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Cisco IP Telephony Solution Reference Network Design
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Chapter 3 Voice Gateways
Fax and Modem Support
dial-peer voice 100 pots
application mgcpapp
port 1/0/0
!
!
Gateway Controlled with Capability Exchange Through H.245 or Session Definition Protocol (SDP)
The following characteristics apply to this method of configuring T.38 fax relay:
·
T.38 capability is exchanged between gateways. A Network Services Engine (NSE) message is sent
on the RTP stream from the terminating gateway to signal the originating gateway to switch to T.38
fax relay upon detection of a fax tone. Because the NSE message is sent on the RTP stream, it is
transparent to call control signaling.
·
Cisco CallManager cannot support this capability exchange with either H.323 or MGCP. Therefore,
you must use a configuration command to force T.38 fax relay even though T.38 capability is not
exchanged.
·
There are three fallback mechanism to choose from:
Cisco fax relay (default)
Fax pass-through
None
The following example illustrates this type of configuration:
H.323
!
dial-peer voice 1000 voip
destination-pattern 1T
session target ipv4:10.10.10.1
modem passthrough mode nse codec g711ulaw
!
! To enable T.38 fax relay and fall back to Cisco fax relay when
! T.38 fax negotiation fails. This is the default case.
fax protocol t38 fallback cisco
!
dial-peer voice 1001 voip
destination-pattern 2T
session target ipv4:10.10.10.2
modem passthrough mode nse codec g711ulaw
!
! To enable T.38 fax relay and fall back to fax passthrough when
! T.38 fax negotiation fails.
fax protocol t38 nse fallback pass-through
!
dial-peer voice 1002 voip
destination-pattern 3T
session target ipv4:10.10.10.3
modem passthrough mode nse codec g711ulaw
!
! This CLI is needed when talking to MGCP endpoint where CA/GK
! doesn't support T.38 fax relay such as CCM.
fax protocol t38 nse force fallback none
!
!