Fax Troubleshoot Guide - H.323

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Updated:February 5, 2015
Document ID:118726

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Introduction

This document describes one of the most effective approaches to troubleshoot fax, which includes these steps:

  1. Split the call into two legs.
  2. Identify the protocol (SIP/H.323/SCCP/MGCP) on each leg.
  3. Choose a leg and then check if the call is incoming or outgoing on that leg and if the gateway/endpoint associated is a terminating gateway (TGW) or originating gateway (OGW) correspondingly.

You can split a fax call into four parts:

  1. Set up the voice call
    • Off-hook, Dial, Ring, Answer
    • Calling (CNG) and Called Equipment Identification (CED) Tones
  2. Switchover
    • Codec upspeed/correction
    • Voice Activation Detection (VAD) disabled on DSP
    • Jitter buffer transitions from adaptive to a fixed optimum value
  3. Pre-message procedures
    • Fax Terminal Identification
    • Capabilities exchange and setting
    • Training
  4. In-message and post message procedures
    • Transmission of pages
    • Error detection and correction (ECM)
    • End of message and page confirmation
    • Call Disconnect, On-hook

This call flow includes the messages to look for when H.323 is the protocol identified. There are corresponding sections based on whether your endpoint is a TGW or OGW.

Note: In the table in the next section, both T.38 Relay and Passthrough were tested simultaneously and differences between G3 and SG3 have been pointed out.

TGW - Fax Call Incoming on H.323 Leg

Note that:

  • T.38 - Delay<1000ms, Jitter<300ms, Packet loss should be NONE unless T.38 with redundancy.
  • Passthrough - Delay<1000ms, Jitter<30ms, Packet loss should be NONE.
  • Protocol Based switchover - This is standard based.
  • NSE Based switchover - This is proprietary and works only between Cisco voice gateways.
Passthrough T.38 Relay

GW-------------------------CUCM/GW
<-----------H.225 Setup-------------
------------H.225 Callproc--------->
------------H.225 Alerting---------->


Check for VTSP shows:
Fax Relay=DISABLED - 'fax rate disabled' set (dial-peer) Primary Fax Protocol=IGNORE_FAX_RELAY, Fallback Fax Protocol=IGNORE_FAX_RELAY
Fax Relay CM Suppression :=ENABLED, Fax Relay ANS Suppression :=DISABLED

GW-------------------------CUCM/GW
<-----------H.225 Setup-------------
------------H.225 Callproc--------->
------------H.225 Alerting---------->


Check for VTSP shows:
Fax Relay=ENABLED
Primary Fax Protocol=T38_FAX_RELAY, Fallback Fax Protocol=NONE_FAX_RELAY
Fax Relay CM Suppression :=ENABLED, Fax Relay ANS Suppression :=DISABLED
Protocol Based NSE Based Protocol Based NSE Based

GW-------CUCM/GW

----H.225 Connect---->

<----H.245 TCS-------

-----H.245 TCS-------->

-----H.245 MSD------>

-----H.245 TCSAck-->

 <---H.245 MSD-------

----H.245 MSDAck-->

<--H.245 TCSAck----

<--H.245 MSDAck----

<--H.245 OLC---------

----H.245 OLC-------->

----H.245 OLCAck--->

<--H.245 OLCAck-----
 

Note: In case of Fast Start (FS), Open Logical Channel (OLC) would have been exchanged in setup and connect/callproc.

GW-------CUCM/GW

---H.225 Connect--->

<----H.245 TCS-------

RTPAudioTelephony
EventCapability

 

----H.245 TCS-------->

RTPAudioTelephony
EventCapability

 

----H.245 MSD------->

----H.245 TCSAck--->

 <----H.245 MSD------

--H.245 MSDAck--->

<---H.245 TCSAck---

<---H.245 MSDAck--

<----H.245 OLC-------

------H.245 OLC----->

---H.245 OLCAck--->

<--H.245 OLCAck----



Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW---------CUCM/GW

------H.225 Connect--->

<-----H.245 TCS---------

application t38fax :
    {
      t38FaxProtocol udp : NULL
      t38FaxProfile
      {
        fillBitRemoval FALSE
        transcodingJBIG FALSE
        transcodingMMR FALSE
        version 0
        t38FaxRate
Management transferredTCF : NULL
        t38FaxUdp
Options
        {
          t38FaxMax
Buffer 200
          t38FaxMaxDatagram 320
          t38FaxUdpEC t38UDPRedundancy : NULL
        }



-----H.245 TCS-------->

application t38fax :
    {
      t38FaxProtocol udp : NULL
      t38FaxProfile
      {
        fillBitRemoval FALSE
        transcodingJBIG FALSE
        transcodingMMR FALSE
        version 0
        t38FaxRate
Management transferredTCF : NULL
        t38FaxUdp
Options
        {
          t38FaxMaxBuffer 200
          t38FaxMax
Datagram 72
          t38FaxUdpEC t38UDPRedundancy : NULL
        }



-----H.245 MSD------->

-----H.245 TCSAck---->

 <---H.245 MSD--------

-----H.245 MSDAck--->

<---H.245 TCSAck------

<---H.245 MSDAck-----

<-----H.245 OLC---------

-------H.245 OLC------->

-----H.245 OLCAck--->

<----H.245 OLCAck-----



Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW--------CUCM/GW

-----H.225 Connect---->

<----H.245 TCS----------

RTPAudioTelephony
EventCapability



------H.245 TCS------->

RTPAudioTelephony
EventCapability



-----H.245 MSD------->

-----H.245 TCSAck--->

 <----H.245 MSD-------

----H.245 MSDAck--->

<----H.245 TCSAck----

<---H.245 MSDAck----

<----H.245 OLC---------

-------H.245 OLC------>

----H.245 OLCAck---->

<---H.245 OLCAck-----



Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW-------------------------CUCM/GW

<========AUDIO==========>



Audio call established at this stage, but as FAX machines talk they exchange tones in the audio call.

