I have an application sending DTMF Tones out of a VIC3-2 E/M card on a Cisco 2901 Router. The tail end of each tone is coming out distorted and is unable to be decoded at times. The tones are being sent in band in the g.711 voice stream. We are unable to use RTP NTE or any other type of DTMF relay for this specific application.
I took a network capture of the in band DTMF tones going to the router and the tones seem to be consistent and clear when playing back through wireshark. They just sound distorted even when listening with a butt set off of the 4-wire interface.
I have attached a network capture sending 911. Sent from 192.168.13.101 to 192.168.13.8. There are two streams, but they are going to different VIC Ports. It sounds the same out of both ports.
***Update 2/4/2015 After analysis, it turns out that the tail end "distortion" is actually echo. The VIC Card is sending out the tone perfectly and then echoing the last 45 ms of audio outbound on the 4-wire interface. We tried enabling echo cancel, but that doesn't help. The echo does not exist in the capture. Any thoughts on how to get rid of the echo?
***Update 2/9/2015 Maybe this tidbit will stir up some ideas since I have not received any feedback yet. I suspect the culprit to be a Cisco feature called PLC (Packet loss concealment). The gateway is taking the last couple or three audio packets and replaying them out of the VIC card and lower levels. The audio tapers to silence. The intent of this feature is to make voice sound less choppy, but it obviously causes problem when generating in-band voice DTMF. The only IOS commands that I can find relating to this feature involve a fax setting. I do not have any fax configs in my setup so that does not apply to me. Does anyone know of a way to disable PLC?
Not too much to my config, but here is the relevant part:
voice-port 0/1/0
operation 4-wire
signal lmr
no echo-cancel enable
voice-detection
no comfort-noise
dial-peer voice 1 pots
corlist outgoing port1
service hookflash
destination-pattern 2111111
port 0/1/0
I appreciate any suggestions.
