Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Fax T.38 support on TDM/IP GW

Hello gents,

I have a customer which I need to provide a Trunk SIP to Service Provider. The connection to PBX (Alcatel OXE R10) will be done using E1 interfaces (alcatel is not IP Enable).

I have a Cisco 3945 ISRG2 that make conversion between E1 (ISDN) interfaces and SP SIP network.

Fax is an important subject for the customer and I have some questions on this item:

- for T.38 fax support, I need to have DSP configured  (T.38 is medium complexity codec).

- T.38 work in fax-relay mode, correct?

- They have SG3 fax. To enable fax calls at SG3 speed, I need to support T38 v3.0 (on all endpoints / GW) and enable fax pass-through. Correct?

Issue is that I don't support fax pass-through on SP Voice network.

So final question is: do I support SG3 fax calls (maybe by lowering speed to G3) if I configure T38 fax-relay on my GW? Is there any consideration I need to take into account (DSP, Codec...)?

For example, on my GW, I will have 240 simultaneous voice calls and 80 fax calls.

for DSP calculation, I have take 320 low complexity calls and added 80 transcoding sessions to medium complexity.

Is it correct or do I need to select 240 low complexity calls and 80 medium complexity calls and no transcoding (DSP for transcoding included in medium complexity calls)?

Thanks for your help.

Everyone's tags (3)

Fax T.38 support on TDM/IP GW


     You can try the follow lines of config on your ISR, these are for a gateway running H.323.  what is your current DSP/PVDM ration? -

voice service voip

allow-connections h323 to h323

fax protocol t38 nse ls-redundancy 0 hs-redundancy 0 fallback cisco


modem passthrough nse codec g711ulaw

mgcp sdp simple

mgcp fax t38 ecm

mgcp fax t38 inhibit

mgcp fax t38 gateway force

I almost always force my SG3 fax calls back to G3.  If you want to do this as well add the following

voice service voip

fax-relay sg3-to-g3


Please rate helpful posts!

Please rate helpful posts
CreatePlease to create content