Initial T.30 tones (Cannot be seen in debugs as these are always sent in RTP.)



G3 FAX:

<<<<<<<<<<CNG<<<<<<<<<<<

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.



>>>>>>>>>>CED>>>>>>>>>>>

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15 Hz with phase reversal every 450 ms.


SG3 FAX:
<<<<<<<<<<CNG<<<<<<<<<<<

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.



>>>>>>>>>>ANSAM>>>>>>>>>

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15 Hz with phase reversal every 450 ms..



<<<<<<<<<<CM<<<<<<<<<<<<

>>>>>>>>>>JM>>>>>>>>>>>>

<<<<<<<<<<CJ<<<<<<<<<<<<

V.34 Initialization (Phases 2-4)



The TGW waits to detect V.21 Preamble in the tones. It finds it in CED tone (G3) or ANSAM (SG3). Once it detects the V.21 Flag, it initiates switchover.



Check for VTSP shows:
Event=E_DSMP_DSP_MODEM_TONE



One of the tasks in switchover is to make the Jitter buffer transitions from adaptive to a fixed optimum value.

Fax passthrough uses the last voice mode setting before the switchover for jitter or playout buffers. Enter the show voice port X/X/X command in order to check the current values of playout delay.

GW-------------------------CUCM/GW

<========AUDIO==========>



Audio call established at this stage, but as FAX machines talk they exchange tones in the audio call.

Initial T.30 tones (Cannot be seen in debugs as these are always sent in RTP.)



G3 FAX:

<<<<<<<<<<CNG<<<<<<<<<<<

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.



>>>>>>>>>>CED>>>>>>>>>>>

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15 Hz with phase reversal every 450 ms.



SG3 FAX:
<<<<<<<<<<CNG<<<<<<<<<<<

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.



>>>>>>>>>>ANSAM>>>>>>>>>

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15 Hz with phase reversal every 450 ms.



Cisco gateways only support G3 fax calls with T.38. In order to properly handle the higher speeds of SG3 calls, modem passthrough must be used.

No V.34 Initialization (Phases 2-4) exists. The initial V.8 Phase I also does not complete. OGW squelchs the CM tone and as SG3 is backward compatible with G3 fax standard, the FAX machines failover to G3.



>>>>>>>>>CED>>>>>>>>>>>

2100 Hz tone that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path.



The TGW waits to detect V.21 Preamble in the tones. It finds it in CED tone (G3) or ANSAM (SG3). Once it detects the V.21 Flag, it initiates switchover.



Check for VTSP shows:
VTSP: Event=E_DSMP_DSP_FAX_TONE



Check for DSMP shows:
E_DSM_CC_MC_START



Check for CCAPI shows:
CCAPI:Caps(Codec=T38Fax(0x10000), Fax Rate=FAX_RATE_14400(0x80),
Fax Version:=0, Vad=OFF(0x1),



One of the tasks in switchover is to make the Jitter buffer transitions from adaptive to a fixed optimum value.

T.38 uses 300 ms fixed jitter or playout buffers. Enter the playout-delay fax 100 command under voice port in order to reduce the buffer time if the delay is high. Enter the show voice port X/X/X command in order to check the current values of playout delay.
Protocol Based NSE Based Protocol Based NSE Based

GW--------CUCM/GW

-H.245 RequestMode>

{
      sequenceNumber 1
      requestedModes
      {

        {

          {
            type audioMode : g711Ulaw64k : NULL

<--H.245 RequestModeAck---

    {
      sequenceNumber 1
      response willTransmitMost
PreferredMode : NULL
    }

-----H.245 CLC------->

-----H.245 OLC------>
{  

forwardLogicalChannel
Number 2
      forwardLogicalChannel
Parameters
      {
        dataType audio
Data &colon; g711Ulaw64k : 20
        multiplexParameters h2250LogicalChannel
Parameters :
        {
          sessionID 1
          mediaControlChannel unicastAddress : iPAddress :
          {
            network '04040413'H
            tsapIdentifier 17849
          }
          silenceSuppression FALSE
        }

<----H.245 CLC--------

---H.245 CLCAck---->
<-----H.245 OLC-------

{

forwardLogicalChannel
Number 2
      forwardLogicalChannel
Parameters
      {
        dataType audio
Data &colon; g711Ulaw64k : 20
        multiplexParameters h2250LogicalChannel
Parameters :
        {
          sessionID 1
          mediaControlChannel unicastAddress : iPAddress :
          {
            network '04040419'H
            tsapIdentifier 17205
          }
          silenceSuppression FALSE
        }

---H.245 OLCAck---->

<--H.245 CLCAck--
<--H.245 OLCAck---

 

show call active voice brief will not show change



 Note: CUCM does not support the H.245 Request Mode for Passthrough. If the TGW tries to initiate switchover by sending H.245 RequestMode for Passthrough to CUCM, CUCM replies back with RequestModeReject.

