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Fax and Modem

Two mechanisms are available to deploy fax and modem services over an IP network: fax and modem relay and fax and modem pass-through. The support varies from one Cisco platform to another and is highly dependent on the underlying voice-signaling protocol, whether it is SCCP, MGCP, or H.323.

Fax and modem pass-through capabilities are supported in all the gateways deployed in the XYZ network. Also with this model, pass-through V.34 modem speed can be guaranteed. Refer to the following URL for more details on fax support on various voice gateways:

http://www.cisco.com/en/US/tech/tk652/tk777/technologies_tech_note09186a0080159cf3.shtml

While using fax and modem pass-through, you have to force the codec to be G.711. XYZ will not be able to use the up-speed feature of the pass-through mode because the CAC mechanism that is currently available in CallManager cannot dynamically adjust the bandwidth that is deducted. All fax and modem endpoints will belong to the device pool DP-MOH-FAX (refer to Table 6-20). That will force the call to be negotiated as a G.711 call from the start of the call and deduct the right amount of G.711 bandwidth from the CAC mechanism.

Note

Implementing fax up-speed allows the use of high-compression codecs (such as G.729) for voice calls. However, when certain fax tones are detected, the codec is "up-speeded" or changed to G.711.

With XYZ, at the central sites in San Jose and Sydney, the incoming fax call from the PSTN comes from the WS-X6608-T1 and WS-X6608-E1 gateways. The fax machines are connected to the VG248 gateways. At the remote sites, the incoming fax calls arrive on the MGCP gateways.

Example 6-5 shows the configuration required for the MGCP Cisco IOS voice gateway to support the fax and modem pass-through features. This configuration is required on all the remote-site gateways.

Example 6-5. Cisco IOS MGCP Gateway Configuration for Fax and Modem Pass-Through Features

Mgcp
! Enables MGCP support on the gateway                                          
mgcp call-agent 10.1.2.1 service-type mgcp version 0.1
! Point the MGCP daemon on the gateway to a call control agent i.e. CallManager
no ccm-manager fax protocol cisco
mgcp modem passthrough voip mode nse
! Enables peer-to-peer RTP NSE (Named Signaling Events) to coordinate          
! the following between the originating and terminating gateways:              
! codec switchover and the disabling of                                        
! the echo canceller and voice activity detection (VAD).                       
mgcp modem passthrough voip codec g711ulaw
no mgcp timer receive-rtcp
mgcp sdp simple
mgcp fax t38 inhibit
! This is used to disable MGCP support for T.38 fax.                           
! By default, T.38 is enabled.                                                 
!
mgcp profile default
ccm-manager fallback-mgcp
ccm-manager mgcp
! This command is required for the CallManager                                 
! to be able to control the gateway.                                           
!
dial-peer voice 100 pots
! Fax machine                                                                  
application mgcpapp
! This voice port is controlled by CallManager via MGCP                        
destination-pattern 5611
port 1/0/0

After you configure the fax settings on the MGCP gateway for fax pass-through, you can verify the settings by using the show mgcp command on the voice gateway, as shown in Example 6-6.

Example 6-6. Verifying the Fax Pass-Through Configurations on the Cisco IOS MGCP Gateways

#Show mgcp
MGCP voip modem passthrough mode: NSE, codec: g711ulaw, redundancy: DISABLED,
MGCP voaal2 modem passthrough disabled
MGCP voip modem relay: Disabled.                                             
MGCP T.38 Fax is DISABLED
MGCP T.38 Fax ECM is DISABLED
MGCP T.38 Fax NSF Override is DISABLED
MGCP T.38 Fax Low Speed Redundancy: 0MGCP T.38 Fax High Speed Redundancy: 0
MGCP Upspeed payload type for G711ulaw: 0, G711alaw: 8

Figure 6-20 shows the configuration of the Catalyst WS-X6608 gateway, which is a non-IOS-based MGCP gateway to enable fax pass-through. The same configuration settings are required even with the 6624 FXS card.

Figure 6-21 shows the configuration of VG248, which is an SCCP-based gateway to enable fax pass-through. When configuring the VG248 for fax, observe the following:

If Cisco fax relay is enabled on the VG248 and the far-end gateway supports only fax passthrough, the VG248 can negotiate fax pass-through.

Disable call waiting on the port that is enabled for fax.

The VG248 has a legacy mode setting for fax pass-through so that it is backward compatible with old devices. If the other end gateway is a Cisco IOS router, change the pass-through signaling parameter to IOS Mode (refer to Figure 6-21). Maintain the passthrough signaling as IOS Mode throughout the network on all nonIOS gateway configurationsthat is, VG248, 6608, and 6624 if all the other voice gateways in your network are based on Cisco IOS.