HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 345

Standard Bit Rate
G.728 16 Kbps
G.729 8 Kbps
N o t e
Codec (Sample Time)
2.5 ms
10 ms
The codec dictates how often the voice stream is sampled, and together with
the bit rate, the size of each voice sample. For example, G.722 has a 20 ms
codec and 64 Kbps rate. Therefore, each sample is 160 bytes. (64,000 bits/
second x .02 seconds = 1280 bits. 1280 bits/8 = 160 bytes.) This sample size is
relatively large. Others, however, are very small. For example, a G.729 sample
size might be only 10 bytes.
The voice payload for VoIP packets is divisible by the sample size. Some VoIP
applications combine several samples into a single packet to reduce overhead.
For example, G.728 packets often include four samples.
Calculate the packets transmitted per second by dividing the bit rate by the
number of bits (not bytes) in the total voice payload. The router must forward
this many frames across the WAN connection every second to maintain an
acceptable QoS.
To determine how much bandwidth the router needs to forward the required
number of frames per second, first calculate the total size of a frame. Add the
number of bits in the frame and packet headers to the number of bits in the
voice payload. Then multiply the total size of the frames in bits by the rate of
packets per second. This is the minimum bandwidth required per call over the
WAN connection.
Table 7-9 shows example calculations for common sample sizes with several
VoIP standards.
Setting Up Quality of Service
Sample Size
• 5 bytes
• often more than one
sample per packet—
for example, 4 samples
per packet for 20 bytes
• 10 bytes
• often more than one
sample per packet—
for example, 2 samples
per packet for 20 bytes
Configuring LLQ
Packets Per Second
100
50
7-33