G3 FAX:

GW-------CUCM/GW

====NSE192=====>

Upspeed Codec and Switch to Passthrough Mode.

Check for VTSP shows:
E_DSM_CC_
MODIFY_MEDIA_IND

debug voip rtp session named event

Pt:100    Evt:192     Pkt:00 00 00  <Snd>>>



<===NSE192======



Check for VTSP shows:
E_DSMP_DSP_
REPORT_PEER
_TO_PEER

_MSG

debug voip rtp session named event

<<<Rcv> Pt:100    Evt:192     Pkt:00 00 00



SG3 FAX:

GW------CUCM/GW

====NSE192=====>

Upspeed Codec and Switch to Passthrough Mode.



Check for VTSP shows:
E_DSM_CC_
MODIFY_MEDIA
_IND

debug voip rtp session named event

Pt:100    Evt:192    Pkt:00 00 00  <Snd>>>



<===NSE192=====



Check for VTSP shows:

E_DSMP_DSP_
REPORT_PEER
_TO_PEER

_MSG

debug voip rtp session named event:

<<<Rcv> Pt:100    Evt:192     Pkt:00 00 00


====NSE193====>

Detect phase reversal of ANSam Disable ECAN.



Check for VTSP shows:

E_DSM_CC_
MODIFY_MEDIA_IND

debug voip rtp session named event

Pt:100    Evt:193     Pkt:00 00 00  <Snd>>>



<===NSE193=====



Check for VTSP shows:
E_DSMP_DSP_
REPORT_PEER_
TO_PEER

_MSG

debug voip rtp session named event

<<<Rcv> Pt:100    Evt:193     Pkt:00 00 00

Note: NSE-194 is triggered by a local detection of 4 seconds of silence or carrier loss detection. This message instructs the remote gateway to return to voice mode. Basically, all the changes made by NSE-192 and NSE-193 are undone.



show call active voice brief shows: MODEMPASS nse

GW---------CUCM/GW

-H.245 RequestMode->

type dataMode :
  {
    application t38fax :
     {
      t38FaxProtocol udp : NULL
      t38FaxProfile
           {
        fillBitRemoval FALSE
        transcodingJBIG FALSE
        transcodingMMR FALSE
        version 0
        t38FaxRateManagement transferredTCF : NULL
        t38FaxUdpOptions
        {
          t38FaxMaxBuffer 200
          t38FaxMaxDatagram 72
          t38FaxUdpEC t38UDPRedundancy : NULL
        }
      }
    }
    bitRate 144

<H.245 RequestModeAck-

      {
      sequenceNumber 1
      response willTransmitMost
PreferredMode : NULL
    }

-----H.245 CLC--------->

-----H.245 OLC--------->

 forwardLogicalChannel
Parameters
{
    dataType data &colon;
    {
         application t38fax :
         {
           t38FaxProtocol udp : NULL
        t38FaxProfile
        {
          fillBitRemoval FALSE
         transcodingJBIG FALSE
         transcodingMMR FALSE
         version 0
         t38FaxRateManagement transferredTCF : NULL
         t38FaxUdpOptions
         {
           t38FaxMaxBuffer 200
           t38FaxMaxDatagram 72
           t38FaxUdpEC t38UDPRedundancy : NULL
         }
       }
     }
     maxBitRate 144
    }
    multiplexParameters h2250LogicalChannel
Parameters :
    {
      sessionID 3
      mediaControlChannel unicastAddress : iPAddress :
      {
        network '04040413'H
        tsapIdentifier 17351
      }
      silenceSuppression FALSE
    }



<----H.245 CLC---------

-----H.245 CLCAck--->
<-----H.245 OLC--------

 forwardLogicalChannel
Parameters
  {
    dataType data &colon;
    {
     application t38fax :
     {
       t38FaxProtocol udp : NULL
       t38FaxProfile
       {
         fillBitRemoval FALSE
         transcodingJBIG FALSE
         transcodingMMR FALSE
         version 0
         t38FaxRateManagement transferredTCF : NULL
         t38FaxUdpOptions
         {
         t38FaxMaxBuffer 200
         t38FaxMaxDatagram 320
         t38FaxUdpEC t38UDPRedundancy : NULL
         }
       }
     }
      maxBitRate 144
   }
   multiplexParameters h2250LogicalChannel
Parameters :
   {
    sessionID 3
     mediaControlChannel unicastAddress : iPAddress :
     {
       network '04040419'H
       tsapIdentifier 18463
     }
     silenceSuppression FALSE
   }

---H.245 OLCAck----->

<---H.245 CLCAck-----
<---H.245 OLCAck-----



show call active voice brief shows: t38

G3 FAX:

GW--------CUCM/GW

====NSE200=====>

Transition from voice mode to T.38



Check for VTSP shows:E_DSM_CC_MODIFY_
MEDIA_IND

debug voip rtp session named event

Pt:100    Evt:200     Pkt:00 00 00  <Snd>>>



<===NSE201======

T.38 ACK received, instructs TGW to start T.38 session



Check for VTSP shows:E_DSMP_DSP_
REPORT_PEER_
TO_PEER

_MSG

debug voip rtp session named event

<<<Rcv> Pt:100    Evt:201     Pkt:00 00 00

SG3 FAX:

As you spoof SG3 to G3 by squelching the CM tone, there is no SG3 FAX scenario in T38 relay.



