Cisco IOS gateways with Cisco IOS Release 12.4.4T and later offer support for Super Group 3 (SG3) fax transmissions when T.38 or Cisco fax relay are configured; however, only Group 3 speeds are negotiated.
If it is necessary to transport SG3 high-speed faxes at their native speeds, then modem passthrough must be used. With the release of Cisco IOS version 15.1.1T, a new feature will provide native support for SG3 faxes by T.38 fax relay.
See detailed asnwer here.. I dont think Super G4 is supported though. Cant see any reference to it.
If you are talking ATAs - then definately not. Expect v.slow fax... like 9600k.
IOS based gateways have lots of features (in t.38 mode) aimed at preventing faxes from negotiating complex connections, forcing them to negotiate G3 rather than Super G3, disabling ECM etc. so although you will get better performance from a properly configured IOS gateway over an ATA, I would always set expections quite low.
If speed is really important it's probably best to look at a proper server fax solution rather than hoping to push the performance of a fax machine over IP.
It'll save some trees as well :-)
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One of the problems I'm getting, is that many standard fax machines try and use Super G3/G4 and won't reliably down-speed to 14.4 Kb/s. This isn't with just cheap fax machines, but also higher cost/spec fax machines too.
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