Whenever you encounter a Fax over IP (FoIP) problem, the following troubleshooting model should always be employed to quickly hone in on the problem cause. This troubleshooting model is very basic and breaks down a FoIP call into three distinct stages as shown in Figure 1. Once you narrow the problem down to one of these three stages then the troubleshooting tools and techniques for that stage can be employed to efficiently resolve the issue.
FIGURE 1:FoIP Troubleshooting Stages
The three troubleshooting stages are voice media, switchover, and fax media. You always start with Voice Media and move your way down. If Voice Media is not working then it is pointless to implement troubleshooting techniques and debugs for the Switchover stage. Each of these stages is covered below in their own subsection.
Stage 1: Voice Media
All fax calls start off as a plain, regular VoIP calls. If a VoIP call cannot be setup between the fax end points then fax will never work. Therefore, the first stage of troubleshooting any fax problem is to make sure that VoIP works. It is surprising how many fax problems are not really fax problems at all but VoIP issues. If fax devices are replaced with analog handsets, then you should be able to successfully make VoIP calls. If you are sure that VoIP is working then you should move on to the next troubleshooting stage, Switchover.
If VoIP calls are not working between fax endpoints, then you need to ignore fax and troubleshoot the problem as a VoIP call setup problem. There are numerous VoIP troubleshooting documents and references available (especially compared to FoIP) so the tools and tips for troubleshooting this stage of a FoIP problem will not be covered comprehensively here. Instead, just some common problems and issues to look at are listed.
Check the call routing and make sure that the call is actually reaching the destination endpoint. This may involve reviewing that dial peers are configured correctly on the voice gateways and/or that Cisco Unified Communications Manager (CUCM) is also routing calls correctly. The voice media can be setup using call control protocols such as H.323, SIP, MGCP, and SCCP.
Use the appropriate debug commands to look at the call control messages and confirm that the call is setting up and that the voice media stream is being negotiated successfully. For example,
debug h225 asn1anddebug h245 asn1forH.323
debug ccsip messagesfor SIP
debug mgcp packetsfor MGCP
Stage 2: Switchover
At some point during the voice call, a transition or switchover must occur where the voice media stream is changed over to a fax media stream. This switchover can only occur after the voice media stage has been successfully completed. If a switchover does not occur then fax calls will almost always fail as fax requires unique transport mechanisms that are not typically employed for voice calls. Therefore, ensuring that a switchover occurs and that it is successful is a critical stage when troubleshooting a FoIP problem.
Fax switchovers fall into one of three categories, NSE-based, protocol-based, or RTP Payload Type (PT). NSEs or Named Signaling Events are Cisco proprietary messages sent within the voice media stream for signaling certain events like fax and modem switchovers. For NSE-based and RTP PT switchovers, the actual switchover messages are sent within the voice media stream. Protocol-based switchovers use the call control protocol to initiate the switchover and this is more of a standards-based switchover method. In the case of fax passthrough, you have NSE-based passthrough and protocol-based fax pass-through as shown in Figure 2.
FIGURE 2:Fax Passthrough Switchovers
For NSE-based passthrough, the switchover messages can be viewed using the command debug voip rtp session named-event and for protocol-based pass-through you must use the appropriate debug command for the configured call control protocol. So, for fax pass-through with H.323, you would use the command debug h245 asn1 and look for an H.245 Request Mode message. For SIP you would use the command debug ccsip messages and look for a re-INVITE message.
In the case of fax relay, two relay protocols exist along with three switchover mechanisms. These fax relay protocols and their switchovers are highlighted in Figure 3 and include protocol-based T.38 fax relay, NSE-based T.38 fax relay, and Cisco fax relay with its unique RTP PT switchover.
FIGURE 3:Fax Relay Switchovers
debug h245 asn1 for H.323 and look for the H.245 Request Mode for T.38 fax relay