Fax, Modem, and Text Support over IP Configuration Guide, Cisco IOS Release 15M&T
Configuring Fax Pass-Through
Configuring Fax Pass-Through
Last Updated: December 4, 2012
This chapter describes the configuration of fax pass-through. With fax pass-through, modulated fax information from the PSTN is passed in-band over a voice speech path in an IP network. Fax pass-through disables compression, echo cancellation, and issues redundant packets to ensure complete transmission.
Your software release may not support all the features documented in this chapter. For the latest feature information and caveats, see the release notes for your platform and software release.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn . An account on Cisco.com is not required.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for Configuring Fax Pass-Through
Before you configure fax pass-through, perform the following tasks:
Restrictions for Configuring Fax Pass-Through
Restrictions for fax pass-through are as follows:
Subsystems are defined for these platforms, starting from port 0 and grouping consecutive ports 36 at a time. There are 3 subsystems per dfc108 card on the Cisco AS5400 (3 times 36 for a total of 108 ports) and 9 subsystems on the Cisco AS5850 tetryl card (9 times 36 for a total of 324 ports). The limitations are as follows:
Examples of fax pass-through sessions mixed with a high load voice session type are as follows:
The Cisco AS5400 and Cisco AS5850 have a capability to transmit 20-ms packets and receive 10-ms packets, which significantly improves performance over what can currently be handled with 10-ms packets in both directions. Currently, other Cisco universal gateway implementations may have an outgoing packet size limitation that imposes the use of 10-ms packets, as opposed to 20-ms packets, which is the optimal setting. This restriction limits the number of ports that can run fax pass-through to 20 per subsystem (10-ms connections only).
Information About Fax Pass-Through
Pass-Through Method of Transport
Fax pass-through takes place when incoming T.30 fax data is not demodulated or compressed for its transit through the packet network. The two endpoints (fax machines or modems) communicate directly to each other over a transparent IP connection. The gateway does not distinguish fax calls from voice calls.
On detection of a fax tone on an established VoIP call, the gateways switch into fax pass-through mode by suspending the voice codec and loading the pass-through parameters for the duration of the fax session. This process, called upspeeding, changes the bandwidth needed for the call to the equivalent of G.711.
With pass-through, the fax traffic is carried between the two gateways in RTP packets using an uncompressed format resembling the G.711 codec. This method of transporting fax traffic takes a constant 64-kbps (payload) stream plus its IP overhead end-to-end for the duration of the call. IP overhead is 16 kbps for normal voice traffic, but when switching to pass-through, the packetization period is reduced from 20 ms to 10 ms, which means that half as much data can be put into each frame. The result is that you need twice as many frames and twice as much IP overhead. For pass-through, the total bandwidth is 64 plus 32 kbps, for a total of 96 kbps. For normal voice traffic, total bandwidth is 64 plus 16 kbps, for a total of 80 kbps.
Packet redundancy may be used to mitigate the effects of packet loss in the IP network. Even so, fax pass-through remains susceptible to packet loss, jitter, and latency in the IP network. The two endpoints must be clocked synchronously for this type of transport to work predictably.
Performance may become an issue. To attempt to mitigate packet loss in the network, redundant encoding (1X or one repeat of the original packet) is used, which doubles the amount of data transferred in each packet. The doubling of packets imposes a limitation on the total number of ports that can run fax pass-through at one time. You can calculate that two voice sessions with no VAD equate to one fax pass-through session with redundancy.
Call Control for Fax Passthrough
Fax pass-through is supported under the following call-control protocols:
In addition, the following information applies to H.323 and SIP fax pass-through.
Fax Pass-Through Signaling Using the Protocol Stack or NSEs
When a fax tone is detected, the originating and terminating gateways need to communicate to each other that they are changing to fax pass-through mode. Gateway signaling of the changeover to fax mode can use either of these methods:
Beginning with Cisco IOS Release 12.2(13)T, you can specify the use of the H.323 or SIP protocol stack to signal the changeover to fax mode. This is enabled with the fax protocol pass-through command.