Note: NSE-202 is a NACK to an NSE-200 message which signifies that the peer gateway cannot process T.38 packets for the call. The call remains in voice mode and does not switch over to T.38.



show call active voice brief shows:

t38

In Passthrough you cannot see any T.30 messages from debugs as all tones go in the RTP like audio with G711ulaw/alaw. However, the FAX tone negotiation remains the same irrespective of relay or passthrough.



GW-------------------------CUCM/GW

>>>>>>>>>CSI>>>>>>>>>>>         (optional)(called subscriber identification)
>>>>>>>>>NSF>>>>>>>>>>>        (optional)(nonstandard facilities)
>>>>>>>>>DIS>>>>>>>>>>>         (digital identification signal)

<<<<<<<<<TSI<<<<<<<<<<<         (optional)(transmitting subscriber identification)
<<<<<<<<<DCS<<<<<<<<<<         (digital command signal)

<++++++++++TCF++++++++++       (high speed)(training check)

>>>>>>>>>>>CFR>>>>>>>>>>    (confirmation to receive)

If you see FTT here that means TCF, training failed, check clocking and slips on T1/E1. In packet captures check TCF should be all 0.

<++++Partial Page RX+++++++       (high speed)
<<<<<<<<PPS/EOM<<<<<<<<         (partial page sent)/(end of message)

>>>>>>>>>MCF>>>>>>>>>>>         (message confirmation)



<++++Partial Page RX+++++++       (high speed)
<<<<<<<<PPS/EOP<<<<<<<<         (partial page sent)/(end of procedure)

>>>>>>>>>MCF>>>>>>>>>>>         (message confirmation)



<<<<<<<<<<DCN<<<<<<<<<<         (disconnect)


Note: ECM is Optional for G3, but Mandatory for SG3. As you can achieve SG3 speeds with passthrough, make sure ECM is enabled on the fax machines for the fax to succeed.
Also, TCF training signal is Required for G3, but is Not applicable for SG3.

Note: For Passthrough, a common channel of 64kbps (g711) is allocated. So, the higher and the lower speeds of the messages becomes irrelevant..

If T38 switchover is succesful, these messages are seen in the debugs:

Check for VTSP shows:
event:E_CC_T38_START



Check for DSMP shows:
E_DSM_CC_MC_LOCAL_DNLD_DONE



Check for CCAPI shows:
Caps(Codec=T38Fax(0x10000), Fax Rate=FAX_RATE_14400(0x80),Fax Version:=0, Vad=OFF(0x1),



debug fax relay t30 all-level-1:
timestamp=1321430729 fr-msg-det NSF
timestamp=1321431129 fr-msg-det CSI
timestamp=1321431879 fr-msg-det DIS
timestamp=1321435719 fr-msg-tx TSI
timestamp=1321436329 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=1321436329 fr-msg-tx good crc, 0 bytes
timestamp=1321436439 fr-msg-tx DCS
timestamp=1321436619 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=1321441499 fr-msg-det CFR
timestamp=1321461449 fr-msg-tx PPS
timestamp=1321461639 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=1321463099 fr-msg-det MCF
timestamp=1321466789 fr-msg-tx DCN
timestamp=1321466869 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=1321466869 fr-msg-tx good crc, 0 bytes



GW-------------------------CUCM/GW

>>>>>>>>>CSI>>>>>>>>>>>         (optional)(called subscriber identification)
>>>>>>>>>NSF>>>>>>>>>>>        (optional)(nonstandard facilities)
>>>>>>>>>DIS>>>>>>>>>>>         (digital identification signal)

<<<<<<<<<TSI<<<<<<<<<<<         (optional)(transmitting subscriber identification)
<<<<<<<<<DCS<<<<<<<<<<         (digital command signal)

<++++++++++TCF++++++++++       (high speed)(training check)

>>>>>>>>>>>CFR>>>>>>>>>>     (confirmation to receive)

If you see FTT here that means TCF, training failed, check clocking and slips on T1/E1. In packet captures check TCF should be all 0.

