Cisco Cisco Customer Voice Portal 8.0(1) Design Guide

On average, a VoiceXML document between the Unified CVP Call Server or Unified CVP 
VXML Server and the gateway is about 7 kilobytes. You can calculate the bandwidth used by 
approximating the number of prompts that will be used per call, per minute. The calculation, for this 
example, is as follows:

7,000 bytes 

∗ 8 bits = 56,000 bits per prompt

(.166 call/second) * (56,000 bit/prompt) * (# of prompts / call) = bps per branch

However, if you are going to use a more complex application that uses many menu prompts (more than 
the average estimated above) or if you want to calculate the bandwidth more exactly, you can use the 
VoiceXML document sizes listed in 

 to calculate the amount of bandwidth needed. The 

document sizes in 

 are measured from the Unified CVP VXML Server to the VXML Gateway.

 

Media files (prompts) can be stored locally in flash memory on each router. This method eliminates 
bandwidth considerations, but maintainability becomes an issue because a prompt that requires changes 
must then be replaced on every router. If the prompts are instead stored on an HTTP media server (or an 
HTTP cache engine), the gateway can locally cache voice prompts once it has initially retrieved the 
prompts. If configured correctly, the HTTP media server can cache many, if not all, prompts, depending 
on the number and size of the prompts. The refresh period for the prompts is defined on the HTTP media 
server. Therefore, the bandwidth utilized would be limited to the initial load of the prompts at each 
gateway, plus periodic updates after the expiration of the refresh interval.

Not caching prompts at the gateway causes significant Cisco IOS performance degradation (as much as 
35% to 40%) in addition to the extra bandwidth usage. For the most current information on configuring 
gateway prompt caching, refer to the latest version of the Configuration and Administration Guide for 
Cisco Unified Customer Voice Portal (CVP), available at 

Assume that there is a total of 50 prompts, with an average size of 50 kB and a refresh interval of 
15 minutes. The bandwidth usage would then be:

(50 prompts) 

∗ (50,000 bytes/prompt) ∗ (8 bits/byte) = 20,000,000 bits

(20,000,000 bits) / (900 secs) = 22.2 average kbps per branch

Root document (one required at beginning of call) 19,000 bytes

Subdialog_start (at least one per call at beginning 
of call)

700 bytes

Query gateway for Call-ID and GUID (one 
required per call)

1,300 bytes

Menu (increases in size with number of menu 
choices)

1,000 bytes + 2,000 bytes per menu choice

Play announcement (simple .wav file)

1,100 bytes

Cleanup (one required at end of call)

4,000 bytes