Alternately, you can use the modem passthroughcommand to configure the gateway to use Cisco-proprietary NSEs to signal the switch to pass-through mode. Fax pass-through using NSEs has been available on the Cisco AS5300 since Cisco IOS Release 12.1(3)T and on most other platforms since Cisco IOS Release 12.2(11)T.
If non-Cisco gateways are involved in the fax transmissions, fax pass-through must be used. In all cases, however, T.38 fax relay is the best solution if all of the involved gateways support it.
H.323 or SIP Support of Resource Reservation Protocol
As of Cisco IOS Release 12.2(13)T, H.323 or SIP gateways that are configured for fax pass-through or modem pass-through allow Resource Reservation Protocol (RSVP) bandwidth adjustments when the original voice call is configured to use RSVP. When the original voice codec is restored at the end of the fax session, the original RSVP bandwidth is restored as well. When current bandwidth is unavailable, the fax proceeds at a best-effort rate without RSVP and with no performance guarantees. RSVP bandwidth adjustments for fax transmissions are made as follows:
H.323 Support for Call Admission Control
As of Cisco IOS Release 12.2(13)T, H.323 call admission control (CAC) adjustments are allowed in the case of fax pass-through and modem pass-through. An H.323 gateway that uses a gatekeeper requests the following bandwidths from the gatekeeper when codec changes are necessary:
If the gatekeeper accepts the bandwidth changes, the session is permitted to continue over the fax codec (G.711). If the gatekeeper rejects the bandwidth increase, the fax codec is terminated and the gateway uses the configured fax protocol fallback or the original voice codec, in which case the fax transfer fails.
How to Configure H.323 and SIP Fax Pass-Through
For H.323 and SIP networks, fax pass-through is configured on gateway dial peers.
The purpose of these tasks is to configure VoIP dial peers for fax or modem pass-through, one at a time or globally. If both methods are used, an individual dial-peer configuration takes precedence over the global configuration, which means that a call matching a particular dial peer tries first to apply the fax method that was configured individually on that dial peer. If no individual dial peer configuration was made, the router uses the global configuration.
When configuring dial peers, you have the choice of specifying fax pass-through or modem pass-through for the pass-through method. If you use the fax protocol pass-through command to specify fax pass-through as the method, the gateway uses the H.323 or SIP protocol stack to signal the changeover to fax mode. If you use the modem passthroughcommand to specify modem pass-through as the method, the gateway uses NSEs for fax changeover signaling.
Configuring One or More Individual VoIP Dial Peers
Use this task to enable fax pass-through on individual dial peers. Select the fax protocol pass-through command or the modem passthrough command, but not both.
Configuring VoIP Dial Peers Globally
If you are adding fax pass-through capability previously defined VoIP dial peers, you can configure them globally in voice-service configuration mode.
Alternately, you can configure fax pass-through VoIP dial peers one at a time by following the instructions in the Configuring One or More Individual VoIP Dial Peers.
When using the voice service voip and modem passthrough nsecommands on a terminating gateway to globally set up fax pass-through with NSEs, you must ensure that each incoming call is associated with a VoIP dial peer to retrieve the global fax configuration. Associate calls with dial peers using the incoming called-number command to specify a sequence of digits that incoming calls can match. You can ensure that all calls match at least one dial peer by using the following commands:
Router(config)# dial-peer voice tag voip Router(config-dial-peer)# incoming called-number .
How to Configure MGCP Fax Pass-Through
Use the following steps to configure MGCP fax pass-through on voice gateways. You must have the same configuration on the originating and terminating gateways.
Before You BeginSUMMARY STEPS
Identify endpoints and configure the MGCP application as described in the MGCP and Related Protocols Configuration Guide.