<++++Partial Page RX+++++++       (high speed)
<<<<<<<<PPS/EOM<<<<<<<<         (partial page sent)/(end of message)

>>>>>>>>>MCF>>>>>>>>>>>         (message confirmation)



<++++Partial Page RX+++++++       (high speed)
<<<<<<<<PPS/EOP<<<<<<<<         (partial page sent)/(end of procedure)

>>>>>>>>>MCF>>>>>>>>>>>         (message confirmation)



<<<<<<<<<<DCN<<<<<<<<<<         (disconnect)
Protocol Based NSE Based Protocol Based NSE Based

DP level config:

## fax protocol pass-through g711ulaw/g711alaw

## fax rate disable

## fax nsf 000000

DP level config:

## modem passthrough nse codec g711ulaw/g711alaw

## fax rate disable

## fax nsf 000000

DP level config:

## fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback none

## fax nsf 000000        
## fax-relay ecm disable
## fax-relay sg3-to-g3 system
## fax rate 14400

DP level config:

## fax protocol t38 nse force version 0 ls-redundancy 0 hs-redundancy 0 fallback none

## fax nsf 000000        
## fax-relay ecm disable
## fax-relay sg3-to-g3 system
## fax rate 14400

OGW - Fax Call Outgoing on H.323 Leg

Note that:

  • T.38 - Delay<1000ms, Jitter<300ms, Packet loss should be NONE unless T.38 with redundancy.
  • Passthrough - Delay<1000ms, Jitter<30ms, Packet loss should be NONE.
  • Protocol Based switchover - This is standard based.
  • NSE Based switchover - This is proprietary and works only between Cisco voice gateways.
Passthrough T.38 Relay

GW-------------------------CUCM/GW
-----------H.225 Setup------------->
<------------H.225 Callproc---------
<------------H.225 Alerting----------


Check for VTSP shows:
Fax Relay=DISABLED - 'fax rate disabled' set (dial-peer) Primary Fax Protocol=IGNORE_FAX_RELAY, Fallback Fax Protocol=IGNORE_FAX_RELAY
Fax Relay CM Suppression :=ENABLED, Fax Relay ANS Suppression :=DISABLED

GW-------------------------CUCM/GW
-----------H.225 Setup------------->
<------------H.225 Callproc---------
<------------H.225 Alerting----------


Check for VTSP shows:
Fax Relay=ENABLED
Primary Fax Protocol=T38_FAX_RELAY,
Fallback Fax Protocol=NONE_FAX_RELAY
Fax Relay CM Suppression :=ENABLED, Fax Relay ANS Suppression :=DISABLED
Protocol Based NSE Based Protocol Based NSE Based

GW--------CUCM/GW

<---H.225 Connect-----

-----H.245 TCS-------->

<---H.245 TCS---------

<---H.245 MSD--------

<---H.245 TCSAck----

 ----H.245 MSD------->

<---H.245 MSDAck---

-----H.245 TCSAck--->

-----H.245 MSDAck-->

-----H.245 OLC------->

<---H.245 OLC--------

<---H.245 OLCAck---

----H.245 OLCAck--->
 

Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW-----------CUCM/GW

<------H.225 Connect----

--------H.245 TCS-------->

RTPAudioTelephony
EventCapability

 

<------H.245 TCS----------

RTPAudioTelephony
EventCapability

 

<------H.245 MSD--------

<------H.245 TCSAck-----

 -------H.245 MSD------->

<------H.245 MSDAck---

--------H.245 TCSAck--->

--------H.245 MSDAck-->

--------H.245 OLC-------->

<------H.245 OLC---------

<------H.245 OLCAck----

--------H.245 OLCAck--->

 

Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW---------CUCM/GW

<----H.225 Connect-----

------H.245 TCS-------->

application t38fax :
  {
    t38FaxProtocol udp : NULL
    t38FaxProfile
    {
      fillBitRemoval FALSE
      transcodingJBIG FALSE
      transcodingMMR FALSE
      version 0
      t38FaxRate
Management transferredTCF : NULL
      t38FaxUdpOptions
      {
        t38FaxMaxBuffer 200
        t38FaxMaxDatagram 320
        t38FaxUdpEC t38UDPRedundancy : NULL
      }



<-----H.245 TCS--------

application t38fax :
  {
    t38FaxProtocol udp : NULL
    t38FaxProfile
    {
      fillBitRemoval FALSE
      transcodingJBIG FALSE
      transcodingMMR FALSE
      version 0
      t38FaxRate
Management transferredTCF : NULL
      t38FaxUdpOptions
      {
        t38FaxMaxBuffer 200
        t38FaxMax
Datagram 72
        t38FaxUdpEC t38UDPRedundancy : NULL
      }



<-----H.245 MSD-------

<-----H.245 TCSAck---

 ------H.245 MSD------>

<-----H.245 MSDAck--

-------H.245 TCSAck--->

------H.245 MSDAck-->

------H.245 OLC------->

<----H.245 OLC---------

<----H.245 OLCAck----

-----H.245 OLCAck--->
Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW----CUCM/GW

<-H.225 Connect---

---H.245 TCS---->

RTPAudioTelephony
EventCapability

 

<---H.245 TCS-----

RTPAudioTelephony
EventCapability

 

<---H.245 MSD----

<--H.245 TCSAck-

 ---H.245 MSD--->

<-H.245 MSDAck-

---H.245 TCSAck->

-H.245 MSDAck->

---H.245 OLC----->

<--H.245 OLC------

<-H.245 OLCAck--

--H.245 OLCAck->
Note: In case of FS, OLC would have been exchanged in setup and connect/callproc.

GW-------------------------CUCM/GW

<========AUDIO==========>



Audio call established at this stage, but as FAX machines talk they exchange tones in the audio call.

Initial T.30 tones (Can not be seen in debugs as these are always sent in RTP.)



G3 FAX:

>>>>>>>>>>CNG>>>>>>>>>>>

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.



