Fax T.38 support on TDM/IP GW

[vanwitt]

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.

[tonyperla]

Hi,

     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

h323

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

Thanks,
Tony

Please rate helpful posts!