FAX-MGCP Troubleshoot Guide
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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 Media Gateway Control Protocol (MGCP) is the protocol identified. There are corresponding sections based on whether your endpoint is a TGW or OGW.
TGW - Fax Call Incoming on MGCP 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 | ||
|---|---|---|---|
| Protocol Based | NSE Based | Protocol Based | NSE Based |
Fax Passthrough Protocol Based is not supported with MGCP. |
GW---------------CUCM/GW <------------CRCX------------- X: 1f L: p:20, a:pCMU, s:off, t:b8M: recvonly R: D/[0-9ABCD*#] Q: process,loop ------------200 OK-----------> I: 3 m=audio 18138 RTP/AVP 0 100 <-----------MDCX------------ I: 3 v=0 Check for VTSP shows: ------------200 OK------------> |
GW--------CUCM/GW <--------CRCX---------- X: 1f L: a:image/t38, s:off, t:b8, fxr/fx:t38 ---------200 OK--------> v=0 <--------MDCX-------- I: 3 Check for VTSP shows: ---------200 OK--------> |
GW------------CUCM/GW <----------CRCX------------ X: 1f L: p:20, a:PCMU, s:off, t:b8 ------------200 OK----------> I: 4 a=X-cpar: a=rtpmap: <----------MDCX----------- I: 4 Check for VTSP shows: -----------200 OK---------->
|
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: 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) 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: 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 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 |
Fax Passthrough Protocol Based is not supported with MGCP. |
G3 FAX: GW---------------CUCM/GW ======NSE192======> Upspeed Codec and Switch to Passthrough Mode. Check for VTSP shows: debug voip rtp session named event: Pt:100 Evt:192 Pkt:00 00 00 <Snd>>> <====NSE192======= Check for VTSP shows: _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: debug voip rtp session named event: Pt:100 Evt:192 Pkt:00 00 00 <Snd>>> <=====NSE192======== Check for VTSP shows: _MSG debug voip rtp session named event:
Detect phase reversal of ANSam Disable ECAN. Check for VTSP shows: debug voip rtp session named event: Pt:100 Evt:193 Pkt:00 00 00 <Snd>>> <=====NSE193======== Check for VTSP shows: _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 ----------NTFY---------> N: ca@209.165.201.3:2427 O: FXR/t38(start) <--------200OK--------- <--------MDCX--------- I: 3 ----------200OK--------> v=0 show call active voice brief shows: t38
|
G3 FAX: GW-----------CUCM/GW =====NSE200======> Transition from voice mode to T.38 Check for VTSP shows: 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: _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. how call active voice brief shows: |
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 ir-relevant. |
If T38 switchover is succesful, these messages are seen in the debugs: Check for VTSP shows: Check for DSMP 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 |
| N/A | "conf t" level config: ## no mgcp default-package fxr-package ## no mgcp package-capability fxr-package ## mgcp modem passthrough voip mode nse ## mgcp modem passthrough voip codec g711ulaw ## mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no ccm-manager fax protocol ## no mgcp fax t38 ecm |
"conf t" level config: ## no ccm-manager fax protocol Cisco ## mgcp fax t38 ls_redundancy 0 ## mgcp default-package fxr-package ## mgcp package-capability fxr-package ## no mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no mgcp fax t38 ecm ## mgcp fax-relay sg3-to-g3 ## mgcp fax rate 14400 |
"conf t" level config: ## no ccm-manager fax protocol Cisco ## mgcp fax t38 gateway force ## mgcp fax t38 ls_redundancy 0 ## no mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no mgcp fax t38 ecm ## mgcp fax-relay sg3-to-g3 ## mgcp timer nse-response t38 200 ## mgcp fax rate 14400 |
OGW - FAX Call Outgoing on MGCP 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 | ||
|---|---|---|---|
| Protocol Based | NSE Based | Protocol Based | NSE Based |
Fax Passthrough Protocol Based is not supported with MGCP. |
GW--------------CUCM/GW <-----------CRCX------------- X: 1 L: p:20, a:PCMU, s:off, t:00M: recvonly R: D/[0-9ABCD*#] Q: process,loop -----------200 OK------------> I: 2 m=audio 18138 RTP/AVP 0 100 <----------MDCX------------- I: 2 v=0
Check for VTSP shows: -----------200 OK-----------> |
GW--------------CUCM/GW <-----------CRCX------------- X: 1 ------------200 OK-----------> I: 2 v=0 <----------MDCX------------ I: 2 L: p:20, a:PCMU, s:off, t:b8, fxr/fx:t38 Check for VTSP shows: -------------200 OK-----------> |
GW-------CUCM/GW <-------CRCX----------- X: 1 --------200 OK--------> I: 3v=0 c=IN IP4 209.165.201.1 m=audio 16724 RTP/AVP 0 100 a=rtpmap:100 X-NSE/8000 a=fmtp:100 200-202 a=X-sqn:0 a=X-cap: 1 audio RTP/AVP 100 a=X-cpar: a=rtpmap:100 X-NSE/8000 a=X-cpar: a=fmtp:100 200-202 a=X-cap: 2 image udptl t38 <-------MDCX--------- I: 3 Check for VTSP shows:
--------200 OK---------> |
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: 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: 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 |
Fax Passthrough Protocol Based is not supported with MGCP. |
G3 FAX: GW--------------CUCM/GW <=====NSE192====== Upspeed Codec and Switch to Passthrough Mode. Check for VTSP shows: _MSG debug voip rtp session named event: <<<Rcv> Pt:100 Evt:192 Pkt:00 00 00 =====NSE192=======> Check for VTSP shows: 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: _MSG debug voip rtp session named event: <<<Rcv> Pt:100 Evt:192 Pkt:00 00 00 ======NSE192======> Check for VTSP shows: debug voip rtp session named event: Pt:100 Evt:192 Pkt:00 00 00 <Snd>>>
Disable ECAN. Check for VTSP shows: _MSG debug voip rtp session named event: <<<Rcv> Pt:100 Evt:193 Pkt:00 00 00 ======NSE193=======> Check for VTSP shows: 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 <------------MDCX----------- I: 2 ------------200OK------------> v=0 show call active voice brief shows: t38
|
G3 FAX: GW--------CUCM/GW <===NSE200===== Transition from voice mode to T.38 Check for VTSP shows: _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: debug voip rtp session named event: 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 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 succesful, 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) |
||
| Protocol Based | NSE Based | Protocol Based | NSE Based |
| N/A | conf t" level config: ## no mgcp default-package fxr-package ## no mgcp package-capability fxr-package ## mgcp modem passthrough voip mode nse ## mgcp modem passthrough voip codec g711ulaw ## mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no ccm-manager fax protocol |
conf t" level config: ## no ccm-manager fax protocol Cisco ## mgcp fax t38 ls_redundancy 0 ## mgcp default-package fxr-package ## mgcp package-capability fxr-package ## no mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no mgcp fax t38 ecm ## mgcp fax-relay sg3-to-g3 ## mgcp fax rate 14400 |
conf t" level config: ## no ccm-manager fax protocol Cisco ## mgcp fax t38 gateway force ## mgcp fax t38 ls_redundancy 0 ## no mgcp fax t38 inhibit ## mgcp fax t38 nsf 000000 ## no mgcp fax t38 ecm ## mgcp fax-relay sg3-to-g3 ## mgcp timer nse-response t38 200 ## mgcp fax rate 14400 |
Debugs to Collect
- debug vpm all (in case of FXS)
- debug isdn q931 (in case of PRI)
- debug mgcp packet
- 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 |
30-Jan-2015 |
Initial Release |
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