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New Member

Modem Passthrough vs T.38 on Cisco GW ATA187 VG224

In my experiemce there are always issues with T.38 and ATA 187 devices and VG's.

This is when ywe have this setup PSTN----PRI-ISDN------GW------SW----CUCM

Could also be FXO doesnt matter still have issues.

NOTE This dicussion relates to this architecture PSTN----PRI-ISDN------GW------SW----CUCM

I find T.38 to be very incosistant and unreliable when there is POTS and ATA 187 and also VG224,

Ive even tried T.38 fallback none with modem passthough under global config and found that doesnt work well (consistantly) and we get  failures.

What I have also tried and found is if we have T.38 configured  with ulaw and  fallback none

then modem passthrough under Voice Service VoIP - the calls dont connect always either.

What I have found to be seemless and work

creating dedicated dial-peer for the actuall faxes and doing fax/modem passthough on these with G711ulaw as codec.

Given CUCM only allows setting of ulaw - there is no point sending alaw or setting in voice class or dial-peer point to CUCM  at it or the Cisco IP to analogue gatereways dont transmit or take a setting for alaw?

In summary Id ask

  1. Is it preferebale to only use T.38 when both call legs are IP ?
  2. Note I have tested "PSTN----PRI-ISDN------GW------SW----CUCM" when the other end is a IP brooktrout stack and  T.38 seem ok no problems
  3. All the Cisco docs I remember seeing relating to T.38 show VoIP legs and no POTS leg


New Member

Modem Passthrough vs T.38 on Cisco GW ATA187 VG224

From what I've been able to gather, getting FoIP to work well is more of an art that a science.  For what its worth we've had the best luck with straight modem passthrough without T.38 being involved at all.