Modem Call PCM Trace Analysis

Analyzing PCM traces of modem calls is more difficult than fax calls. This difficulty has to do with the more complex negotiations and full duplex nature of most modem calls. However, some basic analysis can still take place to help you troubleshoot modem problems.

After a modem PCM trace has been loaded into an audio program such as Audacity, you will see the modem negotiation at the beginning of the call. Depending on the modulation and supported negotiation procedures, this negotiation can be quick or somewhat long. Figure 12-27 shows a V.34 negotiation procedure where the four phases of a V.34 negotiation are noted.

Figure 12-27 PCM Trace of a V.34 Negotiation and Its Phases

Figure 12-27 PCM Trace of a V.34 Negotiation and Its Phases

Experienced modem experts can listen to the trace in Figure 12-27 and hear whether there are problems in certain phases. They also will view the phases in the frequency spectrum for an even more detailed analysis. However, for our simplified purposes, knowing that the PCM trace in Figure 12-27 exemplifies a good V.34 negotiation allows for basic comparisons with other V.34 negotiations. Any V.34 PCM trace with a successful negotiation should bear a strong resemblance to Figure 12-27. For more information about the V.34 phases, see the section "Modem Call Analysis" in Chapter 1.

When problems occur in the V.34 negotiations, the phase where the problem is occurring will be longer than usual, taking up more time as messages and other signals are repeated. Sometimes, the best way to identify these problems is by listening to the trace, where repetitive signals are easier to detect.

In many cases, the modem call negotiates correctly, but problems occur later in the communication. One of the most troublesome issues for a modem connection is retrains. Retrains stop all data flow and interrupt the modem connection for as long as it takes for a complete new negotiation to take place. Figure 12-28 shows a V.34 retrain occurring. For more information about retrains, see the section "Retrains and Speedshifts" in Chapter 1.

Figure 12-28 PCM Trace of a V.34 Retrain

Figure 12-28 PCM Trace of a V.34 Retrain

In Figure 12-28, you see a PCM trace of a retrain occurring during the data communication portion of a V.34 call. Notice that the call is already in data mode when the retrain occurs and that the call returns to data mode after the retrain is finished. While the retrain is occurring, all data communication between the modems is discontinued.

Unlike Figure 12-27, which contains Phase 1 of the V.34 negotiation, the retrain in Figure 12-28 does not have the first phase. All retrains begin at Phase 2, and only in the initial V.34 modem negotiation will you see the Phase 1 negotiation.

PCM traces for modems are truly necessary for only the most complex cases, but their usefulness in identifying problems is undoubtedly without equal. See the section "Advanced Troubleshooting for Modem Relay" earlier in this chapter for additional tips to resolve any problems found in a PCM trace. As emphasized previously, confirming proper line conditions and levels for the connection between the modem and the Cisco voice gateway should be one of the first troubleshooting steps.

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