Fax-SIP Troubleshoot Guide
Available Languages
Contents
Introduction
This document describes one of the most effective approaches to troubleshoot fax, which includes these steps:
- Split the call into two legs.
- Identify the protocol (SIP/H.323/SCCP/MGCP) on each leg.
- 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:
- Set up the voice call
- Off-hook, Dial, Ring, Answer
- Calling (CNG) and Called Equipment Identification (CED) Tones
- Switchover
- Codec upspeed/correction
- Voice Activation Detection (VAD) disabled on DSP
- Jitter buffer transitions from adaptive to a fixed optimum value
- Pre-message procedures
- Fax Terminal Identification
- Capabilities exchange and setting
- Training
- 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 Session Initiation Protocol (SIP) is the protocol identified. There are corresponding sections based on whether your endpoint is a TGW or OGW.
TGW - Fax Call Incoming on SIP 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 Check for VTSP shows: Fax Relay=DISABLED - 'fax rate disabled' |
GW-------------------------CUCM/GW Check for VTSP shows: Fax Relay=ENABLED |
||
|
Protocol Based
|
NSE Based
|
Protocol Based
|
NSE Based
|
GW------CUCM/GW ---200OK+SDP-----> v=0 <----ACK+SDP----- v=0 Note: In case of EO, a similar SDP would have been received with INVITE. |
GW---------CUCM/GW ------200OK+SDP------> v=0 <------ACK+SDP-------- v=0 Note: In case of EO, a similar SDP would have been received with INVITE. |
GW------------CUCM/GW -------200OK+SDP--------> v=0o=CiscoSystemsSIP-GW-UserAgent 0 6060 IN IP4 209.165.201.2 s=SIP Call c=IN IP4 209.165.201.2 t=0 0 m=audio 17924 RTP/AVP 0 c=IN IP4 209.165.201.2 a=rtpmap:0 PCMU/8000 a=ptime:20 <-------ACK+SDP--------- v=0 Note: In case of EO, a similar SDP would have been received with INVITE. |
GW------CUCM/GW -----200OK+SDP----> v=0 <-----ACK+SDP------ v=0 Note: In case of EO, a similar SDP would have been received with INVITE. |
GW-------------------------CUCM/GW <========AUDIO==========> Audio call established at this stage, but as the fax machines talk they start to 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 the fax machines talk they start to exchange tones in the audio call. Initial T.30 tones (Cannot be seen in debugs as these are always sent in RTP.)
<<<<<<<<<<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) 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: Check for CCAPI 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 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 ---INVITE+SDP----> v=0 a=rtpmap:0 PCMU/8000
<--200OK+SDP----- v=0
--------ACK----------> show call active voice brief will not show change
|
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 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
Detect phase reversal of ANSam Disable ECAN. Check for VTSP shows: E_DSM_CC_MODIFY_ debug voip rtp session named event: Pt:100 Evt:193 Pkt:00 00 00 <Snd>>> <====NSE193====== Check for VTSP shows: E_DSMP_DSP_REPORT _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 -------INVITE+SDP------> v=0
<-----200OK+SDP--------- v=0 -----------ACK-------------> 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_ debug voip rtp session named event: Pt:100 Evt:200 Pkt:00 00 00 <Snd>>>
T.38 ACK received, instructs TGW to start T.38 session. Check for VTSP shows: E_DSMP_DSP_ _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 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 the 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 64kbps (g711) is allocated. So, the higher and the lower speeds of the messages becomes irrelevant. |
If T38 switchover is successful, these messages are seen in the corresponding debugs: Check for VTSP shows: Check for DSMP shows: Check for CCAPI shows: debug fax relay t30 all-level-1: 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) |
||
|
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 |
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 SIP 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 Check for VTSP shows: |
GW-------------------------CUCM/GW Check for VTSP shows: |
||
|
Protocol Based
|
NSE Based
|
Protocol Based
|
NSE Based
|
GW------CUCM/GW <----200OK+SDP---- v=0 -----ACK+SDP-----> v=0
Note: In case of EO, a similar SDP would have been sent in INVITE. |
GW----------CUCM/GW <-----200OK+SDP------ v=0 -------ACK+SDP-------> v=0 |
GW----------CUCM/GW <------200OK+SDP------ v=0o=CiscoSystems SIP-GW-UserAgent 0 6060 IN IP4 209.165.201.2 s=SIP Call c=IN IP4 209.165.201.2 t=0 0 m=audio 17924 RTP/AVP 0 c=IN IP4 209.165.201.2 a=rtpmap:0 PCMU/8000 a=ptime:20 --------ACK+SDP------> v=0 Note: In case of EO, a similar SDP would have been sent in INVITE. |
GW--------CUCM/GW <-----200OK+SDP----- v=0
v=0 Note: In case of EO, a similar SDP would have been sent in INVITE. |
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 15Hz with phase reversal every 450 ms. >>>>>>>>>>CM>>>>>>>>>>>> <<<<<<<<<<JM<<<<<<<<<<<< >>>>>>>>>>CJ>>>>>>>>>>>> V.34 Initialization (Phases 2-4) The OGW waits for the terminating gateway 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 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) 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 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. |
||
|
Protocol Based
|
NSE Based
|
Protocol Based
|
NSE Based
|
GW------CUCM/GW <---INVITE+SDP---- v=0 a=rtpmap:0 PCMU/8000 ----100TRYING----> ----200OK+SDP----> v=0 <------ACK--------- show call active voice brief will not show change
|
G3 FAX: GW---------CUCM/GW <====NSE192==== Upspeed Codec and Switch to Passthrough Mode.
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_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_ _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>>>
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_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 brie" shows MODEMPASS nse |
GW---------CUCM/GW <-----INVITE+SDP------ v=0 --------100TRYING-----> -------200OK+SDP------> v=0 <--------ACK------------ show call active voice brief will show: t38
Note: Whenever CUCM is involved, for the RE_INVITE in these topologies: |
G3 FAX: GW---------CUCM/GW <====NSE200===== Transition from voice mode to T.38 Check for VTSP shows: E_DSMP_DSP_ _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:
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 signifes that the peer gateway cannot process T.38 packets for the call. The call will remain in voice mode and 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) 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 64kbps (g711) is allocated. So, the higher and the lower speeds of the messages becomes 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: Check for CCAPI 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) |
||
|
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 |
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 ccsip all/messages/verbos
- 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)
Revision History
| Revision | Publish Date | Comments |
|---|---|---|
1.0 |
14-Jan-2015 |
Initial Release |
FeedbackContact Cisco
- Open a Support Case
- (Requires a Cisco Service Contract)