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Walkthrough Wednesdays

DSP confusion

Hi guys,

I have a 2651XM router installed with an ATM-AIM module and a VWIC-1MFT-E1 card. This was decomissioned a while ago but it used to be in the production network and provided PSTN connectivity via an E1 circuit. The question is, that I expected to see the 30 DSPs in the output of "show diags" but it only shows 4. The IPT network uses G.711 to within the site, G.729 over the WAN and uses the E1 for PSTN.


1. Where in the CCM does is it told to use G.711 for talking to the PSTN - i.e. no transcoding required.

2. Why does the "show diags" command show only 4 DSPs? - Logfile attached.

Is there a better command to see how many DSPs there are in the router?



Question 3:

Are DSPs required to talk to the PSTN? Or is it OK just to have the E1 card?


Yes, DSP are required. You have ATM-VOICE-30, see document:

This mode is made for voice over ATM and is quite dated already. I suggest that you either switch to a current platform, that can be small as a 1861, or if you want to reuse the 2600, get an NM-HD-xxx that has no ATM dependencies and uses current compoent as DSP and E1 interface.

Hope this helps, please rate post if it does!

Thanks for that.

The doc says "Three AIMs are described in this data sheet: an ATM AIM called AIM-ATM, a Voice AIM called AIM-VOICE-30".

4 .In the show diag output, it shows ATM AIM. I would have expected it to show ATM-VOICE-30??

5. Can you explain from the logfile, how many simultaneous calls can be made to the PSTN? I'm confused why it shows 4 DSPs as I know that there were more than 10 simultaneous calls made when this was live.

6.And also, why are DSPs are required to communicate with the PSTN? Please explain or provide a URL.

Excuse my ignorance :-)

1 DSP = 4 channels hence 4 DSP's = 16 channels.


when using older IOS, in some cases "show diag" doesn't print the exact part number, but just a somewhat descriptive string as known to the programmer. In your case there can not be confusion because only three variations of the AIM were made: ATM, voice, and ATM + voice.

Each DSP can support multiple calls, so is not that you need to have as many dsp as calls. There is a tool called "DSP calculator" that will exactly tell you how many calls you can support, however it requires registered customer-level access to CCO.

The DSP are required because they do voice packetization for the various codecs that you could choose, tone generation and detection, and many other things.

As a recognition to those providing answers, please rate useful posts!

Thanks guys.

I'll summarise:

1. The module that I have is an AIM-VOICE-30 even though it says ATM-AIM in "show diag".

2. I can make 16 simultaneous calls to the PSTN because each DSP provides 4 channels.

Is this totally correct?

If you could provide a URL that gives detailed info regarding the role of DSPs when communicating with the PSTN, I would appreciate it.

1 yes.

2 according to documentation, up to 30 calls of G711 are supported.

3 I gave you the short answer. If you search for "Ip telephony SRND" or other architecture documents, that would give you the long, long answer.

Thanks for the rating and good luck!

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