<<<<<<<<<<CED<<<<<<<<<<<

2100 Hz tone that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path.


SG3 FAX:
>>>>>>>>>>CNG>>>>>>>>>>>

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.

<<<<<<<<<<ANSAM<<<<<<<<

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15Hz with phase reversal every 450 ms.



>>>>>>>>>>CM>>>>>>>>>>>>

<<<<<<<<<<JM<<<<<<<<<<<<

>>>>>>>>>>CJ>>>>>>>>>>>>

V.34 Initialization (Phases 2-4)

The OGW waits for the TGW to detect V.21 Preamble in the tones. Once TGW detects the V.21 Flag, it initiates switchover.

One of the tasks in switchover is to make the Jitter buffer transitions from adaptive to a fixed optimum value.

Fax passthrough uses the last voice mode setting before the switchover for jitter or playout buffers. Enter the show voice port X/X/X command in order to check the current values of playout delay.

GW-------------------------CUCM/GW

<========AUDIO==========>



Audio call established at this stage, but as FAX machines talk they exchange tones in the audio call.

Initial T.30 tones (Can not be seen in debugs as these are always sent in RTP.)


G3 FAX:

>>>>>>>>>>CNG>>>>>>>>>>>

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.

<<<<<<<<<<CED<<<<<<<<<<<

2100 Hz tone that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path.

SG3 FAX:
>>>>>>>>>>CNG>>>>>>>>>>>

1100 Hz, every 3 seconds for .5 seconds. Indicates a calling nonspeech terminal.

<<<<<<<<<<ANSAM<<<<<<<<

2100 Hz tone as CED, but amplitude modulated by a sine wave at 15Hz with phase reversal every 450 ms.



>>>CM>>X



Cisco gateways only support G3 fax calls with T.38. In order to properly handle the higher speeds of SG3 calls, modem passthrough must be used.

No V.34 Initialization (Phases 2-4) exists. The initial V.8 Phase I also does not complete. OGW squelchs the CM tone and as SG3 is backward compatible with G3 fax standard, the FAX machines failover to G3.



<<<<<<<<<<CED<<<<<<<<<<<

2100 Hz tone that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path.



The OGW waits for the TGW to detect V.21 Preamble in the tones. Once TGW detects the V.21 Flag, it initiates switchover.



One of the tasks in switchover is to make the Jitter buffer transitions from adaptive to a fixed optimum value.

T.38 uses 300 ms fixed jitter or playout buffers. Enter the playout-delay fax 100 command under voice port in order to reduce the buffer time if the delay is high. Enter the show voice port X/X/X command in order to check the current values of playout delay.
Protocol Based NSE Based Protocol Based NSE Based

GW--------CUCM/GW

<-H.245 RequestMode-

{
      sequenceNumber 1
      requestedModes
      {

        {

          {
            type audioMode : g711Ulaw64k : NULL

--H.245 RequestModeAck--->

    {
      sequenceNumber 1
      response willTransmitMost
PreferredMode : NULL
    }

 

<---H.245 CLC---------

<---H.245 OLC---------

 {
 forwardLogicalChannel
Number 2
      forwardLogicalChannel
Parameters
      {
        dataType audioData
&colon; g711Ulaw64k : 20
        multiplexParameters h2250LogicalChannel
Parameters :
        {
          sessionID 1
          mediaControlChannel unicastAddress : iPAddress :
          {
            network '04040413'H
            tsapIdentifier 17849
          }
          silenceSuppression FALSE
        }

---H.245 CLC-------->

<---H.245 CLCAck----
-----H.245 OLC------->

{

forwardLogicalChannel
Number 2
      forwardLogicalChannel
Parameters
      {
        dataType audioData
&colon; g711Ulaw64k : 20
        multiplexParameters h2250LogicalChannel
Parameters :
        {
          sessionID 1
          mediaControlChannel unicastAddress : iPAddress :
          {
            network '04040419'H
            tsapIdentifier 17205
          }
          silenceSuppression FALSE
        }

<---H.245 OLCAck----

-----H.245 CLCAck--->
-----H.245 OLCAck--->

 

show call active voice brief will not show change



Note: CUCM does not support H.245 Request Mode for Passthrough. If the TGW tries to initiate switchover by sending H.245 RequestMode for Passthrough to CUCM, CUCM replies back with RequestModeReject.

 

 

 

G3 FAX:

GW-----------CUCM/GW

<====NSE192=====

Upspeed Codec and Switch to Passthrough Mode.

Check for VTSP shows:
E_DSMP_DSP_REPORT_
PEER_TO_PEER

_MSG

debug voip rtp session named event:

<<<Rcv> Pt:100    Evt:192     Pkt:00 00 00

====NSE192=====>

Check for VTSP shows:
E_DSM_CC_MODIFY_
MEDIA_IND

debug voip rtp session named event:

Pt:100    Evt:192    Pkt:00 00 00  <Snd>>>



SG3 FAX:

GW-----------CUCM/GW

<====NSE192=====

Upspeed Codec and Switch to Passthrough Mode.

