CVP "wan-delay-ringback" Configuration in the Survivability.tcl Script to Mitigate Network Latency Effects

Introduction

This document describes the problem and solution when the network latency between a Customer Voice Portal (CVP) server and Voice XML (VXML) gateway approaches or exceeds 200ms round trip.

Problem

The network latency between a CVP server and VXML gateway is expected to be much less than 200ms round trip. If the round trip time approaches or exceeds this, CVP Interactive Voice Response (IVR) performance is severely impacted as follows:

• Call drops.
• No VXML app audio played.
• The microapp works fine, but VXML apps (such as the helloworld app) experience no audio or audio delays over 5 seconds.

Solution

The network latency section in the CVP 8 Solution Reference Network Design (SRND) describes a potential workaround to reduce the effect of delayed audio from a VXML application due to network latency between the VXML server and the VXML gateway. This workaround is to configure the survivability Tool Command Language (tcl) script and activate the "wan-delay-ringback" feature.  A configuration example is shown in this survivability.tcl script snippet:

The survivability.tcl script indicates "wan_delay_ringback" is a global variable similar to the "ccb" variable.  Therefore, you need to use the parameter 'keyword' configuration in order to provide a value for this service parameter.

application

 service cvp-survivability flash:survivability.tcl

  paramspace english language en

  paramspace english index 0

  param ccb id:192.168.249.10

  paramspace english location flash

  param wan-delay-ringback 1

  paramspace english prefix en

 !

The survivability.tcl script also suggests that the "ringback.wav" file needs to be stored in the ingress gateway flash where the survivability application is located. This allows the script to play a ringback.wav file when the parameter has been set to 1.

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