This document describes one of the most effective approaches to troubleshoot fax, which includes these steps:
You can split a fax call into four parts:
This call flow includes the messages to look for when Skinny Call Control Protocol (SCCP) is the protocol identified. There are corresponding sections based on whether your endpoint is a TGW or OGW.
Note that:
Passthrough
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T.38 Relay
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Protocol Based
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NSE Based
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Protocol Based
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NSE Based
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Protocol Based switchover is not supported with SCCP. |
GW-----------------CUCM/GW <--------SelectSoftKeys--------- <-------CallStateMessage------ CallStateMsg Info: RINGIN-------OffHookMessage-----> sccp_send_offhook_v1 <---OpenReceiveChannel---- -OpenReceiveChannelAck-> <--StartMediaTransmission-- Check for VTSP shows: |
Protocol Based switchover is not supported with SCCP. |
GW-----------------CUCM/GW <--------SelectSoftKeys--------- <-------CallStateMessage------ CallStateMsg Info: RINGIN-------OffHookMessage-----> sccp_send_offhook_v1 <---OpenReceiveChannel---- -OpenReceiveChannelAck-> <-StartMediaTransmission- 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 Fax Parameters Set By= SCCP Call Type |
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 that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path. SG3 Fax: 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 that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path. SG3 Fax: 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) exist, 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: 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 to reduce the buffer time if the delay is high. Enter the show voice port X/X/X in order to check the current values of playout delay. |
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Protocol Based
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NSE Based
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Protocol Based
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NSE Based
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Protocol Based switchover is not supported with SCCP. | G3 Fax: GW--------------------CUCM/GW =======NSE192========> Upspeed Codec and Switch to Passthrough Mode. Check for VTSP shows: E_DSM_CC_MODIFY_ debug voip rtp session named event: Pt:100 Evt:192 Pkt:00 00 00 <Snd>>> <======NSE192========= Check for VTSP shows: E_DSMP_DSP_REPORT_ _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
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 |
Protocol Based switchover is not supported with SCCP. | G3 Fax: GW--------------------CUCM/GW =======NSE200========> Transition from voice mode to T.38 Check for VTSP shows: E_DSM_CC_MODIFY_ 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_ _MSG debug voip rtp session named event: <<<Rcv> Pt:100 Evt:201 Pkt:00 00 00 SG3 Fax: As you squelch the CM tone in order to spoof SG3 to G3, 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 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) 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. >>>>>>>>>MCF>>>>>>>>>>> (message confirmation) <++++Partial Page RX+++++++ (high speed) >>>>>>>>>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 64 kbps (g711) is allocated. So, the higher and the lower speeds of the messages become irrelevant. |
If T38 switchover is successful, these messages are seen in the corresponding debugs: Check for VTSP shows: event:E_CC_T38_START Check for DSMP shows: debug fax relay t30 all-level-1: GW-------------------------CUCM/GW >>>>>>>>>CSI>>>>>>>>>>> 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. >>>>>>>>>MCF>>>>>>>>>>> (message confirmation) <++++Partial Page RX+++++++ (high speed) >>>>>>>>>MCF>>>>>>>>>>> (message confirmation) <<<<<<<<<<DCN<<<<<<<<<< (disconnect) |
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Protocol Based
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NSE Based
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Protocol Based
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NSE Based
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N/A | voice service voip level config: ## fax protocol none ## modem passthrough nse codec g711ulaw |
N/A | voice service voip level config: ## fax protocol t38 nse force version 0 ls-redundancy 0 hs-redundancy 0 fallback none ## fax-relay sg3-to-g3 |
Note that:
Passthrough
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T.38 Relay
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Protocol Based
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NSE Based
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Protocol Based
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NSE Based
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Protocol Based switchover is not supported with SCCP. |
GW----------------------CUCM/GW -------OffHookMessage-------> --KeypadButtonMessage----> <-------CallStateMessage------ CallStateMsg Info: PROCEED<----OpenReceiveChannel----- --OpenReceiveChannelAck--> <--StartMediaTransmission-- <-------CallStateMessage------ CallStateMsg Info: RINGOUT Check for VTSP shows: <-------CallStateMessage------ CallStateMsg Info: CONNECTED |
Protocol Based switchover is not supported with SCCP. |
GW----------------------CUCM/GW -------OffHookMessage-------> --KeypadButtonMessage----> <-------CallStateMessage------ CallStateMsg Info: PROCEED<----OpenReceiveChannel----- --OpenReceiveChannelAck--> <--StartMediaTransmission-- <-------CallStateMessage------ CallStateMsg Info: RINGOUT Check for VTSP shows: <-------CallStateMessage------ CallStateMsg Info: CONNECTED |
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 that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path. SG3 Fax: 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 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 (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 that lasts between 2.6 - 4.0 seconds. Disables echo suppressors in the transmission path. SG3 Fax: 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>>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) exist, the initial V.8 Phase I also does not complete. OGW squelches 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 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. |
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Protocol Based
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NSE Based
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Protocol Based
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NSE Based
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Protocol Based switchover is not supported with SCCP. |
G3 Fax: GW--------------------CUCM/GW <=======NSE192======== Upspeed Codec and Switch to Passthrough Mode. Check for VTSP shows: E_DSMP_DSP_REPORT_ _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_INDdebug 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_ _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_INDdebug 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_ _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_INDdebug 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 |
Protocol Based switchover is not supported with SCCP. |
G3 Fax: GW--------------------CUCM/GW <=======NSE200======== Transition from voice mode to T.38 Check for VTSP shows: E_DSMP_DSP_REPORT_ _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_INDdebug voip rtp session named event: SG3 Fax: As you squelch the CM tone in order to spoof SG3 to G3, 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 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) 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. <<<<<<<<<MCF<<<<<<<<<<< (message confirmation) ++++Partial Page RX++++++> (high speed) <<<<<<<<<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. Note: For Passthrough a common channel of 64 kbps (g711) is allocated. So, the higher and the lower speeds of the messages become irrelevant. |
If T38 switchover is successful, these messages are seen in the corresponding debugs: Check for VTSP shows: Check for DSMP shows: debug fax relay t30 all-level-1: timestamp=352613376 fr-msg-det PPS GW-------------------------CUCM/GW <<<<<<<<<CSI<<<<<<<<<<< (optional)(called subscriber identification) 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. <<<<<<<<<MCF<<<<<<<<<<< (message confirmation) ++++Partial Page RX++++++> (high speed) <<<<<<<<<MCF<<<<<<<<<<< (message confirmation) >>>>>>>>>DCN>>>>>>>>>>> (disconnect) |
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Protocol Based | NSE Based | Protocol Based | NSE Based |
N/A | voice service voip level config: ## fax protocol none ## modem passthrough nse codec g711ulaw |
N/A | voice service voip level config: ## fax protocol t38 nse force version 0 ls-redundancy 0 hs-redundancy 0 fallback none ## fax-relay sg3-to-g3 |
Revision | Publish Date | Comments |
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1.0 |
15-Jan-2015 |
Initial Release |