Information Technology Reference
In-Depth Information
The full definition of each of these parameters was discussed in Chapter 6, “Understand-
ing the CME Dial-Plan.” This is half of the puzzle: We know how fast and consistent our
voice traffic must travel across the network. The other half of the puzzle is how much
voice traffic must travel across the network. You can find this out based on two factors:
the voice CODEC you are using for the calls and how many concurrent calls you plan to
support. Table 15-3 shows the average bandwidth usage for the two most popular codecs
used in a Cisco VoIP network.
Table 15-3
Average Bandwidth Utilization for G.711 and G.729A
Key
To p i c
Codec
Packetization Interval
Bandwidth Per Call
G.711
20 ms
80 kbps
G.711
30 ms
74 kbps
G.729A
20 ms
24 kbps
G.729A
30 ms
19 kbps
Keep in mind that these are simply average values. After you develop the ninja skills
needed for CCNP Voice, you can calculate these values down to the bit-level for your spe-
cific environment. The methods you need to do this are covered in the CVOICE Autho-
rized Self-Study Guide, which you can find at
www.ciscopress.com/bookstore/
Tip:
The packetization interval represents how much audio is included per packet. The
larger your packetization interval, the more audio data you put into each packet. The more
audio data you put in each packet, the less packets you send (thus, the slight bandwidth
savings by choosing 30 ms packetization intervals). The default packetization interval on
Cisco routers is 20 ms.
Now, we can put the two puzzle pieces together. We know the requirements for high-qual-
ity audio and we know how much bandwidth each call consumes. Now, we need to provi-
sion QoS to ensure our switches and routers are able to guarantee priority queuing for our
voice call bandwidth.
After you configure QoS, keep a proactive eye on the network to ensure that it meets
quality standards. Although there are many sophisticated (and expensive!) tools available
to help measure and monitor the voice traffic crossing the network, you actually get a ba-
sic monitoring utility each time you purchase a Cisco IP Phone. Whenever a phone is on
an active call, you can press the question-mark button (help menu) twice to retrieve call
statistics, as shown in Figure 15-3.
The call statistics include:
Codec
■
Packet size
■
Received and transmitted packets
■
Average and maximum jitter
■
Lost packets
■
