Cisco PCM Capture Tool

The Cisco PCM capture tool requires you to enable a feature known as the HPI logger. The HPI logger will then stream the DSP trace information to a file. The steps for enabling the HPI logger on Cisco IOS gateways are as follows:

Step 1 Configure the HPI capture buffer size using the IOS configuration command voice hpi capture buffer size, where size refers to the capture buffer size. This capture buffer is used internally for the DSP messages, and a typical value is 1000000.

Step 2 Configure the HPI capture destination with the configuration command voice hpi capture destination location. This command specifies the file location of the HPI logger file. If you want to store the file on the router's internal flash, you can use the configuration command voice hpi capture destination flash:pcm.dat, where pcm.dat is the arbitrary name selected in this example for the HPI logger file itself. For storing the file on a TFTP server, the configuration command voice hpi capture destination tftp://ip_addressfpcm.dat can be used, where ip_address is the IP address of the TFTP server. If a UNIX-based server is being used with the TFTP option, remember to touch the file on the server and do a chmod to give write permission. An FTP server is also a valid location by using the command voice hpi capture destination ftp:// username:[email protected]_address/pcm.dat. The username and password refer to the login credentials for the FTP server itself. If you are obtaining PCM captures for only one or just a few short calls, often the flash: location is the most convenient provided that there is enough space. For capturing many calls or for calls of a long duration, the TFTP and FTP options are preferred because they are less likely to run into file-size restrictions.

Step 3 Enable the PCM capture tool for a specific voice port using the command test voice port port pcm-dump caplog bit_index duration time. The port parameter defines the specific port or timeslot in the case of a digital interface for the PCM capture. The bit_index defines the bit values for the three locations in the DSP where the capture file is taken: Rin, Sin, and Sout. The Rin location is the incoming PCM information from the IP side before it is processed and played out the voice port by the DSP. Sin is the inbound PCM coming into the voice port before it is processed by the DSP. And, Sout is this same PCM stream after is has been processed by the DSP. The best approach is to grab all of these captures by setting bit_ index to a value of 7. The last parameter is the duration, with a time value in seconds. Setting time to a value of 0 keeps the PCM capture tool running indefinitely. To disable the PCM capture tool, use the command test voice port port pcm-dump disable.

TIP The timeslot of a digital interface that are you wanting to enable a Cisco PCM capture for must be formatted using a particular syntax. The trick for providing the timeslot information in the proper format is to use the same designation for the port as seen with the command show voice call summary.

As mentioned in Table 12-15, the Cisco PCM capture tool does not officially support fax or modem relay calls. The most common problem with using the Cisco PCM capture tool with fax and modem relay calls is that not all the PCM traces from the various DSP locations (such as Rin, Sin, and Sout) are fully captured. Even though your mileage may vary when using the Cisco PCM capture tool for fax and modem relay calls, the tool works well for passthrough calls.

Although the Cisco PCM capture tool can extract PCM captures from single or multiple calls, realize that you need to know which voice port or timeslot the call will occur on ahead of time. Otherwise, by the time you see the call on a specific port or timeslot and enable the Cisco PCM capture tool, you will have missed the beginning of the call. Therefore, when dealing with digital interfaces and multiple timeslots, you might need to busy out timeslots on the digital interface to force the call to a particular timeslot for a successful PCM capture.

You can track the function of the PCM capture tool while it runs with the command show voice hpi capture. This command will let you know the status of the HPI capture buffer and the status of the file transfer to the specified capture destination.

After your capture file has been obtained at your capture destination, this file can be decoded only by a Cisco TAC engineer because of its proprietary nature and the special program needed to decode it.

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