Check for VTSP shows:
E_DSMP_DSP_REPORT_
PEER_TO_PEER

_MSG

debug voip rtp session named event:

<<<Rcv> Pt:100    Evt:192     Pkt:00 00 00

====NSE192=====>

Check for VTSP shows:
E_DSM_CC_MODIFY_
MEDIA_IND

debug voip rtp session named event:

Pt:100    Evt:192    Pkt:00 00 00  <Snd>>>


<====NSE193======

Disable ECAN.

Check for VTSP shows:E_DSMP_DSP_REPORT_
PEER_TO_PEER

_MSG

debug voip rtp session named event:

<<<Rcv> Pt:100    Evt:193     Pkt:00 00 00

====NSE193======>

Check for VTSP shows:
E_DSM_CC_MODIFY_
MEDIA_IND

debug voip rtp session named event:

Pt:100    Evt:193    Pkt:00 00 00  <Snd>>>



Note: NSE-194 is triggered by a local detection of 4 seconds of silence or carrier loss detection. This message instructs the remote gateway to return to voice mode. Basically, all the changes made by NSE-192 and NSE-193 are undone.

show call active voice brief shows:
MODEMPASS nse

GW----------CUCM/GW

<-H.245 RequestMode-

type dataMode :
{
   application t38fax :
  {
    t38FaxProtocol udp : NULL
    t38FaxProfile
    {
      fillBitRemoval FALSE
      transcodingJBIG
FALSE
      transcodingMMR FALSE
      version 0
      t38FaxRate
Management transferredTCF : NULL
      t38FaxUdpOptions
      {
       t38FaxMaxBuffer 200
        t38FaxMaxDatagram 72
        t38FaxUdpEC t38UDPRedundancy : NULL
      }
    }
  }
  bitRate 144

-H.245 RequestModeAck->

  {
   sequenceNumber 1
   response willTransmitMost
PreferredMode : NULL
    }

 

<---H.245 CLC----------

<---H.245 OLC----------

 forwardLogicalChannel
Parameters
  {
    dataType data &colon;
    {
      application t38fax :
      {
       t38FaxProtocol udp
: NULL
       t38FaxProfile
       {
         fillBitRemoval FALSE
         transcodingJBIG FALSE
         transcodingMMR FALSE
         version 0
         t38FaxRateManagement transferredTCF : NULL
         t38FaxUdpOptions
         {
           t38FaxMaxBuffer 200
           t38FaxMaxDatagram 72
           t38FaxUdpEC t38UDPRedundancy : NULL
          }
       }
    }
     maxBitRate 144
   }
   multiplexParameters h2250LogicalChannel
Parameters :
   {
     sessionID 3
     mediaControlChannel unicastAddress : iPAddress :
     {
       network '04040413'H
       tsapIdentifier 17351
     }
     silenceSuppression FALSE
   }

----H.245 CLC--------->

<---H.245 CLCAck-----
-----H.245 OLC-------->

 forwardLogicalChannel
Parameters
{
     dataType data &colon;
     {
     application t38fax :
    {
     t38FaxProtocol udp
: NULL
    t38FaxProfile
     {
       fillBitRemoval FALSE
       transcodingJBIG FALSE
       transcodingMMR FALSE
       version 0
      t38FaxRateManagement transferredTCF : NULL
       t38FaxUdpOptions
       {
         t38FaxMaxBuffer 200
         t38FaxMaxDatagram 320
         t38FaxUdpEC t38UDPRedundancy : NULL
         }
       }
      }
     maxBitRate 144
   }
   multiplexParameters h2250LogicalChannel
Parameters :
   {
     sessionID 3
     mediaControlChannel unicastAddress : iPAddress :
     {
       network '04040419'H
       tsapIdentifier 18463
     }
     silenceSuppression FALSE
   }

<---H.245 OLCAck----

-----H.245 CLCAck-->
-----H.245 OLCAck-->

 

show call active voice brief shows: t38

G3 FAX:

GW---CUCM/GW

<==NSE200====

Transition from voice mode to T.38

Check for VTSP shows:
E_DSMP_DSP_
REPORT_PEER_
TO_PEER

_MSG

debug voip rtp session named event:

<<<Rcv> Pt:100    Evt:200     Pkt:00 00 00

==NSE201===>

T.38 ACK received, instructs TGW to start T.38 session

Check for VTSP shows:
E_DSM_CC_
MODIFY_MEDIA_
IND

debug voip rtp session named event:
Pt:100    Evt:201    Pkt:00 00 00  <Snd>>>



SG3 FAX:

As you spoof SG3 to G3 by squelching the CM tone, there is no SG3 FAX scenario in T38 relay.



Note: NSE-202 is a NACK to an NSE-200 message that signifies that the peer gateway
cannot process T.38 packets for the call. The call remains in voice mode and does 
not switch over to T.38.

show call active voice brief shows:

t38

In Passthrough you cannot see any T.30 messages from debugs as all tones go in the RTP like audio with G711ulaw/alaw. However, the FAX tone negotiation remains the same irrespective of relay or passthrough..

GW-------------------------CUCM/GW

<<<<<<<<<CSI<<<<<<<<<<<         (optional)(called subscriber identification)
<<<<<<<<<NSF<<<<<<<<<<<        (optional)(nonstandard facilities)
<<<<<<<<<DIS<<<<<<<<<<<         (digital identification signal)

>>>>>>>>>TSI>>>>>>>>>>>>        (optional)(transmitting subscriber identification)
>>>>>>>>>DCS>>>>>>>>>>>        (digital command signal)

++++++++++TCF+++++++++>         (high speed)(training check)

<<<<<<<<<<CFR<<<<<<<<<<        (confirmation to receive)

If you see FTT here that means TCF, training failed, check clocking and slips on T1/E1. In packet captures check TCF should be all 0.