[sample-duration [10 |
Configuration Examples for Fax Pass-Through
H.323 Fax Pass-Through Example
The following example show a configuration for fax pass-through with H.323 support.
version 12.2 service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname 5850 ! no logging buffered no logging rate-limit ! resource-pool disable dial-tdm-clock priority 1 trunk-slot 1 port 0 spe link-info poll voice 5 spe default-firmware spe-firmware-1 ip subnet-zero ip cef distributed ip ftp username mgcusr ip ftp password lab no ip domain lookup ip host colos_tftp 10.100.00.00 ip host brios 255.255.255.255 ip dhcp smart-relay ! isdn switch-type primary-net5 ! voice service voip h323 modem passthrough nse codec g711alaw redundancy sample-duration 20 ! no voice hpi capture buffer no voice hpi capture destination ! mrcp client session history duration 0 mrcp client session history records 0 memory check-interval 3600 memory validate-checksum 7200 redundancy no keepalive-enable mode classic-split ! controller E1 0/0 pri-group timeslots 1-31 ! dial-peer voice 5001 pots incoming called-number 550 destination-pattern 800 direct-inward-dial port 0/0:D prefix 800 ! dial-peer voice 500 voip incoming called-number 800 destination-pattern 550 session target ipv4:10.100.00.00 fax rate disable codec g726r32 ! gateway ! line con 0 exec-timeout 0 0 logging synchronous line aux 0 exec-timeout 0 0 logging synchronous line vty 0 4 password lab no login line 2/00 5/323 flush-at-activation no modem status-poll no modem log rs232
SIP Fax Pass-Through Example
The following configuration example shows fax pass-through with SIP support.
version 12.3 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname 2691 ! boot-start-marker boot system flash:c2691-ipvoice-mz.andante_0224 boot-end-marker ! logging buffered 100000 debugging enable password lab ! no aaa new-model ! resource manager ! memory-size iomem 25 clock timezone PST -8 clock summer-time PDT recurring no network-clock-participate slot 1 voice-card 1 codec complexity high dspfarm ! ip subnet-zero ip cef ! no ip dhcp use vrf connected ! no ip domain lookup no ftp-server write-enable isdn switch-type primary-qsig ! voice service voip fax protocol pass-through g711ulaw sip ! controller T1 1/0 framing sf linecode ami ! controller T1 1/1 framing sf linecode ami ! interface FastEthernet0/0 ip address 10.20.109.104 255.255.0.0 duplex auto speed auto ! interface FastEthernet0/1 no ip address shutdown duplex auto speed auto ! ip default-gateway 10.20.0.1 ip classless ip route 0.0.0.0 0.0.0.0 10.20.0.1 ! no ip http server ! control-plane ! dial-peer voice 2000 voip fax rate disable fax protocol pass-through g711alaw ! line con 0 exec-timeout 0 0 line aux 0 line vty 0 1 exec-timeout 0 0 password lab login line vty 2 4 login ! ntp clock-period 17180778 ntp server 10.10.254.253 prefer ! end
MGCP Fax Pass-Through Example
The following example shows an MGCP gateway configured for fax pass-through.
version 12.2 no parser cache service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname fptrtr ! voice-card 1 ! ip subnet-zero ! no ip domain lookup ! isdn switch-type primary-5ess ! voice call carrier capacity active ! mta receive maximum-recipients 0 ! ccm-manager mgcp no ccm-manager fax protocol cisco ! controller T1 1/0 framing esf clock source line primary linecode b8zs ds0-group 0 timeslots 1 type fxs-loop-start ! controller T1 1/1 framing esf linecode b8zs ds0-group 0 timeslots 1 type e&m-wink-start ! interface Ethernet0/0 ip address 10.3.222.6 255.255.0.0 ip helper-address 10.3.222.1 no ip mroute-cache half-duplex ! interface Ethernet0/1 shutdown ! ip default-gateway 10.3.0.1 ip classless ip route 192.168.254.0 255.255.255.0 10.3.0.1 ip http server ! call rsvp-sync ! voice-port 1/0:0 ! voice-port 1/1:0 ! voice-port 3/0/0 ! voice-port 3/0/1 ! mgcp mgcp call-agent 10.3.222.1 service-type mgcp version 0.1 mgcp modem passthrough voip mode nse mgcp package-capability rtp-package mgcp fax t38 inhibit ! mgcp profile default ! dial-peer cor custom ! dial-peer voice 3641 pots application mgcpapp port 3/0/0 ! dial-peer voice 3643 pots application mgcpapp port 1/1:0 ! line con 0 exec-timeout 0 0 line aux 0 line vty 0 4 exec-timeout 0 0 password cisco login ! ! end
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