Attacking the Problem

Fax, modem, and text problems in VoIP networks can range from simple issues to complex issues that may require a substantial amount of troubleshooting. However, with the appropriate troubleshooting methodology, you can attain more expedient resolutions.

People do not always troubleshoot problems in the same exact way. Different strategies, points of view, and levels of experience are just some of the factors that determine a person's troubleshooting methodology.

When it comes to troubleshooting fax, modem, and text problems, years of troubleshooting experience by a few Cisco TAC (Technical Assistance Center) engineers has led to the development of a specific troubleshooting methodology. This methodology provides a systematic and efficient troubleshooting approach that can assist you in achieving rapid fax, modem, and text problem resolutions.

Based on a "divide and conquer" notion of being able to narrow the problem down to a well-defined part of the fax, modem, or text call, this methodology offers a simple means to quickly hone in on the root cause of the problem. Figure 12-1 illustrates the recommended methodology for efficiently resolving fax, modem, and text issues.

Figure 12-1 shows two fax machines connected over an IP network via two voice gateways, but modems or text telephony devices could replace the fax machines in this illustration, too. The main concept demonstrated here is the systematic breakdown to the troubleshooting of fax, modem, and text problems.

As you look at Figure 12-1 from top to bottom, you will see a step-by-step troubleshooting methodology numbered one through five. While you are tackling fax, modem, and text problems, this model intuitively leads you to a resolution in a logical, orderly manner. After a troubleshooting step or section has been completed, its components can be eliminated, narrowing the scope of the problem.

When using the methodology illustrated in Figure 12-1, the first place to start is with fundamental troubleshooting. Fundamental troubleshooting encompasses a number of quick checks and tests that if performed early can curtail a lot of frustration later on. Some of these checks are global in scope and include testing voice calls in place of fax calls, testing calls over the public switched telephone network (PSTN), and reviewing the configuration of the gateways. Foundational items such as these are discussed in detail in the next section, "Fundamental Troubleshooting."

Figure 12-1 Recommended Troubleshooting Methodology for Fax, Modem, and Text Problems

1. Fundamental Troubleshooting

2. Telephony Troubleshooting

3. IP Troubleshooting

Unified CM

3. IP Troubleshooting

Unified CM

4. Troubleshooting the Switchover Signaling

5. Passthrough and Relay Troubleshooting

The next area to examine after fundamental troubleshooting is telephony troubleshooting. The telephony portion of a problem involves the analog or digital POTS (plain old telephone service) connection between the voice gateway and the fax/modem/text device. Issues or impairments on the telephony side can be a source of call degradation and failures. Troubleshooting the telephony portion of a fax/modem/text call can be found in the "Telephony and IP Troubleshooting" section of this chapter.

IP troubleshooting is the next step in the fax, modem, and text troubleshooting methodology identified in Figure 12-1. The IP portion of a call lies between the voice gateways, and this is an environment not inherently designed for modulated communications. Therefore, effective problem resolution within the IP network from a fax/modem/text perspective is important. More information about IP troubleshooting is covered in the section "Telephony and IP Troubleshooting."

If you have not identified the cause of the problem after examining the components in the first three troubleshooting areas, the next area to analyze is the switchover signaling. The switchover signaling is responsible for the transition from voice mode to the appropriate passthrough or relay mode. Confirming that a successful switchover has occurred on the gateway itself is critical to resolving fax and modem issues. In many cases, troubleshooting the switchover signaling is further complicated by the presence of other devices, such as Cisco Unified Communications Manager (Unified CM), in the signaling path. A number of techniques are available for validating a correct switchover on voice gateways and Unified CM and these techniques are discussed in the section "Troubleshooting the Switchover Signaling."

The last step in the troubleshooting methodology diagrammed in Figure 12-1 is the troubleshooting of the passthrough and relay protocols themselves and their underlying data. Quite a few debugs are included in this section, as well as some advanced troubleshooting techniques that are usually needed for only the most complex problems. These advanced techniques may require third-party tools to assist in the proper capture and analysis of the problem. The section "Passthrough and Relay Troubleshooting" covers these techniques along with the appropriate gateway debugs.

The five troubleshooting steps that have just been discussed divide a fax, modem, or text problem into distinct segments. This allows the segment where the problem is occurring to eventually be isolated.

With the continued application of the progressive troubleshooting methodology outlined in Figure 12-1, you will find yourself honing in on fax, modem, and text problems much quicker. Familiarity and practice with this methodology develops a troubleshooting intuition that eventually enables you to manipulate this methodology in a manner that is appropriate for your experience level.

For example, a fax problem may be presented in a manner where you are comfortable that the call connects fine but you feel that there is a problem in the transition to T.38. At this point, skipping to the section "Troubleshooting the Switchover Signaling" may make sense. Just be aware that jumping around too much defeats the purpose of this methodology, which is built upon a systematic elimination of problem areas.

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