++++Partial Page RX++++++>        (high speed)
>>>>>>>>>PPS/EOM>>>>>>>        (partial page sent)/(end of message)

<<<<<<<<<MCF<<<<<<<<<<<         (message confirmation)

++++Partial Page RX++++++>       (high speed)
>>>>>>>>>PPS/EOM>>>>>>>       (partial page sent)/(end of message)

<<<<<<<<<MCF<<<<<<<<<<<        (message confirmation)

>>>>>>>>>DCN>>>>>>>>>>>        (disconnect)



Note: ECM is Optional for G3, but Mandatory for SG3. As you can achieve SG3 speeds with passthrough, make sure ECMis enabled on the FAX machines for the FAX to succeed.
Also, TCF training signal is Required for G3, but is Not applicable for SG3.



Note: For Passthrough a common channel of 64kbps (g711) is allocated. So, the higher and the lower speeds of the messages becomes irrelevant.

If T38 switchover is succesful, these messages are seen in the corresponding debugs:


Check for VTSP shows:
event:E_CC_T38_START



Check for DSMP shows:
E_DSM_CC_MC_LOCAL_DNLD_DONE



Check for CCAPI shows:
Caps(Codec=T38Fax(0x10000), Fax Rate=FAX_RATE_14400(0x80),Fax Version:=0, Vad=OFF(0x1),

debug fax relay t30 all-level-1:
timestamp=352583286 fr-msg-tx NSF
timestamp=352583686 fr-msg-tx CSI
timestamp=352583736 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=352583736 fr-msg-tx good crc, 0 bytes
timestamp=352584426 fr-msg-tx DIS
timestamp=352584456 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=352584456 fr-msg-tx good crc, 0 bytes
timestamp=352584906 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=352587656 fr-msg-det TSI
timestamp=352588376 fr-msg-det DCS
timestamp=352594056 fr-msg-tx CFR
timestamp=352594156 FR_GOOD_CRC_LS_DATA 0x0 bytes

timestamp=352613376 fr-msg-det PPS
timestamp=352615656 fr-msg-tx MCF
timestamp=352615776 FR_GOOD_CRC_LS_DATA 0x0 bytes
timestamp=352618716 fr-msg-det DCN

GW-------------------------CUCM/GW

<<<<<<<<<CSI<<<<<<<<<<<         (optional)(called subscriber identification)
<<<<<<<<<NSF<<<<<<<<<<<        (optional)(nonstandard facilities)
<<<<<<<<<DIS<<<<<<<<<<<         (digital identification signal)

>>>>>>>>>TSI>>>>>>>>>>>>        (optional)(transmitting subscriber identification)
>>>>>>>>>DCS>>>>>>>>>>>        (digital command signal)

++++++++++TCF+++++++++>         (high speed)(training check)

<<<<<<<<<<CFR<<<<<<<<<<        (confirmation to receive)

If you see FTT here that means TCF, training failed, check clocking and slips on T1/E1. In packet captures check TCF should be all 0.

++++Partial Page RX++++++>        (high speed)
>>>>>>>>>PPS/EOM>>>>>>>        (partial page sent)/(end of message)

<<<<<<<<<MCF<<<<<<<<<<<         (message confirmation)

++++Partial Page RX++++++>       (high speed)
>>>>>>>>>PPS/EOM>>>>>>>       (partial page sent)/(end of message)

<<<<<<<<<MCF<<<<<<<<<<<        (message confirmation)

>>>>>>>>>DCN>>>>>>>>>>>        (disconnect)
Protocol Based NSE Based Protocol Based NSE Based

DP level config:

## fax protocol pass-through g711ulaw/g711alaw

## fax rate disable

## fax nsf 000000

DP level config:

## modem passthrough nse codec g711ulaw/g711alaw

## fax rate disable

## fax nsf 000000

DP level config:

## fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback none

## fax nsf 000000        
## fax-relay ecm disable
## fax-relay sg3-to-g3 system
## fax rate 14400

DP level config:

## fax protocol t38 nse force version 0 ls-redundancy 0 hs-redundancy 0 fallback none

## fax nsf 000000        
## fax-relay ecm disable
## fax-relay sg3-to-g3 system
## fax rate 14400

Debugs to Collect

  • debug vpm all (in case of FXS)
  • debug isdn q931 (in case of PRI)
  • debug voice ccapi inout
  • debug h225 asn1
  • debug h245 asn1
  • debug cch323 all
  • debug voip vtsp all
  • debug voip dsmp all
  • debug voip hpi all
    debug dsp-resource flex all
  • debug voip dspapi
  • debug fax relay t30 all-level-1
  • debug voip rtp session named-event (in case of NSE based switchover)

Related Information

Revision History

Revision Publish Date Comments
1.0
05-Feb-2015
Initial Release
TAC Authored

Contributed by Cisco Engineers

  • Karan Moudgil
    Cisco TAC Engineer.

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