Determining Bandwidth For Voip - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

Setting Up Quality of Service
Configuring LLQ
Standard Bit Rate
G.711 • 56 Kbps
• 64 Kbps
G.722 • 48 Kbps
• 56 Kbps
• 64 Kbps
G.723.1 • 5.3 Kbps
• 6.3 Kbps
7-32

Determining Bandwidth for VoIP

One of the most common applications for a low-latency queue is VoIP traffic.
You calculate the bandwidth necessary for VoIP traffic by:
1. calculating the bandwidth necessary for one call
2. making adjustments to this calculation according to the capabilities of
your VoIP devices
3. multiplying the per-call bandwidth by the number of calls the router needs
to support at once
Calculating Per-Call Bandwidth. VoIP standards specify the minimum bit
rate necessary for acceptable voice quality. Various standards specify various
rates. (See Table 7-8.)
However, this rate does not correspond exactly to the bandwidth necessary
to maintain voice quality. The rate is that for voice bits, but, because frame
and packet headers add overhead, a connection must provide greater band-
width. VoIP packets are quite small; headers might add as many, or even more,
bytes than the payload of actual voice data.
To calculate the per-call bandwidth for your VoIP application, you must
transform its specified bit rate into a rate of packets per second. This rate
depends on the size of the voice payload, which in turn depends on the codec
your system's application uses.
Table 7-8. VoIP Standards
Codec (Sample Time)
20 ms
20 ms
30 ms
Sample Size
• 140 bytes
• 160 bytes
• 120 bytes
• 140 bytes
• 160 bytes
• 20 bytes
• 24 bytes
Packets Per Second
50
50
33.3