Configuring Cisco Fax Relay

Cisco provides two methods for fax relay. One method is a Cisco-proprietary method called Cisco fax relay, and it is described in this chapter. The second method is based on the ITU-T T.38 standard, and it is described in "Configuring T.38 Fax Relay."

• T.38 fax relay is the default mode for passing faxes through a VoIP network, and Cisco fax relay is the default fax relay type on Cisco voice gateways. This capability has been supported in Cisco IOS Release 11.3 and later releases and is widely available. Cisco fax relay uses Real-Time Transport Protocol (RTP) to transport the fax data.
• Cisco fax relay is configured on the VoIP dial peers that direct calls into and out of the packet network.Cisco fax relay can be configured under the H.323 and Session Initiation Protocol (SIP) call control protocols.

Cisco fax relay supports fax relay packet loss concealment, which is a technique that allows gateways to disregard packet loss rates that might otherwise cause fax failures. High-end fax machines with the memory to store page data often are able to use Error Correction Mode (ECM) for error-free page transmission. When ECM is enabled, a fax page is transmitted in a series of blocks that contain frames with packets of data. After receiving the data for a complete page, a receiving fax machine notifies the transmitting fax machine of any frames with errors. The transmitting fax machine then retransmits the specified frames. This process is repeated until all frames are received without errors. If the receiving fax machine is unable to receive an error-free page, the fax transmission may fail and one of the fax machines may disconnect. On networks that have a packet loss rate greater than 2 per cent, fax transmissions routinely fail when ECM is enabled because of ECM's low tolerance for packet loss.

The Fax Relay Packet Loss Concealment feature allows you to control whether ECM is enabled or disabled for fax transmissions on a VoIP dial peer. By disabling ECM on networks with a large amount of packet loss, you ensure that more fax transmissions are completed, although they may not be totally error-free.

When ECM is disabled, a fax page is transmitted using high-speed modulation in its raw encoded format. When detecting line errors with ECM disabled, the receiving fax machine has three options (in order of increasing severity):

• Respond to page reception with the ReTrain Positive command. This response causes the transmitting fax to go through the training check process before transmitting the next page.
• Respond to the page reception with the ReTrain Negative command. This response causes the transmitting fax to go through the Training Check Frame (TCF) process with a lower modulation scheme.
• Disconnect immediately.

Fax relay ECM is enabled by default. To disable ECM, you use the fax-relay ecm disable command on the VoIP dial peer. After this command is configured, the gateway's Digital Signal Processor (DSP) fax-relay firmware modifies the T.30 Digital Information Signal (DIS) message. This modification is performed on DIS signals in both directions, so that ECM is disabled even when only one gateway is configured to disable ECM.

Disabling of ECM is recommended for dial peers handling fax relay traffic on known lossy networks, especially those with a packet loss rate of 2 percent or greater. The debug fax relay t30 command provides information about the E.164 destination and T.30 messages associated with fax transmissions. Note than an excessive number of simultaneous debug operations can degrade performance.

Super Group 3 (SG3) is a new generation of fax machines that support speeds of up to 33.6 kbps through V.34 half duplex (HD) modulation and V.8 signaling.

SG3 V.8 fax CM message tone suppression enables SG3 fax machines to scale down without end-user interaction and without using the extra bandwidth required by modem pass-through and allows SG3 fax machines to interoperate over a fax-relay network at G3 speeds by blocking the SG3 V.8 CM message, or fax tone, from reaching the called fax machine. This causes the called fax machine to time out on the ANSam tone and scale down to G3 speeds by initiating V.21 negotiations.

SG3 V.8 fax CM message tone suppression supports both the one-gateway and two-gateway solutions:

• With a one-gateway solution, the gateway on one end of the call can be configured to suppress the SG3 V.8 fax CM message independently of the gateway on the other end of the call. The one-gateway solution suppresses the fax CM tone on both TDM and IP interfaces (TI C5510 DSPs only), and can interoperate with third-party gateways when the fax CM tone suppression gateway is the originating gateway. A one-gateway solution
• With a two-gateway solution, the gateways on both ends of the call must have this feature enabled. The two-gateway solution suppresses the fax CM tone only on the TDM interface (TI C5421 and TI C549 DSPs). Both gateways must support this feature to interoperate at G3 speeds, or the fax tone suppression gateway must be the originating gateway.

Note	If both the originating gateway and the terminating gateways are configured for V.8 fax CM message suppression, the suppression occurs on the originating gateway.

1.    enable

2.    configure terminal

3.    dial-peer voice tag voip

4.    fax protocol {cisco | none| system}

5.    fax rate {12000 | 14400 | 2400 | 4800 | 7200| 9600 | disable | voice} [bytes rate]

6.    fax-relay ecm disable

7.    fax nsf word

8.    fax-relay sg3-to-g3 system

Use the following steps to configure Cisco fax relay globally for VoIP dial peers.

