUCM 7.1(3) via an ICT (they wanted it that way so I built it that way. I am also using both MGCP and H323 gateways and all work fine through this setup. Be aware that there is a bug in IOS 12.4(24)T2 that says t.38 is enabled but it doesnt work. Some small note on the MGCP config for this, you also need to issue the command "mgcp default-package fxr-package" for the fax capabilities to be enabled by default for the gateway. If you have already done this let me know and I can post my configs and ICT stuff etc if needed.

safety2008 Mon, 08/09/2010 - 08:19
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Many thanks. Would like to see your configs as I am kind of a newbie to

this voip world and def newbie to setting up gateways.. thank you so



Tracy Larson Wed, 08/11/2010 - 07:34
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you bet, let me know if you need anything else.

Michael Hanes Tue, 08/10/2010 - 10:14
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I sent you a Private Message to see if you wanted to get together on a quick conference call and go through your scenario. It might be quicker to explain everything in a live conversation. I can then post what we discuss and any solutions back to this thread. Let me know if you are interested.



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