Note	Fax relay parameters that are set for an individual dial peer under the dial-peer voice command take precedence over global settings made under the voice service voip command.

1.    enable

2.    configure terminal

3.    voice service voip

4.    fax protocol {cisco | none}

5.    fax-relay sg3-to-g3

SG3 V.8 fax CM message suppression is enabled by default and does not appear in the running configuration. To view the configuration for:

• H.323 and SIP--Use the show dial-peer voice tag command.
• MGCP--Use the show mgcp command.

Router# show dial-peer voice 2000
VoiceOverIpPeer2000
        peer type = voice, information type = voice,
        description = `',
        tag = 2000, destination-pattern = `',
        answer-address = `', preference=0,
        CLID Restriction = None
        CLID Network Number = `'
        CLID Second Number sent
        CLID Override RDNIS = disabled,
        source carrier-id = `', target carrier-id = `',
        source trunk-group-label = `',  target trunk-group-label = `',
        numbering Type = `unknown'
        group = 2000, Admin state is up, Operation state is up,
        incoming called-number = `2...', connections/maximum = 0/unlimited,
        DTMF Relay = disabled,
        modem transport = relay, nse, payload type = 100, codec = g711alaw, , ga
		teway-controlled,
        URI classes:
            Incoming (Called) =
            Incoming (Calling) =
            Destination =
        huntstop = disabled,
        in bound application associated: 'DEFAULT'
        out bound application associated: ''
        dnis-map =
        permission :both
        incoming COR list:maximum capability
        outgoing COR list:minimum requirement
        Translation profile (Incoming):
        Translation profile (Outgoing):
        incoming call blocking:
        translation-profile = `'
        disconnect-cause = `no-service'
        advertise 0x40 capacity_update_timer 25 addrFamily 4 oldAddrFamily 4
        type = voip, session-target = `ipv4:10.2.109.103',
        technology prefix:
        settle-call = disabled
        ip media DSCP = ef, ip signaling DSCP = af31,
        ip video rsvp-none DSCP = af41,ip video rsvp-pass DSCP = af41
        ip video rsvp-fail DSCP = af41,
        UDP checksum = disabled,
        session-protocol = cisco, session-transport = system,
        req-qos = best-effort, acc-qos = best-effort,
        req-qos video = best-effort, acc-qos video = best-effort,
        req-qos audio def bandwidth = 64, req-qos audio max bandwidth = 0,
        req-qos video def bandwidth = 384, req-qos video max bandwidth = 0,
        RTP dynamic payload type values: NTE = 101
        Cisco: NSE=100, fax=96, fax-ack=97, dtmf=121, fax-relay=122
               CAS=123, ClearChan=125, PCM switch over u-law=0,A-law=8
        RTP comfort noise payload type = 19
        fax rate = fax,   payload size =  20 bytes
        fax protocol = system
        fax-relay ecm enable
        Fax Relay SG3-to-G3 Enabled (by system configuration)
        fax NSF = 0xAD0051 (default)
        codec = g729r8,   payload size =  20 bytes,
        Media Setting = flow-through (global)
        Expect factor = 10, Icpif = 20,
        Playout Mode is set to adaptive,
        Initial 60 ms, Max 250 ms
        Playout-delay Minimum mode is set to default, value 40 ms
        Fax nominal 300 ms
        Max Redirects = 1, signaling-type = cas,
        VAD = enabled, Poor QOV Trap = disabled,
        Source Interface = NONE
        voice class sip url = system,
        voice class sip rel1xx = system,
        redirect ip2ip = disabled
        probe disabled,
        voice class perm tag = `'
        Time elapsed since last clearing of voice call statistics never
        Connect Time = 0, Charged Units = 0,
        Successful Calls = 0, Failed Calls = 0, Incomplete Calls = 0
        Accepted Calls = 0, Refused Calls = 0,
        Last Disconnect Cause is "",
        Last Disconnect Text is "",
        Last Setup Time = 0.

When SG3 V.8 fax CM message suppression is not enabled, the running configuration showsno mgcp fax-relay sg3-to-g3, as shown in mgcp section of the following example:

Router# show runnning config
Building configuration...
Current configuration : 3231 bytes
!
! No configuration change since last restart
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Router
!
mgcp
mgcp call-agent ccm service-type mgcp version 0.1
mgcp dtmf-relay voip codec all mode out-of-band
mgcp rtp unreachable timeout 1000 action notify
mgcp modem relay voip mode nse gw-controlled
mgcp package-capability rtp-package
no mgcp package-capability res-package
mgcp package-capability sst-package
no mgcp package-capability fxr-package
mgcp package-capability pre-package
no mgcp timer receive-rtcp
mgcp sdp simple
mgcp fax t38 inhibit
no mgcp fax-relay sg3-to-g3
mgcp rtp payload-type g726r16 static
!
mgcp profile default